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Formation of an Isomeric Mixture of Dienynes Instead of a Diallene

Formation of an Isomeric Mixture of Dienynes Instead of a Diallene molbank Communication Formation of an Isomeric Mixture of Dienynes Communication Formation Instead of a Diallene of an Isomeric Mixture of Dienynes Instead of a Diallene Susanne M. Petrova and Leiv K. Sydnes * Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway; Susanne M. Petrova and Leiv K. Sydnes * susannapetrova@gmail.com Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway; susannapetrova@gmail.com * Correspondence: leiv.sydnes@uib.no; Tel.: +47-55-583-450 * Correspondence: leiv.sydnes@uib.no; Tel.: +47-55-583-450 Received: 17 April 2020; Accepted: 7 May 2020; Published: 11 May 2020 Received: 17 April 2020; Accepted: 7 May 2020; Published: 11 May 2020 Abstract: Attempts to convert 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne to a symmetric allene by Abstract: Attempts to convert 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne to a symmetric allene by reduction with lithium aluminum hydride failed. Instead reduction accompanied by isomerization reduction with lithium aluminum hydride failed. Instead reduction accompanied by isomerization occurred and afforded 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne as a mixture of three isomers in occurred and a orded 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne as a mixture of three isomers in 63% 63% total isolated yield. total isolated yield. Keywords: conjugated diyne; LAH reduction; diacetal; pent-1,2,3,4-tetraene intermediate Keywords: conjugated diyne; LAH reduction; diacetal; pent-1,2,3,4-tetraene intermediate 1. Introduction 1. Introduction Since the synthesis of 3,3,4,4-tetraethoxybutyne (TEB) was reported some 15 years ago [1,2,3], Since the synthesis of 3,3,4,4-tetraethoxybutyne (TEB) was reported some 15 years ago [1–3], many many of its chemical properties have been uncovered and used to prepare a range of chemical of its chemical properties have been uncovered and used to prepare a range of chemical compounds compounds with rich structural diversity [4,5,6,7,8,9,10,11,12,13,14,15]. Among the most densely with rich structural diversity [4–15]. Among the most densely functionalized molecules made is functionalized molecules made is 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne (1), which has one ketal 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne (1), which has one ketal moiety in propargylic position to each moiety in propargylic position to each of the triple bonds [16]. The compound therefore belongs to a of the triple bonds [16]. The compound therefore belongs to a group of compounds that can undergo group of compounds that can undergo SN2′ reactions by nucleophilic attack of the triple bond, which S 2 reactions by nucleophilic attack of the triple bond, which is accompanied by C–C bond migration is accompanied by C–C bond migration that leads to the release of a leaving group from the that leads to the release of a leaving group from the propargylic carbon and formation of an allene propargylic carbon and formation of an allene moiety. The most common leaving groups are moiety. The most common leaving groups are alkanoates [17–19], bromide [20,21], and chloride [22,23], alkanoates [17,18,19], bromide [20,21], and chloride [22,23], but examples involving alkoxides have but examples involving alkoxides have also been published [24–27]. As for the nucleophiles, both also been published [24,25,26,27]. As for the nucleophiles, both carbanions and hydride have been carbanions and hydride have been applied [17–27]. applied [17,18,19,20,21,22,23,24,25,26,27]. On this basis, we envisaged that 1 might be used as a substrate to make a functionalized diallene by On this basis, we envisaged that 1 might be used as a substrate to make a functionalized diallene two S 2 reactions, one at each of the propargylic moieties, using hydride as a nucleophile (Scheme 1). by two SN2′ reactions, one at each of the propargylic moieties, using hydride as a nucleophile (Scheme Lithium aluminum hydride (LAH) was deemed to be a suitable reagent [23] but, as reported here, 1). Lithium aluminum hydride (LAH) was deemed to be a suitable reagent [23] but, as reported here, when the reaction was performed, the expected product was not obtained; instead, an isomeric mixture when the reaction was performed, the expected product was not obtained; instead, an isomeric of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) was the only product formed. mixture of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) was the only product formed. CH CH OCH CH 2 3 2 3 CH CH O 3 2 CH(OCH CH ) 2 3 2 O CH CH O 3 2 1 8 • (CH CH O) HC 3 2 2 CH CH O OCH CH 3 2 3 4 5 6 2 3 1) LAH 2 7 2) H O CH CH O OCH CH 2 CH CH O 3 2 2 3 CH(OCH CH ) 3 2 2 3 2 OCH CH O 2 3 CH CH 2 3 OCH CH (CH CH O) HC 3 2 2 2 3 Scheme 1. Expected and obtained products from lithium aluminum hydride (LAH) reduction of 1. Scheme 1. Expected and obtained products from lithium aluminum hydride (LAH) reduction of 1. 2. Results and Discussion 2. Results and Discussion The reaction was carried out under anhydrous conditions in refluxing diethyl ether using six The reaction was carried out under anhydrous conditions in refluxing diethyl ether using six equivalents of hydride with respect to diallene formation. The reaction was monitored by TLC and equivalents of hydride with respect to diallene formation. The reaction was monitored by TLC and when quenched and worked up after 1 h, three products were detected and subsequently isolated by Molbank 2020, 2020, x; doi: www.mdpi.com/journal/molbank Molbank 2020, 2020, M1133; doi:10.3390/M1133 www.mdpi.com/journal/molbank Molbank 2020, 2020, M1133 2 of 6 Molbank 2020, 2020, x 2 of 6 when quenched and worked up after 1 h, three products were detected and subsequently isolated by flash chromatography. All the products had the same molecular weight as the expected product, but flash chromatography. All the products had the same molecular weight as the expected product, but their IR spectra did not show any absorption in the allene region (1955–1925 cm ) [28], which rules out −1 their IR spectra did not show any absorption in the allene region (1955–1925 cm ) [28], which rules 1 1 that diallene formation had occurred. However, absorptions in the 1680–1620 cm and 840–790 cm −1 out that diallene formation had occurred. However, absorptions in the 1680–1620 cm and 840–790 regions indicate the presence of trisubstituted alkenes [28], and this requires the presence of a C–C triple −1 cm regions indicate the presence of trisubstituted alkenes [28], and this requires the presence of a bond to be compatible with the determined molecular weight. Considering the symmetry of the starting C–C triple bond to be compatible with the determined molecular weight. Considering the symmetry material, the triple bond would be expected to be symmetrically substituted, and this would explain the of the starting material, the triple bond would be expected to be symmetrically substituted, and this absence of an absorption in the 2270–2120 cm region [28]. In order to determine whether this was a −1 would explain the absence of an absorption in the 2270–2120 cm region [28]. In order to determine reasonable assumption, the Raman spectrum of 2 was recorded. To our satisfaction, a strong absorption whether this was a reasonable assumption, the Raman spectrum of 2 was recorded. To our appeared at 2186 cm . This observation made the structure elucidation fairly straightforward when −1 satisfaction, a strong absorption appeared at 2186 cm . This observation made the structure 1 13 H- and C-NMR data were considered, and the three compounds, isolated in 26%, 34% and 3% yield, 1 13 elucidation fairly straightforward when H- and C-NMR data were considered, and the three were proved to be the (Z,Z), (E,Z) and (E,E) isomers of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne, 2a, compounds, isolated in 26%, 34% and 3% yield, were proved to be the (Z,Z), (E,Z) and (E,E) isomers 2b and 2c, respectively. of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne, 2a, 2b and 2c, respectively. The stereoisomers could be di erentiated by a detailed study of the 3.4–5.5 ppm region of their The stereoisomers could be differentiated by a detailed study of the 3.4–5.5 ppm region of their proton NMR spectra, shown in Figure 1. The (Z,Z) and (E,E) isomers both exhibit C2 symmetry, and proton NMR spectra, shown in Figure 1. The (Z,Z) and (E,E) isomers both exhibit C2 symmetry, and the methine protons at C-1 and C-8 consequently have the same chemical shift, as do the olefinic the methine protons at C-1 and C-8 consequently have the same chemical shift, as do the olefinic protons at C-3 and C-6, and the methylene groups in the ethoxy groups attached to C-2 and C-7. These protons at C-3 and C-6, and the methylene groups in the ethoxy groups attached to C-2 and C-7. isomers will therefore show the same number of signals in the 3.4–5.5 ppm region. The asymmetry of These isomers will therefore show the same number of signals in the 3.4–5.5 ppm region. The the corresponding (E,Z) isomer results in twice the number of signals in this region, giving rise to the asymmetry of the corresponding (E,Z) isomer results in twice the number of signals in this region, middle spectrum in Figure 1, which interestingly is almost identical to that obtained when the top and giving rise to the middle spectrum in Figure 1, which interestingly is almost identical to that obtained the bottom spectra are combined. when the top and the bottom spectra are combined. 1 1 Figure Figure 1. 1. The The 5 5.5–3.4 .5–3.4 ppm region of the ppm region of the H-N H-NMR MR spectra of spectra t of he three isome the three isomers rs of 2 of . The si 2. The gnal signals s in the in the 4.6– 4.6–3.4 3.4 ppm re ppm gion are due to region are due the methylene to the methylene moieties in moieties the in ethoxy groups the ethoxy groups attached to attached C-1 to , C-2, C-7, C-1, C-2, C-7, and C-8. To and C-8. denote the two To denote the two hydrogen ato hydrogen atoms ms in th in the e methylene group i methylene groupn in EtO attache EtO attached d to C-n, the to C-n, the following following notat notation ion is is used: used: 2 2H-(OEt-n). H-(OEt-n). The assignments of the proton spectra of 2a, 2b and 2c shown in Figure 1 were arrived at by using The assignments of the proton spectra of 2a, 2b and 2c shown in Figure 1 were arrived at by information harvested from the DEPT-90, DEPT-135, HSQC, and HMBC spectra of each of the isomers. using information harvested from the DEPT-90, DEPT-135, HSQC, and HMBC spectra of each of the The DEPT and HSQC spectra confirmed the presence of the methyl, methylene, and methine protons, isomers. The DEPT and HSQC spectra confirmed the presence of the methyl, methylene, and methine the olefinic CH groups, and the quaternary carbon atoms, whereas the HMBC spectrum showed the protons, the olefinic CH groups, and the quaternary carbon atoms, whereas the HMBC spectrum correlation between hydrogen and carbon atoms two and three bonds apart. The HMBC spectra can showed the correlation between hydrogen and carbon atoms two and three bonds apart. The HMBC ther spectra ca efore be n theref used ore be used to to assign the a H-1, ssign the H- H-8 and 1, H-3, H-8 a H-6 nd H- singl 3, H-6 singl ets, which ets, which is clearly is cl illustrated early illustra by ted the spectrum for 2c (Figure S13). In this spectrum, the proton singlet at 5.47 ppm correlates through three by the spectrum for 2c (Figure S13). In this spectrum, the proton singlet at 5.47 ppm correlates through bonds three bonds with the with the met methylene hylene carbon carbon atoms at atoms at 63.1 ppm 63.1 ppm due t due to theoethoxy the ethoxy groups groups at at C-1 C-1 and and C-8 C-8 and and through two bonds with the quaternary carbon atoms at 161.9 ppm (C-2 and C-7); thus, this Molbank 2020, 2020, M1133 3 of 6 Molbank 2020, 2020, x 3 of 6 through two bonds with the quaternary carbon atoms at 161.9 ppm (C-2 and C-7); thus, this proton singlet proton sing is duele to t is d H-1u and e to H-8. H-1 This and H- assignment 8. This asis sign further ment is supported further supported by by correlationscorrelations between between the proton singlet at 4.95 ppm and the olefinic quaternary carbons at 161.9 ppm (C-2 and C-7) and the acetal the proton singlet at 4.95 ppm and the olefinic quaternary carbons at 161.9 ppm (C-2 and C-7) and carbon the acet atoms al carat bon at 98.4om ppm s at (C-1 98.4and ppm C-8). (C-1 and C-8). In order to di erentiate between the (Z,Z) and (E,E) isomers, the chemical shifts of the methine In order to differentiate between the (Z,Z) and (E,E) isomers, the chemical shifts of the methine hydr hydrogen ogen at at C-1 C-1 (and C (and C-8) -8) and and the the meth methylene ylenepr proton otonss i innthe the ethoxy group ethoxy group attached attached to C-2 (a to C-2 (and nd C-7 C-7)) in in the two iso the two isomers mers were co were compar mpared. D ed. Due ue to the anisot to the anisotr ropy of opy of the tripl the triple e bond a bond ttached to C- attached to C-3 3 (and C (and - C-6), 6), protons protons cis cis to the to theacety acetyleni lenicc moiety moiety will be will be de deshielded, shielded, whe wher re eas as those those in in tr trans ansposition position will will be be shielded shielded [[29 29]. ]. Thus, si Thus, since nce the methyl the methylene ene qu quartet artet inin the the bottom spectr bottom spectr um a um ppea appears rs at aat lower a lowe fiel rd tha fieldn than in the top spectrum, 4 in the top spectrum, .44 ppm compa 4.44 ppm compar red to 3ed .84to ppm, a 3.84 ppm, nd the methi and the ne si methine nglet in the bottom singlet in the spectrum bottom spectr appear um s at appears a higher at f aihigher eld than field in tthan he toin p sp theect top rum spectr , 4.72 um, ppm 4.72 coppm mpared compar to 5.ed 47 p top5.47 m, th ppm, e botthe tom bottom spectrum belongs to spectrum belongs (Z,Zto )-2( (Z,Z 2a) a )-2 n( d the top spectrum 2a) and the top spectr to ( um E,Eto )-2( (E,E 2c). )- 2 (2c). The The complete absence complete absence in in the reaction m the reaction mixtur ixture of e of the the exp expected ected diallene diallene and and any any of th of thee by-products by-products this this cum cumulene ulene conceiv conceivably ably could could have have given under given under the rea the rc eaction tion and and work-up work-up condi conditions tions [30,31 [30 ,3– 2,33] 33] indicates indicates tha thatt 1 1 is is not not at attacked tacked init initially ially atat C-C-4 4 (and (and C-5) C-5). . A like A ly likely explanat explanation ion for thfor is isthi ths atis ththat e 1,1,the 2,2- 1,1,2,2-tetraethoxyethyl tetraethoxyethyl groups gr aoups ttached to the ends of attached to the the ends bu of tathe -1,3buta-1,3-diyne -diyne moiety ren moiety der itr s t ender ermini its re termini latively relatively electroposit electr ive and ma opositiveke i andnmake itial at initial tack of C-3 attack of (and C-3C (and -6) m C-6) ore f mor avo erfaavorable. ble. It is a Itlsis o c also oncconceivable eivable that that coordin coora dination tion of one or several oxygen of one or several oxygen atoms to specie atoms to species s derived derived from LAH from at so LAH me stage at some will stage facilitate will facilitate the same sort the same of atta sort ck of (see attack Scheme 2) (see Scheme . This l2 ea ).ds This to form leads ation to formation of a pent-1 of,2,3,4-tetraene a pent-1,2,3,4-tetraene derivative derivative (3), which i (3),s unst whichab isle unstable in the presence in the presence of a hyd of a hydride ride sourc sour e ce and and reacts reactsquick quickly ly to g to give ive d dienyne ienyne 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne ( (22 ) ) (Scheme (Scheme 2). Thus, th 2). Thus, thee two hydrog two hydrogens ens replacin replacingg the the two two ethoxy ethoxy group groupssin in the conv the conversion ersion o off1 1to to 22ar are a e attached ttached by hydri by hydride de a attack ttack inin a a S SN 22′fashion, fashion,not not b byy reaction reaction wi with th wa water ter during during the the quenching quenching of the re of the reacti action. In on. In accor accor dance dance w with ith th this, is, quenchin quenchingg usin usingg D D2 O O did did not not lead leato d t deuterium o deuterium incorporation incorporatin ion 2.in Traces 2. Trof acany es of a intermediate ny intermedia werete not were not det detected, neither ected, spectr neither spectroscopically n oscopically nor by TLC. or by TLC. Scheme 2. Possible mechanism for formation of dienyne 2 from diyne 1 by two subsequent S 2 Scheme 2. Possible mechanism for formation of dienyne 2 from diyne 1 by two subsequent SN2′ reactions with hydride as a nucleophile. reactions with hydride as a nucleophile. 3. Materials and Methods 3. Materials and Methods 3.1. General 3.1. General The chemicals were obtained from commercial suppliers and used without further purification. The chemicals were obtained from commercial suppliers and used without further purification. Thin-layer chromatography (TLC) was performed using pre-coated aluminum TLC plates (Alugram, Thin-layer chromatography (TLC) was performed using pre-coated aluminum TLC plates (Alugram, 0.20 mm Silica Gel 60 F ) and eluting with a 95:5 mixture of hexanes and ethyl acetate. The R values 254 f 0.20 mm Silica Gel 60 F254) and eluting with a 95:5 mixture of hexanes and ethyl acetate. The Rf values were determined after the liquid front had migrated 4–5 cm. Visualization of the chromatograms was were determined after the liquid front had migrated 4–5 cm. Visualization of the chromatograms was done with phosphomolybdic acid (NH ) MoO 4H O) in ethanol followed by heating. Flash-column 4 4 4 2 done with phosphomolybdic acid (NH4)4MoO4·4H2O) in ethanol followed by heating. Flash-column chromatography (FC) was performed manually using Silica Gel from Fluka Analytical (230–400 mesh) chromatography (FC) was performed manually using Silica Gel from Fluka Analytical (230–400 and eluting with a 80:20 mixture of hexanes and ethyl acetate. NMR spectra were recorded on a mesh) and eluting with a 80:20 mixture of hexanes and ethyl acetate. NMR spectra were recorded on 1 13 Bruker Biospin AV500 instrument (500 MHz for H, 125 MHz for C) in CDCl as solvent, using the 1 13 a Bruker Biospin AV500 instrument (500 MHz for H, 125 MHz for C) in CDCl3 as solvent, using the 1 13 solvent peaks as references in both H- and C-NMR spectra (7.26 and 77.16 ppm, respectively). The 1 13 solvent peaks as references in both H- and C-NMR spectra (7.26 and 77.16 ppm, respectively). The chemical shifts are reported in ppm, the coupling constants (J) in Hz, and the multiplicity is given as s chemical shifts are reported in ppm, the coupling constants (J) in Hz, and the multiplicity is given as (singlet), d (doublet), t (triplet), and m (multiplet). Infrared (IR) spectra were recorded on a Nicolet s (singlet), d (doublet), t (triplet), and m (multiplet). Infrared (IR) spectra were recorded on a Nicolet Protege 460 FT-IR spectrophotometer with an attenuated total reflectance (ATR) unit attached. Samples Protege 460 FT-IR spectrophotometer with an attenuated total reflectance (ATR) unit attached. were analyzed neat on a ZnSe crystal, and absorption peaks are reported in wavenumbers (cm ) and Samples were analyzed neat on a ZnSe crystal, and absorption peaks are reported in wavenumbers −1 (cm ) and characterized as strong (s), medium (m), weak (w) and broad (br). Raman spectra were TM recorded on a Kaiser RamanRxn1 instrument operated under standard conditions. High-resolution Molbank 2020, 2020, M1133 4 of 6 characterized as strong (s), medium (m), weak (w) and broad (br). Raman spectra were recorded on a TM Kaiser RamanRxn1 instrument operated under standard conditions. High-resolution mass spectra (HRMS) were obtained on a Jeol AccuTOFTM mass spectrometer operated in the ESI mode under standard conditions. 3.2. Reduction of 1,1,2,2,7,7,8,8-Octaethoxyocta-3,5-diyne (1) A two-necked, round-bottom flask, equipped with a magnetic stirring bar, a condenser with a CaCl tube, and a septum, was charged with dry diethyl ether (6 mL) and LAH (0.099 g, 2.6 mmol). The suspension was cooled (ice/water) and a solution of 1 (0.399 g, 0.87 mmol) in dry diethyl ether (3.0 mL) was added dropwise with a syringe. The bath was removed, and the resulting mixture was stirred under reflux for 1 h. The mixture was then added to some ice and when the ice had melted, a saturated solution of Rochelle salt was added. The hydrolysate was extracted with diethyl ether (4 10 mL); the combined organic phases were dried (Na SO ), filtered, and concentrated under 2 4 vacuum on a rotary evaporator. The residue contained three products (TLC), which were isolated by flash chromatography and proved to be isomers of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) based on the following spectroscopic and spectrometric data. (Z,Z)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2a): Yellowish liquid (0.090 g, 26%); R 0.48; FT-IR (film): 2976 (s), 2930 (m), 2883 (m), 2171 (w), 1649 (m), 1621 (m), 1180 (s), 1110 (s), 1049 (s), 913 (s), 841 max 1 1 (m), 793 (m) cm ; H-NMR (CDCl , 500 MHz):  5.23 (s, 2H, 2 CH(OEt) ), 4.72 (s, 2H, 2 -CH=), 4.44 (q, 3 2 4H, J = 7.0 Hz, 2 OCH CH ), 3.65-3.48 (m, 8H, 4 OCH CH ), 1.30 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.21 2 3 2 3 2 3 (t, 12H, J = 7.0 Hz, 4 OCH CH ) ppm; C NMR (CDCl , 125 MHz):  158.6 (2C), 100.1 (2C), 88.4 (2C), 2 3 3 + + 85.7 (2C), 66.6 (2C), 62.0 (4C), 15.6 (2C), 15.3 (4C) ppm; HRMS Calcd for C H O Na [M + Na] m/z 20 34 6 393.22531, found m/z 393.22555. (E,Z)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2b): Yellowish liquid (0.110 g, 34%); R 0.36; FT-IR (film): 2976 (s), 2930 (m), 2882 (m), 2187 (w), 1641 (m), 1612 (m), 1252 (s). 1110 (s), 1049 (s), 839 (m), 793 max 1 1 (m) cm ; H-NMR (CDCl , 500 MHz):  5.43 (s, 1H, CH(OEt) ), 5.26 (d, 1H, J = 2.6 Hz, -CH=), 4.94 (d, 3 2 1H, J = 2.6 Hz, -CH=), 4.72 (s, 1H, CH(OEt) ), 4.45 (q, 2H, J = 7.0 Hz, OCH CH ), 3.83 (q, 2H, J = 7.0 Hz, 2 2 3 OCH CH ), 3.76-3.49 (m, 8H, 4 OCH CH ), 1.36 (t, 3H, J = 7.0 Hz, OCH CH ), 1.31 (t, 3H, J = 7.0 Hz, 2 3 2 3 2 3 OCH CH ), 1.24 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.22 (t, 6H, J = 7.0 Hz, 2 OCH CH ) ppm; C NMR 2 3 2 3 2 3 (CDCl , 125 MHz):  162.2 (1C), 158.4 (1C), 100.1 (1C), 98.4 (1C), 88.1 (1C), 87.7 (1C), 85.9 (1C), 84.5 (1C), 66.5 (1C), 64.1 (1C), 63.2 (2C), 62.1 (2C), 15.6 (1C), 15.32 (2C), 15.27 (2C), 14.3 (1C) ppm; HRMS + + Calcd for C H O Na [M + Na] m/z 393.22531, found m/z 393.22566. 20 34 6 (E,E)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2c): Yellowish liquid (0.009 g, 3%); R = 0.20; FT-IR (film):  2976 (s), 2930 (m), 2881 (m), 2172 (w), 1626 (s), 1254 (s), 1198 (s), 1110 (s), 1048 (s), 956 (s), max 1 1 794 (s) cm ; H-NMR (CDCl , 500 MHz):  5.47 (s, 2H, 2 CH(OEt) ), 4.95 (s, 2H, 2 -CH=), 3.84 (q, 4H, 3 2 J = 7.0 Hz, 2 OCH CH ), 3.77-3.55 (m, 8H, 4 OCH CH ), 1.37 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.25 2 3 2 3 2 3 (t, 12H, J = 7.1 Hz, 4 OCH CH ) ppm; C NMR (CDCl , 125 MHz):  161.9 (2C), 98.4 (2C), 87.3 (2C), 2 3 3 + + 84.4 (2C), 64.1 (2C), 63.1 (4C), 15.3 (4C), 14.4 (2C) ppm. HRMS Calcd for C H O Na [M + Na] m/z 20 34 6 393.22531, found m/z 393.22567. 1 13 Supplementary Materials: The following are available online, Figures S1–S9: IR, H-NMR, and C-NMR spectra of compounds 2a, 2b, and 2c; Figures S10–S13: DEPT-90, DEPT-135, HSQC, and HMBC spectra of 2c; Figure S14: Raman spectrum of a mixture of compounds 2a, 2b, and 2c. Author Contributions: Conceptualization, L.K.S.; methodology, investigation, S.M.P.; writing—original draft preparation, L.K.S.; writing—review and editing, L.K.S. and S.M.P.; supervision, project administration, funding acquisition, L.K.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by University of Bergen, the Munin Foundation, and Norges Forskningsråd (Research Council of Norway). Acknowledgments: We thank Bjarte Holmelid (University of Bergen) for technical assistance. Valuable comments from the reviewers are also highly appreciated. Molbank 2020, 2020, M1133 5 of 6 Conflicts of Interest: The authors declare no conflict of interest. References 1. Sydnes, L.K. Formation of Acetylenes by Ring Opening of 1,1,2-Trihalocyclopropanes. Eur. J. Org. Chem. 2000, 3511–3518. [CrossRef] 2. Kvernenes, O.H.; Sydnes, L.K. Synthesis of 2-Chloroacrolein Diethyl Acetal (2-Chloroprop-2-enal, Diethyl Acetal. Org. Synth. 2005, 83, 184–192. 3. Sydnes, L.K.; Holmelid, B.; Kvernenes, O.H.; Sandberg, M.; Hodne, M.; Bakstad, E. Synthesis and some chemical properties of 3,3,4,4-tetraethoxybut-1-yne. Tetrahedron 2007, 63, 4144–4148. [CrossRef] 4. Sydnes, L.K.; Valdersnes, S. Recent advances in the synthesis of carbohydrate analogues. Pure Appl. Chem. 2007, 79, 2137–2142. [CrossRef] 5. Sydnes, L.K.; Holmelid, B.; Myagmasuren, S.; Hanstein, M. New regiospecific synthesis of tri- and tetrasubstituted furans. J. Org. Chem. 2009, 74, 3430–3443. [CrossRef] 6. Sydnes, L.K.; Isanov, R.; Sengee, M.; Livi, F. 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Toward the synthesis of modified carbohydrates by conjugate addition of propane-1,3-dithiol to , -unsaturated ketones. Helv. Chim. Acta 2012, 95, 2099–2122. [CrossRef] 13. Leiren, M.K.; Valdersnes, S.; Sydnes, L.K. Selective transformations of a diprotected 2-oxo-butanedial. Helv. Chim. Acta 2013, 96, 1841–1850. [CrossRef] 14. Nes, I.; Sydnes, L.K. Formation of N-heterocycles from 1,1-diethoxy-5-hydroxyalk-3-yn-2-ones. Synthesis 2014, 47, 89–94. 15. Isanov, R.; Holmelid, B.; Törnroos, K.W.; Sydnes, L.K. Synthesis of (E)-1,1-diethoxy-3-(3-hydroxy-3-arylfuro [2,3-b]quinoxalin-2(3H)-ylidene)propan-2-ones via acid-catalyzed stereoselective 5-Exo-Dig cyclization. J. Heterocycl. Chem. 2015, 52, 711–718. [CrossRef] 16. Holmelid, B.; Sydnes, L.K. Synthesis of 1,1,2,2,7,7,8,8-Octaethoxyocta-3,5-diyne. Molbank 2015, 2015, M840. [CrossRef] 17. Nantz, M.H.; Bender, D.M.; Janaki, S. A convenient terminal allene synthesis from propargylic acetates. Synthesis 1993, 6, 577–578. [CrossRef] 18. Haces, A.; Van Kruchten, E.M.G.A.; Okamura, W.H. On the stereochemistry of organocopper mediated conversion of propargylic esters to allenes. Israel J. Chem. 1985, 26, 140–146. [CrossRef] 19. Elsevier, C.J.; Stehouwer, P.M.; Westmijze, H.; Vermeer, P. Anti-stereoselectivity in the palladium(0)-catalyzed conversion of propargylic esters into allenes by phenylzinc chloride. J. Org. Chem. 1983, 48, 1103–1105. [CrossRef] 20. Mae, M.; Hong, J.A.; Xu, B.; Hammond, G.B. Highly Regioselective Synthesis of gem-Difluoroallenes through Magnesium Organocuprate SN2 Substitution. Org. Lett. 2006, 8, 479–482. [CrossRef] 21. Soler-Yanes, R.; Arribas-Alvarez, I.; Guisan-Ceinos, M.; Bunuel, E.; Cardenas, D.J. NiI Catalyzes the Regioselective Cross-Coupling of Alkylzinc Halides and Propargyl Bromide to Allenes. Chem. Eur. J. 2017, 23, 1584–1590. [CrossRef] [PubMed] 22. Kalli, M.; Landor, P.D.; Landor, S.R. Allenes. XVII. Reaction of dialkyl(lithio)copper reagents with 1-bromoallenes, 1-iodoallenes, and 3-chloro-1-alkynes. J. Chem. Soc. Perkin Trans. 1 1973, 1347–1349. [CrossRef] Molbank 2020, 2020, M1133 6 of 6 23. Jacobs, T.L.; Wilcox, R.D. Dehalogenation of Propargyl and Allenyl Halides. II. J. Am. Chem. Soc. 1964, 86, 2240–2247. [CrossRef] 24. Cowie, J.S.; Landor, P.D.; Landor, S.R. Allenes. XIV. Preparation of -allenic alcohols from the mono-O-(tetrahydropyran-2-yl) derivatives of 1,4-butynediols. J. Chem. Soc. Perkin Trans. 1 1973, 720–724. [CrossRef] 25. Alexakis, A.; Marek, I.; Mageney, P.; Normant, J.F. Mechanistic Aspects on the Formation of Chiral Allenes from Propargylic Ethers and Organocopper Reagents. J. Am. Chem. Soc. 1990, 112, 8042–8047. [CrossRef] 26. Alexakis, A. Stereochemical Aspects on the Formation of Chiral Allenes from Propargylic Ethers and Epoxides. Pure Appl. Chem. 1992, 64, 387–392. [CrossRef] 27. Boreux, A.; Lonca, G.H.; Riant, O.; Gagosz, F. Synthesis of Trifluoromethylallenes by Gold-Catalyzed Rearrangement Propargyl Benzyl Ethers. Org. Lett. 2016, 18, 5162–5165. [CrossRef] 28. Bellamy, L.J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: London, UK, 1980; Volume 2. 29. Becker, E.D. High Resolution NMR: Theory and Applications, 3rd ed.; Academic Press: San Diego, CA, USA, 2000; pp. 83–117. 30. Skatteboel, L.; Solomon, S. Thermally induced reactions of some novel allenes. J. Am. Chem. Soc. 1965, 87, 4506–4513. [CrossRef] 31. Mühlstädt:, M.; Graefe, J. Synthesen cycloaliphatischer Ketone aus Cyclodecatrien-(1,5,9). II. Herstellung von Cyclononen durch Hydratisierung cyclischer Allene. Chem. Ber. 1967, 100, 223–227. [CrossRef] 32. Ho , S.; Brandsma, L.; Arens, J.F. Conversions of allenyl ethers. Rec. Trav. Chim. Pay-Bas 1968, 87, 1179–1184. [CrossRef] 33. Pasto, D.J.; Kong, W. Study of the substituted vinylallene-methylenecyclobutene electrocyclic equilibria. Comparison with the butadiene-cyclobutene and bisallene-bismethylenecyclobutene electrocyclic equilibria. J. Org. Chem. 1989, 54, 4028–4033. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Molbank Multidisciplinary Digital Publishing Institute

Formation of an Isomeric Mixture of Dienynes Instead of a Diallene

Petrova, Susanne M.; Sydnes, Leiv K.
Molbank , Volume 2020 (2) – May 11, 2020
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molbank Communication Formation of an Isomeric Mixture of Dienynes Communication Formation Instead of a Diallene of an Isomeric Mixture of Dienynes Instead of a Diallene Susanne M. Petrova and Leiv K. Sydnes * Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway; Susanne M. Petrova and Leiv K. Sydnes * susannapetrova@gmail.com Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway; susannapetrova@gmail.com * Correspondence: leiv.sydnes@uib.no; Tel.: +47-55-583-450 * Correspondence: leiv.sydnes@uib.no; Tel.: +47-55-583-450 Received: 17 April 2020; Accepted: 7 May 2020; Published: 11 May 2020 Received: 17 April 2020; Accepted: 7 May 2020; Published: 11 May 2020 Abstract: Attempts to convert 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne to a symmetric allene by Abstract: Attempts to convert 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne to a symmetric allene by reduction with lithium aluminum hydride failed. Instead reduction accompanied by isomerization reduction with lithium aluminum hydride failed. Instead reduction accompanied by isomerization occurred and afforded 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne as a mixture of three isomers in occurred and a orded 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne as a mixture of three isomers in 63% 63% total isolated yield. total isolated yield. Keywords: conjugated diyne; LAH reduction; diacetal; pent-1,2,3,4-tetraene intermediate Keywords: conjugated diyne; LAH reduction; diacetal; pent-1,2,3,4-tetraene intermediate 1. Introduction 1. Introduction Since the synthesis of 3,3,4,4-tetraethoxybutyne (TEB) was reported some 15 years ago [1,2,3], Since the synthesis of 3,3,4,4-tetraethoxybutyne (TEB) was reported some 15 years ago [1–3], many many of its chemical properties have been uncovered and used to prepare a range of chemical of its chemical properties have been uncovered and used to prepare a range of chemical compounds compounds with rich structural diversity [4,5,6,7,8,9,10,11,12,13,14,15]. Among the most densely with rich structural diversity [4–15]. Among the most densely functionalized molecules made is functionalized molecules made is 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne (1), which has one ketal 1,1,2,2,7,7,8,8-octaethoxyocta-3,5-diyne (1), which has one ketal moiety in propargylic position to each moiety in propargylic position to each of the triple bonds [16]. The compound therefore belongs to a of the triple bonds [16]. The compound therefore belongs to a group of compounds that can undergo group of compounds that can undergo SN2′ reactions by nucleophilic attack of the triple bond, which S 2 reactions by nucleophilic attack of the triple bond, which is accompanied by C–C bond migration is accompanied by C–C bond migration that leads to the release of a leaving group from the that leads to the release of a leaving group from the propargylic carbon and formation of an allene propargylic carbon and formation of an allene moiety. The most common leaving groups are moiety. The most common leaving groups are alkanoates [17–19], bromide [20,21], and chloride [22,23], alkanoates [17,18,19], bromide [20,21], and chloride [22,23], but examples involving alkoxides have but examples involving alkoxides have also been published [24–27]. As for the nucleophiles, both also been published [24,25,26,27]. As for the nucleophiles, both carbanions and hydride have been carbanions and hydride have been applied [17–27]. applied [17,18,19,20,21,22,23,24,25,26,27]. On this basis, we envisaged that 1 might be used as a substrate to make a functionalized diallene by On this basis, we envisaged that 1 might be used as a substrate to make a functionalized diallene two S 2 reactions, one at each of the propargylic moieties, using hydride as a nucleophile (Scheme 1). by two SN2′ reactions, one at each of the propargylic moieties, using hydride as a nucleophile (Scheme Lithium aluminum hydride (LAH) was deemed to be a suitable reagent [23] but, as reported here, 1). Lithium aluminum hydride (LAH) was deemed to be a suitable reagent [23] but, as reported here, when the reaction was performed, the expected product was not obtained; instead, an isomeric mixture when the reaction was performed, the expected product was not obtained; instead, an isomeric of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) was the only product formed. mixture of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) was the only product formed. CH CH OCH CH 2 3 2 3 CH CH O 3 2 CH(OCH CH ) 2 3 2 O CH CH O 3 2 1 8 • (CH CH O) HC 3 2 2 CH CH O OCH CH 3 2 3 4 5 6 2 3 1) LAH 2 7 2) H O CH CH O OCH CH 2 CH CH O 3 2 2 3 CH(OCH CH ) 3 2 2 3 2 OCH CH O 2 3 CH CH 2 3 OCH CH (CH CH O) HC 3 2 2 2 3 Scheme 1. Expected and obtained products from lithium aluminum hydride (LAH) reduction of 1. Scheme 1. Expected and obtained products from lithium aluminum hydride (LAH) reduction of 1. 2. Results and Discussion 2. Results and Discussion The reaction was carried out under anhydrous conditions in refluxing diethyl ether using six The reaction was carried out under anhydrous conditions in refluxing diethyl ether using six equivalents of hydride with respect to diallene formation. The reaction was monitored by TLC and equivalents of hydride with respect to diallene formation. The reaction was monitored by TLC and when quenched and worked up after 1 h, three products were detected and subsequently isolated by Molbank 2020, 2020, x; doi: www.mdpi.com/journal/molbank Molbank 2020, 2020, M1133; doi:10.3390/M1133 www.mdpi.com/journal/molbank Molbank 2020, 2020, M1133 2 of 6 Molbank 2020, 2020, x 2 of 6 when quenched and worked up after 1 h, three products were detected and subsequently isolated by flash chromatography. All the products had the same molecular weight as the expected product, but flash chromatography. All the products had the same molecular weight as the expected product, but their IR spectra did not show any absorption in the allene region (1955–1925 cm ) [28], which rules out −1 their IR spectra did not show any absorption in the allene region (1955–1925 cm ) [28], which rules 1 1 that diallene formation had occurred. However, absorptions in the 1680–1620 cm and 840–790 cm −1 out that diallene formation had occurred. However, absorptions in the 1680–1620 cm and 840–790 regions indicate the presence of trisubstituted alkenes [28], and this requires the presence of a C–C triple −1 cm regions indicate the presence of trisubstituted alkenes [28], and this requires the presence of a bond to be compatible with the determined molecular weight. Considering the symmetry of the starting C–C triple bond to be compatible with the determined molecular weight. Considering the symmetry material, the triple bond would be expected to be symmetrically substituted, and this would explain the of the starting material, the triple bond would be expected to be symmetrically substituted, and this absence of an absorption in the 2270–2120 cm region [28]. In order to determine whether this was a −1 would explain the absence of an absorption in the 2270–2120 cm region [28]. In order to determine reasonable assumption, the Raman spectrum of 2 was recorded. To our satisfaction, a strong absorption whether this was a reasonable assumption, the Raman spectrum of 2 was recorded. To our appeared at 2186 cm . This observation made the structure elucidation fairly straightforward when −1 satisfaction, a strong absorption appeared at 2186 cm . This observation made the structure 1 13 H- and C-NMR data were considered, and the three compounds, isolated in 26%, 34% and 3% yield, 1 13 elucidation fairly straightforward when H- and C-NMR data were considered, and the three were proved to be the (Z,Z), (E,Z) and (E,E) isomers of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne, 2a, compounds, isolated in 26%, 34% and 3% yield, were proved to be the (Z,Z), (E,Z) and (E,E) isomers 2b and 2c, respectively. of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne, 2a, 2b and 2c, respectively. The stereoisomers could be di erentiated by a detailed study of the 3.4–5.5 ppm region of their The stereoisomers could be differentiated by a detailed study of the 3.4–5.5 ppm region of their proton NMR spectra, shown in Figure 1. The (Z,Z) and (E,E) isomers both exhibit C2 symmetry, and proton NMR spectra, shown in Figure 1. The (Z,Z) and (E,E) isomers both exhibit C2 symmetry, and the methine protons at C-1 and C-8 consequently have the same chemical shift, as do the olefinic the methine protons at C-1 and C-8 consequently have the same chemical shift, as do the olefinic protons at C-3 and C-6, and the methylene groups in the ethoxy groups attached to C-2 and C-7. These protons at C-3 and C-6, and the methylene groups in the ethoxy groups attached to C-2 and C-7. isomers will therefore show the same number of signals in the 3.4–5.5 ppm region. The asymmetry of These isomers will therefore show the same number of signals in the 3.4–5.5 ppm region. The the corresponding (E,Z) isomer results in twice the number of signals in this region, giving rise to the asymmetry of the corresponding (E,Z) isomer results in twice the number of signals in this region, middle spectrum in Figure 1, which interestingly is almost identical to that obtained when the top and giving rise to the middle spectrum in Figure 1, which interestingly is almost identical to that obtained the bottom spectra are combined. when the top and the bottom spectra are combined. 1 1 Figure Figure 1. 1. The The 5 5.5–3.4 .5–3.4 ppm region of the ppm region of the H-N H-NMR MR spectra of spectra t of he three isome the three isomers rs of 2 of . The si 2. The gnal signals s in the in the 4.6– 4.6–3.4 3.4 ppm re ppm gion are due to region are due the methylene to the methylene moieties in moieties the in ethoxy groups the ethoxy groups attached to attached C-1 to , C-2, C-7, C-1, C-2, C-7, and C-8. To and C-8. denote the two To denote the two hydrogen ato hydrogen atoms ms in th in the e methylene group i methylene groupn in EtO attache EtO attached d to C-n, the to C-n, the following following notat notation ion is is used: used: 2 2H-(OEt-n). H-(OEt-n). The assignments of the proton spectra of 2a, 2b and 2c shown in Figure 1 were arrived at by using The assignments of the proton spectra of 2a, 2b and 2c shown in Figure 1 were arrived at by information harvested from the DEPT-90, DEPT-135, HSQC, and HMBC spectra of each of the isomers. using information harvested from the DEPT-90, DEPT-135, HSQC, and HMBC spectra of each of the The DEPT and HSQC spectra confirmed the presence of the methyl, methylene, and methine protons, isomers. The DEPT and HSQC spectra confirmed the presence of the methyl, methylene, and methine the olefinic CH groups, and the quaternary carbon atoms, whereas the HMBC spectrum showed the protons, the olefinic CH groups, and the quaternary carbon atoms, whereas the HMBC spectrum correlation between hydrogen and carbon atoms two and three bonds apart. The HMBC spectra can showed the correlation between hydrogen and carbon atoms two and three bonds apart. The HMBC ther spectra ca efore be n theref used ore be used to to assign the a H-1, ssign the H- H-8 and 1, H-3, H-8 a H-6 nd H- singl 3, H-6 singl ets, which ets, which is clearly is cl illustrated early illustra by ted the spectrum for 2c (Figure S13). In this spectrum, the proton singlet at 5.47 ppm correlates through three by the spectrum for 2c (Figure S13). In this spectrum, the proton singlet at 5.47 ppm correlates through bonds three bonds with the with the met methylene hylene carbon carbon atoms at atoms at 63.1 ppm 63.1 ppm due t due to theoethoxy the ethoxy groups groups at at C-1 C-1 and and C-8 C-8 and and through two bonds with the quaternary carbon atoms at 161.9 ppm (C-2 and C-7); thus, this Molbank 2020, 2020, M1133 3 of 6 Molbank 2020, 2020, x 3 of 6 through two bonds with the quaternary carbon atoms at 161.9 ppm (C-2 and C-7); thus, this proton singlet proton sing is duele to t is d H-1u and e to H-8. H-1 This and H- assignment 8. This asis sign further ment is supported further supported by by correlationscorrelations between between the proton singlet at 4.95 ppm and the olefinic quaternary carbons at 161.9 ppm (C-2 and C-7) and the acetal the proton singlet at 4.95 ppm and the olefinic quaternary carbons at 161.9 ppm (C-2 and C-7) and carbon the acet atoms al carat bon at 98.4om ppm s at (C-1 98.4and ppm C-8). (C-1 and C-8). In order to di erentiate between the (Z,Z) and (E,E) isomers, the chemical shifts of the methine In order to differentiate between the (Z,Z) and (E,E) isomers, the chemical shifts of the methine hydr hydrogen ogen at at C-1 C-1 (and C (and C-8) -8) and and the the meth methylene ylenepr proton otonss i innthe the ethoxy group ethoxy group attached attached to C-2 (a to C-2 (and nd C-7 C-7)) in in the two iso the two isomers mers were co were compar mpared. D ed. Due ue to the anisot to the anisotr ropy of opy of the tripl the triple e bond a bond ttached to C- attached to C-3 3 (and C (and - C-6), 6), protons protons cis cis to the to theacety acetyleni lenicc moiety moiety will be will be de deshielded, shielded, whe wher re eas as those those in in tr trans ansposition position will will be be shielded shielded [[29 29]. ]. Thus, si Thus, since nce the methyl the methylene ene qu quartet artet inin the the bottom spectr bottom spectr um a um ppea appears rs at aat lower a lowe fiel rd tha fieldn than in the top spectrum, 4 in the top spectrum, .44 ppm compa 4.44 ppm compar red to 3ed .84to ppm, a 3.84 ppm, nd the methi and the ne si methine nglet in the bottom singlet in the spectrum bottom spectr appear um s at appears a higher at f aihigher eld than field in tthan he toin p sp theect top rum spectr , 4.72 um, ppm 4.72 coppm mpared compar to 5.ed 47 p top5.47 m, th ppm, e botthe tom bottom spectrum belongs to spectrum belongs (Z,Zto )-2( (Z,Z 2a) a )-2 n( d the top spectrum 2a) and the top spectr to ( um E,Eto )-2( (E,E 2c). )- 2 (2c). The The complete absence complete absence in in the reaction m the reaction mixtur ixture of e of the the exp expected ected diallene diallene and and any any of th of thee by-products by-products this this cum cumulene ulene conceiv conceivably ably could could have have given under given under the rea the rc eaction tion and and work-up work-up condi conditions tions [30,31 [30 ,3– 2,33] 33] indicates indicates tha thatt 1 1 is is not not at attacked tacked init initially ially atat C-C-4 4 (and (and C-5) C-5). . A like A ly likely explanat explanation ion for thfor is isthi ths atis ththat e 1,1,the 2,2- 1,1,2,2-tetraethoxyethyl tetraethoxyethyl groups gr aoups ttached to the ends of attached to the the ends bu of tathe -1,3buta-1,3-diyne -diyne moiety ren moiety der itr s t ender ermini its re termini latively relatively electroposit electr ive and ma opositiveke i andnmake itial at initial tack of C-3 attack of (and C-3C (and -6) m C-6) ore f mor avo erfaavorable. ble. It is a Itlsis o c also oncconceivable eivable that that coordin coora dination tion of one or several oxygen of one or several oxygen atoms to specie atoms to species s derived derived from LAH from at so LAH me stage at some will stage facilitate will facilitate the same sort the same of atta sort ck of (see attack Scheme 2) (see Scheme . This l2 ea ).ds This to form leads ation to formation of a pent-1 of,2,3,4-tetraene a pent-1,2,3,4-tetraene derivative derivative (3), which i (3),s unst whichab isle unstable in the presence in the presence of a hyd of a hydride ride sourc sour e ce and and reacts reactsquick quickly ly to g to give ive d dienyne ienyne 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne ( (22 ) ) (Scheme (Scheme 2). Thus, th 2). Thus, thee two hydrog two hydrogens ens replacin replacingg the the two two ethoxy ethoxy group groupssin in the conv the conversion ersion o off1 1to to 22ar are a e attached ttached by hydri by hydride de a attack ttack inin a a S SN 22′fashion, fashion,not not b byy reaction reaction wi with th wa water ter during during the the quenching quenching of the re of the reacti action. In on. In accor accor dance dance w with ith th this, is, quenchin quenchingg usin usingg D D2 O O did did not not lead leato d t deuterium o deuterium incorporation incorporatin ion 2.in Traces 2. Trof acany es of a intermediate ny intermedia werete not were not det detected, neither ected, spectr neither spectroscopically n oscopically nor by TLC. or by TLC. Scheme 2. Possible mechanism for formation of dienyne 2 from diyne 1 by two subsequent S 2 Scheme 2. Possible mechanism for formation of dienyne 2 from diyne 1 by two subsequent SN2′ reactions with hydride as a nucleophile. reactions with hydride as a nucleophile. 3. Materials and Methods 3. Materials and Methods 3.1. General 3.1. General The chemicals were obtained from commercial suppliers and used without further purification. The chemicals were obtained from commercial suppliers and used without further purification. Thin-layer chromatography (TLC) was performed using pre-coated aluminum TLC plates (Alugram, Thin-layer chromatography (TLC) was performed using pre-coated aluminum TLC plates (Alugram, 0.20 mm Silica Gel 60 F ) and eluting with a 95:5 mixture of hexanes and ethyl acetate. The R values 254 f 0.20 mm Silica Gel 60 F254) and eluting with a 95:5 mixture of hexanes and ethyl acetate. The Rf values were determined after the liquid front had migrated 4–5 cm. Visualization of the chromatograms was were determined after the liquid front had migrated 4–5 cm. Visualization of the chromatograms was done with phosphomolybdic acid (NH ) MoO 4H O) in ethanol followed by heating. Flash-column 4 4 4 2 done with phosphomolybdic acid (NH4)4MoO4·4H2O) in ethanol followed by heating. Flash-column chromatography (FC) was performed manually using Silica Gel from Fluka Analytical (230–400 mesh) chromatography (FC) was performed manually using Silica Gel from Fluka Analytical (230–400 and eluting with a 80:20 mixture of hexanes and ethyl acetate. NMR spectra were recorded on a mesh) and eluting with a 80:20 mixture of hexanes and ethyl acetate. NMR spectra were recorded on 1 13 Bruker Biospin AV500 instrument (500 MHz for H, 125 MHz for C) in CDCl as solvent, using the 1 13 a Bruker Biospin AV500 instrument (500 MHz for H, 125 MHz for C) in CDCl3 as solvent, using the 1 13 solvent peaks as references in both H- and C-NMR spectra (7.26 and 77.16 ppm, respectively). The 1 13 solvent peaks as references in both H- and C-NMR spectra (7.26 and 77.16 ppm, respectively). The chemical shifts are reported in ppm, the coupling constants (J) in Hz, and the multiplicity is given as s chemical shifts are reported in ppm, the coupling constants (J) in Hz, and the multiplicity is given as (singlet), d (doublet), t (triplet), and m (multiplet). Infrared (IR) spectra were recorded on a Nicolet s (singlet), d (doublet), t (triplet), and m (multiplet). Infrared (IR) spectra were recorded on a Nicolet Protege 460 FT-IR spectrophotometer with an attenuated total reflectance (ATR) unit attached. Samples Protege 460 FT-IR spectrophotometer with an attenuated total reflectance (ATR) unit attached. were analyzed neat on a ZnSe crystal, and absorption peaks are reported in wavenumbers (cm ) and Samples were analyzed neat on a ZnSe crystal, and absorption peaks are reported in wavenumbers −1 (cm ) and characterized as strong (s), medium (m), weak (w) and broad (br). Raman spectra were TM recorded on a Kaiser RamanRxn1 instrument operated under standard conditions. High-resolution Molbank 2020, 2020, M1133 4 of 6 characterized as strong (s), medium (m), weak (w) and broad (br). Raman spectra were recorded on a TM Kaiser RamanRxn1 instrument operated under standard conditions. High-resolution mass spectra (HRMS) were obtained on a Jeol AccuTOFTM mass spectrometer operated in the ESI mode under standard conditions. 3.2. Reduction of 1,1,2,2,7,7,8,8-Octaethoxyocta-3,5-diyne (1) A two-necked, round-bottom flask, equipped with a magnetic stirring bar, a condenser with a CaCl tube, and a septum, was charged with dry diethyl ether (6 mL) and LAH (0.099 g, 2.6 mmol). The suspension was cooled (ice/water) and a solution of 1 (0.399 g, 0.87 mmol) in dry diethyl ether (3.0 mL) was added dropwise with a syringe. The bath was removed, and the resulting mixture was stirred under reflux for 1 h. The mixture was then added to some ice and when the ice had melted, a saturated solution of Rochelle salt was added. The hydrolysate was extracted with diethyl ether (4 10 mL); the combined organic phases were dried (Na SO ), filtered, and concentrated under 2 4 vacuum on a rotary evaporator. The residue contained three products (TLC), which were isolated by flash chromatography and proved to be isomers of 1,1,2,7,8,8-hexaethoxyocta-2,6-dien-4-yne (2) based on the following spectroscopic and spectrometric data. (Z,Z)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2a): Yellowish liquid (0.090 g, 26%); R 0.48; FT-IR (film): 2976 (s), 2930 (m), 2883 (m), 2171 (w), 1649 (m), 1621 (m), 1180 (s), 1110 (s), 1049 (s), 913 (s), 841 max 1 1 (m), 793 (m) cm ; H-NMR (CDCl , 500 MHz):  5.23 (s, 2H, 2 CH(OEt) ), 4.72 (s, 2H, 2 -CH=), 4.44 (q, 3 2 4H, J = 7.0 Hz, 2 OCH CH ), 3.65-3.48 (m, 8H, 4 OCH CH ), 1.30 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.21 2 3 2 3 2 3 (t, 12H, J = 7.0 Hz, 4 OCH CH ) ppm; C NMR (CDCl , 125 MHz):  158.6 (2C), 100.1 (2C), 88.4 (2C), 2 3 3 + + 85.7 (2C), 66.6 (2C), 62.0 (4C), 15.6 (2C), 15.3 (4C) ppm; HRMS Calcd for C H O Na [M + Na] m/z 20 34 6 393.22531, found m/z 393.22555. (E,Z)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2b): Yellowish liquid (0.110 g, 34%); R 0.36; FT-IR (film): 2976 (s), 2930 (m), 2882 (m), 2187 (w), 1641 (m), 1612 (m), 1252 (s). 1110 (s), 1049 (s), 839 (m), 793 max 1 1 (m) cm ; H-NMR (CDCl , 500 MHz):  5.43 (s, 1H, CH(OEt) ), 5.26 (d, 1H, J = 2.6 Hz, -CH=), 4.94 (d, 3 2 1H, J = 2.6 Hz, -CH=), 4.72 (s, 1H, CH(OEt) ), 4.45 (q, 2H, J = 7.0 Hz, OCH CH ), 3.83 (q, 2H, J = 7.0 Hz, 2 2 3 OCH CH ), 3.76-3.49 (m, 8H, 4 OCH CH ), 1.36 (t, 3H, J = 7.0 Hz, OCH CH ), 1.31 (t, 3H, J = 7.0 Hz, 2 3 2 3 2 3 OCH CH ), 1.24 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.22 (t, 6H, J = 7.0 Hz, 2 OCH CH ) ppm; C NMR 2 3 2 3 2 3 (CDCl , 125 MHz):  162.2 (1C), 158.4 (1C), 100.1 (1C), 98.4 (1C), 88.1 (1C), 87.7 (1C), 85.9 (1C), 84.5 (1C), 66.5 (1C), 64.1 (1C), 63.2 (2C), 62.1 (2C), 15.6 (1C), 15.32 (2C), 15.27 (2C), 14.3 (1C) ppm; HRMS + + Calcd for C H O Na [M + Na] m/z 393.22531, found m/z 393.22566. 20 34 6 (E,E)-1,1,2,7,8,8-Hexaethoxyocta-2,6-dien-4-yne (2c): Yellowish liquid (0.009 g, 3%); R = 0.20; FT-IR (film):  2976 (s), 2930 (m), 2881 (m), 2172 (w), 1626 (s), 1254 (s), 1198 (s), 1110 (s), 1048 (s), 956 (s), max 1 1 794 (s) cm ; H-NMR (CDCl , 500 MHz):  5.47 (s, 2H, 2 CH(OEt) ), 4.95 (s, 2H, 2 -CH=), 3.84 (q, 4H, 3 2 J = 7.0 Hz, 2 OCH CH ), 3.77-3.55 (m, 8H, 4 OCH CH ), 1.37 (t, 6H, J = 7.0 Hz, 2 OCH CH ), 1.25 2 3 2 3 2 3 (t, 12H, J = 7.1 Hz, 4 OCH CH ) ppm; C NMR (CDCl , 125 MHz):  161.9 (2C), 98.4 (2C), 87.3 (2C), 2 3 3 + + 84.4 (2C), 64.1 (2C), 63.1 (4C), 15.3 (4C), 14.4 (2C) ppm. HRMS Calcd for C H O Na [M + Na] m/z 20 34 6 393.22531, found m/z 393.22567. 1 13 Supplementary Materials: The following are available online, Figures S1–S9: IR, H-NMR, and C-NMR spectra of compounds 2a, 2b, and 2c; Figures S10–S13: DEPT-90, DEPT-135, HSQC, and HMBC spectra of 2c; Figure S14: Raman spectrum of a mixture of compounds 2a, 2b, and 2c. Author Contributions: Conceptualization, L.K.S.; methodology, investigation, S.M.P.; writing—original draft preparation, L.K.S.; writing—review and editing, L.K.S. and S.M.P.; supervision, project administration, funding acquisition, L.K.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by University of Bergen, the Munin Foundation, and Norges Forskningsråd (Research Council of Norway). Acknowledgments: We thank Bjarte Holmelid (University of Bergen) for technical assistance. Valuable comments from the reviewers are also highly appreciated. 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