Natural acetylenes. Part 53. A route to C5, C6, and C7cis- andtrans-alk-3-en-1-ynesviatheir trimethylsilyl derivatives

摘要

1978 1485 Natural Acetylenes. Part 532 A Route to C5,C6,and C, cis-and trans-Alk-3-en-I -ynes via their Trimethylsilyl Derivatives By lain W. Farrell, Milton T. W. Hearn. and Viktor Thaller,” The Dyson Perrins Laboratory, Oxford University, Oxford OX1 3QY cis-and trans-?-Trimethylsilyl-pent-3-en-l-yne, -hex-3-en-l -yne, and -hept-3-en-l -yne have been prepared by fractional distillation of their cis-trans-mixtures and shown to provide a convenient route for the synthesis of the corresponding stereoisomeric terminal acetylenes. PENT-and HEX-~-EN-~-YNES have been frequently re-quired in the synthesis of natural acetylenes, e.g. trans-pentenyne (la) for the C,, alcohol (2) from dahlias and cis-hexenyiie (lb) €or the acetylenic fury1 ketone (3)present in the broad bean.4 The need to synthesise labelled 14-cis-and 14-trans-dehydrocrepenynate (4)for biosynthetic experiments necessitated the preparation of cis-and tram-heptenynes (lc) and led to a convenient general route to the stereoisomeric C,--C, alkenynes.Alk-3-en-l-ynes can be synthesised in a number of ways, sonw leading predominantly to one of the stereo- isomer^.^ Most commonly the cis-trans-mixture is ob- tained either from alk-3-ynyl tosylate~*>~.~ (this route has Part 52, I. W. Farrell, V. Thaller, and J. L. Turner, J.C.S. Pfvkin 1, 1977, 1886. A more detailed account of part of the work described in this paper is in the D.Phi1. Thesis of I. W. Farrell, Oxford, 1977. C. Chin, I. C. Cutler, Sir Ewart R. H. Jones, J. Lee, S.Safe, and V. Thaller, .J. Clzem. SOC.(C), 1970, 314. * F. C. Falvcett, D. M. Spencer, R. L. Wain, A. G. Fallis, Sir Ewart H.H. Jones, M. Le Quan, C. B. Page, V. Thaller, D. C. Shubrook, and 1’. M. Whitham, J. Chem. SOC.(C), 1968, 2455. been used in the present investigation) or from alk-2-ynyl ethers8 The mixtures can be separated into stereo- isomers by fractional distillation, as described for the C, and C, compounds, or by g.1.c. as described for the C, corn~ound.~Neither separation is particularly con-venient because of the very close b.p.s of the stereo- isomers, the losses incurred by the high volatility of the alkenynes, and their instability. Silylation of the ethynyl group is known both to stabilise terminal acetylenes and to increase their b.p.s.1° The cis-trans-mixtures of the alkenynes (la-c) were therefore silylated via their Grignard or lithio derivatives, most efficiently (ca.8076 yields) when n-butyl-lithium Cf.E. J. Corey and R. A. Ruden, Tett,ahedyon Letteys, 1973, 1495. G. Eglinton and M. C. Whiting, J. Chem. Soc., 1950, 3650. A. Butenandt and E. Hecker, Angew. Chem., 1961, 73,349. L. Brandsma, ‘ Preparative Acetylenic Chemistry,’ Elsevier, Amsterdam, 1971, p. 124. J. L. H. Allan and M. C. Whiting, J. Chem. Soc., 1953, 3314. lo Cf. R. Eastmond, T. R. Johnson, and D. R. M. Walton, Tetrahedron, 1972, 28, 4601. was added to the mixture of trimethylsilyl chloride and the cis-trans-alkenyne (cf. silylation of benzenes ll). The resulting trimethylsilyl derivatives (5a-c) were easy RCH=CH*C-CH (1) a; R = Me b; R=Et c; R = Prn t t,tMeCH=CH*C-C,*CH=CH,*CH(OH)-CH,,*CH=CH, J.C.S.Perkin I distillation on a spinning-band column using p-cymene as chaser and gave cis- l-Erimethylsilyl~ent-3-en-l-yne cis-(5a) (5.38g, 43.5 ; contaminated with 2.5 trans-isomer), b.p. 62" at 46 mniHg (Found: C, 70.1; H, 10.4. C,H,,Si requires C, 69.5; H, 10.lyo), nDl9 1.4575, tn (48.5 "C) 3.0 min, vmax,(CC14)3 030, 2 150, and 2 120 cm-l, vma,.(CS,) 1 250, 840br, and 720 cm-l, ~(Ccl,) 9.98 (9 H, s, SiMe,), 8.3 (3 H, dd, J 7 and 1.5 Hz, CH,*CH=CH), 4.8 (1 H, dq, J 11and 1.5 (2) C r-o--1 t EtCH=CH-C-C*CO*C=CH*CH=C*CH=CH*CO,i'vIe(3) CPr*CH=CH*C-C*CH,CH-CH*CH,,*CO,hIe(4) RCH=CH*C-C*SiMe, (5)a; R = Me b; R=Et c; R=Pr to handle and the b.p.s of the stereoisomers differed sufficiently in each case (by ca.6 "C) to make their separation by fractional distillation convenient (a spin- ning band column was used). The cis-and trans-isomer fractions of (5a-c) were characterised and their stereo- chemical purity (294)was determined by g.1.c. The cis-and trans-alkenynes (la-c) could be regenerated as ethereal solutions by cleaving the carbon-silicon bond with silver nitrate l2 or fluoride the cleavage pro- ceeding in 85-90y0 yields. For example, by using the fluoride ion cleavage, an ethereal solution of the cis-and trans-hexenynes (1b) was obtained from the cis-trans-alkenyne mixture in 60 overall yield.EXPERIMENTAL For spectral techniques see Part 41.13 Spinning band distillations were carried out in a Nester- Faust Annular Still (NFA-200) operating at 7 000 rev. min-l. For g.1.c. a Pye 104 series 24 instrument with 10 poly- ethylene glycol succinate on Embacel (60-100 mesh) packed in a 150 x 0.3 cm column (N, flow 45 ml min-l) was used. Tetrahydrofuran (THF) was purified by refluxing with Na wire and benzophenone prior to distillation and was stored over Linde 4 8, molecular sieve. Solutions of BuIlLi in hexane were standardised by the double titration method using benzyl chloride. (a) Silylation via the Li Salts.-cis- and trans- l-Trimethyl- silylpent-3-en- l-yne (5a). cis-trans-Pent-3-en- l-yne (la) (4.5 g,68 mmol; 55 cis-isomer) and Me,SiCl (7.49 g, 69 mmol) in anhydrous THF (50ml) were cooled (Me,CO-solid CO,) under N,.BunLi in hexane (42.5 ml, 68 mmol) was added dropwise to the stirred solution (the red-brown colour of the acetylide ion was briefly seen). After the addition was completed a fine white precipitate slowly appeared. The stirred mixture was allowed to warm to 20 "C over 12 h. It was then poured into an excess of dil. HC1 and extracted with Et,O. The extract was dried (MgSO,) and concentr- ated under N, at 760 mmHg. The residue was fractionally distilled and gave cis-trans- l-trimethylsilylpent-3-en- l-yne (8.4 g,89; 56 cis-isomer), b.p. 68-75' at 50 mmHg. The mixture of isomers (12.35 g) was separated by fractional l1 R. Naylor, Tetrahedvon Letters, 1975, 435.l3 €I. M. Schmidt and J. F. Arens, Rec. Trav. chim., 1967, $6. 1138. Hz, CH,*CH=CH), and 4.2 (1 H, dq, J 11 and 6.5 Hz, CH,CH=CH), m/e 138 (M+,500/6),123 (loo), 107 (8), 95 (27), 81 (lo), 69 (22), and 67 (22); and the trans-isomer trans- (5a)l (3.9 g, 29y0; contaminated with 6 cis-isomer), b.p. 68" at 46 mmHg (Found: C, 69.5; H, 10.1. C,H,,Si requires C, 69.5; H, 10.1), nD1@ 1.4625, tR (48 "C) 4.4 min, vma,(CC14) 3 030, 2 170, 2 130, and 955 cm-l, v,,.(CS,) 840br cm-l, ~(Ccl,) 9.9 (9 H, s, SiMe,), 8.3 (3 H, dd, J 7 and 2 Hz, CH,*CH=CH), 4.7 (1 H, dq, J 16 and 2 Hz, CH,*CH=CH), and 4.0 (1 H, dq, J 16 and 6.5 Hz, CH,-CH=CH), in/e 138 (M', 25y0), 123 (loo), 109 (7), 107 (6), 97 (7), 95 (12), 83 (lo), 6 (15), and 59 (15).An intermediate fraction (0.9g, 7.376) had a cis : trans ratio of ca. 2 : 1. cis-and trans-l-Trimethylsilylhex-3-en-1-yne (5b). Analogously, cis-trans-hex-3-en- 1-yne (lb) (7.6 g, 95 mmol; 55 cis-isomer), Me,SiCl (10.9 g, 100 mmol), and BunLi in hexane (59 ml, 95 mmol) in THF (100 ml) gave, after distil- lation, cis-trans-l-Me3Si-hex-3-en-l-yne(5b) (1 1.4 g, 7976 ; 53 cis-isomer), b.p. 58-72" at 12 mniHg. Separation of the mixture (23 g) gave cis-l-tri~etlzylsiZyl~e~-3-en-l-yne cis-(5b) (10.7 g, 46.5 ; contaminated with 90 conversion into trans-hex-3-en-l-yne (<2 cis-isomer) ; no other product was detected. The mixture was poured into dil. HC1 (10 ml) and extracted with Et,O. The extract was washed successively with dil. HC1 and brine, and dried (MgSO,). Filtration gave an Et,O solution of trans-hex-3-en-1-ync. We thank I.C.I. for a Fellowship (toM. T. W. H.),and the S.R.C. for a studentship (to I. W. I?.) and research grant support. 8/122 Received, 25th January, 19781