Other Six-membered Ring Synthesis

Decorzant, W. Giersch, and G. Ohloff, Helv. Chim. Acta, 1981,64,1387. 

A rapid if not very efficient (23—46% yields) entry to cyclohex-3-en-1-ones (170) is by condensations between a-lithio-nitriles and buta-1,3-dienes.Two closely related methods involving Michael additions of 2-(2-lithiophenyl)ethyl halides leading to tetralin derivatives have been reported. Thus, condensation of (171) with the diester (172) results in the formation of the lignan precursor (173),while the aryl-lithium (174) reacts similarly with unsaturated sulphones (175 n = 1 or 2) to give (176) in high yields. A further example of the 

Allenyl cations derived from a-chloroacetylenes add to cycloalka-1,3-dienes to give the [2 -i- 4]cycloadducts (179) in variable yields,and photoadducts of 

Treatment of the enone (185 n = 2, R = H) with Me2CuLi results in the formation of (187) this rarely observed reaction involving replacement of a ketone oxygen by two methyl groups presumably proceeds via the lactone (186). 

Further work from Vollhardt s group on cobalt-catalysed cycloadditions of olefins and acetylenes has led to a synthesis of cyclohexadiene derivatives (188 n = 0—2) by addition of u-enynes to trimethyl silylacetylenes the reactions can also be carried out in an intramolecular sense, leading to tricyclic compounds. The novel tricyclic compound (189) has been obtained from naphthoquinone by a [2 + 2]addition of l,l-bis(silylmethyl)ethylene followed by treatment with a Lewis acid. A key step in the first total synthesis of a decipiene diterpene is the formation of tricyclic ketone (190) by an intramolecular aldol condensation, which required the use of methanolic barium oxide.  

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Other Six-membered Ring Synthesis.— The course of intramolecular aldol condensation of heptane-2,6-diones can be completely changed by varying the reaction conditions (Scheme 14). 

Other Six-membered Ring Synthesis.— The aldolization of 1,5-diones to cyclo-hexenones is one of the commonest methods for elaborating six-membered rings. Whereas Baldwin and Wilkinson have demonstrated the use of the 1,3-dioxolenone (117) as a formylacetone equivalent to elaborate the appropriate keto-aldehyde for six-membered-ring aldolization (Scheme 14),Hosami et have described an alternative route to 1,5-diones that is based on a Michael... 

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