Elucidafion of Origin of"Syn-Effect" and Application to Organic Synthesis

研究了LDA对[3-取代（E）-1-丙烯基]环氧乙烷1，4-消去开环生成2，4-二烯醇的立体化学。2，4-二烯醇的Z/E比随丙烯基3位取代基的不同而变化。用σ→π ~* 相互作用解释了这一现象，并研究了（E）-1-对甲苯磺酰基-1，3-丁二烯与胺的亲核加成反应。所得烯丙基砜的Z/E比随胺、溶剂、温度和浓度而变化。当二乙胺浓度较低、反应温度较高时，优先得到相应的空间位阻不佳的（Z）-4-氨基-2-丁烯基砜。研究了（E）-2，4-异戊烯酸乙酯与胺的亲核加成反应，发现酯基取代对甲苯磺酰基（Ts）作为共轭吸电子基团，实现了类似的高（Z）-选择性.在δ-苄氧基取代的乙烯基环氧乙烷中，[1，2]-Wittig重排在1，4-消除开环后进行，完全立体选择性地生成（E，Z）-2，4-二烯基-1，6-二醇。以α，α-二甲基烯丙基砜为原料，依次进行1，4-消去开环-[1，2]-Wittig重排反应，得到相应的（Z）-二烯醇。

Stereochemistry of the 1,4-eliminative ring opening of [3-substituted (E) -1-propenyl]oxiranes to the corresponding 2,4-dienyl alcohols by LDA was investigated. The Z/E ratios of the resulting 2, 4-dienyl alcohols varied with the substituents at 3-position of the propenyl group. This phenomenon was discussed based on the concept of a "syn-effect," which is most primarily rationalized by a σ→π* interaction.Stereochemistry of nucleophilic addition of amines to (E) -1-tosyl-l,3-butadiene was investigated. The Z/E ratios of the resulting allylic sulfones varied with amines, solvents, temperature, and concentration. When diethylamine was reacted in low concentration at higher temperature, the corresponding sterically unfavorable (Z) -4-amino-2-butenyl sulfone was preferentially obtained. Stereochemistry of nucleophilic addition of amines to ethyl (E)-2,4-pentadienoate, which possesses an ester group as a conjugated electron-withdrawing group instead of p-toluenesulfonyl(Ts) group, was also investigated to realize similar high (Z)-selectivity. The predominant formation of (Z)-isomers in both cases was rationalized by a "syn-effect," which might be mainly due to n/σ→π* interaction and/or 6r-electron homoaromaticity.In the case of δ-benzyloxy substituted vinyloxiranes, [1,2]-Wittig rearrangement proceeded following the initial 1,4-eliminative ring opening to give a (E,Z) -2,4-dienyl 1,6-diols in a completely stereoselective manner. The sequential 1,4-eliminative ring opening-[1,2]-Wittig rearrangement also proceeded using α, α-dimethyl allylic sulfones to give the corresponding (Z)-dienyl alcohols.