Studies on the Mechanism of the [3+2] Annulation Based on the Brook Rearrangement

在[3+2]环法制备功能化环戊烯醇的过程中，β -杂原子取代丙烯基硅烷与酮烯醇酯反应的产物分布高度依赖于β -取代基。因此，与(E)-和(Z)- - -苯基硫代衍生物的观察结果相反，在这些衍生物中，无论酰基硅烷的几何形状如何，都能以几乎相同的比例得到异构环戊烯醇，而三甲基硅基衍生物则能以取决于乙烯基硅烷几何形状的比例得到单个环戊烯醇和未环烯基硅醚。为了使这些结果合理化，我们假设了一个反应过程，其中包括两个相互竞争的途径，这取决于丙烯基硅烷中β取代基的α -碳稳定能力；(a)中间体离域烯丙基阴离子的分子内醛醇反应，1,2-加合物的Brook重排产物；(b)由1,2-加合物经Brook重排/环丙烷化顺序生成的环丙酸酯的氧阴离子加速乙烯基环丙烷重排。为了支持这一观点，我们利用烯酸锂与丙烯基硅烷与醋酸t-丁酯的反应，比较了苯硫代和三甲基硅基对α -碳离子的稳定能力，提供了1,2加成产物和布鲁克重排/烯丙基重排产物的比例，反映了b取代基对α -碳离子稳定能力的差异。利用四种异构体乙烯基环丙酯与MeLi （2.2eq）的反应，研究了低温氧阴离子加速乙烯基环丙烷环戊烯重排，得到环戊醇和未环烯醇硅基醚的比例取决于乙烯基硅烷的几何形状。

Product distributions in the reactions of beta-heteroatom-substituted acryloylsilanes with ketone enolates, which was used in [3+2] annulation for preparation of functionalized cyclopentenols, highly depend upon the beta-substituent. Thus, in contrast to the observation with (E)- and (Z)-beta-phenylthio derivatives in which isomeric cyclopentenols were obtained in almost same ratio irrespective of the acylsilane geometry, the trimethylsilyl derivative afforded a single cyclopentenol and uncyclized enolsilyl ether in the ratio depending on the vinylsilane geometry. In order to rationalize these results, we postulated a reaction course which involves two compet-ing pathways depending of the alpha-carbanion-stabilizing ability of the beta-substituents in the acryloylsilane ; (a) intramolecular aldol reaction of delocalized allylic anion in termediate, Brook rearrangement product of 1,2-adduct, and (b) oxyanion-accelerated vinylcyclopropane rearrangement of cylcopropanolate which is derived from the 1,2-adduct via Brook rearrangement/cyclopropanation sequence. To get support to this proposal, we compared alpha-carbanion-stabilizing ability of the phenythio and the trimethylsilyl groups using the reactions of the acryloylsilanes with lithium enolate to t-butyl acetate, providing 1,2-addition product and Brook rearrangement/allylic rearrangement product in the ratio reflecting the difference of alpha-carbanion stabilizing ability of the b-substituent. And we examined low-temperature oxyanion accelerated vinylcyclo-propane-cyclopentene rearrangement using the reaction of four isomeric vinylcyclopropyl acetates with MeLi (2.2eq), affording the cyclopentenol and the uncyclized enol silyl ether in the ratio depending on the vinylsilane geometry.