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

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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.