On the mechanism of base-induced rearrangements of epoxides to ketones:: a formal synthesis of (S)-physoperuvine

The enantioselective alpha-deprotonation-rearrangement of trans- and cis-5-silyloxycycloheptene oxides (trans-8 and cis-8) using organolithiums in the presence of(-)-sparteine 16 or (-)-alpha-isosparteine 17 to give predominantly 4-silyloxycycloheptanone [(R)-9 (up to 87% ee), a known intermediate in the synthesis of (S)-physoperuvine (10), and (S)-9 (up to 71% ee), respectively] is described; the results indicate an alpha-ring opening mechanism. (C) 1999 Elsevier Science Ltd, All rights reserved.