The diastereoselectivity of electrophilic attack on trigonal carbon adjacent to a stereogenic centre

Abstract

This paper summarises the work described in the nine papers immediately preceding it. The diastereoselectivity of attack on a trigonal carbon adjacent to a stereogenic centre is governed by the general rule expressed most simply by the drawing 34, except when the medium-sized group on the stereogenic centre is methyl at the same time as the substituent cis to the stereogenic centre is a hydrogen atom, when reaction may or may not take place in the sense of the drawing 35. When the large group on the stereogenic centre is a silyl group, the diastereoselectivity of electrophilic attack is high, as shown by enolate alkylations and aldol reactions, and the reactions of allylsilanes with osmium tetroxide, peracid, the Simmons–Smith reagent and boranes. The protodesilylation of allylsilanes can be used to control the geometry of trisubstituted double bonds, notably those exocyclic to a ring, provided that the medium-sized group on the stereogenic centre is an isopropyl group. Two regiocontrolled syntheses of allylsilanes that are secondary at both ends of the allylic system are reported in the last two papers. The first of the two syntheses gives largely trans-allylsilanes, but the second provides good control, of double bond geometry with either the cis- or the trans-configuration. Both syntheses are amenable to the synthesis of homochiral allylsilanes.