Pummerer rearrangement, vinylogous

A highly stereoselective vinylogous Pummerer rearrangement of a optically active sulfoxide has been shown to proceed by a [l,4]-migration of the sulfinyl oxygen atom via an intermediate intimate ion pair (Scheme 12).16 The reaction occurs with high enantioselectivity when o-sulfinyl-substituted benzyl carbanions are treated with trimethylsilyl halides. 

The Pummerer rearrangement of sulfoxides with acid anhydrides has been extensively utilized as a method for the synthesis of a-substituted sulfides. When a,(3-unsaturated sulfoxides are used, the initial formed oxysulfonium ion may undergo two different pathways the additive Pummerer reaction or the vinylogous Pummerer reaction. The following sections will consider examples from both pathways. 

Scheme 20.16 Application of vinylogous Pummerer rearrangement in the total synthesis of lyconadin A.

One of the most successful variants of the Pummerer rearrangement is the corresponding vinylogous reaction. In this modification, unsaturated sulfoxides 58 are activated by electrophiles, just as with saturated substrates. The resulting conjugated thionium ions 60 have two electrophilic positions, giving one of two products, that is, the a- or y-isomer (Scheme 20.1 SI. 

More recently, two distinct mechanistic pathways for the Pummerer rearrangement have been described. These are the vinylogous Pummerer pathway, and the additive Pummerer sequence. The vinylogous pathway... 

The use of trimethylsilyl halides has been expanded further to afford optically pure benzyl alcohols through a stereoselective vinylogous Pummerer rearrangement. The reaction proceeds via a l,4 migration of the sufinyl oxygen atom to give (i )-benzyl alcohols 30a-f with > 98% ee. 

An additive Pummerer rearrangement is an apt description of the course of the reaction between AcCl and the vinyl sulphoxide (62). Hot AcgO converts (62) into products (63) and (64) of a vinylogous Pummerer rearrangement. ... 

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