Some Uses of Silicon Compounds in Organic Synthesis

The stereochemistry with which allylsilanes react with electrophiles is reviewed. Stereochemically defined allylsilanes (14 and 15) can be prepared by the stereospecific anti displacement of tertiary allylic acetates (9-12) with the phenyldimethylsilyl-cuprate reagent (13). These allylsilanes react with electrophiles with an overall anti bias. In protonation (Scheme 13) and especially in acylation (Scheme 15), the anti bias is not strong enough completely to overcome the axial and equatorial preferences shown by the electrophiles. In epoxidation, however, the anti bias nearly completely overcomes the inherent axial preference of this reaction. The method can be applied in other systems: for example, endo2-propenyl-exo-2-norbornanol (42) can be easily prepared from norbornanone. INTRODUCTION The complete control of stereochemistry is a major goal in organic synthesis. The problem is especially acute when chiral centres are neither adjacent nor in ring systems, and remote chiral control is therefore particularly challenging. One way of achieving this kind of control is to move chiral information one, two, or three places along a chain using a stereospecific reaction. Thus, it has long been established (Ref. 1) that the 5N2' reaction can have overall syn selectivity, and can therefore pass chirality three places along a carbon chain. The problem is that the 5N2' reaction is rare (5N2 reactions in allylic systems are far more common that 5N2 and the syn preference is neither strong nor reliable (Ref. 2). However, the Claisen rearrangement can be viewed as a variation of the 5N2' reaction; it is regiochemically and stereochemically reliable (Scheme 1), and it has therefore found much use SCHEME 1 H in synthesis precisely to move stereochemical information along a carbon chain (Ref. 3). However, it is limited in the range of substituents and functionality that it can set up, and it would clearly be useful to have other methods as reliable as this. One possible candidate is the 5E2' reaction, except that this has received almost no attention until very recently. THE STEREOCHEMISTRY OF THE SE2' REACTION OF ALLYLSILANES The 5E2' reaction of allylsilanes (Scheme 2) is regiochemically reliable (Ref. 4). What is needed is much more information about its stereochemistry. The problem has been the dearth SCHEME 2