Trans-3-trimethylsilyl allylic alcohols

The Claisen rearrangement has been used to prepare j8,y-unsaturated amides, largely as the E-isomers, from 3-(trimethylsilyl)allyl alcohols and amide acetals (Scheme 48). Overall yields and stereoselectivities are excellent. Conditions have been found under which the stereochemical outcome of the related ynamine-Claisen rearrangement can be controlled. Thus, the kinetically favoured intermediate is the E-enamine (140) which leads to the "trans product (142), whereas the Z-isomer (141) is thermodynamically favoured and gives the cw ... 

Epoxidation of acyclic allyl alcohols with peracid and Mo/TBHP displays an opposite stereospecificity to that for the V/TBHP system. Trimethylsilyl-substituted allylic alcohols give t/zreo-epoxyalcohols with MCPBA and erythro-alcohols with VO(acac)a-TBHP, with high stereoselectivity. In the stereospecific epoxidation of cis- and trans-allyl alcohols, formation of a transition state is assumed with the development of two H bonds between the hydrogen atom of the hydroxy group of the allyl alcohol and the oxygen of the peracid, and between the hydrogen of the peracid OH and the oxygen of the ether 10. An analysis of the diastereometric transition-state interactions for stereoselective epoxidation of acyclic allylic alcohols has been published. A conformational effect may be responsible for the unexpected cis major product in Eq. 2. 

An important clue as to how this could be done came from work done by Henbest and coworkers [2]. This group compared the diastereoselectivity of peracid oxidation reactions of 3-hydroxy and 3-acyloxycyclohex-2-enes (Scheme 8.2). When the alcohol was capped by an acetate group, the trans addition product predominated. Better selectivity was later obtained by placing a larger trimethylsilyl group on the allylic alcohol [3]. In both cases, the source of the selectivity could be ascribed to the approach of the reagent from the least hindered side of the molecule anti to OR) put another way, the approach from one face was slowed relative to the other (Scheme 8.2a). 

Unique to E-vinyl derivatives of 9-BBN is their Grignard-like addition to aldehydes [6] to afford stereodefined alcohols. trans-E-Vinyl-9-BBN synthesis via dehydroborylation [7] process is extended to the synthesis of trans-2-tnmeth-ylsilyl vinylborane [8]. trans-2-Trimethylsilylvinyl-9-BBN adds cleanly to aldehydes (Eq. 6.17) to provide pure frans-3-trimethylsilyl(TMS) allylic alcohols in excellent yields (Table 6.18) [8]. ... 

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