ASYMMETRIC INDUCTION IN THE WITTIG-STILL REARRANGEMENT OF ETHERS CONTAINING AN ALLYLIC STEREOCENTER - DIASTEREOCONTROL BY ALLYLIC OXYGEN

Stannylated allyl ethers 13, 15, 19, and 21 bearing an allylic center of chirality are submitted to the Still variant of the [2,3] Wittig rearrangement. In these reactions alkoxy groups at the chiral center exert stereocontrol through asymmetric induction. With only few exceptions, the syn‐configurated rearrangement product (27) is formed in preference to the anti epimer (28). syn: anti ratios are often >90: <10 and repeatedly ≥98: ≤2 (27/28f,g,m,n). The dependence of asymmetric induction from the structure of the starting material is investigated. The formation of syn product is particularly favorable in unhindered substrates (Scheme 4) with Z rather than E configuration (Table 5). Allylic MOMO and MEMO groups induce higher syn:anti ratios than allylic alkoxy groups (Scheme 5). Anionic allylic directing groups effect modest levels of anti selectivity (Scheme 6). Cyclic n‐membered ethers can rearrange with more (n = 5) or less (n = 6) syn selectivity than open‐chain analogs (Scheme 7). – The magnitude of the vicinal coupling constant 3J between β‐ and γ‐protons may be used to assign the stereochemistry in epimeric syn and anti γ‐alkoxy alcohols. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim