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 3 J between β‐ and γ‐protons may be used to assign the stereochemistry in epimeric syn and anti γ‐alkoxy alcohols.