Enantioselective BIMP-catalysed [2,3]-Wittig Rearrangements of Oxindole-Derived Allylic and Propargylic Ethers

Abstract

The organocatalytic enantioselective [2,3]-Wittig rearrangement of a range of oxindole-derived allylic and propargylic ethers using bifunctional iminophosphorane (BIMP) organosuperbase catalysts has been investigated, generating 3-hydroxyoxindole derivatives in high stereoselectivity (>35 examples, up to 99:1 er). In the allylic ether series, substituent variation has a significant effect upon product diastereoselectivity, with excellent enantioselectivity observed in all cases. The incorporation of a C(3)-fluorine substituent leads to improved diastereoselectivity, giving products bearing a stereogenic tertiary fluoride, in up to 94:6 dr and 98:2 er. In the propargylic series, the [2,3]-rearrangement generates α-allenyl alcohols in excellent yield and enantioselectivity (up to 98% yield, 99:1 er) at 60 ˚C, with a range of substituents around the oxindole core, as well as alkyl and aryl-substitution at the alkyne functionality demonstrated. Functionalisation of the α-allenyl alcohol to spirocyclic oxindoles without loss of stereochemical integrity has also been demonstrated.