Investigation of the Chemo- and Stereoselectivity of the Ketene-Claisen Rearrangement

Abstract

A novel type of ketene-Claisen rearrangement in which the precursor of the rearrangement is generated in situ by reaction of optically active allyl thioethers with dichloroketene is described. A characteristic feature of this rearrangement is the excellent chemoselectivity in favor of allyl thioethers vs. allyl ethers, i.e., exclusive chirality transfer of the allylic sulfur moiety is observed with 12, 13, and 25- 27. The cyclic, optically active allyl thioethers (+)-(R)-4 and (-)-(S)-4 and the open-chain allyl thioethers 11-13 rearrange with in situ generated dichloroketene to the optically active thioesters (-)-(S)-28, (+)-(R)-28, and 31-33, respectively. A chirality-transfer of > 99% in the cyclic cases (+)-(R)-4 and (-)-(S)-4, and 96-98% in the open-chain cases 11-13 is observed. Furthermore, the dichloroketene-Claisen rearrangement is characterized by a high asymmetric 1,2-induction. The chiral allylic sulfides 25-27 give the optically active thioesters 36-38 with a 1,2-induction > 99% as determined by NMR-shift experiments.