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

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.