ACYCLIC STEREOSELECTION IN THE ORTHO ESTER CLAISEN REARRANGEMENT

The ortho ester Claisen rearrangement of trisubstituted allylic alcoho ls exhibits significant levels of diastereoselection. In E allylic alc ohols, a 1,3-diaxial interaction develops in the chairlike transition state leading to the anti isomer, rendering the reaction syn selective by a factor of 3-5 to 1. In Z allylic alcohols, the 1,3-diaxial inter action develops in the transition state leading to the syn isomer, gen erating an anti:syn selectivity of 6-15 to 1. The relative stereochemi stry of the syn isomer was confirmed independently by the synthesis of the mycotoxin botryodiplodin.