| The acid-catalyzed condensation of ;The syn stereochemistry of three condensation products has been proven by independent synthesis, X-ray crystallographic analysis, and NMR analysis of trans-fused lactones obtained by sodium borohydride reduction. The net reaction of the rearrangement-hydrolysis is the diastereoselective (typically ;The mechanism for the orthoester condensation most likely consists of a series of equilibria involving the reversible, acid-catalyzed formation of an oxonium ion from the ;The stereochemistry of the condensation is proposed to arise from a rapid, diastereoselective Claisen rearrangement of an allyl vinyl orthoester intermediate. The proven stereochemistry of three orthoester condensation products is consistent with a chair transition state. |
