Abstract
Treatment of ß,ß-disubstituted ,ß-unsaturated acid chlorides with Et3N in the presence of an alcohol leads to mixtures of vinylketenes, which were trapped as the ß,y-unsaturated ester. These studies establish that deprotonation occurs largely on the less substituted 7-carbon (CH3 > CH2 > CH) due to the greater kinetic acidity of protons on a less substituted alkyl group. The stereochemistry of the a,d-unsaturated acid chloride has no effect on the mixture of vinylketenes formed. This result is in marked contrast to the deprotonation of ,ß- unsaturated esters, acids, and amides with lithium bases, which occurs regiospecifically on the 7-carbon syn to the carbonyl group. Vinylketenes 5, 11, 16, and 21 undergo an intramolecular [2 + 2] cycloaddition with the terminal double bond to give adducts 6, 12, 17 and 18, and 23 with an exocyclic double bond. In two cases, cycloadducts 14 and 25 with an endocyclic double bond were also isolated. These minor adducts, which were formed only in cases where cyclization should be slow, appear to be formed by interconversion of vinylketenes by protonation-deprotonation prior to cyclization.