A Highly Stereoselective and Completely Regiospecific Method for the Dehydration of β-Hydroxy Esters via β-Alanoxy Enolates. Application to the Synthesis of Trisubstituted Olefins and Two Ant Mandibular Gland Secretions

The treatment of a β-hydroxy ester with aluminum ethoxide or diethoxyaluminum chloride, followed by warming with added lithium diisopropylamide, produces the corresponding α,β-unsaturated ester in moderate yield (44-56%). This process is completely regiospecific (no β, isomers formed) and in suitably substituted cases is completely stereospecific, producing only the E olefin isomer. It is conceived that the reaction proceeds through a β-alanoxy enolate intermediate, similar to that postulated in the lithium aluminum hydride reduction of β-dicarbonyl enolates; the stereoselectivity results from the minimization of 1,4 interactions in a boat cyclohexane-like transition state. This alanoxy enolate dehydration process has been applied in the stereo- and regiospecific syntheses of two ant mandibular gland secretions.