High Asymmetric Induction in<i>Lewis</i>Acid-Promoted Intramolecular Ene-Type Reactions: A diastereo- and enantioselective synthesis of (+)-α-allokainic acid. Preliminary Communication
The monocyclic amino diacid (+)-α-allokainic acid 1 has been prepared enantioselectively from the ester of cis-β-chloroacrylic acid and (−)-8-phenylmenthol by a series of four synthetic operations in over 15% yield. The crucial step is the intramolecular ‘ene-type’ reaction of the (Z)-diene 4 which on treatment with a mild Lewis acid undergoes a highly accelerated, dia- and enantiostereoselective cyclization