ENANTIOSELECTIVE SYNTHESIS OF (+)-α-ALLOKAINIC ACID BY ASYMMETRIC LEWIS ACID-MEDIATED INTRAMOLECULAR ENE REACTION

Article abstract of DOI:10.1016/S0040-4020(01)82424-8

(+)-α-Allokainic acid ((+)-1) has been prepared for the ester of cis-β-chloroacrylic acid and (-)-8-phenylmenthol by a sequence of four synthetic operations.The crucial step 8d-14d (Scheme 4) is a 100percent diastereo- and 90percent enantioselective intramolecular ene reaction proceeding at -35 degC on treatment of 8d with Me2AlCl.Saponification of 14d regenerated the auxiliary chiral alcohol and yielded (+)-1 on subsequent decarboxylation.In the analogous cyclization 11d-15d the sense of asymmetric induction (78percent d.e.) was opposite as confirmed by the conversion of 15d to (-)-α-allokainic acid.