STEREOSPECIFIC ALKYLATION OF THE SCHIFF-BASE ESTER OF ALANINE WITH 2-SUBSTITUTED-(E)-VINYL AND 2-SUBSTITUTED-(Z)-VINYL BROMIDES - AN EFFICIENT SYNTHESIS OF 2-METHYL-(E)-3,4-DIDEHYDROGLUTAMIC ACID, A POTENT SUBSTRATE-INDUCED IRREVERSIBLE INHIBITOR OF L-GLUTAMATE-1-DECARBOXYLASE