ASYMMETRIC-SYNTHESIS VIA HETEROCYCLIC INTERMEDIATES .39. ASYMMETRIC-SYNTHESIS OF (ENANTIOMERICALLY AND DIASTEREOMERICALLY VIRTUALLY PURE) METHYL 2-AMINO-4,5-EPOXY-3-HYDROXY-ALKANOATES AND METHYL 2-AMINO-3-HYDROXY-4,5-METHYLENE-ALKANOATES BY THE BISLACTIMETHER METHOD