A reaction of methy (4R,5R)-4,5-epoxy-2(E)-hexenoate (1) with N-methylbenzylamine gave a diastereomerically pure (3S,4R,5R)-7 and (4S,5R)-8. The former was chemoenzymatically converted to (-)-osmundalactone (12) which is an aglycone of osmundalin. On the other hand, direct conjugated addition of dimethylamine to methyl (4S,5S)-4,5-epoxy-2(E)-hexenoate (1) followed by treatment with MeOH at 40 degrees C exclusively provided (4R,5S)-17 which was converted into L-forosamine (19).