A structure 3 erroneously deduced for the marine fungal metabolite leptosphaerin on the basis of spectroscopic evidence and an ambiguous x-ray analysis was disproved by synthesis. An alternative formulation 1, including its absolute configuration, was confirmed by a stereospecific synthesis that began with condensation of the acetonide 8 of (R)-glyceraldehyde with the dianion from (Z)-N-methyl-2-benzyloxy-3-phenylthioacrylamide (34). The derived α-hydroxy lactone 43 was converted to the enamide moiety of leptosphaerin via decomposition of azidolactone 45. Leptosphaerin is thus the γ lactone of (4S,5R)-2-acetamido-4,5,6-trihydroxy-2-hexenoic acid and, as such, is a derivative of a D amino hexose.