The structure and absolute configuration of natural lentiginosine isolated from plant sources was determined to be (1S,2S,8aS)-1,2-dihydroxyindolizidine ((+)-4) on the basis of the synthesis of both enantiomers (+)-4 and (-)-4 and their inhibition of amyloglucosidases. Alkaloid (+)-4 was derived from (L)-(+)-tartaric acid via a highly stereo- and regioselective 1,3-dipolar cycloaddition of (3S,4S)-3 ,4-bis[(tert-butyldiphenylsilyl)oxy]-1-pyrroline N-oxide to methylenecyclopropane, followed by thermal rearrangement of the adduct into (1S,2S,8aS)-1,2-[(tert-butyldiphenylsily)oxy]octahydroindolizin-7-one. The enantiomer (-)-4 was derived in the same way from (D)-(-)-tartaric acid. Both (+)-4 and (-)-4 displayed inhibition specificity for amyloglucosidases, being inactive toward 17 other glycosidases. With amyloglucosidase from Aspergillus niger, synthetic (+)-4 displayed inhibition (K-i = 2 mu M) 5 times stronger than that reported for natural lentiginosine, 35 times that measured for (-)-4, and twice that of castanospermine. Alkaloid (+)-4 is thus the most potent and specific competitive inhibitor of amyloglucosidases among azasugars and their analogues.