N-Benzyloxycarbonyl-2,5-dideoxy-2,5-imino-3,4-O-isopropylidene-L-ribose 12a has been converted into (1R,2S,6R, 7S,7aS)-5 and (1R, 2S, 6S, 7R, 7aR)-1,2,6,7-tetrahydroxypyrrolidin-5-ones 6 and (1R,2S,6S,7S,7aS)-7 and (1R,2S,6R,7R,7aS)-1,2,6,7-tetrahydroxypyrrolizidines 8 following stereoselective paths. These new compounds have been assayed for their inhibitory activities towards 25 glycosidases. Pyrrolizidines 7 and 8 are moderate but selective inhibitors of amyloglucosidase from Rhizopus mold (7: IC50 = 130 muM, K-i = 120 muM; 8: IC50 = 200 muM, K-i = 180 muM, mixed type of inhibition). (C) 2003 Elsevier Ltd. All rights reserved.