Abstract: Novel bicyclic tetrahydroxylated pyrrolizidines are disclosed which are inhibitors of glycosidase enzymes. A preferred inhibitor is 1.alpha., 2.alpha., 6.alpha., 7.alpha., 7.alpha..beta.-1,2,6,7-tetrahydroxypyrrolizidine. It is synthesized from D-glycero-D-gulo-heptono-1,4-lactone.Novel Intermediate compounds prepared during this synthesis are 7-O-tert-butyldiphenylsilyl-2,3:5,6-di-O-isopropylidene-D-glycero-D-gulo-h ept ono-1,4-lactone and 1.alpha., 2.alpha., 6.alpha., 7.alpha., 7.alpha..beta.-1,2:6,7-di-O-isopropylidene-1,2,6,7-tetrahydroxypyrrolizidi ne.