Mannostatin A (1) is a naturally-occurring alpha-mannosidase inhibitor whose carbocyclic structure represents a significant departure from known alkaloid-based glycosidase inhibitors. The total synthesis of 1 [(1R,2R,3R,4R,5S)-1-(methylthio)-2,3,4-trihydroxy-5-aminocyclopentanel, as well as those of several analogs and derivatives, was devised in order to probe structure-activity relationships in this family of N-linked glycoprotein biosynthesis inhibitors. The synthetic strategy features an asymmetric hetero Diels-Alder reaction and a highly syn-stereoselective alkene dihydroxylation using OsO4. Both N-benzylmannostatin and mannostatin sulfone exhibit good competitive inhibition of jack bean alpha-mannosidase (K(I) = 380 +/- 81 and 126 +/- 16 nM, respectively), although not as potent as that of 1. interestingly, enantiomerically pure 3,4-bis-epi-mannostatin is also a modest competitive inhibitor of jack bean alpha-mannosidase (K(I) of 16 +/- 2 muM), comparable in activity to 1-deoxymannojirimycin. Synthetic samples of both diastereomeric sulfoxides of 1 exhibit activity comparable to that of 1. Finally, epoxide 41, the 3,4-anhydro derivative of 1, was synthesized and shown to inactivate jack bean alpha-mannosidase in a time-dependent manner (K(I) = 153 +/- 26 muM; k(inact)/K(I) = 160 min-1 M-1).