(1 S ,2 S ,3 R ,6 R )‐6‐Aminocyclohex‐4‐ene‐1,2,3‐triol (= (−)‐Conduramine B‐1) Is a Selective Inhibitor of α ‐Mannosidases. Its Inhibitory Activity Is Enhanced by N ‐Benzylation

(−)‐ and (+)‐Conduramine B‐1 ((−)‐ and (+)‐ 5 , resp.) have been derived from (+)‐ and (−)‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one (‘naked sugars’ of the first generation). Although (−)‐ 5 imitates the structure of β ‐glucosides, it does not inhibit β ‐glucosidases but inhibits α ‐mannosidases selectively. N ‐Benzylation of (−)‐ 5 improves the potency of conduramine B‐1 as α ‐mannosidase inhibitor and also generates compounds inhibiting β ‐glucosidases. For instance, (−)‐ N ‐benzyl‐conduramine B‐1 ((−)‐ 19a ) is a competitive inhibitor of β ‐glucosidase from almonds ( IC 50  = 32 μ M , K i  = 10 μ M ) and a weak inhibitor of α ‐mannosidases from jack bean ( IC 50  = 171 μ M ) and from almonds ( IC 50  = 225 μ M ) whereas (−)‐ N ‐(4‐phenylbenzyl)conduramine B‐1 ((−)‐ 19g ) is a good inhibitor of α ‐mannosidase from jack beans ( IC 50  = 29 μ M , K i  = 4.8 μ M ) and a weaker inhibitor of β ‐glucosidase from almonds ( IC 50  = 32 μ M , K i  = 7.8 μ M ) ( Table 1 ).