α-Homonojirimycin (1) is an aza-sugar, whose oxygen atom within the monosaccharide ring is replaced with an imino moiety; it has been shown to significantly affect sugar chain processing in vivo. 1 is an excellent glycosidase inhibitor. The mechanism for its inhibition has been explained as that a glutamic acid or asparagine residue of the glycosidase forms an ionic bond with an imino moiety of the aza-sugar.¹⁾ Moreover, 1 is also used as a leading compound of effective agents for the treatment of Fabry disease (disease causing a glyco sphingolipid lysosomal storage disorder), which is caused by an active enzyme deficiency (α-galactosidase A).²⁾