NEW MATERIAL: The compound of formula I and formula II [wherein R is H, alkyl, alkenyl, aryl, acyl, etc.; R1 is H; R2 is H, F, formula III (wherein R10 is alkyl); R3 is OH or, together with R2, represents formula IV (wherein R6, R7 each are alkyl or, together with R1, represents epoxide); R4, R5 are each H, OH protecting group; R11 is H, OH; R21 is H, OH, F or, together with R11, represents keto; R31 is OH, benzyl, formula V (wherein R101 is alkyl, aryl or the like) etc., or, together with R21, represents formula VI (wherein R61, R71 are each alkyl); R41, R51 are each H, OH protecting group].

EXAMPLE: N-Butyl-1,2-dideoxy-2-fluoronojirimycin.

USE: A glycosidase inhibitor.

PROCESS: The compound of formula VII (wherein R8 and R9 are each H, alkyl or aryl) is obtained, e.g. by allowing N-protected-1-deoxynojirimycin compound, after its 4-OH and 6-OH protected, to react with dialkyl tin oxide and allowing the obtained cyclic starenin derivative to react with an acylation agent, a benzylation agent or an allylation agent.