To obtain a bicyclic heterocyclic compound which is useful as a raw martial for antimicrobial compounds by reaction of a specific halohbydroxynitrile with hydrochloric acid and an alcohol, allowing the product to react with an aminoacetal, then effecting ring closure, thiolation, halogenation and ring closure reactctions in turns.

A compound of formula I (X1 is a halogen), for example, (R)-4- chloro-3-hydroxybutyronitrile is treated with hydrogen chloride and a compound of the formula: R1-OH (R1 is a 1-6C alkyl) to form an imino ether of formula II, which is allowed to react with an aminoacetal of formula III [R2 is a 1-6C alkyl, a 2-6C alkenyl, a (substituted) aryl; R3 and R4 are 1-6C alkyl] followed by ring closure reaction into an imidazole derivative. The derivative is subjected to thioetherification, halogenation and ring closure reaction in turns and is finally hydrolyzed to give the objective bicyclic heterocyclic compound, for example, 6-mercapto-6,7-dihydro-5H-pyrrolo[1,2-a]imidazole.