Patent ID: 7335778

Claim:
A process for preparing a compound represented by the formula (I): wherein R 1 and R 2 are the same or different from each other, and each represents a hydrogen atom, a benzyl group optionally having a substituent(s) on the benzene ring, a benzhydryl group optionally having a substituent(s) on the benzene ring, or a trityl group; R 3 represents a cyano group, a carboxyl group, an alkoxycarbonyl group, an alkylthiocarbonyl group, or a carbamoyl group optionally having a substituent, or a salt thereof; said method comprising: subjecting a compound represented by the formula (II-a) or a salt thereof to ring transformation by reacting the compound at 0° C. to 200° C.: wherein the symbols have the same meanings as defined above; thereby producing the compound of formula (I).