PURPOSE: To easily obtain a compd. as a sedative, etc., by heating a specified compd.

CONSTITUTION: A compd. of formula I (where R1 is a 1-5C alkyl or an aryl; R2 is a 1-10C alkyl, benzyl, etc.; Ar is an aryl; R4 is H or a 1-10C alkyl; R5 is a 1-10C alkyl and R4 and R5 may bond to each other to form a 5-6C cycloalkyl) is heated to obtain a compd. of formula II. This method includes the resolution of the compd. of formula I by the contact of a mixture of stereoisomers of the compd. with (+)-di-p-toluoyl-D-tartaric acid hydrate, the separation of the stereoisomers and the heating of the separated stereoisomers.