ve, and R2 is benzyl group, R3 is cyano group and R4 is carbamoyl group, or R3 and R4 are taken together to represent a group having the formula: --CO--Y--CH2 CH2 -- in which Y is oxygen atom or --NH--, or a group having the formula: ##STR10## and when R1 and R2 are the same or different and each is an alkyl group having 1 to 3 carbon atoms, R3 and R4 are ##STR11## in which n1 and R6 are as define