with R2,is subjected to an addition reaction with a carbodiimide of the formula R3 --N???C???N--R4in which R3 and R4 independently of one another are an alkyl, cycloalkyl, aralkyl or aryl radical,and the resulting isourea ether of the formula ##STR13## in which R1, R2, R3 and R4 have the meaning indicated above and R5 is hydrogenare reacted with a carboxylic acid of the formula R6 --CO2 Hin which