In which R1 and R2 are alkoxy groups of one to four carbon atoms, R3 and R4 each is hydrogen or an alkoxy group of one to four carbon atoms, and R5 and R6 each is an alkyl group of one to four carbon atoms or an arylalkyl group such as benzyl or R5 taken with R6 and N are pyrrolidinyl, piperidino, or morpholino groups and the pharmaceutically acceptable salts thereof and processes for preparing th