The compounds are of the general formula ##STR2## wherein R.sub.3 represents a hydrogen atom, an alkyl group having 1 to4 carbon atoms or a benzyl group; 1 and m are the same or different and are an integer of from 1 to 4; n is 1 or 2; and the definitions of R.sub.1 and R.sub.2 include groups such as C.sub.1 to C.sub.6 alkyl, C.sub.3 to C.sub.6 cycloalkyl and C.sub.1 toC.sub.6 alkoxy groups.The pr