Compounds of formula I ##STR1## wherein R.sub.1 is free or esterified carboxy, hydroxymethyl or formyl, R.sub.2 and R.sub.4 are each independently of the other aliphatic, cycloaliphatic-aliphatic, araliphatic or heteroarylaliphatic radicals and R.sub.3 is azido, or amino that is aliphatically or araliphatically substituted and/or protected by an amino-protecting group, and the salts thereof, are valuable intermediates in the preparation of medicinal active ingredients, for example of compounds of formula II ##STR2## wherein R.sub.A is an aromatic or heteroaromatic radical, R.sub.2 and R.sub.4 are each independently of the other aliphatic, cycloaliphatic, cycloaliphaticaliphatic or araliphatic radicals, R.sub.3 is unsubstituted or N-mono- or N,N-di-lower alkylated or N-lower alkanoylated amino, and R.sub.B is an aliphatically, cycloaliphatically or heteroaromatically-aliphatically substituted amino group, and the salts thereof, which can be used, for example, as antihypertensives.