The invention relates to a process for preparing compounds of the formula I ##STR1## in which R stands for hydrogen, alkyl or aralkyl and R.sup.1 to R.sup.5 are identical or different radicals, (substituted) alkyl, cycloalkyl or (substituted) aryl or in which the pairs of radicals R.sup.1 and R.sup.2, R.sup.2 and R.sup.3, and R.sup.4 and R.sup.5 together with the carbon atom(s) supporting them form a mono- or bicyclic ring system and the other radicals are hydrogen, by treating a compound of the formula II ##STR2## in which R and R.sup.1 and R.sup.5 have the abovementioned meaning and R.sup.6 and R.sup.7 denote alkyl or aralkyl or together with the nitrogen atom supporting them form a heterocycle which can additionally contain an oxygen atom, with a reducing agent.