A process for preparing compounds of the formula I ##STR1## in which R.sup.1 is hydrogen, an aliphatic group having 1-8 carbon atoms, C.sub.l -C.sub.6 -alkoxycarbonyl, C.sub.l -C.sub.6 -alkylthiocarbonyl or a cyclic ring system having 3-14 ring atoms, and R.sup.2 is hydrogen, an aliphatic group having 1-8 carbon atoms, or R.sup.1 and R.sup.2 together with the carbon atom to which they are bound form a cyclic or bicyclic ring system having 3-14 ring atoms, comprises the preparation of compounds of the formula II ##STR2## in which R.sup.3 and R.sup.4 are readily detachable groups and R.sup.1 and R.sup.2 are as defined above, as starting materials or intermediates and the cyclization of these under suitable reaction conditions to give compounds of the formula I. The intermediates of the formula II are novel.