A process for preparing compounds of the formula I in which R1 is hydrogen, an aliphatic group having 1-8 carbon atoms, C1-C6-alkoxycarbonyl, C1-C6-alkylthiocarbonyl or a cyclic ring system having 3-14 ring atoms, and R2 is hydrogen, an aliphatic group having 1-8 carbon atoms, or R1 and R2 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  in which R3 and R4 are readily detachable groups and R1 and R2 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.