A process for preparing a compound having the formula: ##STR1## wherein R is selected from the group consisting of C.sub.3 -C.sub.6 -cycloalkyl, C.sub.3 -C.sub.6 -cycloalkylmethyl, C.sub.1 -C.sub.6 -alkyl, allyl and 2-methoxyethyl, by reacting: ##STR2## under defined conditions with subsequent stepwise reactions of the respective intermediate compounds with a malonic acid diester, POCl.sub.3, and by heating the final intermediate in a high boiling solvent.