| http://www.w3.org/ns/prov#value | - The cyclization is preferablycarried out using mineral acids such as hydrochloric acid or sulfuric acid.The process of the present invention for preparing compounds of the formula I can be summarized by means of the following reaction scheme, where, for example, R.sup.1 is an alkyl group, R.sup.2 is hydrogen, R.sup.3 is the group H.sub.3 C-CO- andR.sup.4 is an alkoxy group: ##STR11##The yield in t
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