| http://www.w3.org/ns/prov#value | - The formyl acid esters to be used are obtained by various ways disclosed in the literature, thus, e.g., by analogy with the method described in EP 417 597 or DE 2 643 205 of hydroformylation of .alpha.,.beta.-unsaturated esters.The compounds of the formula (III) are likewise known or are obtainable by generally known methods.To facilitate the ring closure reactions, bases such as sodium hydroxide,
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