WE CLAIM: 1. A process for preparing compounds of formula I of the accompanying drawings (where R represents a 3-methyl-2-butenyl or 4-methyl-3-pentenyl) group, which comprises reacting hydrazobenzene or a reactive derivative thereof, with a reactive derivative of malonic acid of formula II (where R is as defined above and the groups X are halogen atoms and lower alkoxy groups), in the presence of a known cyclizing agent, under substantially anhydrous conditions, whereby a compound of formula I is formed.
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