The invention provides the diastereoselective alkylation of optically active nopinone to form the compound of formula (I) according to scheme A below: ##STR00001## in which: --R is a C.sub.5-15 alkyl group; --R1 is especially a C.sub.1-15 alkyl, C.sub.2-15 alkenyl or C.sub.2-15 alkynyl group or a C.sub.5-15 aryl, each group optionally being substituted; and --X is a halogen atom; and the configuration of the compound of formula (I) is either (E) or (Z) or a mixture of the two.The compound of formula (I) is a valuable synthetic intermediate.