100 g. of 2,6-dichloro-4-methoxy-5-methyl-benzyl-triphenylphosphonium chloride, prepared from 52 g. of 2,6-dichloro-4-methoxy-3-methyl-benzyl chloride and 57 g. of triphenylphosphine, are treated with 42 g. of 7-formyl-3-methyl-octa-2,4,6-trien-1-oic acid ethyl ester. After the addition of 120 ml. of 1,2-butylene oxide, the resulting mixture is heated to 80°-85° C. for 4 hours while stirring. The mixture is diluted with toluene/hexane (1:1 parts by volume) and exhaustively extracted with methanol/water (60:40 parts by volume). The resulting solution is evaporated under reduced pressure. The oily residue is purified by adsorption on silica gel using toluene for the elution. 9-(2,6-dichloro-4-methoxy-5-methyl-phenyl)-3,7-dimethyl-nona-2,4,6,8-tetraen-1-oic acid ethyl ester is obtained from the eluate and melts at 100°-101° C. after recrystallization from benzene/hexane (50:50 parts by volume).
