.beta.-Carotene products with a high proportion of the 9(Z) isomer are prepared starting from mother liquors from the industrial preparation of .beta.-ionylideneethyltriarylphosphonium salts (C.sub.15 -triarylphosphonium salts) by Wittig reaction of a C.sub.15 -triarylphosphonium salt which has been enriched in the 9Z isomer directly with .beta.-apo-12'-carotenal or else with 2,7-dimethyl-2,4,6-octatrienedial and subsequently with the C.sub.15 -triarylphosphonium salt and subsequent thermal isomerization.
The present invention relates to a process for preparing quaternary phosphonium salts by reacting a ternary phosphine with an optionally substituted, saturated or monounsaturated or polyunsaturated electrophile having from 3 to 25 carbon atoms in ternary solvent mixtures.
