Preparation of (3R,3'R)-zeaxanthin (I) comprises: (i) reacting (4R)-4-hydroxy-2,2,6-trimethyl-cyclohexanone (II) with dichloromethyl lithium in an inert solvent at -120 to -40 degrees C and warming the reaction mixture to 20-60 degrees C; (ii) reacting the obtained (4R,6R)-1-formyl-2,2,6-trimethyl-7-oxa-bicyclo (2.2.1) heptane of formula (IV) (in isolated form or directly in the reaction mixture) with acetone or a dialkyl (2-oxo-propyl)-phosphonate; (iii) converting the obtained (4R,6R)-1-(3-oxo-but-1-en-1-yl)-2,2,6-trimethyl-7-oxa-bicyclo (2.2.1) heptane of formula (V) into (4R,6R)-1-(3-hydroxy-3-methylpenta-1,4-dienyl)-2,2,6-trimethyl-7-oxa-b icyclo (2.2.1) heptane (VI) by conventional vinylation (or ethynylation followed by partial hydrogenation); (iv) reacting (VI) with a triaryl phosphine (preferably PPh3) and a strong acid to give a mixture of a C15 triaryl phosphonium salt of formula (III) and the isomer of formula (III'); and (v) subjecting 2 moles of the mixture to a double Wittig olefin synthesis reaction with 2,7-dimethyl-octa-2,4,6-triene-1,8-dial (VIII) to give (I). Ar = aryl (preferably optionally substituted phenyl); and X = 1 equivalent of an anion of a strong acid (preferably Cl, Br or HSO4). The preparation of (I) by step (v) is claimed per se. Also claimed are (IV) and (V) and (III') in mixtures with (III).