Recemic 5-hydroxy-2-methylcyclohex-1-enylmethyltriphenylphosphonium bromide, prepared from racemic 5-hydroxy-2-methylcyclohex-1-enylmethanol, gave, in a Wittig reaction with cyclohexene-1-aldehyde, a crystalline conjugated trienol with a chromophore similar to that of the tachysterols. In a similar manner one enantiomorph of 5-hydroxy-2-methylcyclohex-1-enylmethanol provided an optically active phosphonium bromide which, in reaction with 9-formyl-des-AB-cholest-8-ene. gave tachysterol3, isolated as the 4-methyl-3,5-dinitrobenzoate The corresponding ester of 3-epitachysterol3 was similarly obtained. The work constitutes the first purely chemical synthesis of a 9,10-seco-sterol.