Wittig reaction of the vitamin D-derived aldehyde (I) with 3-(methoxycarbonyl)-2-propenyl-1-idene triphenylphosphorane (II) produced the dienoate ester (III). Subsequent addition of ethyllithium to the ester group of (III) furnished the tertiary alcohol (IV). Photochemical isomerization of the 5-6 double bond of (IV) gave rise to the (5Z)-isomer (V), which was finally desilylated using tetrabutylammonium fluoride in THF.