Reduction of vitamin D derivative (I) with NaBH4 in ethanol-THF affords alcohol (II), which is alkylated with propargyl bromide (III) and potassium tert-butoxide in the presence of a crown ether. The resulting propargyl ether (IV) is condensed with hexafluoroacetone, by means of butyl litium, to provide tertiary alcohol (V). This compound is then photochemically isomerized in the presence of anthracene as a sensitizer to give (VI). Final deprotection of TBDMS groups with tetrabutylammonium fluoride in THF yields the title compound.

Vitamin D derivative (I) was condensed with ethyllithium al low temperature to give, after chromatographic separation of isomers, alcohol (II). Alkylation of (II) with 6-bromo-3-ethyl-3-(trimethylsilyloxy)hexane (III) and potassium tert-butoxide in the presence of a crown ether afforded ether (IV), which was then photochemically isomerized in the presence of anthracene as a sensitizer to give (V). Final deprotection of silyl groups of (V) with 5% hydrogen fluoride in ethyl acetate-acetonitrile yielded the title compound.