The disilylated methyl quinicate (I) is oxidized to ketone (II) employing RuCl3 and NaIO4. Subsequent Wittig reaction of ketone (II) with methylenetriphenylphosphorane leads to olefin (III). Reduction of the ester function of (III) is performed by either treatment with DIBAL or, in an improved process, with LiAlH4. The resultant vicinal diol (IV) is then subjected to oxidative cleavage with NaIO4, producing ketone (V). Petrson olefination of (V) with methyl (trimethylsilyl)acetate affords the unsaturated ester (VI), which is further reduced with DIBAL to the allylic alcohol (VII). Tosylation of alcohol (VII), followed by displacement of tosylate (VIII) with the lithium salt of diphenylphosphine generates the allylic phosphine (IX). This is then oxidized by H2O2 to the phosphine oxide (X).
Wittig-Horner coupling of phosphine oxide (X) with the protected 25-hydroxy Grudmann's ketone (XI) produces the target 19-norvitamin D derivative (XII). Finally, complete desilylation of (XII) with a cation-exchange resin in MeOH/benzene leads to the title compound.