合成路线1

Synthesis of ketone (XXIV): Reaction of the known diol (XV) ­ obtained by degradation of vitamin D2 ­ with TsOH in pyridine gives, regioselectively, the monotosylate (XVI), which is protected at the secondary OH group by silylation with TBDMS-Cl and imidazole to yield the silyl ether (XVII). Oxidation of the tosyloxy group of (XVII) with O2 and t-BuOK in DMSO/t-BuOH affords ketone (XVIII), which is reduced with K and i-PrOH providing alcohol (XIX). Condensation of compound (XIX) with ethyl prop-ynoate (XX) by means of NMM in benzene gives the alkoxyacrylate (XXI), which is reduced with H2 over Pd/C in EtOH to yield the alkoxypropionate (XXII). Deprotection of the OH group of (XXII) with HF in acetonitrile affords alcohol (XXIII), which is oxidized with pyridinium dichromate (PDC) in CH2Cl2 providing ketone (XXIV). Wittig-Horner condensation of ketone (XXIV) with phosphine oxide (XIV) by means of BuLi in THF yields the corresponding adduct (XXV), which is submitted to a Grignard reaction with MeLi in THF to give the protected maxacalcitol precursor (XXVI). Finally, this compound is desilylated by means of TBAF in THF.