| Abstract
| - Efficient syntheses of A-ring synthons 24 and 32 are described from hydroxy ester 16, which iseasily available on a preparative scale from (−)-quinic acid. Key features of the syntheses were (a)the ability to selectively perform desilylations in the presence of p-nitrobenzoate esters and (b) theexcellent yield and complete stereospecificity with which the configuration of alcohols 16, 18, and26 could be inverted under Mitsunobu conditions. Thus, A-ring synthons 24 and 32 were bothprepared in 35−38% yield (eight steps) from the common precursor 16. The coupling of A-ringsynthons 24 and 32 with the appropriate CD-ring/side chain fragment 7 provides access to novel6-s-cis locked analogues of steroid hormone 1α,25-dihydroxyvitamin D3: 1α,25-dihydroxy-3-epi-19-nor-previtamin D3 (37) and 1β,25-dihydroxy-3-epi-19-nor-previtamin D3 (38), which are unableto undergo rearrangement to the respective vitamin D form by virtue of the absence of the C-19methyl group. Compounds 37 and 38 can be used as tools for studying the genomic and nongenomicmechanisms of action of the previtamin form of the hormone 1α,25-dihydroxyvitamin D3.
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