Four side chain fluorinated analogues of 1α,25-dihydroxy-19-norvitamin D have been prepared in convergent syntheses using the Wittig-Horner reaction as a key step. Structures and absolute configurations of analogues 3 and 5 were confirmed by X-ray crystallography. All analogues showed high potency in HL-60 cell differentiation and vitamin D-24-hydroxylase (24-OHase) transcription as compared to 1α,25-dihydroxyvitamin D3 (1). Most important is that all of the 20S-configured derivatives (4 and 6) had high bone mobilizing activity in vivo. However, in the 20R series, a 2-methylene group was required for high bone mobilizing activity. A change in positioning of the 20R molecule in the vitamin D receptor when the 2-methylene group is present may provide new insight into the molecular basis of bone calcium mobilization induced by vitamin D.

Six new analogues of 1α,25-dihydroxy-19-norvitamin D3 (3a-4b, 5 and 6) were prepared by a convergent synthesis applying the Wittig-Horner reaction as a key step. The influence of methyl groups at C-22 on their biological activity was examined. It was established that both in vitro and in vivo activity is strongly dependent on the configuration of the stereogenic centers at C-20 and C-22. Introduction of the second methyl group at C-22 (analogues 5 and 6) generates the compounds that are slightly more potent than 1α,25-(OH)2D3 in the in vitro tests but much less potent in vivo. The greatest in vitro and in vivo biological activity was achieved when the C-20 is in the S-configuration and the C-22 is in the R configuration. The building blocks for the synthesis, the respective (20R,22R)-, (20R,22S)-, (20S,22R)- and (20S,22S)-diols were obtained by fractional crystallization of mixtures of the corresponding diastereomers. Structures and absolute configurations of the diols 21a, 21b and 22a as well as analogues 3a, 5 and 6 were confirmed by the X-ray crystallography.