Synthesis and Biological Activity of 1α,25-Dihydroxy-18-norvitamin D<INF>3</INF> and 1α,25-Dihydroxy-18,19-dinorvitamin D<INF>3</INF>

1α,25-Dihydroxy-18-norvitamin D<INF>3</INF> and 1α,25-dihydroxy-18,19-dinorvitamin D<INF>3</INF> were prepared via Wittig−Horner coupling of 25-hydroxy-18-nor Grundmann type ketone with the corresponding A-ring phosphine oxides. Configuration at C-13 in the 18-nor Grundmann type alcohol (C,D-ring synthon), obtained by oxidative degradation of vitamin D<INF>3</INF>, was determined by ¹H NMR spectroscopy and molecular mechanics calculations. Additional proof of the assigned trans-C/D-junction of the key intermediate 18-nor Grundmann type ketone follows from its chiroptical properties (circular dichroism data) and further chemical transformations. 1α,25-Dihydroxy-18-norvitamin D<INF>3</INF> was found more potent than 1α,25-dihydroxyvitamin D<INF>3</INF> in binding to the porcine intestinal vitamin D receptor (5−10×), in differentiation of HL-60 cells (5−10×), and in inhibition of HL-60 proliferation. 1α,25-Dihydroxy-18,19-dinorvitamin D<INF>3</INF> appeared equally active as 1α,25-dihydroxyvitamin D<INF>3</INF> in these activities. In vivo, 1α,25-dihydroxy-18-norvitamin D<INF>3</INF> was only slightly less active than 1α,25-dihydroxyvitamin D<INF>3</INF> in intestinal calcium transport and bone calcium mobilization, while 1α,25-dihydroxy-18,19-dinorvitamin D<INF>3</INF> showed activities 10 times lower. These studies imply that deletion of C-18 does not impair activity of analogs of 1α,25-dihydroxyvitamin D<INF>3</INF>.