Synthesis of (1α)-1,25-Dihydroxyvitamin D3with aβ-Positioned Seven-Carbon Side Chain at C(12)

A convergent synthesis of an analogue of (1α)-1,25-dihydroxyvitamin D 3 (1b) with a C 7 side chain at C(12), i.e., of 5 (Fig.), is described. A key step of the synthesis is the assembly of the triene system by a Pd II-catalyzed ring closure of an enol triflate ('bottom' fragment) followed by coupling of the resulting Pd II intermediate with an alkenylboronate ('upper' fragment) (Scheme 2). The synthetic strategy allows isotopic labelling at the end of the synthesis. Copyright © 2012 Verlag Helvetica Chimica Acta AG, Zurich, Switzerland.