Antiproliferative Hybrid Analogs of the Hormone 1α,25-Dihydroxyvitamin D3:  Design, Synthesis, and Preliminary Biological Evaluation

The combination of 10−12 kbar pressure plus Lewis acidic zinc dichloride promotes highly regioselective and stereoselective Diels−Alder cycloaddition between 3-bromo-2-pyrone, prepared in a new way, and unactivated terminal alkene 4-(tert-butyldimethylsiloxy)-1-butene. The conjugate base of A-ring allylic phosphine oxide 20 adds chemospecifically to the C-8 keto group of some C-8,C-24-diketones to form directly metabolically resistant 24-oxo analogs of 1α,25-dihydroxyvitamin D3 (calcitriol). Several of these new hybrid analogs are as efficacious in vitro as calcitriol at inhibiting growth of murine keratinocytes even at physiologically relevant 10−100 nanomolar concentrations.