INHIBITORS OF 25-HYDROXYVITAMIN D-3-1-ALPHA-HYDROXYLASE - A-RING OXA ANALOGS OF 25-HYDROXYVITAMIN D-3

The most potent inhibitor known of 25-hydroxyvitamin D-3 1 alpha-hydro xylase (1-OHase), a cytochrome P-450 mixed function oxidase involved i n the production of the steroid hormone 1 alpha,25-dihydroxyvitamin D- 3 (2), is 25-hydroxy-3-deoxy-2-oxavitamin D-3(3b). The latter, prepare d previously in relatively low yield, is unusual because it coexists i n nearly equal proportions with its [1,7]-sigmatropic shifted, previta min D-3 tautomer 4b. A more efficient synthesis of this potent inhibit or was developed by applying Trost's enyne Pd(0) cyclization strategy. Besides succeeding in improving the synthesis of 3b/4b, extension of this approach to the synthesis of other related A-ring oxacycles for s tructure-function studies of the 1-OHase system has been successful. T his venture has resulted not only in oxacycles 4a, 3b/4b, and 3c but a lso their 9,11-didehydro counterparts 5a, 5b, and 5c. The analog 5b wa s anticipated to be of particular interest because it represents an an alog of the potent inhibitor 3b, but the presence of the 9,11-double b ond renders it incapable of undergoing a [1,7]-sigmatropic shift to a form resembling 4b. Biological evaluation of 5b revealed it to be a mo re potent inhibitor of 1-OHase than 3b/4b, suggesting that 3b is the l ikely form of the inhibitor 3b/4b. Initial kinetic experiments indicat e that the analogs (3b, 3c, and 5b) tested do not inhibit by direct me chanism-based enzyme inactivation, revealing rather that inhibition of 1-OHase is competitive. Finally, it should be noted that the syntheti c studies described herein provide new information regarding the scope and limitations of the palladium(0)-mediated enyne cyclization strate gy leading to vitamin D molecules.