STUDIES ON VITAMIN-D (CALCIFEROL) AND ITS ANALOGS .37. A-HOMO-11-HYDROXY-3-DEOXYVITAMIN-D - RING SIZE AND PI-FACIAL SELECTIVITY EFFECTS ON THE [1,7]-SIGMATROPIC HYDROGEN SHIFT OF PREVITAMIN-D TO VITAMIN-D

The previtamin D-vitamin D interconversion, a [1,7]-sigmatropic hydrogen shift, provides a convenient system with which to evaluate the structural requirements of this hydrogen migration process. Thus, the A-homoprevitamin D analogues 7a,b and 8a,b were synthesized and their thermal [1,7]-sigmatropic hydrogen rearrangements studied. Relative to the parent previtamin D3 system, the presence of the seven membered A-ring not only accelerates the rate of the rearrangement but also shifts the equilibrium to lie completely in favor of the vitamin. In addition, the 11-hydroxyl group was found to exert a modest effect (approximately 2:1 to approximately 5:1) on the preferred helicity of the antarafacial rearrangement (a pi-facial selectivity effect). In both cases a syn directive effect by the 11-hydroxyl was observed.