New steroid-fused P-heterocycles: Part II. Synthesis and conformational study of oxazaphosphorino[16,17-e]estrone derivatives

16β-Aminomethyl-17β-hydroxyestrone 3-methyl ether 6 and its N-propyl (17), N-benzyl (18) and N-arylmethyl derivatives (19–22) were subjected to ring closure reactions with phenylphosphonic dichloride in order to synthetize P-epimeric oxazaphosphorinanes 23a, 24–29 in which the hetero ring is condensed to ring D of the sterane skeleton. The stereostructures of the products were evaluated by 1H, 13C and 31P NMR spectroscopy. The geometry was optimized by utilizing the B3LYP DFT method. The NMR spectral data and the results of the ab initio calculations demonstrated that the stereostructure of the hetero ring was strongly affected by the rigid sterane framework condensed to it, and the phosphoramidate ring proved to adopt predominantly a distorted-boat conformation, regardless of the P-configuration.