Molecular structure of (±)-3-methoxy-6-oxa-8-isoestra-1,3,5(10)-trien-17β-ol acetate

The molecular structure of (+/-)-3-methoxy-6-oxa-8-isoestra-1,3,5(10)-trien-17 beta-ol acetate C20H26O4 is determined by X-ray structure analysis: sp. gr. P (1) over bar; a = 8.347(2), b = 9.821(2), c = 12.355(2) Angstrom; alpha = 108.08(2)degrees beta = 97.79(2)degrees, and gamma = 109.55(2)degrees; Z = 2; d(calc) = 1.255(1) g/cm(3); and R-w = 0.063 for 1756 nonzero unique reflections with I > 4 sigma(I). It is shown that substitution of the oxygen atom for the 6-CH2 group results in the shortening of the C(5)-O(6) bond length to 1.37 Angstrom, whereas the O(6)-C(7) distance (1.44 Angstrom) corresponds to the normal C-O bond length (1.43 Angstrom). This substitution changes the geometry of the B ring relative to that observed in carbon analogues, thus giving rise to a significant strain in the ring.