An X-ray examination of geometrical factors affecting conjugative overlap between a cyclopropane ring and an adjacent chromophore in two 3,5-cyclosteroid derivatives

Abstract

The structure of 6-oxo-3α,5-cycloandrostan-17-yl p-bromobenzoate, C₂₆H₃₁O₃Br (3), was solved by the heavy-atom method from Weissenberg film data and refined to R 0·17 (1643 observed reflections). The structure of 6-oxo-3β,5-cycloandrostan-17-yl acetate, C₂₁H₃₀O₃(4), was solved by direct-phasing methods from diffractometer data and refined to R 0·10 (1888 observed reflections). Crystal data: (3), monoclinic, space group P2₁, a= 6·45(1), b= 28·69(2), c= 6·690(4)Å, β= 109·97(10)°, Z= 2; (4), orthorhombic, space group P2₁2₁2₁, a= 7·423(1), b= 11·184(1), c= 22·110(1)Å. Both compounds possess the normal steroidal nucleus for rings B, C, and D. The introduction of the C(3)–C(5) bond into the steroidal nucleus changes greatly the general shape of ring A and the two different orientations of the hydrogen at C(3) considerably affect the geometry of the two molecules in the region of the A and B rings. The valency angles at C(5) are distored in both molecules to accommodate the strain of the 3,5-cyclization. The u.v. spectra of the two parent compounds [(1) and (2)] indicate a greater amount of conjugative overlap between the cyclopropane ring and the carbonyl group for (1) than for (2). The spectral differences could not be attributed to a difference in the torsion angle O(1)–C(6)–C(5)–C(3) since these were found to be the same in (3) and (4). They may, however, be due to decreased p character for the C(3)–C(5) bond in compound (4) as the valency angle, C(3)–C(5)–C(6) increases from 114 in (3) to 130° in (4).