Unsaturated steroids. Part 13. Further observations upon the formation of aromatic ring c steroids: X-ray structure of 22.23-dibromo-10-methyl-19-noranthraergosta-5,7,9(10),14-tetraene and of 2β,3α,22,23-tetrabromo-18-nor-17-isoergosta-8(9),11,13(14)-triene.

Abstract

The aromatisation of ring c in 7α,11α,22α,23α-tetrabromo-5α-ergost-8-en-3β-yl acetate (1), with the concomitant elimination of C-18 as methyl halide, has been investigated. The structures and absolute stereochemistry of the two steroids named in the title, and produced in the aromatisation have been defined by X-ray crystallography: the mechanisms of these aromatisations are discussed. Crystals of the anthrasteroid (2) are monoclinic, space group F2 with eight molecules in a cell of dimensions a= 31.38(2), b= 5.918(2), c= 28.469(9)Å, β= 102.20(1)°. Crystals of the 18-nor-17-isoergosta-8,(9),11,13(14)-triene derivative (4) are monoclinic, space group P2₁, with two molecules in a cell of cell dimensions a = 10.512(2), b= 7.682(1), c= 17.148(2)Å, β= 91.59(2)°. Both structures were solved by the heavy atom method and refined by full-matrix least-squares calculations. For (2), R= 0.051, R_w= 0.055 for 853 observed reflections; the corresponding values for (4) are 0.052, 0.055 and 1.136 respectively. In (2), rings A and C adopt symmetrical half-chair conformations (but ring A is disordered over two sites); the aromatic ring B is essentially planar and the cyclopentene ring D has a C-17 envelope conformation. Molecule (4) has ring A in a distorted chair conformation with trans di-axial bromine atoms, ring B is in a sofa conformation, aromatic ring C is planar, and ring D is a C-16 envelope. The bromine-containing side chain is maximally extended in both (2) and (4).