Molecular conformations. Part XII. Conformational distortions in tetracyclic diterpenoids: X-ray diffraction studies of the p-bromobenzenesulphonyl derivatives of beyeran-3α-ol and 7β-hydroxykaurenolide

Abstract

The crystal structures of the p-bromobenzenesulphonyl derivatives of beyeran-3α-ol (3) and 7β-hydroxykaurenolide (4) have been elucidated by three-dimensional X-ray methods. In (3), rings A, B, and C have chair conformations, and a flexing of the perhydrophenanthrene skeleton so that the β-face is concave results in the C(20)⋯ C(18) and C(20)⋯ C(15) diaxial separations on the α-face of the molecule being 3·26 and 3·38 Å. In (4), on the other hand, ring A is very appreciably distorted from ideal chair geometry, with torsion angles which range from 33–67°, and rings B and C adopt distorted twisted boat conformations; these features are attributed to the γ-lactone which bridges rings A and B on the α-face and prevents an A,B,C-chair form of the molecule from flexing like (3) to relieve the C(20)⋯ C(14) diaxial repulsion. The five-membered ring D approximates closely to an envelope form in (3) but is distorted towards a half-chair form in (4). The C(8)–C(14)–C(13) bridging valency angle in the bicyclo[3,2,1]octane moiety is substantially smaller than tetrahedral in both derivatives, 104° in (3) and 100° in (4). Both p-bromobenzenesulphonyl groups have conformations in which the S–OC bond is approximately perpendicular to the plane of the benzene ring; the S–O–C–H torsion angle is 9° in (3) and –22° in (4). Derivative (3) crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 12·454 ± 0·005, b= 27·734 ± 0·011, c= 7·308 ± 0·003 Å, and Z= 4. Derivative (4) crystallizes in the monoclinic space group P2₁ with a= 9·82, b= 11·45, c= 11·33 Å(all ±0·01 Å), β= 111° 40′± 20′, and Z= 2. The atomic co-ordinates were obtained by Fourier and least-squares calculations in which only the bromine atoms were assigned anisotropic thermal parameters, and the absolute configuration of each molecule was established by the anomalous-dispersion method. The final values of R are 10·2% over 2499 reflections (diffractometer) for (3) and 10·8% over 2222 reflections (photographic) for (4).