Structure of olean-12(13)-en-3β-yl acetate

Abstract

fl-Amyryl acetate, C32H5202, M r = 4 6 8 . 8 , monoclinic, P21, a = 7.338 (7), b = 16.466 (6), c = 11.394(3)A, f l=92 .83 (4 ) o, V = 1375.1A 3, Z = 2 , D x = 1.13 gcm -3, Mo Ka, 2 = 0.71073/~, /t = 0.63 cm -~, F(000) = 520, 138 K, final R -0.069 for 140 independent reflections with I > 2tr(/). The steroid rings A, B, D and E have chair conformations and ring C a symmetrical sofa conformation. Only the DIE ring junction is cis. The fl face of the molecule is convex. The bond lengths and angles are in good agreement with those of fl-amyrin analogues, apart from a shortening of bond lengths around C 17. Introduction. 3fl-Hydroxyolean12(13)-ene or fl-amyrin occupies a central position in the chemistry of the triterpenes. It occurs both free and as simple esters in a wide variety of higher plants (De Mayo, 1959) and is the parent compound of innumerable oxygenated analogues (Connolly & Overton, 1972). It is thus surprising that while X-ray structure determinations have been carried out on a number of the more oxygenated analogues of fl-amyrin (Hoge & Nordman, 1974; Roques, Druet & Comeau, 1978; Roques, Declercq & Germain, 1978; Roques, Comeau, Forme, Kahn & Andre, 1977; Mak, Chiang & Chang, 1982; Kitagawa et al., 1982) the parent compound has not been so studied. Other relevant crystal structures that have been determined are those of simple mamyrin esters (a-amyrin differs from fl-amyrin only in the position of the C29 methyl group) and of methyl ursolate (mamyrin-28-carboxylate) (Grynpas & Lindley, 1979; Paton & Paul, 1979). An authentic sample of fl-amyryl acetate was isolated from mistletoe (Viscum album L.) by the procedure of Sakurai & Okumura (1971). Crystals were grown from ethanol. Experimental. Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Mo K~t radiation. Crystal dimensions 0.04 x 0.07 x 0.14 mm. Temperature at crystal site 138 K. Cell parameters based on least-squares fit of 25 independent reflections with 20> 20 °. Intensity data recorded using the og-scan technique with a constant scan speed of 4 ° min -1. Crystal orientation checked every 100 recordings. Three standard reflections measured every 2 h varied by less than 2%, variations were irregular with respect 0108-2701/87/122362-03501.50 to time, intensity data were not corrected for decay. Lp correction, absorption ignored. 2239 independent reflections recorded (20 < 48°), h = 0 to 8, k = 0 to 18, l 1 2 to 13, 832 with I < 2.0tr(I) regarded as unobserved. Structure solved by MULTAN80 (Main et al., 1980). Full-matrix least-squares minimization of ~w(AF) 2, where w -l = tr2(I)/4LpL tr2(I) = t72(1c) +pie, and p = 0 . 0 2 . Isotropic refinement, with H atoms placed geometrically (C--H = 0.95 A), but not refined. Ratio of LS shift to e.s.d. <0.01. Difference Fourier map after the final LS iteration showed maximum residual electron density of 0.34 e ./k -3. The refinement converged to a conventional R = 0.069, wR = 0.050 and S = 1.48.* The absolute configuration could not be determined from the data. Atomic scattering factors were from International Tables for X-ray Crystallography (1974). All computer programs used are part of the Enraf-Nonius SDP-Plus package (Frenz, 1983). Discussion. The structure of fl:amyryl acetate is in general terms close to those already found for the more oxygenated analogues of the fl-amyrin series. Complete structural information is available for gymnemagenin (Hoge & Nordman, 1974), hederagenin (Roques, Druet & Comeau, 1978), 3fl-acetoxyolean12(13)-en-28fl-oic acid (Roques, Declercq & Germain, 1978) and for * Lists of structure factors, H-atom parameters and torsional angles have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 44233 (15 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.