Acta Cryst. (1978). B34, 1311-1316 The Crystal and Molecular Structure of a Mineral Diterpene, Bombiccite, C20H34 BY E. FORESTI SERANTONI, A. KRAJEWSKI AND R. MONGIORGI Istituto di Mineralogia e Petrografia, Universitgt di Bologna, Italy L. RIVA DI SANSEVERINO Istituto di Mineralogia, Petrografia e Geochimica, Universitdt di Palermo, Italy AND G. M. SHELDRICK University Chemical Laboratory, Lensfield Road, Cambridge, England (Received 6 April 1977; accepted 23 August 1977) A natural product found in lignite mines has been assigned the structure ofa-dihydrophyllocladene by crystal structure analysis. The crystals are triclinic, P1, a = 11.39 (7), b = 21.29 (5), c = 7.45 (1)/~,, tt = 94.6 (1), fl = 101.8 (2), y = 81.5 (1) ° and Z = 4. The structure has been solved by direct methods followed iteratively by Fourier syntheses and least-squares refinement of site occupation factors of the resulting peaks. The four independent molecules have the same configuration at the six chiral centres. Rings A, B and C assume the chair conformation with trans junctions, and ring D is an envelope. 1311 Introduction Several organic minerals are in the Mineralogy Museum, University of Bologna, and one of them, simonellite, C t9H24, has already been studied (Foresti & Riva di Sanseverino, 1969). Bombiccite, present as a whitish crust on lignite from Castelnuovo di Valdarno, near Florence (Bombicci, 1869), was named after a former Professor of Mineralogy in Bologna, who encouraged the first chemical studies on the mineral. There was little evidence and much uncertainty about its chemistry (Ciusa & Galizzi, 1921), until a more recent crystallo- graphic study by powder diffractometry (Pellizzer, 1955a) and then by single-crystal data (Pellizzer, 1955b) identified bombiccite with hartite (Strunz, 1974), chemically t~-dihydrophyllocladene (I). The only structure from this family of natural diterpenes included on the Cambridge Crystallographic Data Files is phyllocladan-15-yl bromoacetate (Brown & Hall, 1976). Four other papers on the parent compound kauran (II) (Nakanishi, Goto, Ito, Natori & Nozoe, 1975) are available (Karle, 1972; Hanson, McLaughlin & Sim, 1972; Ferguson & Marsh, 1975; Taylor & (I) (II) Watson, 1976). The diterpenes display extraordinary variations of molecular skeleton and occurrence in nature. Bombiccite particularly has resisted micro- biological degradation. In view both of the uncertainty of its constitution and of the stereochemical problems, an X-ray crystallo- graphic study of bombiccite was started as early as 1969. Experimental Samples of the mineral were collected, dissolved and purified by repeated crystallization from benzene. Plate-like colourless crystals were obtained by slow evaporation. Preliminary crystal data and a full set of intensities were collected by rotation and Weissenberg photo- graphs, using three crystals for the triclinic cell of Table 1. Crystal data for bombiccite C20H34 M r = 274 m.p. 75°C (tt-dihydrophyllocladene 74 °C) a = 11.39 (7)/~ a= 94.6 (1) ° b = 21.29 (5) fl= 101.8 (2) e = 7.45 (1) y= 81.5 (1) V = 1707.7 ,~3 Z = 4 d c = 1.07 g cm -3 d,, = 1.08 g cm -3 (flotation) Space group P1 F(000) -- 616 (from structure solution)