As a part of our ongoing research of the taxane-type constituents from T. baccata L. (Taxaceae) recorded in the Flora of Turkey, 2 alpha, 5 alpha, 10 beta -triacetoxy-14 beta-(2-methyl)-butryloxy-4(20),11-taxadiene (I) and taxusin (II) were isolated from the heartwood, in addition to the other taxoids. Their structures were elucidated using extensive spectroscopic techniques (IR, H-1 NMR, C-13 NMR, DEFT 135, H-1 H-1 COSY, HMQC, HMBC and MS). Molecular mechanic calculations and conformational analysis of I and II have been established using the MM3 program. The most probable conformation for compound I was obtained using semi-empirical molecular orbital methods and X-ray crystal structure results of II. Molecular orbital calculations of the derivative compound 4(20), 11-taxadiene (III) have been carried out to compare semi-empirical and experimental knowledge about molecular geometries in gas phase and solid state. (C) 2001 Elsevier Science B.V. All rights reserved.