The catalytic epoxidat ion by d ioxygen with trans-d ioxorut hen i u m (vi) tet ramesityl porp hyri n [Ru(O),(tmp)] of the acetic esters of cholesterol, 3-epicholesterol and isocholestero1,t as well as of the 7a-epimer of the latter, is P-stereospecific. Substitution by a methyl group on C-6 of pregnenolone acetate $-results in reduced reactivity towards catalytic epoxidation and lower P-stereoselectivity. 1 9-Norsterol esters bearing a double bond at C-8-C-14 or C-14-C-15 are inert towards 0,-RU(O),(tmp) epoxidation. The variable reactivity of these sterol ester substrates is explained by a transition state in which the steroid nucleus approaches the ruthenium-oxo bond approximately perpendicular to the porphyrin ring. The P-selectivity of A5-sterol ester epoxidation is accounted for in terms of this transition state geometry which provides a good fit between the porphyrin catalyst and the steroid substrate when the 9-side faces the 0x0 ligand. On the other hand, reaction on the or-side involves unfavourable steric interactions between axial hydrogen atoms on C -3 and C-7 of the substrate and the porphyrin ring and a mesityl substituent of the catalyst, respectively. The crystal and molecular structures of cholesteryl ethyl carbonate and of its 5,6P-epoxide have been determined by single-crystal X-ray diffraction. The overall conformation of the steroid nucleus is nearly planar in the cholesteryl ester, while it is bent at the junction between rings A and B in the 5,6P-epoxide. This change from pseudo-trans-to cis-stereochemistry of the A-B ring junction upon epoxidation is proposed to amplify the pdiastereofacial selection. Variable temperature 'H N M R spectra indicate that in CD,CI, solution the 5,6pepoxide (not the 5,6a-epoxide) of the cholesteryl acetate coordinates the ruthenium atom of Ru (CO) (tmp) with a nearly perpendicular geometry. These results corroborate the orthogonal substrate approach and the steric origin of the P-stereospecificity in Ru (O),(tmp) -catalysed steroid epoxidations. * This stereochemistry has been found earlier in the structure of 3pacetoxy-5,6~-N-nitroaziridinylcholestane.zz Paper 2/0 12 1 8 I

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