The epothilones structure was confirmed by an X-ray structural analysis, which also gave the absolute configuration of the eight stereogenic centers.
X-ray structural analysis of the epothilone B
The X-ray study shows that intramolecular hydrogen bonds are present between 3-OH and the epoxide oxygen atom, and between 7-OH and the C-5 carbonyl group. The carbon backbone of the macrocycle is largely flat, with maximum deviation from the best-fit plane for C-6 (-0,96 Å) and C-3 (0,85 Å ). The methyl groups C-22, C-25, and C-26 adopt an approximatively axial position and the side chain with the thiazole ring an equatorial position. Only the cis configuration of the epoxide produces strain that the molecule cannot avoid.