Antineoplastic Agents. 36. Crystal and Molecular Structure of the Pseudoguaianolide Autumnolide

A crystal structure analysis of the pseudoguaianolide autumnolide (3) has revealed a conformation of the sevenmem bered ring that is markedly different from that found for the very similar bromohelenalin. The principal difference between the two conformations involves a drastic twist of 111.0° about C6-C7 and associated rotations about all contiguous bonds. As a consequence of this molecular conformation, there is an unusual OH-pπ interaction in autumnolide which is not found in the other pseudoguaianolides. The structure was solved by direct methods analysis of data from a crystal with space group P2₁ and a = 7.595 ± 0.004, b = 13.380 ± 0.009, c = 6.870 ± 0.006 Å, β = 92.87 ± 0.06°, Z = 2, and density ρ_caicd = 1.336 g/cm³. An anisotropic full-matrix least-squares refinement converged to a conventional residual of R = 0.058 for 2086 reflections recorded with Mo Ka radiation on an automatic four-circle diffractometer.