Structure and absolute configuration of plenolin: X-ray analysis of plenolin p-iodobenzoate

Abstract

The structure and absolute configuration of the cytotoxic pseudoguaianolide sesquiterpene lactone, plenolin (II), have been determined by X-ray analysis of the p-iodobenzoate derivative (III). Crystals of (III) are orthorhombic, space group P21212, with a= 9·29(1), b= 26·85(1), c= 8·45(1)A, Z= 4. The structure was solved by the heavy-atom method and the atomic parameters refined by full-matrix least-squares calculations to R 0·041 over 1165 independent reflections from diffractometer measurements. The absolute configuration was established by the anomalous dispersion effect. The cycloheptane ring adopts a twist chair conformation the γ-lactone ring is in an envelope conformation, and the cyclopentenone ring approximates a half-chair form.