Structure and absolute configuration of florilenalin: X-ray analysis of 4-O-acetyl-2-O-p-iodobenzoylflorilenalin

Abstract

X-Ray single-crystal analysis has established the structure and absolute configuration of (IV), the 4-O-acetyl-2-O-p-iodobenzoyl derivative of florilenalin (II), a cytotoxic guaianolide from Helenium autumnale L. Crystals are orthorhombic, space group P212121 with a= 14·40(2), b= 22·62(3), c= 7·37(1)A, Z= 4. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations to R 0·086 over 1109 reflections from photographic data. The absolute configuration was established by the anomalous dispersion effect. The cycloheptane ring adopts a chair conformation, the cyclopentane ring is in an envelope form, and the γ-lactone ring is approximately planar.