Crystal and molecular structure and absolute configuration of Withaperuvin E: a C₂₈ steroidal lactone

Abstract

The withanolide Withaperuvin E, C₂₈H₃₆O₈, has been obtained from the air dried roots of Physalis peruviana and purified by silica gel chromatography. This compound exhibits significant antineo plastic activity. Prismatic pale yellow crystals were obtained from a methanol/acetone solution. The compound crystallizes together with a partially occupied [40(1)%] acetic acid molecule, in space group P2₁2₁2₁, Z = 4, D _c = 1.242 g cm⁻³, with unit, cell parameters a = 11.431(3), b = 14.104(1), c = 17.387(5)Å, V = 2803.2(11)ų, and μ(CuKα) = 0.072mm⁻¹. Through the X-ray analysis the molecule was found to be comprised of a steroidal skeleton with a β-oriented oxygen bridging atoms C(5) and C(6) in ring B. Steroid ring conformations are: A sofa; B sofa; C chair; D intermediate half-chair/sofa; the lactone ring E has a sofa conformation. All rings of the steroid skeleton are trans connected. The absolute configuration of the Withaperuvin E skeleton corresponds to that of naturally occurring steroids, C(18) and C(19) both being β-oriented. The partially occupied acetic acid molecule forms a hydrogen bond to the steroid molecule. Hydrogen bonding also occurs between OH(14) and a symmetry related oxygen O(20) of the steroid.