Molecular conformations. Part VI. The structure of withaferin A: X-ray analysis of withaferin A acetate p-bromobenzoate

Abstract

X-Ray crystal-structure analysis of withaferin A acetate p-bromobenzoate, C₃₇H₄₃BrO₈, has established the constitution and absolute stereochemistry of withaferin A, a novel tumour-inhibiting 5β-steroidal lactone isolated from Acnistus arborescens. The crystals are orthorhombic, of space group P2₁2₁2₁, with eight molecules of the p-bromobenzoate and four molecules of ethyl acetate in a unit cell of dimensions a = 14·74, b = 40·35, c= 12·95 Å. The atomic co-ordinates were determined by Fourier and least-squares calculations. The conformation of ring A is boatlike. Ring B adopts a distorted half-chair form and ring C a chair form. Ring D deviates slightly from the envelope from in which C(13) is the out-of-plane atom.