Stereochemistry of the cucurbitacins. Crystal and molecular structure and absolute configuration of datiscoside bis-(p-iodobenzoate) dihydrate

Abstract

The molecular structure and absolute configuration of datiscoside (Ia) have been determined from a three-dimensional, single-crystal, X-ray analysis of its bis-(p-iodobenzoate) dihydrate (Ib).

Crystals have orthorhombic symmetry, space group P2₁2₁2₁, with a= 19·609(7), b= 31·485(17), c= 8·743(3)Å, Z= 4. The structure was solved by the heavy-atom method, and refined by least-squares methods to R 0·101 for 1627 independent reflections measured by counter diffractometry. The absolute configuration was determined by the method of anomalous dispersion.

The steroidal nucleus of datiscoside has a folded conformation, with a cisoid fusion of rings B and C, and a trans-fusion of rings C and D. Rings A and C are in the chair conformation, while rings B and D are in the half-chair conformation. The previously uncertain configurations at C(2) and C(20) in the cucurbitacins have been unambiguously established. The sugar residue is 2-O-acetyl-6-deoxy-α-L-gluco-hex-3-ulopyranoside.