The structure of hirsutic acid. X-ray analysis of the p-bromophenacyl ester

Abstract

The structure of hirsutic acid has been established by X-rays analysis of the p-bromophenacyl ester. The crystals are orthorhombic, a= 6·49, b= 9·14, c= 35·64 Å, Z= 4, space group P2₁2₁2₁, but irradiation with X-rays initiates an unusual molecular rearrangement without disruption of the crystal structure, and with only minor changes in lattice parameters (to a= 6·56, b= 9·38, c= 35·14 Å) and in the intensities of the reflexions. Two complete sets of intensity data were measured with Cu–K_α radiation: the first consisted of 800 reflexions estimated visually from films, and the second of 1379 (984 observed) reflexions measured with a scintillation counter. Structure analysis by Patterson, electron-density, and least-squares methods (final R= 0·136) indicated that the irradiated crystals contain two different molecules randomly distributed, and established the structure and absolute configuration (anomalous dispersion method) of the p-bromophenacyl ester of hirsutic acid, and of the rearrangement product.

The molecules contain three five-membered rings, each of which has an envelope or slightly distorted envelope conformation. The bond distances, and valency angles in both molecules are normal. The most significant intermolecular distances correspond to O–H ⋯ O hydrogen bonds; these hydrogen bonds are not present in the unirradiated crystals but involve an OH group of the rearrangement product (produced by opening of an epoxide ring of hirsutic acid) and an OH group of the original molecule and/or a carbonyl oxygen of the rearrangement product. All other intermolecular contacts correspond to normal van der Waals interactions.