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 A, Z = 4, space group P2,2121, 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 = 35-1 4 8 ) and in the intensities of the reflexions. Two complete sets of intensity data were measured with Cu-K, 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 = 0.1 36) 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 --0 hydrogen bonds; these hydrogen bonds are not present in the unirradiated crystals but involve a n OH group of the rearrangement product (produced by opening of a n 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.HIRSUTIC ACID is one of the metabolites isolated from Stereum hirsutum.l The present Paper describes the determination of its structure by X-ray analysis of the p-bromophenacyl ester. During the course of the analysis it became apparent that an unusual solid-phase rearrangement was being initiated by the irradiation with X-rays, and that an approximately 1 : 1 mixture of the starting material and the rearranged product was being produced, without disrupting the crystal structure, and with only minor changes in lattice parameters and in the intensities of the X-ray reflexions. In spite of this complication it was possible, with the help of two pieces of chemical evidenceJ2 to determine the structures and absolute configurations of the 9-bromophenacyl ester of hirsutic acid as (I ; R = $-bromophenacyl), and hence of hirsutic acid as (I; R = H ) , and of the rearrangement product as (I1 ; R = $-bromophenacyl).
EXPERIMENTALThe p-bromophenacyl ester of hirsutic acid was prepared by heating equimolar amounts of hirsutic acid and pbromophenacyl bromide under reflux in dry acetone containing potassium carbonate. Recrystallisation from ethanol gave colourless prisms elongated along a, with (001) and ( 010) developed. The density was measured by flotation in aqueous potassium iodide, and the unit-cell dimensions and space group were determined from various rotation, Weissenberg, and precession...
