Santonic acid: catemeric hydrogen bonding in a gamma,epsilon-diketocarboxylic acid

The title compound, (-)-2,3,3a,4,5,6,7,7a-octahydro-alpha,3a,5-trimethyl-6, 8-dioxo-1, 4-methano-1H-indene-1-acetic acid (C15H20O4), exists in the soli d state as a catemer, with the hydrogen bonding following a 2(1) screw axis from the carboxyl to the a-ketone of a neighboring molecule [O ... O = 2.7 52(3) Angstrom]. Two parallel counterdirectional screw-related single-stran d helices pass through the cell in the a direction. Four intermolecular C=O ... H-C close contacts were found, involving all three carbonyl groups.