title: (-)-Parasantonic acid and its enol lactone, (+)-parasantonide: observation of the rare acid-to-acid catemeric hydrogen-bonding mode in a gamma,epsilon-diketocarboxylic acid.
abstract: The title diketo acid, (-)-alpha,3a,7-trimethyl-5,8-dioxo-1,4-ethanoperhydropentalene-1-acetic acid, C(15)H(20)O(4), is shown to aggregate in the solid state as acid-to-acid hydrogen-bonded catemers, whose chains follow 2(1) screw axes from each carboxyl H atom to the C=O group of a neighboring carboxyl group [O.O = 2.672 (4) A and O.H-O = 173 degrees ]. Two parallel counterdirectional screw-related single-strand hydrogen-bonded chains pass through the cell in the a direction. Two intermolecular C=O.H-C close contacts are present in this compound. Both this diketo acid and its enol lactone, (+)-parasantonide [systematic name: (-)-alpha,3a,7-trimethyl-5-oxo-1,4-ethenoperhydropentalene-1,8-carbolactone], C(15)H(18)O(3), have an R configuration at the methylated chiral center adjacent to the carboxyl group, unlike the precursor from which they are derived, viz. (-)-santonic acid.
authors.affiliation: Instituto de Quimica Organica de Sintesis (CONICET-UNR), Facultad de Ciencias Bioquimicas y Farmaceuticas, Suipacha 531, 2000 Rosario, Argentina. jzinczuk@fbioyf.unr.edu.ar
authors.forename: Juan
authors.initials: J
authors.lastname: Zinczuk
authors.forename: Edmundo A
authors.initials: EA
authors.lastname: Ruveda
authors.forename: Hugh W
authors.initials: HW
authors.lastname: Thompson
authors.forename: Roger A
authors.initials: RA
authors.lastname: Lalancette