(-)-Dioxosantadienic acid: hydrogen-bonding patterns in a bicyclic sesquiterpenoid keto acid and its monohydrate

The anhydrous form, (I), of the title compound, (-)-2-(1,2,3,4,4a,7-hexahydro-4a,8-dimethyl-1,7-dioxo-2-naphthyl)- propionic acid, C15H18O4, derived from a naturally occurring sesquiterpenoid, has two molecules in the asymmetric unit, (I) and (I'), differing in the conformations of the saturated ring and the carboxyl group. The compound aggregates as carboxyl-to-ketone hydrogen-bonding catemers [O . . .O = 2.776 (3) and 2.775 (3) Angstrom]. Two crystallographically independent sets of single-strand hydrogen-bonding helices with opposite end-to-end orientation pass through the cell in the b direction, one consisting exclusively of molecules of (I) and the other entirely of (I'). Three C-H . . .O=C close contacts are found in (I). The monohydrate, C15H18O4.H2O, (II), with two molecules of (I) plus two water molecules in its asymmetric unit, forms a complex three-dimensional hydrogen-bonding network including acid-to-water, water-to-acid, water-to-ketone, water-to-water and acid-to-acid hydrogen bonds, plus three C-H . . .O=C close contacts. In both (I) and (II), only the ketone remote from the acid is involved in hydrogen bonding.