(–)-Isosantonic acid: alteration of the hydrogen-bonding mode by conﬁg-urational inversion at a single centre in a (cid:0) , (cid:1) -diketocarboxylic acid

The title diketo acid, (1R,3aS,6aS,7R,9S)-(−)-α,3a,7-tri­methyl-5,8-dioxo-1,4-ethano­perhydro­penta­lene-1-acetic acid (C15H20O4), is shown to aggregate in the crystal structure as acid-to-ketone hydrogen-bonding catemers, whose chains follow 21 screw axes from each carboxyl to a ketone in a neighbouring mol­ecule [O⋯O = 2.7472 (13) A and O⋯H—O = 172.7 (17)°]. Two parallel counterdirectional screw-related single-strand hydrogen-bonding chains pass through the cell in the a direction. Six inter­molecular C—H⋯O=C close contacts are found. Comparisons are drawn with a diastereomer having the opposite configuration at the methyl­ated chiral centre adjacent to the carboxyl group.

The title compound, (I), is a tricyclic diketo acid having the absolute S configuration at C9, whose chirality is independent of the tricyclic framework.Compound (I) is thus the C9diastereomer of (À)-parasantonic acid, (II) (Zinczuk et al., 2004).
Fig. 3 illustrates the packing of (I), involving acid-to-ketone catemers, whose hydrogen-bonding follows a 2 1 screw from each carboxyl to the -ketone (O1) in a neighbour [OÁ Á ÁO = 2.7472 (13) A ˚and O-HÁ Á ÁO = 172.7 ( 17) ].Two parallel counterdirectional chains pass through the cell along the a axis.This mode predominates among chiral non-racemic keto acids, but differs from the rare acid-to-acid hydrogen bonding found in (II).This change results specifically from the inversion at C9 and the extension of O4-H4 at a markedly altered angle relative to the remainder of the molecule in (I) compared with (II).
We characterize the geometry of hydrogen bonding to carbonyls using a combination of HÁ Á ÁO C angle and HÁ Á ÁO C-C torsion angle.In (I), these two angles are 137.7 (5) and À20.1 (8) , respectively.

Experimental
Based on a procedure of Francesconi ( 1895), (À)-santonic acid, derived from (À)--santonin of known absolute stereochemistry, was refluxed in acetic acid for 18 h.The solution was then concentrated and heated for 4 h in vacuo at 448 K, yielding about 10% of the enol lactone of (I) after work-up.Brief acidic hydrolysis then gave (I) (m.p. 424 K), which was recrystallized from diisopropyl ether.Optical-rotation data for (I) were reported by Francesconi (1895).The solid-state (KBr) IR spectrum of (I) has a C O absorption at 1747 cm À1 (strained "-ketone), as well as bands at 1730 and 1685 cm À1 for, respectively, the carboxyl C O lacking hydrogen bonding and the hydrogen-bonded -ketone.In CHCl 3 solution, where dimers predominate, the "-ketone appears at 1741 cm À1 , with a single peak at 1709 cm À1 for the 'normal' absorptions of both the acid and -ketone.The molecular structure of (I), showing displacement ellipsoids drawn at the 50% probability level.

Figure 2
The molecular structure of (À)-parasantonic acid, (II), which differs from (I) only in the configuration around C9.

Figure 3
A partial packing diagram for (I), illustrating the two parallel counterdirectional screw-related single-strand hydrogen-bonded (dashed lines) chains passing through the cell in the a-axis direction.Displacement ellipsoids are drawn at the 40% probability level.H atoms not involved in hydrogen bonding have been omitted.

Data collection
All H atoms were found in electron-density difference maps, placed in calculated positions and allowed to refine freely, both positionally and isotropically.