Santal monohydrate, an isoflavone isolated from Wyethia mollis

The title compound [systematic name: 3-(3,4-dihydroxyphenyl)-5-hydroxy-7-methoxy-4H-chromen-4-one monohydrate], C16H12O6·H2O, is a monohydrate of a natural product santal isolated from Wyethia mollis. In the santal molecule, the dihedral angle between the benzoquinone and dihydroxyphenyl fragments is 53.9 (1)° and an intramolecular O—H⋯O hydrogen bond occurs. In the crystal, O—H⋯O hydrogen bonds link the components into corrugated layers parallel to the ac plane. The short distance of 3.474 (5) Å between the centroids of the benzene rings in neighbouring santal molecules reveals then existence of π–π interactions within the layers.

The title compound [systematic name: 3-(3,4-dihydroxyphenyl)-5-hydroxy-7-methoxy-4H-chromen-4-one monohydrate], C 16 H 12 O 6 ÁH 2 O, is a monohydrate of a natural product santal isolated from Wyethia mollis. In the santal molecule, the dihedral angle between the benzoquinone and dihydroxyphenyl fragments is 53.9 (1) and an intramolecular O-HÁ Á ÁO hydrogen bond occurs. In the crystal, O-HÁ Á ÁO hydrogen bonds link the components into corrugated layers parallel to the ac plane. The short distance of 3.474 (5) Å between the centroids of the benzene rings in neighbouring santal molecules reveals then existence ofinteractions within the layers.

Comment
Santal, C 16 H 12 O 6 , is an isoflavone isolated from Wyethia mollis, a species once used in folk medicine to treat contusions, pain, fevers, and colds. Santal (Figure 1), has a benzoquinone core with an appended dihydroxyphenyl group. The benzoquinone core is substituted with hydroxyl and methoxy substituents. In the santal molecule of the title compound, which is a monohydrate, the flat planes created by the benzoquinone core and the dihydroxyphenyl group are twisted dramatically relative to each other with a dihedral angle of 53.9 (1)°. The torsion angle C11-C4-C5-C13 is 54.1 (5)°.
This twisting breaks conjugation between the rings, but is likely necessitated by steric interactions between O5 and H11.
The molecule stacks together with the benzoquinone rings parallel to each other and with the dihydroxyphenyl rings pointing in toward the center of the unit cell. The crystal structure shows the presence of linking external water molecules. The water interacts uniquely with three separate santal molecules. It acts as a hydrogen bond donor (H1SA) with O5 and as a hydrogen bond acceptor with O4H of a second santal molecule (Table 1). The second hydrogen on the water (H1SB) is stabilized by interaction with the electron rich π system of the dihydroxyphenyl ring of a third santal molecule. Additionally, O4 acts as a hydrogen bond acceptor to O1H in another santal unit. There is an intramolecular hydrogen bond in which the hydroxyl group at O6 acts as the donor and O5 as the acceptor (Table 1).
In the crystal, intermolecular O-H···O hydrogen bonds link all moieties into corrugated layers parallel to ac plane. The short distances of 3.474 (5) Å between the centroids of benzene rings from the neighbouring santal molecules reveal an existence of π-π interactions inside the layers.

Experimental
Santal was isolated as described previously (Waddell et al., 1982). Suitable crystals of the title compound were obtained by slow evaporation of a water solution of the santal.

Refinement
H6 was located in a difference Fourier map and refined freely. H1SA and H1SB (H 2 O) were located in a difference Fourier map and refined with O-H distance restrained to 0.91 (3) Å, with U iso (H)= 1.5U eq (O). All other H atoms were positioned geometrically, with bond distances of 0.85 Å for hydroxyl, 0.98 Å for methyl and 0.95 Å for those bound to aromatic rings and were refined as riding, with U iso (H)= 1.2-1.5U eq of the parent atom.  View of the title compound showing the atomic numbering and 50% probability displacement ellipsoids. Dashed lines denote hydrogen bonds.