Isogentisin (1,3-di­hydroxy-7-methoxy­xanthone)

The crystal structure of isogentisin, C14H10O5, a natural product isolated from Gentiana lutea, has been determined. The phenolic ring system is essentially planar and the displacement of the methoxy substituent from the mean molecular plane is very small. The structure is stabilized by a one-dimensional chain of intermolecular hydrogen bonds.

The crystal structure of isogentisin, C 14 H 10 O 5 , a natural product isolated from Gentiana lutea, has been determined.The phenolic ring system is essentially planar and the displacement of the methoxy substituent from the mean molecular plane is very small.The structure is stabilized by a one-dimensional chain of intermolecular hydrogen bonds.

Comment
Xanthone compounds commonly occur in several higher plant families, such as Gentianaceae, Guttiferae, Moraceae and Polygalaceae.The study of xanthones is interesting both from the chemosystematic and pharmacological point of view.Inhibition of Type A and Type B monoamine oxidases (MAO) by a number of xanthones has been observed (Suzuki et al., 1980(Suzuki et al., , 1981)).Among the xanthones that have been tested, isogentisin revealed potent MAO inhibition (Suzuki et al., 1978).Four ethanolic extracts prepared from leaves, ¯owers and roots of Gentiana lutea were tested for antitubercular activity against Mycobacterium bovis (BCG-strain).The extract obtained from ¯owers showed strong inhibition at a concentration of 1000 mg ml À1 and slight inhibition at 500 mg ml À1 .This activity increased during the various puri-®cation steps, which ®nally led to the isolation of the active compound isogentisin (Menkovic Â et al., 1999).Mutagenicity in the Ames test in Salmonella typhimurium was also shown for isogentisin (Morimoto et al., 1983, Matsushima et al., 1985).Isogentisin was ®rst isolated by Cannonica & Pelizzoni (1955).The present paper presents the ®rst single-crystal X-ray analysis of isogentisin and con®rms that the crystal structure correpsonds to 1,3-dihydroxy-7-methoxyxanthone, (I) (Fig. 1).The 1,3-dihydroxy-7-methoxyxanthone fragment is essentially planar, with the largest displacement within the phenolic ring system of 0.062 (3) A Ê for C1.The methyl group of the methoxy substituent lies close to the mean plane of the molecule, as shown by the torsion angle of C10ÐC9ÐO15ÐC19 of 5.2 (5) .
The packing diagram for isogentisin is shown in Figs. 2 and  3.The crystal structure can be described in terms of parallel molecules stacked along the direction of the a crystallographic axis, with the normal to the plane forming an angle of about 20 relative to it, and an intermolecular separation of about 3.5 A Ê .Within a xanthone unit, an intramolecular hydrogen bond with a length of 1.91 A Ê exists between the hydroxyl H atom H7 and the O5 acceptor of an adjacent carbonyl group.In addition, the same carbonyl O atom participates in a onedimensional intermolecular hydrogen bond with the hydroxyl group on a neighbouring molecule (O5ÐH5 = 1.997A Ê ).The hydrogen-bonding patterns are shown in Fig. 4.

Experimental
Isolation of isogentisin from Gentiana lutea was carried out following a procedure described previously (Menkovic Â, 1997;Menkovic Â et al., 1990).A second view of the packing diagram for isogentisin.

Figure 2
Figure 2One view of the packing diagram for isogentisin.