Methyl 5,7-dihydroxy-2,2,9-trimethyl-6,11-dioxo-6,11-dihydro-2H-anthra[2,3-b]pyran-8-carboxylate

The title compound, C22H18O7, also known as laurentiquinone B, is a new anthraquinone which was isolated from Vismia laurentii, a Cameroonian medicinal plant. The asymmetric unit contains two independent molecules. Each of them contains four fused rings, three of which are coplanar and typical of anthracene, while the heterocyclic rings adopt envelope conformations. Intramolecular O—H⋯O hydrogen bonds result in the formation of two planar rings, which are also almost coplanar with the adjacent rings. In the crystal structure, intermolecular O—H⋯O and C—H⋯O hydrogen bonds link the molecules and a π–π contact is also present [centroid-centroid distance = 3.967 (3) Å].

The title compound, C 22 H 18 O 7 , also known as laurentiquinone B, is a new anthraquinone which was isolated from Vismia laurentii, a Cameroonian medicinal plant. The asymmetric unit contains two independent molecules. Each of them contains four fused rings, three of which are coplanar and typical of anthracene, while the heterocyclic rings adopt envelope conformations. Intramolecular O-HÁ Á ÁO hydrogen bonds result in the formation of two planar rings, which are also almost coplanar with the adjacent rings. In the crystal structure, intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds link the molecules and acontact is also present [centroid-centroid distance = 3.967 (3) Å ].

Comment
Anthraquinones are a class of natural products encompassing several hundreds of compounds. They are found in a large number of plant families particularly in Rubiaceae, Gesneriaceae, Polygonaceae, Guttiferae, fungi or lichen. Anthraquinones can be formed biosynthetically from shikimic acid, α-ketoglutarate and mevalonate or from acetate and malonate along the polyketide pathway (Birch et al., 1965;Shibata & Ikekawa, 1963). Those naturally occurring compounds exhibit some interesting in vivo biological activities such as antimalarial, antileukemic, antibacterial (Adwankar & Chitnis, 1982;Sittie et al., 1999;Rath et al., 1995;Ismail et al., 1997). Several Vismia species are known as sources of anthraquinones (Nagem & de Oliveira, 1997;Nguemeving et al., 2006). They are used in traditional medicine as purgative, tonic or febrifugal agents and also for the treatment of skin diseases (Kerharo, 1974;Macfoy & Sama, 1983). Previous phytochemical investigations of Vismia species have revealed the presence of benzophenones, xanthones, triterpenoids and also anthraquinones (Simmonds et al., 1985, Seo et al., 2000. In a continuation of our search for bioactive compounds from Vismia laurentii, we have isolated from the EtOAc extract of the fruits 5 compounds comprising emodin, isoxanthorin, and three new ones laurentiquinones A, B(1) and C (Noungoue et al., 2008). We reported herein the crystal structure of (1).
The asymmetric unit of the title compound contains two independent molecules, ( In the crystal structure, intermolecular O-H···O and C-H···O hydrogen bonds (Table 1) link the molecules, in which they may be effective in the stabilization of the structure. There also exist a π-π contact between G and H rings, Cg8···Cg7 i [symmetry code: (i) -x, 1 -y, -z, where Cg8 and Cg7 are the centroids of the rings H (C30-C35) and G (C28-C30/C35-C39) may further stabilize the structure, with centroid-centroid distance of 3.967 (3) Å.

Experimental
The fruits of Vismia laurentii were collected from the bank of the Nyong river near Nkolmaka Lake (Endome) in Center Province, Cameroon on 17t h October 2004 by Mr. Nana Victor. A voucher specimen (No. 1882/SRFK) has been deposited in the National Herbarium, Yaounde, Cameroon. Dried fruits (0.988 kg) of V. laurentii were grounded and exhaustively extracted by maceration successively with hexane, ethyl acetate and methanol at room temperature. In each extraction 3x5 L of solvent were used for a period of 3x24 h and the extracts obtained were concentrated to dryness to give green (62.3 g), brown (43.6 g) and brown (22.1 g) crude viscous residues from hexane, EtOAc and MeOH extracts, respectively. The supplementary materials sup-2 EtOAc extract (40 g) was subjected to flash column chromatography on silica gel 60 (0.063-0.200 mm, Merck, 500 g) as a stationary phase eluting with cyclohexane-EtOAc-MeOH mixtures of increasing polarity. Twenty-four fractions of 200 ml each were collected and grouped on the basis of TLC analysis to afford two main fractions A (11.7 g) and B (17.3 g).
Fractions A and B were chromatographed on a silica gel column, using as eluent gradient mixtures of cyclohexane and EtOAc to yield laurentiquinone B (16 mg) in addition to other compounds. Orange-red crystals of the title compound were grown from a hexane-chloroform solution of laurentiquinone B.   Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.