1-Hydroxy-3-(3-methylbut-2-enyloxy)xanthone

In the title compound, C18H16O4, a monoprenylated xanthone, the xanthone skeleton exhibits an essentially planar conformation (r.m.s. deviation 0.0072 Å) and the isoprenyl side chain remains approximately in the mean plane of the xanthone unit, making a dihedral angle of 4.5 (2)°. The hydroxyl group forms an intramolecular O—H⋯O hydrogen bond. Moreover, there is a weak intermolecular C—H⋯O interaction between a ring C atom and the xanthene O atom. In the crystal structure, there are no intermolecular hydrogen bonds and the crystallographic packing is governed by van der Waals forces, leading to an arrangement in which the molecules assemble with their planes parallel to each other, having a separation of 3.6 (3) Å.

In the title compound, C 18 H 16 O 4 , a monoprenylated xanthone, the xanthone skeleton exhibits an essentially planar conformation (r.m.s. deviation 0.0072 Å ) and the isoprenyl side chain remains approximately in the mean plane of the xanthone unit, making a dihedral angle of 4.5 (2) . The hydroxyl group forms an intramolecular O-HÁ Á ÁO hydrogen bond. Moreover, there is a weak intermolecular C-HÁ Á ÁO interaction between a ring C atom and the xanthene O atom. In the crystal structure, there are no intermolecular hydrogen bonds and the crystallographic packing is governed by van der Waals forces, leading to an arrangement in which the molecules assemble with their planes parallel to each other, having a separation of 3.6 (3) Å .

Comment
Prenylated xanthones have been reported to mediate a number of important biological activities, concerning a large variety of targets with therapeutic value. The presence of the prenyl side chains seems to enhance the interaction with biological membranes and with target proteins (Maia et al., 2005 andEpifano et al., 2007) and we plan to further study these kind of interactions.
However, the synthesis of prenylated xanthones usually involves toxic reagents and is considered not only very demanding but also environmentally unfriendly (Castanheiro et al., 2007). We have looked for an alternative method to obtain prenylated xanthones. The title compound was the first example of a prenylated xanthone synthesized by the microwave irradiation method (Castanheiro et al., 2009). In fact, microwave-assisted heating under controlled conditions is an invaluable technology for medicinal chemistry because it often dramatically reduces reaction times.
In the crystal, the title compound molecules are essentially planar (Fig. 1). The isoprenyl side chain adopts a nearly coplanar conformation relatively to the xanthone skeleton (corresponding dihedral angle 4.5 (2)°). This is an exception because in the crystal structures of other prenylated xanthones, the isoprenyl side chain is usually out of the plane of the xanthones moiety (for a review of prenylated xanthone crystal structures see: . Moreover, the hydroxyl substituent bound to C1 forms a strong intramolecular hydrogen bond to O11 [O1-H1A···O11 = 2.5845 (17) Å].
In the crystal structure, the title compound forms stacking planes ( Fig. 2) with intermolecular separation of 3.6 Å. The packing of the molecules is governed by van der Waals forces and there are no intermolecular hydrogen bonds.

Experimental
Prenylation was carried out using prenyl bromide in alkaline medium under microwave irradiation according to the procedure reported by Castanheiro et al. (2009). Single crystals suitable for X-ray crystallographic analysis were grown by recrystallization from slow evaporation of a CH 2 Cl 2 /PE (60-80) solution.

Refinement
Non-hydrogen atoms were refined anisotropically. The H atoms were positioned with idealized geometry using a riding  Fig. 1. The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms.