7β-Hydroxyartemisinin

Crystals of the title compound [systematic name: (3R,6R,7S,8aR,9R,12aR)-7-hydroxy-3,6,9-trimethyloctahydro-3,12-epoxy[1,2]dioxepino[4,3-i]isochromen-10(3H)-one], C15H22O6, were obtained from microbial transformation of artemisinin by a culture of Cunninghamella elegans. The stereochemistry of the compound is consistent with the spectroscopic findings in previously published works. A weak O—H⋯O hydrogen bond occurs in the crystal structure, together with intermolecular C—H⋯O hydrogen bonds.


Comment
The natural occurring sesquiterpene lactone endoperoxide artemisinin has been the subject of extensive research for its effective therapeutic action against multidrug-resistant Plasmodium falciparum strains (Klayman, 1985). Some of the reasons for this are the increasing number of people under risk of contracting malaria, the alarming spread of drug-resistant parasites (TDR, 2007) and the relatively complicated treatment protocols, with so many variables and no effective cure for the several strains of Plasmodium causing the disease (CDC, 2007).
One of the strategies used for increasing the bioavailability of artemisinin is its semi-synthetic transformation through microorganisms (Chen & Yu, 2001;Liu et al., 2006). The metabolites resulting from the action of several enzymes in selected strains of fungi can be further transformed in dimers or attached to other moieties for selective action and/or delivery.
Our group has been studying the microbial transformation of artemisinin for some years (Parshikov et al., 2005; and we follow the numbering system of Blasko & Cordell (1988), and the CA Index Name. Some authors follow a different numbering system and call the title compound, (I), 9β-hydroxyartemisinin, rather than 7β-hydroxyartemisinin. Several well established methods of one-dimensional and two-dimensional NMR have already determined the configuration of artemisinin and most of its derivatives. The crystallographic data confirm the assignment of the chiral centers proposed in a previously published paper (Parshikov et al., 2004). The configuration of the chiral centers in (I) are: C3 R, C5A S, C6 S, C7 S, C8A S, C9 R, C12 S, C12A R.
In the crystal of (I), a weak intermolecular O-H···O hydrogen bond links the molecules into chains and some short C-H···O contacts occur (Table 1).
Experimental 7β-Hydroxyartemisinin was obtained following a method previously published by our group (Parshikov et al., 2004). Well developed fungal mycelia of Cunninghamella elegans ATCC 9245 were removed from the surface of agar slants, suspended in 100 ml of sterilized water, and used to inoculate 1 liter of medium (400 g Sabouraud-dextrose, 300 g sucrose, 200 g peptone, in 20 liters of deionized water) in 4 liter shake flasks. The pH was adjusted to 6.5 using 0.1 N NaOH. Cultures were grown for 48 h on a rotary shaker at 301 K with shaking at 180 rpm. The resulting biomass was used as inocula for 1,000 ml of medium contained in 4 liter shake flasks that were again incubated for 48 h. 10 g of Artemisinin (Mediplantex, Vietnam) were dissolved in 400 ml of methanol (MeOH), filter-sterilized, and 20 ml were added to each flask to make the final concentration 500 mg/l. The cultures were returned to the shaker incubators (180 rpm) for an additional 14 days at 301 K. The cultures were harvested and the broth and mycelia were separated using coarse filter paper in a Büchner funnel. The mycelia were washed with water and discarded. The culture broth was extracted with three equal volumes of ethyl acetate (EtOAc) and evaporated under vacuum. The residues were dissolved in MeOH for analysis.
Colourless needles of (I) were grown by slow evaporation of a solution in absolute ethanol.