Complete Structural Assignment of Serratol, a Cembrane-Type Diterpene from Boswellia serrata, and Evaluation of Its Antiprotozoal Activity

From the dichloromethane extract obtained from the gum resin of Boswellia serrata Roxb. (Burseraceae), a well-known medicinal plant resin (Indian Olibanum), the cembrane-type diterpene serratol was isolated in high yield. Its structure, previously reported without clear specification of double-bond geometry and without specification of stereochemistry, was reanalysed by means of spectroscopic measurements and unambiguously assigned as S(-)-cembra-3E, 7E, 11E-triene-1-ol. Full assignment of all NMR data is reported for the first time. The compound was found to be identical with a cembrenol previously isolated from B. carteri. Serratol was tested for in vitro activity against four protozoan human pathogens, namely, Trypanosoma brucei rhodesiense (East African Human Trypanosomiasis, sleeping sickness), T. cruzi (Chagas' disease), Leishmania donovani (Kala-Azar), and Plasmodium falciparum (Tropical Malaria). It was found active against T. brucei and P. falciparum. These activities were 10- to 15-fold higher than its cytotoxicity against rat skeletal myoblasts. While some reports exist on potential anti-inflammatory activity of Boswellia diterpenes, this is the first report on antiprotozoal activity of such a compound.