The Chemistry of African Croton species

Abstract

The genus Croton is one of the largest of Euphorbiaceae sensu stricto, and consists of over 1300 species of trees, shrubs and herbs that are distributed worldwide in the warm tropics and subtropics. It is reported that 124 Croton species occur in continental Africa whilst a further 156 species are endemic to Madagascar. Another 12 species occur in the Indian Ocean islands of Comoros, Mauritius, Reunion and Sao Tome and Principe [1]. We discuss the chemistry, chemotaxonomic patterns and biological activities of selected compounds from ten African Croton taxa: C. alienus, C. dichogamus, C. gratissimus var. gratissimus, C. megalobotrys, C. megalocarpoides, C. megalocarpus, C. menyhartii, C. pseudopulchellus, C. rivularis and C. sylvaticus. Examples of compounds to be presented include cembranoids (1-3) from C. gratissimus var. gratissimus [2,3], ent-kauranes (4-5) from C. pseudopulchellus [4], ent-clerodanes (6-8) from C. sylvaticus, C. megalocarpus and C. megalocarpoidies, and both halimanes (9) and crotofolanes (10 – 11) from C. dichogamus. Triterpenoids, sesquiterpenoids, flavonoids and cyclohexanol derivatives from Croton will also be discussed. Selected cembranoids from C. gratissimus were tested against a chloroquine-sensitive strain of Plasmodium falciparum (D10) and against the PEO1 and PEO1TaxR ovarian cancer cell lines [2]. Compound 1 showed moderate activity against the PEO1 (IC50= 132 nM) and PEO1TaxR (IC50= 200 nM) ovarian cancer cell lines. Selected ent-kauranoids were tested for their effects on Semliki Forest Virus replication and for cytotoxicity against human liver tumour cells (Huh-7 strain). Other Croton-derived compounds were tested for antimicrobial and antifungal activities [5,6], antiplasmodial activity using two strains of Plasmodium falciparum, antileishmanial activities against Leishmania donovanii [5], and cytotoxic activity against NCI59 cancer cell panels, and colorectal and VERO cancer cell lines.