Volatiles from African species of Croton (Euphorbiaceae), including new diterpenes in essential oil from Croton gratissimus

Abstract

The chemistry of essential oils from the leaves of three African species of Croton (C. gratissimus, C. pseudopulchellus and C. sylvaticus) is comprehensively characterised. Two new volatile diterpenes were isolated from C. gratissimus and the structures assigned using 1D and 2D NMR. One is a furanyl-halimane methyl ester (1) assigned as 12-β-furanyl-halima-5,9-dien-4-methylcarboxylate (gratissihalimanoic ester); the other is an abietane ketone (2) assigned as ent-abiet-8(14), 13(15)-dien-3-one, which we have named gratissimone. High relative abundance of diterpenes in a hydrodistilled essential oil is rare and may be considered an interesting discovery. Known non-volatile diterpenes were also isolated, which were assigned as crotohalimaneic acid (3) and hardwickiic acid (4). All diterpenes occur in fresh leaves prior to distillation and extract into apolar or moderately polar solvents, which demonstrates that the two volatiles are not generated during the hydrodistillation. At this stage it is not clear how widespread this diterpene essential oil chemotype is within the species distribution or if any therapeutic effects can be attributed to them. No antimicrobial activity was observed at 1 mg/ml against a range of bacterial strains.