Stereoselective Oxidation of α-Copaene, a Fire Ant Repellent Sesquiterpene from the Essential Oil of Dipterocarpus turbinatus

Imported fire ants are significant agricultural pests. Repellents can be used to prevent foraging fire ants from entering sensitive areas, including electrical equipment, nursing homes, and hospitals. Bioassay-guided fractionation of the essential oil extracted from gurjun balsam (Dipterocarpus turbinatus) resulted in the identification of (−)-α-copaene (1) as the repellent constituent with a minimum repellent effective dose (MRED) of 15.6 μg/g against both red imported fire ants (Solenopsis invicta) and hybrid imported fire ants (Solenopsis invicta× Solenopsis richteri). Stereoselective oxidation of 1via autoxidation and chemical methods produced (−)-5R-hydroperoxy-α-copaene (2), (+)-3S-hydroperoxycopa-4-ene (3), (−)-α-copaene oxide (4), (+)-β-copaen-4α-ol (5), copaenediol (6), and copaene ketol (7). Reduction of 2and 3with triphenylphosphine afforded (−)-5R-hydroxy-α-copaene (2a) and (+)-3S-hydroxycopa-4-ene (3a), respectively, which led to the structural revision of copa-3-en-2α-ol and copa-2-en-4-ol as 2aand 3, respectively. The configurational assignment of compound 4in the literature was also clarified by the detailed analysis of 2D NMR spectroscopic data. Compounds 2–7showed repellency with MREDs ranging from 3.9 to 15.6 μg/g against hybrid and red imported fire ants, indicating that chemical modification can enhance the repellent effect of (−)-α-copaene.