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 1 via autoxidation and chemical methods produced (-)-5 R -hydroperoxy-α-copaene ( 2 ), (+)-3 S -hydroperoxycopa-4-ene ( 3 ), (-)-α-copaene oxide ( 4 ), (+)-β-copaen-4α-ol ( 5 ), copaenediol ( 6 ), and copaene ketol ( 7 ). Reduction of 2 and 3 with triphenylphosphine afforded (-)-5 R -hydroxy-α-copaene ( 2a ) and (+)-3 S -hydroxycopa-4-ene ( 3a ), respectively, which led to the structural revision of copa-3-en-2α-ol and copa-2-en-4-ol as 2a and 3 , respectively. The configurational assignment of compound 4 in the literature was also clarified by the detailed analysis of 2D NMR spectroscopic data. Compounds 2 - 7 showed 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.