Trifluoromethylketones as possible transition state analog inhibitors of juvenile hormone esterase

A series of compounds containing the trifluoromethylketone group have been synthesized utilizing either a modified Grignard procedure or by reacting selected aliphatic bromides or tosylates with the Collman reagent [Na₂Fe(CO)₄]. When tested in vitro as inhibitors of crude juvenile hormone esterase from the hemolymph of the cabbage looper, Trichoplusia ni (Noctuidae), the most active compounds were trifluoromethylketones possessing either a juvenoid-like structure or a straight aliphatic chain. The logarithm of the inhibitory potency of the aliphatic compounds was proportional to their chain length, up to 1,1,1-trifluorotetradecan-2-one (I₅₀ = 1 × 10^-7 M). This powerful inhibition was found to be highly selective for JHE, reversible, competitive by Lineweaver-Burk analysis, and was characterized by high affinity of the inhibitor for the esterase (K_i = 3.2 × 10^-9 M, K_m JH III = 2 × 10^-7 M). Other trifluoromethylketones were shown to be inhibitors of T. ni α-naphthylacetate esterase and bovine trypsin. By analogy with the mechanism of trypsin action, trifluoromethylketones are probably potent inhibitors due to their resemblance to a tetrahedral transition state on the reaction coordinate to the acylated enzyme.