Kinetics and products of the hydrolysis of 3,4‐dihydroprecocene I 3,4‐epoxide in aqueous organic solvents
The hydrolysis of 3, 4-dihydroprecocene I 3, 4-epoxide (3, 4-dihydro-7-methoxy-2, 2-dimethyl-3, 4-epoxy-2H-benzo[b]pyran), the putative ultimate cytotoxin of the insect growth regulator precocene I (7-methoxy-2, 2-dimethyl-2H-benzo[b]pyran), has been studied and found to exhibit first-order kinetics [k = 0.17 s−1 in 10 mm-phosphate buffer pH 7.0, containing 1, 4-dioxane (1 + 1 by volume), ionic strength 0.1]. Plots of log k versus pH, and k versus buffer concentration, suggest that the reaction is subject to both specific and general acid catalysis. High-performance liquid chromatography showed the reaction products to be predominantly the corresponding stereoisomeric diols (3, 4-dihydro-7-methoxy-2, 2-dimethyl-2H-benzo[b]pyran-3, 4-diol), the trans : cis ratio of which varied from 1.8: 1 to 2.2: 1 but was constant over the pH range 6-8, at a given buffer concentration. The results indicate that acid-catalysed hydration of 3, 4-dihydroprecocene I 3, 4-epoxide is an SN1 reaction, involving a trigonally hybridised carbocation at C4, even at physiological pH. Similar studies on 3, 4-dihydroiso-precocene I 3, 4-epoxide (3, 4-dihydro-6-methoxy-2, 2-dimethyl-3, 4-epoxy-2H-benzo-[b]pyran), a biologically inactive isomer of 3, 4-dihydroprecocene I 3, 4-epoxide suggest that an SNI mechanism also contributes to its hydrolysis, but the rate constant is 4000 times lower than that for 3, 4-dihydroprecocene I 3, 4-epoxide. Knowledge of the reactivity and mechanism of reaction of such compounds forms an important part of the basis for rational prediction of biological activity in precocene analogues, and hence their possible use as pest control agents.