REGIOCHEMICAL VARIATIONS IN REACTIONS OF METHYLCUBANE WITH TERT-BUTOXYL RADICAL, CYTOCHROME-P-450 ENZYMES, AND A METHANE MONOOXYGENASE SYSTEM

Reactions of methylcubane (1) with the tert-butoxyl radical (t-BuO.), with cytochrome P-450 enzymes, and with a methane monooxygenase (MMO) system have been studied. For the purpose of product characterization, authentic samples of 2-methylcubyl and 4-methylcubyl derivatives were prepared. 2-Methylcubanecarboxylic acid (9b) is a new compound prepar ed from cubanecarboxylic acid. The key synthetic reactions were (1) me talation and subsequent iodination of the 2-position of (diisopropylca rbamoyl)cubane to effect the initial functionalization, (2) lithium-fo r-iodine exchange and methylation followed by reduction to give 2-meth yl-1-[(diisopropylamino)methyl]-cubane, and (3) dimethyldioxirane oxid ation of this amine to give 9b. The known 4-methylcubanecarboxylic aci d (9d) was prepared here by a route related to that employed for 9b. R eactions of acids 9b and 9d with methyllithium gave the corresponding methyl ketones which were oxidized by m-chloroperoxybenzoic acid to pr ovide authentic samples of 2- and 4-methylcubanol acetates (3b and 3d) . Reaction of 1 with t-BuO(.) in the presence of 2,2,5,5-tetramethylis oindole-N-oxyl radical (TMIO(.)) at 40-55 degrees C gave mainly cube-s ubstituted products in confirmation of the report (Della, E. W.; Head, N. J.; Mallon, P.; Walton, J. C. J. Am. Chem. Sec. 1992, 114, 10730) that hydrogen atom abstraction by the electrophilic alkoxyl radical at low temperature occurs at the cubyl C-H positions. In a competition e xperiment at 42 degrees C, methylcubane was at least 3.5 times more re active toward t-BuO(.) than cyclohexane, indicating that the cubyl pos itions in 1 are greater than or equal to 40 times more reactive than t he methyl positions in 1 (per hydrogen) toward the alkoxyl radical. Ox idation of 1 by enzymes gave alcohol products that were converted to t heir acetate derivatives for identification and quantitation. Microsom al cytochrome P-450 enzymes from rat and the rat purified P-450 isozym e CYP2B1 hydroxylated 1 at all positions, whereas the reconstituted MM O system from Methylococcus capsulatus (Bath) hydroxylated 1 only at t he methyl position. The differences in regioselectivity suggest that t he transition states for hydrogen abstraction by the alkoxyl radical a nd for enzyme-catalyzed hydroxylation differ considerably. The results are consistent with a model for concerted enzyme catalyzed hydroxylat ion of 1 involving ''side-on'' approach to the C-H bond of substrate.