The mechanism of adduct formation between reduced flavins and arene epoxides

The mechanism of nucleophilic epoxide opening by reduced flavins, a potentially relevant transformation to the carcinogenesis of polycyclic aromatic hydrocarbons, was investigated. In the spontaneous epoxide hydrolysis pH region, the reaction pathway between the epoxide and the dihydroflavin is bimolecular epoxide opening. Chromatographic analysis of the reaction of (+/-)-1a,2,3,7b-tetrahydro-(1aalpha,2alpha,3beta,7balpha)-naphth[1,2-b]oxirene-2,3-diol (1) with 5,10-dihydro-7,8,10-trimethylbenzo[g]pteridine-2,4(1H,3H)-dione (LFH2, dihydrolumiflavin) in 9/1 (v/v) aqueous pH 7.86 Tris buffer-dioxane yielded the flavin N(5) adduct 8 as the only major product. Under the same conditions, (+/-)-1a,2,3,7b-tetrahydro-(1aalpha,2beta,3alpha,7balpha)-naphth[1,2-b]oxirene-2,3-diol (2) gave the N(5) adduct 10, N(3) adducts (the stereoisomers 11a and 12a), and an unknown adduct (possibly the C(4a) adduct), each in similar yields. The tetrahydronaphthalene oxide, (+/-)-1a,2,3,7b-tetrahydro-(1aalpha,7balpha)-naphth[1,2-b]oxirene (3), gave the N(5) adduct 14 and the C(4a) adduct 16, in approximately a 2/1 ratio. The C(4a) adduct was not stable, however, and was transformed to a secondary adduct. A comparison of the absorption spectra indicated that the outcome of 1 and 3 with FMNH2 Was similar to that for LFH2. The reaction of 2 with FMNH2 gave an adduct, assigned to that of a C(4a) adduct, in addition to the N(5) and N(3) adducts. While the reactions of 1 (pH 5.1) and 2 (pH 4.3) with LFH2 under acidic conditions gave only the N(5) adducts, that of 3 at pH 6.6 gave the same adducts as observed at pH 7.86. Most of these adducts have been isolated. All of the adducts (except 12a) exhibited trans stereochemistry with respect to epoxide opening. Rate constants for these reactions were determined by chromatographic monitoring of the epoxide disappearance and for these epoxides in the spontaneous region (pH approximately 7.5) are in the range of 0.1-0.9 M-1 s-1. The magnitude of these rate constants indicates that the rate constant for nucleophilic, bimolecular attack by the dihydroflavin on these epoxides is comparable to that of the thiolate anion (rate constants for the 2-thioethanol thiolate opening of 1 and 2 are 0.27 and 0.85 M-1 s-1, respectively, under conditions comparable to this study: Becker, A. R.; Janusz, J. M.; Bruice, T. C. J. Am. Chem. Soc. 1979, 101, 5679-5687). The behavior of the epoxides with the dihydroflavins provides one of the few quantitative estimates of the nucleophilic capacity of the dihydroflavin and confirms the perception of the dihydroflavin as an exceptionally reactive nucleophile.