Oxidation-Active Flavin Models: Oxidation of R-Hydroxy Acids by Benzo-dipteridine Bearing Metal-Binding Site in the Presence of Divalent Metal Ion and Base in Organic Solvents Hideaki Ohshiro, † Keita Mitsui, † Nobuyuki Ando, † Yoichi Ohsawa, † Wataru Koinuma, † Hirobumi Takahashi, † Shin-ichi Kondo, † Tatsuya Nabeshima, ‡ and Yumihiko Yano* ,† Contribution from the Department of Chemistry, Gunma UniVersity, Kiryu, Gunma 376-8515, Japan and the Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ReceiVed March 14, 2000 Abstract: The oxidizing ability of benzo-dipteridine bearing a bipyridin-6-ylmethyl moiety (4) was found to be increased with Zn 2+ by ∼10 3 -fold for sulfite addition in MeOH and ∼10 2 -fold for oxidation of an NADH model in MeCN. It was found for the first time that 4 is able to oxidize R-hydroxy acids to R-keto acids in the presence of a divalent metal ion such as Zn 2+ , Co 2+ , and Ni 2+ and an amine base in MeCN or t-BuOH, whereas benzo-dipteridine having a bipyridin-5-ylmethyl moiety (3) is unable to oxidize them under the same conditions. The oxidation reaction was kinetically investigated including the kinetic isotope effect for deuterated mandelic acids (k H /k D ) 2.1-3.7) and the Hammett plots for substituted mandelic acids (V-shaped plots). In the reaction of R-substituted R-hydroxy acids such as R-methyl mandelic and benzylic acids with 4, novel oxidative decarboxylation was found to take place, giving acetophenone and benzophenone, respectively. The oxidation mechanism for mandelic acid was proposed to proceed via a ternary complex of 4‚Zn 2+ ‚PhCH(OH)CO 2 - , in which R-oxyanion of mandelate attacks C(4a)-position of 4 to form an adduct followed by 1,2-elimination to afford benzoyl formate and 2e-reduced 4. The roles of the metal ion were proposed as follows; (i) activation of 4, (ii) substrate-binding site, and (iii) activation of the bound R-hydroxy acid by lowering pK a ’s of R-OH and R-CH. This is a first example that a flavin model oxidizes R-hydroxy acids in the presence of a metal ion. Introduction Flavin coenzymes such as FMN and FAD exhibit diverse redox functions in biological systems, in which manifestation of the functions is modulated through interactions with apo- proteins. Although biochemical and chemical investigations on flavoenzymes have been extensively conducted, 1 still much remains to be clarified in molecular levels. Model study has made a significant contribution to mechanistic understanding of flavin catalysis. 2 We considered that oxidation-active flavin mimics are quite useful not only for the mechanistic study but also for exploitation of new flavin-mediated oxidation reactions because the oxidizing ability of conventional flavin mimics reported thus far is fairly weak. We have successfully exploited highly oxidation-active flavin mimics by chemical modification of an isoalloxazine ring. 3 Among them, benzo-dipteridine (2), which is ∼10 7 times more reactive than a conventional flavin model for reactions involving nucleophilic attack at the C(4a)- position, is quite useful. It should be noted that the reactivities of the oxidation-active flavin mimics are correlated with their redox potentials in aqueous solution. 3a By employing 2, we reported oxidative dealkylation of N-nitrosamines (a metabolic model of N-nitrosamines), 3c oxidation of sulfite ion (an APS reductase model), 3d and oxidation of o-aminophenol (an isophe- noxazine synthase model). 3f To construct more sophisticated model systems, 4 the so-called “artificial flavoenzymes”, how- ever, pertinent arrangements of reaction-promoting factors, such as a substrate-binding site, a substrate-activating and transition- state stabilizing factors, are necessary. These might be achieved by covalent and noncovalent functionalization of the oxidation- active flavin mimic. Namely, we can introduce a functional group into a benzo-dipteridine skeleton through covalent bonds and also through noncovalent bonds by employing a function- alized flavin receptor which binds the flavin model close to the functional group. 5 We selected a metal ion as the functionality, since some flavoenzymes require a metal ion as a prosthetic group. 6 For example, D-lactate dehydrogenases from bacterial and mammalian sources, which oxidize D-lactate to pyruvate, * To whom correspondence should be addressed: yano@chem.gunma- u.ac.jp. † Gunma University. ‡ University of Tsukuba. (1) (a) Ghisla, S.; Massey, V. Eur. J. Biochem. 1989, 181,1-17. (b) Muller, F., Ed. Chemistry and Biochemistry of FlaVoenzymes; CRC Press, Boston, 1991; Vols. I-III. (2) (a) Bruice, T. C. Acc. Chem. Res. 1980, 13, 256-262. (b) Walsh, C. Acc. Chem. Res. 1980, 13, 148-156. (c) Bruice, T. C. Isr. J. Chem. 1984, 24, 54-61. (3) (a) Yano, Y.; Nakazato, M.; Sutoh, S.; Vasquez, R.; Kitani, A.; Sasaki, K. J. Chem. 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(d) Kajiki, T.; Kuroi, T.; Moriya, H.; Hoshino, K.; Kondo, S.-I.; Nabeshima, T.; Yano, Y. J. Org. Chem. 1999, 64, 9679-9689. 2478 J. Am. Chem. Soc. 2001, 123, 2478-2486 10.1021/ja0009121 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/27/2001