Asymmetric catalysis by flavin-dependent halogenases

Abstract

In nature, flavin-dependent halogenases (FDHs) catalyze site-selective chlorination and bromination of aromatic natural products. This ability has led to extensive efforts to engineer FDHs for selective chlorination, bromination, and iodination of electron rich aromatic compounds. On the other hand, FDHs are unique among halogenases and haloperoxidases that exhibit catalyst-controlled site selectivity in that no examples of enantioselective FDH catalysis in natural product biosynthesis have been characterized. Over the past several years, our group has established that FDHs can catalyze enantioselective reactions involving desymmetrization, atroposelective halogenation, and halocyclization. Achieving high activity and selectivity for these reactions has required extensive mutagenesis and mitigation of problems resulting from hypohalous acid generated during FDH catalysis. The single-component flavin reductase/FDH AetF is unique among the wild type enzyme we have studied in that it provides high activity and selectivity toward several asymmetric transformations. These results highlight the ability of FDH active sites to tolerate different substrate topologies and suggest that they could be useful for a broad range of oxidative halogenations.

Keywords: atroposelective; desymmetrization; halocyclization; halogenase; kinetic resolution.

FIGURE 1

(A) Flavin-dependent halogenase (FDH)-catalyzed aromatic C-H halogenation. (B) A simplified mechanism for the FDH RebH (FRED = flavin reductase; Dn = electron-donating group). (C) General schemes for potential modes of enantioselective FDH catalysis.

FIGURE 2

(A) Representative substrate scope of flavin-dependent halogenase (FDH)-catalyzed desymmetrization of methylenedianilines; Conv. versus RebH indicates the fold improvement in conversion relative to RebH. (B) Stereoretentive Larock cyclization of 1a to generate an α-chiral indole.

FIGURE 3

(A) Representative substrate scope of flavin-dependent halogenase (FDH)-catalyzed atroposelective kinetic and dynamic kinetic resolution. ^a1 mol% 3-T. ^b2.5 mol % 3-T. ^cStarting material exists as stable atropisomers. ^d20 equiv. NaBr, 5 mol% 3-T. (B) Stereoretentive Suzuki cross-coupling and Knorr pyrrole synthesis.

FIGURE 4

(A) General scheme for olefin halocyclization (5-exo- and 6-endo-trig pathways refer to the case were n = 1). (B) Representative substrate scope of flavin-dependent halogenase (FDH) catalyzed halolactonization. ^aA 87:13 mixture of E/Z-16 was used. ^bVariant 4PL + E461G was used. ^c100 equiv. NaCl was used.

FIGURE 5

(A, B) Representative substrate scope of flavin-dependent halogenase (FDH) catalyzed haloetherification. ^aA 64:36 mixture of E/Z-22 was used. ^bVariant 2RFQ F111S was used. ^cNaCl was used instead of NaBr. ^dVariant 3LR was used. ^eVariant 4PL was used.

FIGURE 6

(A) Simplification of flavin-dependent halogenase (FDH) biocatalysis using the single component flavin reductase/FDH AetF. (B) Reprsentative AetF substrate and reaction scope. ^a10 equiv. NaBr was used. ^b10 equiv. NaI was used.