Gold-Catalyzed One-Pot Synthesis of Polyfluoroalkylated Oxazoles from N-Propargylamides Under Visible-Light Irradiation

A gold-catalyzed synthesis of polyfluoroalkylated oxazoles from N-propargylamides under visible-light irradiation has been developed. These reactions display excellent compatibility of radicals and gold catalysts under visible-light irradiation. Mechanistic experiments indicate that polyfluoroalkyl iodides play a dual role in enhanced compatibility of radicals and gold catalysts through assisted protodeauration of vinyl gold and reactivated the gold catalyst. In addition, PPh₃AuNTf₂ not only activates N-propargylamide to generate vinyl gold intermediate, but also greatly promotes homolysis of polyfluoroalkyl iodides under blue light irradiation.     

Characterization of the two-component, FAD-dependent monooxygenase SgcC that requires carrier protein-tethered substrates for the biosynthesis of the enediyne antitumor antibiotic C-1027

C-1027 is a potent antitumor antibiotic composed of an apoprotein (CagA) and a reactive enediyne chromophore. The chromophore has four distinct chemical moieties, including an ( S)-3-chloro-5-hydroxy-beta-tyrosine moiety, the biosynthesis of which from l-alpha-tyrosine requires five proteins: SgcC, SgcC1, SgcC2, SgcC3, and SgcC4; a sixth protein, SgcC5, catalyzes the incorporation of this beta-amino acid moiety into C-1027. Biochemical characterization of SgcC has now revealed that (i) SgcC is a two-component, flavin adenine dinucleotide (FAD)-dependent monooxygenase, (ii) SgcC is only active with SgcC2 (peptidyl carrier protein)-tethered substrates, (iii) SgcC-catalyzed hydroxylation requires O 2 and FADH 2, the latter supplied by the C-1027 pathway-specific flavin reductase SgcE6 or Escherichia coli flavin reductase Fre, and (iv) SgcC efficiently catalyzes regioselective hydroxylation of 3-substituted beta-tyrosyl-S-SgcC2 analogues, including the chloro-, bromo-, iodo-, fluoro-, and methyl-substituted analogues, but does not accept 3-hydroxy-beta-tyrosyl-S-SgcC2 as a substrate. Together with the in vitro data for SgcC4, SgcC1, and SgcC3, the results establish that SgcC catalyzes the hydroxylation of ( S)-3-chloro-beta-tyrosyl-S-SgcC2 as the final step in the biosynthesis of the ( S)-3-chloro-5-hydroxy-beta-tyrosine moiety prior to incorporation into C-1027. SgcC now represents the first biochemically characterized two-component, FAD-dependent monooxygenase that acts on a carrier-protein-tethered aromatic substrate.     

Regiospecific chlorination of (S)-beta-tyrosyl-S-carrier protein catalyzed by SgcC3 in the biosynthesis of the enediyne antitumor antibiotic C-1027

C-1027 is a potent antitumor antibiotic composed of an apo-protein and a reactive enediyne chromophore. The chromophore consists of four different chemical subunits including an (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety, the biosynthesis of which from l-alpha-tyrosine is catalyzed by six proteins, SgcC, SgcC1, SgcC2, SgcC3, SgcC4, and SgcC5. Biochemical characterization of SgcC3 unveiled the following: (i) SgcC3 is a flavin adenine dinucleotide (FAD)-dependent halogenase; (ii) SgcC3 acts only on the SgcC2 peptidyl carrier protein-tethered substrates; (iii) SgcC3-catalyzed halogenation requires O2 and reduced FAD and either the C-1027 pathway-specific flavin reductase SgcE6 or E. coli flavin reductase (Fre) can support the SgcC3 activity; (iv) SgcC3 also efficiently catalyzes bromination but not fluorination or iodination; (v) SgcC3 can utilize both (S)- and (R)-beta-tyrosyl-S-SgcC2 but not 3-hydroxy-beta-tyrosyl-S-SgcC2 as a substrate. These results establish that SgcC3 catalyzes the third enzymatic transformation during the biosynthesis of the (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety of C-1027 from l-alpha-tyrosine. SgcC3 now represents the second biochemically characterized flavin-dependent halogenase that acts on a carrier protein-tethered substrate. These findings will facilitate the engineering of new C-1027 analogs by combinatorial biosynthesis methods.