A mild
and efficient protocol for the intramolecular aromatic C–H
oxygenation of 2-biphenylcarboxylic acids has been achieved via iron
photocatalysis. The 2-biphenylcarboxylic acids with a diverse array
of substituents at both phenyl rings could furnish the oxygenation
products in good to excellent yields. We speculate that the aryl carboxylate–iron(III)
complexes should generate the aroyloxy radicals and iron(II) upon
visible light irradiation.
A novel strategy for de novo synthesis of pyridines featuring an unprecedented α-addition of aldehyde and enamide to isonitrile as a key step is described. Under mild conditions, a cascade reaction involving Zn(OTf)2-promoted [1 + 5] cycloaddition of isonitrile with N-formylmethyl-substituted enamide, facile aerobic oxidative aromatization and intermolecular acyl transfer from the pyridinium nitrogen to the 5-hydroxy oxygen, and finally acylation of the 4-amino group by an external acyl chloride efficiently afforded 2-substituted 4-acylamino-5-acyloxypyridines in good to excellent yields.
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