Synthesis and properties of chromono[2,3-b]pyridines

Chromono[2,3-b]pyridines were synthesized by cyclization of 2-phenoxynicotinic acids. Their UV spectra are due to - ^* transitions and, in contrast to the spectrum of xanthone, are of high intensity. In concentrated sulfuric acid the chromono[2,3-b]pyridines form a doubly charged pyridinium-pyrylium ion, and the reaction is accompanied by an increase in the intensity of the long-wave bands (330–340 nm) in the UV spectrum. The spectrophotometrically determined pK_a values of 8-R-chromono[2,3-b]pyridines range from –8.62 to –9.81, depending on the substituent in the 8 position, and correlate with Hammett _p constants.Translated from Khimiya Geterotsiklicheskikh Soedinenii No. 9, pp. 1235–1238, September, 1976.