Structure et réactivité des benzoxazoles: étude par résonance magnétique nucléaire du carbone-13

Abstract

Thirty-four benzoxazoles, oxazole, and 10 o-aminophenols have been studied by 13C nmr spectroscopy. All the signals have been attributed from substituent effects. The structure of products obtained by nitration of benzoxazole have been determined by 13C nmr. The shifts induced by substitution at the 2 position have been discussed as a function of an empirical model using the structural parameters F, R, and Q*. Azido–tetrazole equilibria (N3 in position 2) and prototropic tautomerism (NH2, OH, and SH in position 2) have been also discussed. The chemical shifts and coupling constants of oxazole and unsubstituted benzoxazole have been compared.

A carbon-13 nuclear magnetic resonance study of a series of 5-substituted benzimidazoles

“…[I] indicate that these results compare favorably with those obtained for other benzazole systems (4,5,6,10). For the ipso carbon, C-5, the correlation is not quite as good; this is consistent with the results observed for the Z-substituted benzimidazoles (4).…”

Naturally Occurring Labdane Diterpene and Benzofuran from Globba pendula

“…13 C (125 MHz, CD 3 OD) and1 H NMR (500 MHz, CD 3 OD) data and HMBC of compound 2.Phytochemical investigation on Globba pendulaNatural Product Communications Vol. 4(8) 2009 1033 coupling pattern of which suggested the presence of a 1,2-disubstituted benzene ring system.…”

Synthesis of β-D-ribofuranosylbenzazoles using nitrile oxide addition chemistry

“…The carbon NMR spectrum in CDCl 3 showed characteristic signals for the benzoxazole moiety [162.5 (C-2), 152.0 (C-7a), 141.7 (C-3a), 126.9 (C-5), 125.7 (C-6), 121.7 (C-4) and 112.0 ppm (C-7)], which were very similar to those previously reported for 2-pyranosylbenzoxazoles 7, 8 and are typical of 2-alkylsubstituted benzoxazoles. 30 The presence of the perbenzoylribofuranose unit was evident in both the 13 C and 1 H NMR spectra. The reaction pathway is believed to proceed by initial dehydrochlorination of the hydroximoyl chloride 15 by one equivalent of the 2-aminophenol to generate the nitrile oxide 14, followed by nucleophilic addition of the anilino group of a second equivalent of 2-aminophenol to form the amidoxime 20; nucleophilic displacement of hydroxylamine by the phenolic group then leads to the benzoxazole, as illustrated in Scheme 3.…”