The spectra, ionization, and deuteriation of oxazoles and related compounds

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Abstract
The 1H n.m.r. spectra of oxazole, its derivatives, and some related compounds in non-polar, polar, and protonating media are discussed. The hitherto undetermined pKa values for oxazole (0·8, now measured by the chemical shifts of H-2 in acidic media) and its derivatives show that the nucleus is far less basic than previously supposed, primarily owing to the powerful inductive effect of the oxygen atom. The ultraviolet spectra of 2,5- and 2,4-diphenyloxazole indicate through-conjugation only in the former. This fact and other data suggest that oxazoles should be considered more as conjugated dienes than as fully aromatic compounds. The rates for the 2-deuteriation of simple oxazoles increase with the alkalinity; oxazole also undergoes 5- but not 4-deuteriation under extreme conditions. On these grounds, the electron-distribution of the three carbon atoms of oxazole is clearly in the order 4 > 5 > 2. This is confirmed by the chemical shifts of the respective protons and the pKa values of 2-, 4-, and 5-carboxyoxazole. Isoxazole (pKa–2·03) is a much weaker base than previously reported.

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