Five-membered ring systems isoxazoles

Many other reactions of pyrylium salts have been described, converting this readily accessible conjugated C5-synthon into monocyclic aromatic systems with five-membered rings (2-acylfurans, isoxazoles, pyrazoles), or aromatic bicyclic systems (naphthalenes, indolizines),... 

When sulfur or oxygen atoms are present in a five-membered ring, they provide the pyrrole-like lone pair and any nitrogen atoms are necessarily pyridine-like. The four most important systems are oxazole 17 and thiazole 18 with 1,3-related heteroatoms and isoxazole 19 and isothiazole 20 with an N-0 or N-S bond. 

Electrophilic Reactions. Further studies aimed at the correlation of the reactivity of five-membered heteroaromatic rings towards electrophiles have been reported. Acid-catalysed hydrogen exchange rates were measured (n.m.r.) for isoxazole, isothiazole, and their 3- and 5-methyl derivatives. Extrapolated values for the rate constants at 100 °C and pH 0 were obtained and compared with those for other ring systems. Quantitative effects of the methyl groups reflect the relative degree of bond fixation, and indicate that the aromaticity increases in the series isoxazole, pyrazole, and isothiazole. ... 

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