Five-Membered Heterocycles Containing One Heteroatom

In the thiophene series a Wolff-Kishner reaction on the 3-aroyl compound 379, in addition to the expected reduction product, affords the pyrazole 381. A typical rearrangement of the hydrazone 380, followed 

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As shown in Scheme 2, two heteroatom-carbon bonds are constructed in such a way that one component provides both heteroatoms for the resultant heterocycle. By variation of X and Z entry is readily obtained into thiazoles, oxazoles, imidazoles, etc. and by the use of the appropriate oxidation level in the carbonyl-containing component, further oxidized derivatives of these ring systems result. These processes are analogous to those utilized in the formation of five-membered heterocycles containing one heteroatom, involving cyclocondensation utilizing enols, enamines, etc. 

Five-membered Heterocycles containing One Heteroatom Pyrrole, Furan and Thiophene... 

From five-membered heterocycles containing one heteroatom 91... 

The order of azole reactivity will parallel that found for the analogous five-membered heterocycles containing one heteroatom (Chapter 6) [i.e., NH(R) > O > Se > S]. 

Cross-coupling reactions of thiophenes by C-H functionalization have been extensively reviewed in the year 2014 (14ASC17). One important feature is a discussion of recent progress made in direct arylation and heteroarylation reactions involving thiophenes and other five-membered heterocycles containing one heteroatom. 

The rates of nitration of isothiazole (and its 3- and 5-methyl derivatives) have been measured. While isothiazole and its 3- and 5-methyl derivatives are nitrated as free bases, the 3,5-dimethyl-compound is nitrated as the conjugate acid. Isothiazoles are nitrated in the 4 position 10 times slower than benzene. The order of reactivity for nitration is pyrazole > isothiazole > isoxazole, in contrast to that of five-membered heterocycles containing one heteroatom, which for trifluoroacetylation is pyrrole > furan > thiophen. ... 

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