Heteroarynes five-membered

Thus, rutroheterocycles are importaru synthoas of five-membered heteroarynes h cyclo idchdQa reacdoas, which are geaedilly chfficidt to be geaerated ... 

Thus, nitroheterocycles are important synthons of five-membered heteroarynes in cycloaddition reactions, which are generally difficult to be generated.38... 

Unlike its predecessor, this review also includes heteroarynes, though the treatment is less exhaustive than previous comprehensive reviews of six-membered and five-membered heteroarynes. 

The literature prior to approximately 1981-2 has been thoroughly and exhaustively reviewed " consequently, this section will deal only with literature since then, and only with the major types of six- and five-membered heteroarynes. 

Most experimental evidence for the existence of five-membered heteroarynes is questionable indeed it was stated in 1967 that no unambiguous evidence is yet available for the formation of dehydro derivatives of five-membered aromatic heterocycles , and the picture has not changed greatly since then, despite a major (with 591 references) and excellent review on the subject. The only possible exception is for 2,3-didehydrothiophene 787 proposed as an intermediate in the pyrolysis of 2,3-thiophenedicarboxylic anhydride and a recent re-investigation using matrix isolation techniques found no direct... 

To date, then, the unequivocal identification of a five-membered heteroaryne remains a challenge. 

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