Pyridynes, hetarynes

In an extensive investigation of the behavior of halogeno-pyridines and -quinolines towards potassium amide in liquid ammonia, tendencies to react according to the hetaryne type and other mechanisms were compared. The following results may be mentioned. 3-Fluoropyridine does not react via 3,4-pyridyne, but yields fluoro derivatives of 4,4 - and 2,4 -bipyridine. 3,6-Dibromopyridine is converted first into 6-bromo-3,4-pyridyne and not into 6-amino-3-bromopyridine. 2-Bromoquinoline yields 2-methylquinazoline together with 2-aminoquinohne. ... 

Hetarynes (dehydroaromatic heterocycles) can also act as dienophiles in Diels-Alder reactions. 2 This capacity and the inherent regiochemical problems are illustrated by two syntheses of the alkaloid ellip-ticine, which both feature a [4 + 2] cycloaddition of 3,4-pyridyne (533) (Scheme 126). 2 The first approach used the triazenylcarboxylic acid (534) as a precursor for hetaryne (533), which then underwent an addition to pyranoindolone (535) spontaneous CCh extrusion of the initial cycloadducts (536 X = N, Y = CH) and (536 X = CH, Y = N) gave directly a 50 50 mixture of ellipticine (537) and its undesired regioisomer (538). 2 ... 

Arynes and hetarynes (heterocyclic arynes)67-72 such as 25 and 26 are named benzyne or dehydrobenzene and 3,4-pyridyne or 3,4-dehydropyridine, respectively. 

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