Heterocycles as inner dienes in cycloaddition

Because of the strong rr-deficiency of most six-membered heteroaromatic compounds, cycloadditions of this type belong to Diels-Alder reactions with inverse electron demands in other words, they are LUMO -HOMOp, controlled reactions (for review see (B-87MI 502-08)). Acceptor substituents in the heterocyclic diene and donor substituents in the dienophile accelerate the reaction, as shown by kinetic data (83TL1481,84TL2541,90TL6851). 

Condensed heteroaromatic cations are reactive in [2 + 4] cycloaddition reactions with inverse electron demand. For instance, 2-benzopyrylium salts (389) react with vinyl ethyl ether to afford 

Phthalazines undergo [2 + 4] cycloaddition with enamines to give naphthalene derivatives. Pyrimidines also react as azadienes in reactions with enamines, e.g. (398 — 399). 

4-Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene, 

In most Diels-Alder reactions, dienophile addition occurs across the 3- and 6-positions of the 

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