Heterocycles as inner-ring dienes

8 Reactions with Cyclic Transition States 3.3.1.8.1 Heterocycles as inner ring dienes [Pg.328]

Furan has much greater reactivity in cycloaddition reactions compared to pyrrole and thiophene the latter is the least reactive diene. A-Substituted pyrroles often show enhanced diene character compared with the parent heterocycle. [Pg.328]

Conversely, furans with electron-withdrawing groups e.g. CHO, CN, C02Me) in the 2-position show reduced Diels-Alder reactivity. Although furan-2-carboxaldehyde is a poor diene the related N,N-dimethylhydrazones do take part in reactions with a range of dienophiles including maleic anhydride, maleimides, and fumaronitrile (88JOC1199). [Pg.328]

Electron-releasing groups in the 3- or 4-position appear to have little effect on the diene character of the furan ring. Although 3,4-dimethoxyfuran readily undergoes [4 + 2] cycloaddition with maleic [Pg.328]

Pyrroles and indoles can give a wide variety of cycloaddition reactions and this area has seen vigorous activity since 1990 because of the potential of methodology for the synthesis of complex indole alkaloids. The reactions of pyrroles with dienophiles generally follow two different pathways involving either a [4 + 2] cycloaddition or a Michael-type addition to a free a-position of the pyrrole ring. Pyrrole itself gives a complex mixture of products with maleic anhydride or maleic acid. [Pg.329]

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