Diels-Alder cycloaddition 4//-furo indoles

The furo[3,4-h]indole analogs serve as fused indolo-2,3-qumodimethanes and have been widely used in Diels-Alder cycloaddition reactions. Recent publications have reviewed the various methods developed for the synthesis of furo[3,4-b]indoles [126], as well as the pioneering work on the utility of these precursors in cycloaddition reactions to produce various heterocychc derivatives [127]. The early application of [4-1-2] cycloaddition reactions with furo[3,4-b]indoles to the synthesis of natural products was highlighted by the first regioselective Diels-Alder synthesis of ellipticine (385), which involved a cycloaddition reaction of the furoindole 382 with a dihydropyridone derivative 381 as the key step (Scheme 86) [128]. 

In addition to the above intermolecular reactions, the intramolecular Diels-Alder cycloadditions of furo[3,4-b]indoles have also been reported. Herndon and coworkers [131] used the coupling reaction of the Af-benzylindolynal derivative 396 with Fischer carbenes 397a-b to obtain the carbazoles 398a-b, presumably via the... 

Generally, cycloadditions represent powerful reactions for construction of heterocycles. Tandem intramolecular Diels-Alder/retro-Diels-Alder reaction sequences were applied in the syntheses of many A,B-diheteropentalenes <1996GHEC-II(7)1>. Gribble and co-workers <1998SL1061> reported new syntheses of pyrrolo[3,4-, ]indoles 426, benzo[4,5]furo[2,3-f]pyrroles 429, and benzo[4,5]thieno[2,3-4pyrroles 430 using the 1,3-dipolar cycloaddition... 

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