The pyrano[4,3-b]indol-3-ones (7) are stable indole-2,3-quinodimethane type dienes, which undergo Diels-Alder reaction with alkynes t o give, after loss of carbon dioxide, carbazoles. The reactivity of the diene is increased by the presence of the electron-withdrawing t-butoxycarbonyl group on the indole nitrogen. The pyranoindolones (7) are less reactive, and exhibit the opposite regiochemistry in their Diels-Alder reactions than the isomeric pyrano [3,4-b] indol-3-ones (1 ). Factors which affect the regiochemistry of the Diels-Alder reaction are discussed.Over the last 5 years we have been interested in the synthesis and Diels-Alder reactivity of the pyrano[3,4-b]indol-3-one system (l), stable compounds which contain the indole-2,3quinodimethane diene. Thus we have described their cycloaddition to simple alkynes to give, after loss of carbon dioxide, carbazoles,' and the steric and electronic factors which influence the regioselectivity of the Diels-Alder reaction.2 We have also described intramolecular Diels-Alder reaction^,^ and the use of pyrano[3,4-b]indol-3-ones in the synthesis of the carbazole alkaloids carbazomycin A and B, and hyella~ole.~ It was, therefore, of interest to investigate the isomeric pyrano[4,3blindol-3-one diene system (2) and how the reversal of the carbon dioxide bridge would affect the regioselectivity of the Diels-Alder reaction.
194 ChemInform Abstract The pyranoindolones (I) undergo Diels-Alder reaction with a variety of alkynes such as (II) to form, after loss of CO2, the carbazoles (III) or/and (IV). The reactivity of the diene is increased by the presence of the electron-withdrawing groups at the indole nitrogen. Factors which affect the regiochemistry of the Diels-Alder reaction are discussed. The pyranoindolones (I) yield (III) as major products, whereas isomeric pyranoindolones such as (V) yield (IV)-analogues of type (VI) as major product.
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