In situ vinylindole synthesis: Diels–Alder reactions with N-phenylmaleimides, 1-tetralones and 4-chromanones to give annulated tetrahydrocarbazoles

An acid-catalyzed condensation of indole 4 with 1-tetralones 5a-f formed a vinylindole which reacted in situ with N-phenylmaleimides 7a-g. This cascade of condensation and Diels-Alder reaction was performed in IPA (isopropyl alcohol) and in presence of p-TsOH (p-toluenesulfonic acid) at reflux to produce annulated tetrahydrocarbazoles 8a-g and 10a-e. Similarly, 9a-g were synthesized by substituting 4-chromanone 6 for 5a-f. The 1-tetralone derived products 8a-g and 10a-e were obtained in higher yields (19-79%) compared to 4-chromanone derived 9a-g (6-28%), likely due to additional side reactions in case of chromanones. To establish the structure and stereochemistry of products, the crystal structure of 10c was obtained, confirming endo-Diels-Alder addition.