Multicomponent Reactions of Indole, Ethyl Glyoxylate and Anilines: From Friedel–Crafts to Aza‐Diels–Alder Reactions Catalysed by Scandium Triflate

The multicomponent reaction (MCR) of indole ( 1 ), ethyl glyoxylate ( 2 ) and 3,4‐dimethoxy‐ or 3,4‐methylenedioxyanilines ( 3a , b ) give, in analogy to Friedel–Crafts alkylation of indole, expected acetates 4a , b . When the reactions are catalysed by scandium triflate, however, a completely different reaction pathway is followed and two pairs of diastereomeric aza‐Diels–Alder adducts ( 7a , b and 8a , b ) are isolated, which result from the reactions in which the ethyl 2‐(arylimino)acetates (azomethynes of 2 with 3a , b ) behave as heterodienes and indole is the dienophile. These products, whose structures were confirmed by X‐ray crystal structure analysis of 7a , are not derived from the scandium‐catalysed rearrangement of 4a , b , because when these latter are treated with Sc(OTf) 3 , the rearrangement produces acetates 5a , b only. The limits of the aza‐Diels–Alder reaction were investigated by performing the MCR on substituted anilines 3f – k . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)