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Multicomponent Reactions of Indole, Ethyl Glyoxylate and Anilines: From Friedel–Crafts to Aza‐Diels–Alder Reactions Catalysed by Scandium Triflate
Author(s) -
Desimoni Giovanni,
Faita Giuseppe,
Mella Mariella,
Toscanini Marco,
Boiocchi Massimo
Publication year - 2009
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200900054
Subject(s) - chemistry , trifluoromethanesulfonate , scandium , indole test , friedel–crafts reaction , diels–alder reaction , medicinal chemistry , diastereomer , adduct , organic chemistry , catalysis , stereochemistry
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)