Coumarin-fused azadienes: new dienes for the IEDDA reaction

Abstract

Coumarin-fused 1-azadienes were prepared via a condensation between 3-formylcoumarin and a variety of amines. Coumarin-fused 2-azadienes were prepared via a condensation of 3-aminocoumarin and a variety of aldehydes. A new synthesis of 3-aminocoumarin was developed. -- 1,2-Imine addition products were observed when 1-azadiene 184 was reacted with enamine 134. This reaction resulted in the formation of reduced aldol condensation product 242 (37%) and the desired Diels-Alder adduct 243 (1% yield). When 1-azadienes 186 and 187 were reacted with enamine 134, aldol condensation product 250 was formed (53% yield with 186 and 77% yield with 187). When 2-azadiene 222 was reacted with enamine 134, compound 252 (95%), which is also the result of 1,2-addition, was formed. -- Using modified Povarov reaction conditions with the 2-azadienes, cycloaddition products were isolated. Two procedures were employed. The first one involved the use of a preformed 2-azadiene, a catalytic amount of Yb(OTf)₃, a non-enamine dienophile in acetonitrile. The other procedure was a three-component (in situ generated diene) method that entailed the combination of 3-aminocoumarin 212, a catalytic amount of Yb(OTf)₃, a non-enamine dienophile and an aldehyde. An endo and exo isomer was obtained in most cases with selectivity ranging from >95 : 5 in favor or exo to >95 : 5 in favor of endo. Two of the Povarov adducts were oxidized with bromine to produce pyrido[2,3-c ]coumarins.