The Vilsmeier reaction in the synthesis of 3-substituted [1]Benzopyrano[4,3-b]pyridin-5-ones. An unusual pyridine ring closure

Article abstract of DOI:10.1002/jhet.5570320221

Starting from 4-chlorocoumarin-3-carbaldehyde (1) and Wittig phosphoranes 2a-d the title compounds 6a-c have been synthesized via a four-step sequence. The intermediate 4-alkylamino-3-vinylcoumarins 5a-k have been prepared by the reaction of 4-chloro-3-vinylcoumarins 3a-d with primary amines 4a-h. The coumarin derivatives 5 (except 5k) underwent an unusual pyridine ring closure under Vilsmeier conditions to form the benzopyrano[4,3-b]pyridines 6. When the aminoaldehydes 7 were treated with the Wittig reagent 2b the fused N-alkyl-2(1H)-pyridinones 8 have been obtained as expected.