The vilsmeier reaction in the synthesis of 3‐substituted [1]benzopyrano[4,3‐<i>b</i>]pyridin‐5‐ones. An unusual pyridine ring closure

Abstract

Abstract 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 (1 H ) ‐pyridinones 8 have been obtained as expected.