THE VILSMEIER REACTION IN THE SYNTHESIS OF 3-SUBSTITUTED [1]BENZOPYRANO[4,3-B]PYRIDIN-5-ONES - AN UNUSUAL PYRIDINE RING-CLOSURE

Starting from 4-chlorocoumarin-3-carbaldehyde (1) and Wittig phosphora nes 2a-d the title compounds 6a-c have been synthesized via a four-ste p sequence. The intermediate 4-alkylamino-3-vinylcoumarins 5a-k have b een prepared by the reaction of 4-chloro-3-vinylcoumarins 3a-d with pr imary 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.