A new synthetic approach to N-substituted 1,4-dihydropyridines

Some novel N-substituted 1,4-dihydropyridines (DHPs) (15a–d) have been synthesized by reaction of 2-amino-5-formyl-4H-pyran (10) with primary amines. Formation of 1,4-DHPs involves ring cleavage of the 4H-pyran ring by nucleophilic attack of the respective amine and subsequent 6-exo-dig cyclization. Treatment of the pyran system 10 with hydrazines under the same reaction conditions leads, however, to the corresponding hydrazone derivatives 12a,b. Two different reaction routes are observed depending whether the amine or hydrazine derivative is used as nucleophilic reagent. A competition between 1,4 versus 1,2 addition reaction pathway is proposed.