Multisubstituted Pyridines from 1,2,4-Triazines by Diels-Alder Chemistry

Significance

Two methods for the preparation of multisubstituted pyridines, which involve Diels-­Alder reaction of 1,2,4-triazines with in situ generated enamines, are presented. The development of a solvent-free microwave-promoted procedure is a noteworthy feature of the enamine method. Pyrrolidine was shown to be the optimal secondary amine for the in situ-generated method while N-methylethylenediamine served as the best diamine for the tethered imine-enamine procedure. Both cyclic and acyclic ketones were shown to give highly substituted pyridines in reasonable to good yields.