Stereoselective Synthesis of Pyridinones: Application to the Synthesis of (−)-Barrenazines

The stereoselective synthesis of pyridinones was accomplished by the nucleophilic addition of Grignard reagents to a chiral pyridinium salt
derived from 4-methoxypyridine. This methodology was applied to an expedient synthesis of (−)-barrenazine A and B. After N-functionalization
and 1,4-reduction of the pyridinone system, the corresponding α-amino piperidinones readily undergo dimerization to give the hexahydrodipyridinopyrazine skeleton of the barrenazine alkaloids.