Conjugate Addition Reactions of <i>N</i>‑Carbamoyl-4-pyridones and 2,3-Dihydropyridones with Grignard Reagents in the Absence of Cu(I) Salts

<i>N</i>-Boc- and <i>N</i>-ethoxycarbonyl-4-pyridones and the resulting 2,3-dihydropyridones undergo 1,4-addition reactions with Grignard reagents in the presence of chlorotrimethylsilane (TMSCl) or BF₃·Et₂O in excellent yields. Copper catalysis is not required, and mechanistic considerations suggest that the reaction is proceeding by a conjugate addition pathway rather than by a pathway involving 1,2-addition to an intermediate pyridinium ion. TMSCl-mediated conjugate addition of Grignard reagents to 2-substituted-2,3-dihydropyridones gives the <i>trans</i>-2,6-disubstitued piperidinones stereoselectively, while cuprate reagents give either the <i>trans</i> or <i>cis</i> diastereomers or mixtures