Scalable synthesis of functionalised 2-pyridones via [4+2] cycloaddition reactions of 2-pyrazinones and alkynylboronates

The multigram synthesis of N-protected 2-pyridone boronic acid derivatives via a [4+2] cycloaddition of alkynylboronates with 2-pyrazinones is presented. The reactions are highly chemoselective, and generally highly regioselective although trimethylsilyl-substituted alkynylboronates have proven to be an exception. Nonetheless, in this latter case, separation of regioisomers was successfully accomplished via high performance counter-current chromatography allowing isolation of analytically pure 2-pyridones. Further derivatisations of trimethylsilyl-substituted 2-pyridone boronates were performed providing access to a selection of functionalised scaffolds.