Synthesis of Annulated Pyridines by Intramolecular Inverse-Electron-Demand Hetero-Diels-Alder Reaction under Superheated Continuous Flow Conditions

Pyrimidine alkynes can be transformed into the corresponding annulated pyridines efficiently in flow. The superheating of organic solvents far beyond their boiling point enables toxic and difficult to workup solvents such as nitrobenzene or chlorobenzene, which are usually employed for these reactions, to be replaced by less harmful ones like toluene. The relative rate of reactivity for a series of structurally close starting materials was investigated and a scalable flow process was developed, providing facile access to a series of novel annulated pyridine building blocks. The effect of thermal volume expansion of solvents under superheated conditions was found to be significant and influenced the residence times considerably. To obtain meaningful and accurate residence times, flow rates need to be corrected for volume expansion. To avoid confusion with residence times, t(R), calculated from nominal flow rates, we would propose to use for such corrected residence times the term effective residence time, t(R,eff).