Highly efficient thermal cyclization reactions of alkylidene esters in continuous flow to give aromatic/heteroaromatic derivatives

Intramolecular thermal cyclization and benzannulation reactions of the Gould-Jacobs and Conrad-Limpach types were performed in a designed continuous flow reactor system at temperatures in the range of 300-360 °C and under high pressure conditions (100-160 bar) with very short residence times (0.45-4.5 min) in tetrahydrofuran as a low-boiling point solvent. Substituted heteroaromatic compounds including pyridopyrimidinones and hydroxyquinolines were synthesized in moderate to high yields. Application of the reaction conditions also allows the synthesis of naphthol and biphenyl derivatives. The procedure involves an easy work-up and the non-batchwise preparative synthesis method is suitable for automation.