One‐Pot Metal‐Free Synthesis of 3‐Substituted 4‐Hydroxyisocoumarins via Oxa‐Hauser–Kraus Reaction of Sulfonyl Phthalide With Glyoxal Derivatives

The [4 + 2] Hauser–Kraus (H–K) annulation strategy has been employed for the first time for the synthesis of 4‐hydroxyisocoumarins. This method involves reaction of sulfonyl phthalide with aryl glyoxal under mild conditions (Cs 2 CO 3 , THF, rt) to generate 3‐acyl 4‐hydroxyisocoumarin derivatives. Good substrate scope, scalability, and further transformations of the products have been demonstrated. Calculation of highest occupied molecular orbital–lowest unoccupied molecular orbital energies, electrophilicity and nucleophilicity indices, as well as transition state energies by density functional theory provided insights into the probable mechanism of the reaction and the observed selectivity.