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DOI: 10.1055/sos-SD-014-00294

Williams, A. C.; Camp, N.Science of Synthesis, (200314374.

The acid-catalyzed reaction between a phenol and a β-oxo ester or malonic acid derivative remains one of the most widely used preparative approaches to the synthesis of 2H-1-benzopyran-2-ones, despite being known for over 120 years. A historical and mechanistic perspective may be found in an earlier review, which also summarizes the major classes of coupling reagent described at that time.[‌171‌] A wide range of catalysts has been described, the most important being sulfuric acid (Schemes 24 and 25), zinc(II) chloride (Scheme 26), sulfonic acids and sulfonic acid resins (Schemes 27 and 34), hydrogen chloride and fluoride (Scheme 28), aluminum trichloride (Scheme 29), phosphoryl chloride and phosphoric acid derivatives (Schemes 30 and 31), montmorillonite (Scheme 32), and trifluoroacetic acid (Scheme 33). Some of the more recent developments are the use of microwave reactors and clay catalysts (alone or in combination with the use of microwaves).[‌172‌‌175‌] By appropriate choice of coupling partner all of the available positions may be functionalized, provided sufficient consideration is given to the potential acid-lability of the substituents: yields are generally good to excellent.[‌41‌,‌50‌,‌57‌‌59‌,‌62‌‌64‌,‌161‌,‌171‌‌236‌]