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DOI: 10.1055/sos-SD-014-00294

Williams, A. C.; Camp, N.Science of Synthesis, (200314380.

In Scheme 31 examples of the reactions of phenols and anisoles with 3-oxoalkanoic acid derivatives, catalyzed by phosphoric acid and its derivatives, to give 2H-1-benzopyran-2-ones 9698 are given.[‌199‌,‌226‌‌230‌] A difference in the regiochemistry of the cyclizations has been observed between polyphosphoric acid (as in the formation of 97) and polyphosphate ester (as in the formation of 98), the origin of which is unclear.

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