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14.2.1.1.2.2 Method 2: Ring Closure of 3-Aryl-1-(2-hydroxyaryl)prop-2-en-1-ones (2′-Hydroxychalcones) to Flavylium Salts

DOI: 10.1055/sos-SD-014-00073

Nógrádi, M.Science of Synthesis, (200314220.

2-Hydroxychalcones are readily available by condensation of 2-hydroxyacetophenones and aryl aldehydes. Ring closure in the presence of triethyl orthoformate proceeds via a 2-aryl-2,3-dihydro-4H-benzopyran-4-one. It is accompanied by dehydrogenation and leads to 2-aryl-4-ethoxy-1-benzopyrylium salts 31 in 2555% yield (Scheme 36),[‌137‌,‌138‌] while borohydride reduction followed by dehydration, acid treatment, and reoxidation with benzo-1,4-quinone gives 2-aryl-1-benzopyrylium salts unsubstituted at C4.[‌48‌,‌139‌]

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