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3.6.13.1.3.2 Method 2: Synthesis of Dicoumarins via Cyclization–Homocoupling Using tert-Butyl Hydroperoxide

DOI: 10.1055/sos-SD-103-00038

Hopkinson, M. N.; Gouverneur, V.Science of Synthesis Knowledge Updates, (20112117.

The use of the strong oxidizing agent tert-butyl hydroperoxide enables the synthesis of 2H,2′H-[3,3′-bibenzopyran]-2,2′-diones (dicoumarins) 37 directly from aryl propynoate esters 36 via a gold(III)-catalyzed cyclization–oxidative homocoupling cascade process (Scheme 16).[‌67‌] When reacted with tetrachloroauric acid (5 mol%) in the presence of the oxidant (5 equiv) in 1,2-dichloroethane, ester 36 (R1 = t-Bu; R2 = R3 = H) undergoes a 6-endo-dig cyclization followed by homocoupling to deliver 2H,2′H-[3,3′-bibenzopyran]-2,2′-dione 37 (R1 = t-Bu; R2 = R3 = H) as the major product in 55% isolated yield. The formation of 2H-benzopyran-2-one 38 (R1 = t-Bu; R2 = R3 = H) resulting from cyclization–protodeauration is not completely suppressed and this product can be isolated in 32% yield. In the absence of the oxidant, this compound is the major product (85% yield using 5 mol% AuCl3/AgOTf). Notably, subjecting 2H-benzopyran-2-one 38 (R1 = t-Bu; R2 = R3 = H) to the optimized reaction conditions with tert-butyl hydroperoxide does not lead to the corresponding dimer 37. The reaction is tolerant of a range of substituents on the benzene ring although electron-withdrawing groups such as cyano substituents inhibit cyclization.

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