The Isolation of a Bicyclo[3.2.1] Intermediate During Formation of Benzotropolones, a Common Nucleus Found in Black Tea Pigments: Theaflavins

The coupling of 5-methylpyrogallol (6) with 4-methyl-o-quinone (7) in an aprotic solvent (CH2Cl2) efficiently afforded a key intermediate 8 having a bicyclo[3.2.1] ring whose structure had been proposed for fifty years. Subsequent ring-opening of 8 by the addition of H2O, air oxidation and decarboxylation produced the benzotropolone 10 in good yield.