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

Abstract

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.

Key words

bicyclic compounds - oxidations - benzotropolones - quinones - theaflavins

References

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12

Bicyclo intermediate (8): mp 169 °C. ¹H NMR (500 MHz, d ₆-acetone): δ = 2.30 (d, 3 H, J = 0.7 Hz, Me), 2.35 (d, 3 H, J = 1.4 Hz, Me), 4.37 (d, 1 H, J = 1.1 Hz, 5-H), 5.92 (quin, 1 H, J = 1.4 Hz, 3-H), 6.08 (br s, 1 H, OH), 6.62 (d, 1 H, J = 0.5 Hz, 8-H), 7.43 (br s, 1 H, OH), 7.93 (1 H, br s, OH). ¹³C NMR (100 MHz, d ₆-acetone): δ = 17.53 (Me), 23.90 (Me), 57.42 (C-5), 92.31 (C-8), 117.51 (C-7), 123.00 (C-4), 124.90 (C-11), 128.65 (C-13), 128.97 (C-6), 145.23 (C-9), 166.67 (C-10), 191.20 (C-2), 197.67 (C-12). HRMS (EI, 70 eV): m/z calcd for C₁₄H₁₂O₅: 260.0685; found: 260.0693.

13

We consider that this condensation reaction is not a Diels-Alder-type coupling but ionic coupling of o-quinone with phenol. The details will be reported in the future.