Effective Enzymatic Synthesis of Benzotropolone and Formation Mechanism of Its Bicyclo[3.2.1]octane‐type Intermediate

Abstract In this study, 1′,2′‐dihydroxy‐3,4‐benzotropolone (BTP), the simplest theaflavins (TFs) derivative, is synthesized via the oxidative coupling of pyrogallol (PG) and catechol (CC). We show that low temperature was beneficial to the production of BTP. A possible generation mechanism of the key bicyclo[3.2.1]octane‐type intermediate (BOI) during the formation process of BTP was subsequently proposed. Our analysis based on quantum chemical calculations indicated that three types of tautomers of PG quinones should be present in the reaction solution. Among them, PG quinone tautomer III (PGQT‐III) with two carbanions might be one of the direct substrates for BOI synthesis, which underwent 1,6‐addition reaction to C3‐C4 bond of ortho‐quinone with strong electrophilicity leading to the key intermediate. Furthermore, the presence of PGQT‐III was also supported by the retrosynthetic analysis of BOI. The findings not only shed light on the production mechanism of BTP, but also create new prospects in synthesizing TFs with controllable products.