Читать книгу Recent Advances in Polyphenol Research - Группа авторов - Страница 39

This annulation method was also applied to the synthesis of a trimeric natural product, (+)‐cinnamtannin B1 (62) (Figure 2.27) (Ito et al. 2014), which is known as a rare sweet polyphenol. To realize the convergent assembly of the trimeric scaffold, the orthogonal coupling strategy was adapted by employing monomers 34, 58, and 52. Annulation of dioxy electrophilic unit 34 with sulfide 58 was carried out by using BF₃·OEt₂ to give the coupling product 59 in 91% yield. The arylthio group remained intact under the hard Lewis acidic conditions. Next, the soft activation of dimeric sulfide 59 was conducted by using I₂ and Ag₂O, enabling the reaction with the nucleophilic epicatechin unit 52 to give trimer 60 in 96% yield. After two‐step deprotection via 61, (+)‐cinnamtannin B1 (62) was obtained in 70% overall yield.