Recent Advances in Polyphenol Research. Группа авторов

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Recent Advances in Polyphenol Research - Группа авторов


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href="#ulink_9ca86ae0-bf52-5db4-b0c0-b78c7478fe5d">Figure 2.6 shows some of the tetramers with A‐type structure, which are homo‐ or hetero‐oligomers (Nonaka et al. 1983; Morimoto et al. 1985; Morimoto et al. 1987; Balde et al. 1995; Nam et al. 2017). It should be noted that the molecular diversity arises not only from the elements stated before, but also from the inclusion of the enantiomeric flavan‐3‐ols (e.g. ent‐AZ, ent‐EC, ent‐CA in pavetannin C5) as constituent monomers. The diversity will increase in the future, and some of these compounds may show potentially significant biological activities.

      2.3.1 Hypothetical Biosynthetic Routes

Schematic illustration of Mayer's PA (procyanidin A2): the terminological origin of A/B-type structures. Schematic illustration of the structures of the tetramers with A-type linkages. Schematic illustration of two plausible biosynthetic pathways forming the A-type structure.

      2.3.2 Retrosynthesis

      Route I is relevant to the Path I biosynthesis discussed in Section 2.3.1 (see Figure 2.7), disconnecting the C–O bond i in A to B with a single connection and the C(2) cation center, which could be traced back to a B‐type structure B' as a precursor. In executing the synthesis, this approach has an advantage, that the corresponding B‐type structures are synthetically well accessible (Ohmori et al. 2004, 2011; Oyama et al. 2008; Kozikowski and Tückmantel 2009; Saito et al. 2009; Yano et al. 2012; Makabe 2013). However, a concern is that the site‐specific oxidation at the C(2) benzylic center on the upper flavan unit may be challenging.

Schematic illustration of retrosynthetic analyses of the A-type structure.

      Route III corresponds to another biomimetic pathway (Path II, Figure 2.7), based on the two‐bond disconnection at bonds i and iii in A, assuming a formal [3+3]‐cycloaddition of a dicationic species F and a nucleophilic partner G. As the possible synthetic equivalents to the key dicationic species F, one could conceive flavylium salt F' or flavan unit F″ with two leaving groups at the C(2) and C(4) positions. This approach would realize direct conversion to the key bicyclic skeleton. If flavylium salt F' were used, the enantiocontrol would inevitably pose a serious problem. In contrast, use of the flavan unit F″ could achieve a stereoselective reaction, as will be discussed later (see Section 2.3.5).

      2.3.3 Oxidative Conversion from B‐type PAs (Route I)

      2.3.4 Approaches via an Acyclic Precursor (Route II)


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