Recent Advances in Polyphenol Research. Группа авторов

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with the natural symmetrical 3‐deoxyanthocyanidins such as luteolinidin and apigeninidin, the 6,8 rearrangement is not observed. The 6,8 rearrangement in the flavylium cation was first reported (Jurd 1963) for 4’,5,7,8‐tetrahydroxyflavylium and for 6/8‐C‐glycosyl‐3‐deoxyanthocyanidins (Bjorøy et al. 2009). More recently, a complete study extended to the other species of the multistate present in moderately acidic medium was reported (Scheme 1.13), R=Br (Cruz et al. 2016, 2017).

      In the case of the 6,8 rearrangements for R=CH3 or phenyl (Basílio et al. 2017) the 8 derivative is the only one observed and equilibrium in moderately acidic medium is established between A8 and Ct (c. 20%). The pH‐dependent equilibrium between flavylium cation and a mixture of quinoidal base (major species) and trans‐chalcone was previously reported for 5‐deoxyanthocyanidins (Sousa et al. 2014; Brouillard et al. 1982). At moderately acidic pH values there is some spectral evidence that the two quinoidal base isomers are in equilibrium, but the spectral variations are relatively small. In order to unveil the two isomers other strategies should be used; see below.

      Source: Adapted from Cruz et al. 2016. © 2016 John Wiley & Sons.

Schematic illustration of the absorption spectra of equilibrated solutions of the compound 8-bromo,4′,5,7 trihydroxyflavylium R equals Br at pH equals 3.7.

      Source: Adapted from Cruz et al. 2016. © 2016 John Wiley & Sons.

      1.4.3.1 Unveiling the 6,8 Rearrangement Through Host‐Guest Complexation

      The host‐guest strategy was used to form the 6‐phenyl isomer from the 8‐phenyl‐5,7‐dihydroxyflavylium profiting from the capacity of cucurbit[7]uril to complex preferentially the former (Basílio et. al 2017) as well as for 6,8‐bromo‐apigeninidin (Basílio et al. 2016).

Schematic illustration of 1H NMR of 6-bromo-5,7-dihydroxyflavylium and its 8-bromo isomer in the presence of cucurbit[7]uril.

      Source: Adapted from Basílio et al. 2017. © 2017 John Wiley & Sons.

Schematic illustration of the discrimination, isolation, and 6,8 rearrangement of 6-bromo-5,7 dihydroxyflavylium and its 8-bromo isomer in the presence of cucurbit[7]uril.

      Source: Adapted from Basílio et al. 2017. © 2017 John Wiley & Sons.

      The multistate of chemical species followed by anthocyanins is not restricted to this family of compounds. Besides those of natural origin such as anthocyanidins and deoxyanthocyanidins, anthocyanins have been a source of bio‐inspiration for the synthesis of new derivatives that permit us to achieve complexity at the bottom, through this multistate of chemical species. The multistate of chemical species can be operated by pH changes, light, temperature, and host‐guest chemistry. The light response is particularly challenging, since there are two photochromic systems: (i) the ring opening of the hemiketal to produce cis‐chalcone, and (ii) cis‐trans isomerization of the chalcones. The multistate response can be modulated by host‐guest chemistry, increasing the potentialities of the system as photochromic materials, or in the domain of the health sciences.

      1 Basílio, N., Cruz, L., Freitas, V., et al. (2016). A multistate molecular switch based on the 6,8 rearrangement in bromo‐apigeninidin operated with pH and host–guest inputs. Journal of Physical Chemistry, 120 (29): 7053–7061.

      2 Basílio,


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