Phloroglucinolic A-rings

In reactions at pH 11.0, no phloroglucinol A-ring can be detected after only 1 hour at 100 °C, and there is little change in the amount of methylol or formation of methylene bridge after 6 hours of heating. The methylene signals that are... 

Anthocyanins are highly reactive in aqueous medium and can undergo a nucleophilic attack on the electron-deficient carbons 2 and 4 of the pyranic ring or an electrophilic attack on the carbons 6 and 8 of the phloroglucinol A-ring, probably in the noncharged hemiacetal and quinonoidal forms (see Fig. 3.3). The hydroxyl groups may also react with electrophilic species. 

The NMR spectra of profisetinidin polymers are quite distinct from pro-cyanidin polymers in several respects (34). In particular, C-4 for a fisetinidol-4 unit occurs at J 130 and J 110 respectively, well upfield and downfield respectively from the corresponding resonances, J 155 and d 97, in catechin-4 units (34). NMR is, however, less useful for assigning relative stereochemistry than for Type 1 polymers. Type 2 oligomers have a low degree of interflavanoid bond stereospecificity (15, 119), in contrast to Type 1 polymers (i.e. 4a or 4)8), and this leads to extra spectral complexities. In addition, the B units of quebracho and wattle tannins have a phloroglucinol A-ring oxidation pattern, whereas the T and M units are of the resorcinol A-ring pattern (15). 

A series of very unusual flavanones has been detected in the genera Baeckea, Luma, and Melaleuca, all belonging to the family Myrtaceae (Table 15.7). The compounds are based on 6-C-methylpinocembrin with an unusual substituent at C-8, including a methylated phloroglucinol-based ring structure fused by a 6-carbon heterocyclic ring (which bears either... 

Proanthocyanidins are plant phenolic biopolymers that consist of flavanoid monomer units. Two major classes of proanthocyanidins occur those that possess a resorcinol-pattern A-ring (Figure 1) and those that possess a phloroglucinol-pattern A-ring. The latter are by far the most common, occurring in a high proportion of monocotyledonous and dicotyledonous plants (1,2). The resorcinol-pattern proanthocyanidins are confined to a few genera of tropical or subtropical hardwoods and associated shrubby species (2), but are economically important, since the internationally commercially predominant wattle (3,4) and quebracho... 

Figure 3. Polyphenolic patterns occurring in tannins. Key XVII, resorcinol A-pyrogallol B XVIII, resorcinol A-catechol B XIX, phtoroglu-cinol A-pyrogallol B and XX, phloroglucinol A-catechol B ring systems.

Consideration of the generalized aflatoxin structure represented in (I) leads one to the inescapable conclusion that at the core of the system is the A-ring in the form of a phloroglucinol nucleus, in which each of the pendant phenolic oxygen atoms is uniquely differentiated. Furthermore, two of the three carbon sites on this nucleus are differentially substitued with carbon moieties. This constitutes a potentially difficult synthetic circumstance in that not only must a high degree of substitution be provided for, but also a certain amount of regiochemical control must... 

It remains unclear whether the basic 13-carbon benzophenone is constructed (from the shikimate and acetate pathways) and then prenylated and cyclized or the B-ring is derived from phloroglucinol and later the 7-membered carbon substructure (A-ring) is added. Given the structural diversity of benzophenones, their biosynthesis is suggestive of multiple pathways. Further research is needed to fully understand the biosynthesis of benzophenones. 

Dihydrochalcones [e.g., (12) (Fig. 11.2)] are derived from chalcones by reduction of the a,3 double bond. As in the case of chalcones, the A ring is derived from acetate and has a phloroglucinol oxidation pattern. Similarly, the B ring is derived from a phenylpropanoid precursor. These compounds are comparatively rare they occur as both aglycones and as glycosides. Dihydrochalcones are colorless and are not easily detected (Fig. 11.25). 

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