Pyrone-like structures

Suarez, D., Menendez, J.A., Fuente, E., and Montes-Moran, M.A. (2000). Pyrone-like structures as novel oxygen-based organic superbases. Angew. Chem., Int. Ed. Engl, 39, 1320-23. 

Pyrone-like structures are also abundant on the carbon surfaces. These are combinations of non-neighboring carbonyl and etlier oxygen atoms at the edges of the graphene layers, as... 

Fig. 8. Redox reactions between several pyrone-like structures and anthraquinone (representative of quinoid structures on caitoon surfaces) [68], Reprinted with permission from M.A. Montes-Moran, D. Suarez, J.A. Menendez and E. Fuente, Carbon, 42 (2004) 1219...

The alternative structural situation of an exocyclic methylene group in conjugation with a ring oxygen is found in methylene-4-pyrans (vinylogous pyrones), in which a 4-pyrone-like resonance [230] occurs. These compounds are little known. Their synthesis... 

The chemical nature of acidic and basic oxides on carbons has been widely investigated. The structure of basic oxides was first ascribed to cromene-like structures fl09] and later to pyrone-type groups [110]. Other workers [17,111] assumed that carbon basicity of the Lewis type can be associated with the high concentration of n electrons on basal graphene layers where k bonding of water molecules may occur in oxygen-free media ... 

Tickle, 1899). Which of the two oxygens was the site of proton addition became clear only when Hantzsch (1919) demonstrated the close optical analogy between the salts and methiodides of dimethyl-pyrone and pyridinium salts and argued that a benzene-like ring arises in the pyroxonium salts [191]. The discussion of the structure... 

PKSs are characterized by their ability to catalyze the formation of polyketide chains from the sequential condensation of acetate units from malonate thioesters. In plants they produce a range of natural products with varied in vivo and pharmacological properties. PKSs of particular note include acridone synthase, bibenzyl synthase, 2-pyrone synthase, and stilbene synthase (STS). STS forms resveratrol, a plant defense compound of much interest with regard to human health. STS shares high sequence identity with CHS, and is considered to have evolved from CHS more than once. ° Knowledge of the molecular structure of the CHS-like enzymes has allowed direct engineering of CHS and STS to alter their catalytic activities, including the number of condensations carried out (reviewed in Refs. 46, 51, 52). These reviews also present extensive, and superbly illustrated, discussions of CHS enzyme structure and reaction mechanism. 

Further analysis of the raspberry PKS genes has shown that the proteins coded by the five genes have the seven conserved cysteines at positions 30,60,84,130,164,190 and 341. Cysteines at these conserved locations are likely to have structural functions, because in addition to the CHS from which the consensus sequence derives, they were found at identical or similar locations in Arachis hypogea STS, in the Gerbera hybrida 2-pyrone synthase (2PS), in the Ruta graveolens acridone synthase (ACS), the Rheum palmatum benzalacetone synthase (BAS) and in the aleosone synthase (ALS) (Abe et al., 2004). 

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