4-Pyrone structure

The 2-hydroxy-4-pyrone structure 25b was initially assigned to isodehy-droacetic acid [63JCS(C)4483]. However, an IR study at high dilution showed a broad absorption (3200-3000 cm" ) due to intramolecular hydrogen bonding only possible in structure 25a (87T5245). 

If no olefins are present, triacetic acid lactone 1 and its methyl ether 21a react photochemically to afford a mixture of compounds without pyrone structure (70CJC237, 70CJC2645). 

There are only a few leads in the fungicide area worth mentioning in this chapter. Phenoxan (18) [45], a secondary metabolite isolated from Polyganium sp. strain PI VO 19 with an oxazole-y-pyrone structure is discussed as a first example (Fig. 

Early biosynthetic studies on 138 involved [ " C]acetate incorporation into the pyrone structure, but the observed distribution of label made it difficult to rationalize in terms of the normal pathways of polyketide biosynthesis [160]. Instead, smdies conducted on 138 enriched biosynthetically with [1- C], [2- C], and [l,2- C]acetate supported the first proposal for the biosynthesis of 138. Thus, NMR spectrum of 138 labeled from [l- C]acetate showed strong enhancement relative to natural abundance of C-2, C-4, C-6, and C-9, whereas the [2- C]acetate-enriched spectrum showed enhancement of C-3, C-5, C-7, C-8, and C-10 and a coupling between C-3 and C-8, indicative of a head-to-head... 

Khellin (33), an antispasmodic drug that comprises approximately 1% of the dry weight of plants of Ammi visnaga (Apiaceae), has a pyrone structure reminiscent of that of many linear furanocoumarins (Fig. 9.6). This compound acts directly on smooth muscle, is a bronchial dilator, and is used in the treatment of asthma (Sneader, 1985). 

The forth synthetic strategy is to utilize pyrone structure to trigger intramolecular Diels-Alder reaction [30]. In this strategy, we can construct the core structure of Maoecrystal V via compound 1.7.26. As shown in Fig. 1.18, natural product Maoecrystal V can be transformed to compound 1.7.24 via simple functional group transformations, and compound 1.7.24 can be prepared by compound 1.7.25 via ring-opening lactonization. Compound 1.7.25 can be constructed by compound 1.7.26 via intramolecular Diels-Alder reaction of pyrone. The precursor (1.7.26)... 

Altholactone (56), found in the bark of unnamed Polyalthia (Annonaceae), has a unique tetrahydrofuro-pyrone structure (69). This compound is bioge-netically related to a number of a-pyrones with aromatic substituents, found in families of Lauraceae, Annonaceae, or Piperaceae. It can reasonably be regarded as an oxygenated and cyclized derivative of goniothalamin (57), distributed in several species of Goniothalamus (Annonaceae) (56, 69). 

Flavones are natural yellows of a phenyl benzo-pyrone structure, present in plants as glycosides or tannic acid esters. Quercetin and its derivative quercitrin and some other derivatives are ap-... 

A positively charged heteroatom can be balanced by a negatively charged substituent, and if they are oriented favourably relative to each other the charges may formally cancel. Thus, the 2- and 4-pyridone structures (26, 27 X = NH) and the corresponding pyrones... 

A different type of tautomeric relationship exists between compounds of types 323 and 324. Both types of structure can be isolated, pyridones (324, Z = N—Me) and pyrones (324, Z = 0) being formed when 323 (Z = N—Me or 0) is heated with palladium on charcoal in ethylene glycol. Similar isomerizations in the quinol-4-one series have been reported."... 

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

By 0-acylation with 2-methyl-1,3-dioxolenium fluoroborate, which reacts as 0-acetyl ethylene oxide, 2,6-dimethyl-4-pyroiie is converted into 4-acetoxy-2,0-dimethylpyrylium fluoroborate (24, Y = 0, R = Ac, X=BT 4). The alleged compound with this structure which has been obtained from 8 and acetyl fluoroborate is, in fact, the BF3-complex of the pyrone. 

Synthesis, structure, and transformations of a-pyrone derivatives, viz., xanthy-rones, glaucyrones, and chelated magnesium enolates 98T8243. 

The general features of this elegant and efficient synthesis are illustrated, in retrosynthetic format, in Scheme 4. Asteltoxin s structure presents several options for retrosynthetic simplification. Disassembly of asteltoxin in the manner illustrated in Scheme 4 furnishes intermediates 2-4. In the synthetic direction, attack on the aldehyde carbonyl in 2 by anion 3 (or its synthetic equivalent) would be expected to afford a secondary alcohol. After acid-catalyzed skeletal reorganization, the aldehydic function that terminates the doubly unsaturated side chain could then serve as the electrophile for an intermolecular aldol condensation with a-pyrone 4. Subsequent dehydration of the aldol adduct would then afford asteltoxin (1). 

Dipoles can be embedded in heterocyclic structures, just as dieneunits are present in pyrones and other ring structures (see p. 491). N-Substituted pyridinium-3-ols can be deprotonated to give 3-oxidopyridinium betaines that have 1,3-dipolar character.142... 

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