2-Pyrones benzene ring

Coumarins belong to a group of compounds known as the benzopyrones, all of which consist of a benzene ring joined to a pyrone. They may also be found in nature, in combination with sugars, as glycosides. They can be categorized as simple fura-nocoumarins, pyranocoumarins, and coumarins substituted in the pyrone ring (Murray and others 1982). 

From the table it is apparent that the excitation spectra of fur-an and pyrone both lie above the UV-A region (320-400 nm) in energy. Fusing the systems to a benzene ring reduces the excita-... 

Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly benzopyran or benzopyrylium systems. 

As for pyrones. the nature of the nucleophile (hard or soft) determines whether reactions occur at the carbonyl group or at the atom of the enone system. Electrophiles attack in I he benzene ring. 

In benzo- and phenyl-pyridines and in phenylpyridine 1-oxides, electrophilic substitution usually takes place in the benzene ring. In benzo-pyridones, -pyrones and -pyridine A-oxides, electrophilic substitution often occurs in either the benzene or the heterocyclic ring depending on the conditions sometimes mixtures are formed (see Section 3.2.3.2.1). 

Fused benzene rings aid nucleophilic attack on pyridines, pyridinium and pyrylium ions, and pyrones the loss of aromaticity involved in the formation of the initial adduct is less in monobenzo derivatives and still less in linear dibenzo derivatives than in monocyclic compounds. For the same reason, the tendency for this initial adduct to re-aromatize is less for benzopyridines. Fused benzene rings also influence the point of attack by nucleophilic reagents attack rarely occurs on a carbon atom shared with a benzene ring. Thus, in linear dibenzo derivatives, nucleophilic attack is at the y-position (157). 

If the hetero ring is in the form of a pyridone, pyrone or A-oxide, or contains a strongly electron-donating substituent (OR or NR2), electrophilic substitution into the hetero ring can compete with substitution into a fused benzene ring. In some such compounds, substitution occurs entirely in the... 

The phthaleins derived from di- or polyhydric phenols are all anhydrides formed by the elimination of water from two phenolic hydroxyls, attached to two different benzene rings. These anhydride phthaleins are known as pyronines, since they contain, like the pyrones, a six-membered oxygen-containing ring. 

First, a compound released by many cut plants, especially grasses, with a fresh delightful smell usually called new mown ha/. Coumarin is also present in some herbs such as lavender. It contains a benzene ring and an a-pyrone fused together. 

Products of the phenylpropanoid pathway. These compounds, commonly referred to as polyphenols, are derived from cinnamic acid and include the xanthones and flavonoids. The basic structural unit of the flavonoid family (Fig. 1) comprises two benzene rings linked through a heterocyclic pyran or pyrone ring (C ring) variations in the C ring and the extent of hydroxylation define the major classes [4]. 

Synthesis of pyrans and benzene rings from pyrones... 

CHEMICAL REACTIONS Reactions of the carboxyl group Esters formation and reactions Acyl chlorides formation and reactions Decarboxylation Miscellaneous reactions Reactions of the pyrone carbonyl group Reactions of the pyrone ring Reactions of the benzene ring Conversion to other heterocycles... 

The products of catalytic reduction of chromone-2-carboxylic acids or esters depend partly on the catalyst used, the amount of hydrogen present and the conditions of reaction. Ethyl chromone-2-carboxylate is reduced (in 86 per cent yield) to the corresponding chromanone in the presence of Raney nickel and hydrogen at a pressure of 50 Ib/in [184]. The 5-hydroxy analogue is similarly reduced at 80°C and 30 Ib/in in 44 per cent yield [8]. A nitro group on the benzene ring is more readily hydrogenated than the pyrone double bond (10 atmospheres, platinum-charcoal or palladium-charcoal catalyst) [24,40]. 

Chemically, all flavonoids possess a Ce—C3—Ce backbone, with two benzene rings (A and B) connected via a heterocyclic pyrane or a pyrone ring (C) (Rgure 4.1). 

Another feature of 2-pyrone 87 is its ability to undergo Diels-Alder reactions with acetylenes. The cycloadducts decarboxylate spontaneously to form benzene rings bearing the CF3 group. The substitution pattern is determined by the regioselectivity of the [4-1-2] cycloaddition step. Thus, the reaction of 87 with l-(NJ -diethylamind)-l-propyne takes place at 0 °C to produce 92 as a single isomer. Less electron rich acetylenes require heating at 140-200 C. Treatment of 87 with acetylene at 200 C leads to 93, while with dimethyl acetylenedicarboxylate triester 94 is formed [37] (Scheme 28). 

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... 

The presence of a benzene unit fused to a pyrone ring affects the chemistry of both coumarins and chromones, but there are subtle differences in reactivity between the two types of compounds. 

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