Oxygen containing heterocycles coumarin

Tetrahydrobenzopyrans, chromenes, coumarins, xanthenes, and so on are some of the most important oxygen-containing heterocyclic compounds. Several synthetic methods are available for these compounds. This part of the chapter deals with the synthesis of these heterocyclic compounds using nano-ZnO as a reusable heterogeneous catalyst. 

Halophenol derivatives are promising substrates for the synthesis of oxygen-containing heterocyclic compounds. As early as 1989, the carbony-lation of 2-iodophenol with phenyl acetylene or norbornadiene was realized. Coumarin and aurone can be prepared under the same conditions from different substrates (Scheme 2.22). 

This method has opened a very precious and useful gateway to the preparation of a series of an important oxygen-containing heterocyclic compounds, namely, coumarin and its derivatives. In this article, we try to highlight the various aspects, issues, and applications of this reaction with the hope of winning the attentions and considerations of synthetic organic chemists. 

There is also a great, number of oxygen-containing heterocyclic compounds that fluoresce. Coumarins and flavonoids are the two largest classes of oxygen heterocycles. Coumarins fluoresce more intensely under basic conditions where flavones fluoresce weakly. In 30% sulfuric acid solution, flavones fluoresce intensely and coumarins do not. 

Phenol and its derivatives are the most common substrates for the construction of oxacycles in this approach. In 2000, Fujiwara and coworkers reported a palladium-catalyzed intramolecular addition reaction of aryl alkynoates to construct coumarins in a mixture of trrfluoroacetic acid and CH2CI2 at room temperature (Scheme 3.6) [21]. This method is simple and efficient to give the desired oxygen-containing heterocycles in high yields. However, this palladium-catalyzed C-H olefination reaction is only constrained to the electron-rich aromatic compounds owing to the inherent limitation of electrophilic metalation process. 

Coumarin. The coumarin moiety is found in a number of important drugs. This is a bicyclic heterocycle containing two six-membered rings and two oxygens, one endo-cyclic, one exocyclic. Since the coumarin contains an intramolecular lactone ester, it undergoes hydrolysis to yield a carboxylic acid and a phenol. 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

Some natural complex matrices do not need sample preparation prior to GC analysis, for example, essential oils. The latter generally contain only volatile components, since their preparation is performed by SD. Citrus oils, extracted by cold-pressing machines, are an exception, containing more than 200 volatile and nonvolatile components. The volatile fraction represents 90-99% of the entire oil, and is represented by mono- and sesquiterpene hydrocarbons and their oxygenated derivatives, along with aliphatic aldehydes, alcohols, and esters the nonvolatile fraction, constituting 1-10% of the oil, is represented mainly by hydrocarbons, fatty acids, sterols, carotenoids, waxes, and oxygen heterocyclic compounds (coumarins, psoralens, and polymethoxylated flavones—PMFs) [92]. 

The major part of this review concerns chromones which possess a carboxyl or related group at C-2 but a few derivatives of the 3-carboxylic acids are known and discussed. Compounds in which the heterocyclic oxygen or the pyrone carbonyl oxygen is replaced by sulphur are also mentioned. The chemistry of chromones has been reviewed up to 1948 [1] and a survey (without references) of the synthesis of coumarins and chromones of therapeutic interest appeared in 1970 [2], Naturally-occurring chromones are covered in a chapter of a book by Dean [3]. The chromone-2- and -3-carboxylic acids have not been reviewed previously, although an early review of chromones [4] contains a short section on chromone carboxylic acids. 

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