Heterocyclic compounds, aromatic coumarins

These observations indicated that an intermolecular double condensation to give a bis N-(methylene-4-oxocoumarinyl)-l,4 aromatic diamine had occurred. Data from the elemental analysis indicated that the calculated and observed values were within the acceptable limits ( 0.4%) and in conformity with the assigned structure. In the addition of molar equivalents of 1,4-aromatic binucleophilic compounds to compound 72 we did not observe any heterocyclic compounds resulting from the further intermolecular nucleophilic attack on the single condensation product. Since the condensation of 3-(dimethylaminomethylene)-chromane-2,4-dione with aromatic binucleophilic compounds is the only route to the new coumarinic compounds, this represents a useful synthetic method. 

Preparation of an appropriately disubstituted aromatic compound is the first step in the synthesis of fused aromatic heterocyclic compounds. Thus a method for the synthesis of benzofurans, coumarins etc. involves the preparation of the salicyl aldehydes as the first step. Standard methods are then used to add one or more carbon atoms and to effect cyclisation to complete the synthesis. 

A wide assortment of electron transfer photosensitizers for diaryiiodonium salts have been described in the journal and patent literature. Among these are polynuclear aromatic hydrocarbons with three or more rings such as anthracene, alkoxyanthracenes, pyrene, and perylene heterocyclic compounds of low basicity such as carbazoles " and phenothiazines aromatic ketones such as benzophenone, Michler s ketone, and thioxanthone and substituted thiox-anthones coumarins phenanthrene-9,10-quinone Mannich bases and (dimethylamino)benzylidyne compounds. In addition, the use of dyes such as eosine, acridine orange, acridine red, and benzoflavin has been employed to provide photosensitization in the visible region of the spectrum. 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

The available evidence suggests that the heterocyclic ring in compounds (19), (28) and (206) is not markedly aromatic. Much of the chemistry of coumarins can be rationalized in terms of a benzo-fused enol lactone structure. 

Retrosynthetic analysis of the target coumarin skeleton provides two major synthetic routes (Figure 10.1). Route A requires an aromatic o-hydroxy carbonyl compound and a two-carbon fragment observed in Knoevenagel and Perkin reactions reflecting the [4h-2] approach for the construction of six-membered heterocycles. Route B represents the reaction between phenols and three carbon fragments associated with the Pechmann cyclization. In recent years, newer synthetic methodologies have also been applied to the synthesis of a variety of coumarins that have avoided the use of concentrated sulfuric acid. The present chapter reviews various... 

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. 

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