Coumarins and chromones

Over the years the literature is filled with examples where the initial characterization was incorrect. One example is illustrated below. In 1940, Sethna and Shah presumed that they synthesized coumarins 42 and 43 from a reaction between P-orcacetophenone (44) and its 4-0-methyl ether 45 under standard Kostanecki-Robinson conditions, respectively. Three decades later Bose and Shah synthesized coumarin 43 via another route and concluded that the initial assignment made by Sethna and Shah was incorrect. After the Bose and Shah findings were published, Ahluwalia and Kumar concluded that the Sethna and Shah products were actually chromones 46 and 47 based on proton NMR data and chemical derivatization. Despite these shortcomings, the Kostanecki-Robinson reaction remains an effective method for formation of both coumarins and chromones. 

Some differences between the chemical reactions of coumarins and chromones... 

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. 

Spectral data for substituted coumarins and chromones are widely spread in the literature, and references should be sought from the review literature. 

A number of natural flavonoids, coumarins and chromones have a prenyl (3-methylbut-2-enyl) side chain which is capable of cyclization on to an oxygen atom to form a pyran ring (77HC(31)633). Prenylation is effected by reaction with either 3-methylbut-2-enyl (prenyl) bromide or 2-methylbut-3-en-2-ol in the presence of a base or a Lewis acid. When this reaction is applied to flavonoids, the prenyl group has a choice of two benzene rings and... 

Several reports describing the 170 NMR of pyrans and derivatives have appeared, including a study in which the natural abundance 170 NMR data for lactones such as pyranone were collected and the relationships between 170 chemical shifts and structure were discussed of <1989H(29)301>. It is possible to distinguish between polyfunctionalized coumarins and chromones by 170 NMR <1993CPB211>. 

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. 

Further comparison between PCM and MPE was reported in the paper by Rinaldi et al. [26] using several multipole moment distribution types. Again, an excellent agreement was obtained. The authors reported examples for which the computational time with the MPE method represents only 30% of the time required for the equivalent computation with PCM (for details on code versions, see that paper). MPE and PCM results are also found comparable for IR properties [91] as well as for UV spectra [91,92] of substituted coumarins and chromones at the TD-DFT level. 

In a simplistic way, phenolics can be classified according to the number of constitutive carbon atoms in the basic skeleton. With a basic skeleton of Cg, there are simple phenols and benzoquinones with Cg-Ci, there are hydroxybenzoic acids such as gallic acid with Cg-C2, there are the phenylacetic acids and with Cg-Cs, there is a larger class including hydroxycinnamic acids, coumarines, and chromones. Sometimes, these structures... 

C-Substitution of coumarins and chromones has been observed in both rings in strongly acidic media, in which presumably it is a hydroxy-benzopyrylium cation that is attacked, substimtion takes place at C-6, for example nitration. This can be contrasted with the dimethylaminomethylation of chromone, iodin-ation of flavones or the chloromethylation of coumarin where hetero-ring substitution takes place, presumably via the non-protonated (non-complexed) heterocycle (CAUTION CH2O/HCI also produces some CICH2OCH2CI, a carcinogen). 

Both coumarin and chromone are converted by diborane then alkaline hydrogen peroxide into 3-hydroxy-chroman." " Catalytic reduction of conmarin or chromone satnrates the C-C double bond." For both systems, hydride reagents can of course react either at carbonyl carbon or at the conjugate position and mixtures therefore tend to be prodnced. Zinc amalgam in acidic solntion converts benzopyrones in 4-unsubstituted benzopyrylinm salts." ... 

There are three important ways of putting together 1-benzopyryliums, coumarins, and chromones all begin with phenols. The isomeric 2-benzopyrylium and isocoumarin nuclei require the construction of an ortAo-carboxy- or ort/jo-formyl-arylacetaldehyde (homophthalaldehyde). 

This reaction has been used to prepare both coumarin and chromone derivatives. 

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