Coumarins and Isocoumarins

Cylindrocarpon olidum, a fungus isolated from the root knot nematode Meloidogyne incognita, contains the antifungal 8-chlorocannabiorcichromenic acid (1574) (1497). 

The mycotoxin ochratoxin A (AAA) (7), which is a possible human carcinogen, continues to receive extensive attention due to its presence in a myriad of foods and beverages (1520, 1521) and its well-established toxicity (teratogenicity, mutagenicity, immunotoxicity, genotoxicity, and carcinogenicity) (1522-1524). Major sources of ochratoxin A are grapes, must, and wine (1525-1533), cereals (1534), beer (1535,1536), dried fruit (1537), roasted coffee (1538), and cocoa products and chocolate (1539). 

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The IR spectra of coumarin and isocoumarin show carbonyl frequencies characteristic of lactones. There is no evidence to suggest that contributions from betaines (207) and (208) are significant. Moreover, EHT MO calculations predict high ir-bond orders for the carbonyl groups in these molecules. IR data and C=0 7r-bond orders are presented in Table 10. 

Coumarins and isocoumarins appear to be of varied origins. Simple coumarins, such as umbelliferone, are formed by the shikimic acid pathway in which hydroxylation of p-hydroxycinnamic acid occurs. Other coumarins, for example alternariol (690), are derived from a polyketide unit, as are a number of chromanones, chromones, pyranones and isocoumarins (B-78MI22400). The biosynthesis of 5-hydroxy-2-methylchromone has been shown to involve the chromanone (60JCS654). However, isocoumarins are also derived from the mixed acetate-shikimate route, through initial cyclization of the polyketide and subsequent lactonization. 

The study of this cycloaddition and its mechanism is complicated by the formation in small yield of additional photoproducts. Irradiation of phenanthraquinone (308) in, for example, 2-methylpropene affords the oxetane (309), arising by 1,2-cycloaddition of the alkene to the carbonyl, in addition to the expected 1,4-dioxene (310).334 Oxetanes are the principal products of photoaddition of phenanthraquinone to benzofuran, furano[3,2-y]coumarin, and isocoumarin.329 A further product has the structure (311), and is undoubtedly the... 

Coumarins and Isocoumarins.- Several new or improved syntheses of coumarins have been reported. A new one-pot synthesis under mild conditions promises to be a widely applicable route from salicylaldehydes or 2-hydroxyaryl alkyl (or aryl) ketones (145 R1 = H, alkyl, OH, N02 R2=H, Me or Ph) and trimethylsilylketene... 

Coumarins (and isocoumarins) are quantitatively hydrolysed to give yellow solutions of the salts of the corresponding cis cinnamic acids (coumarinic acids) which cannot be isolated since acidification brings about immediate relactonisation prolonged alkali treatment leads to isomerisation and the formation of the trans acid (coumaric acid) salt. 

Coumarins, and isocoumarins, react with Grignard reagents, as do esters, and can give mixtures of products, resulting from ring opening of the initial carbonyl adduct the reaction of coumarin with methylmagnesium iodide illustrates this. ... 

Coumarins and Isocoumarins.—Ketene intermediates have been detected at — 190 °C during the photocyclization of 2,6-dichIoiocinnamates to 5-chlorocou-marins. 

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