Oxygenated Coumarins

Kayser, O. and Kolodziej, H. (1995) Hi ly oxygenated coumarins from Pelargonium sidoides. Phytochemistry, 39,1181-85. 

After the separation of furanocoumarin, spectroscopic methods can be used to identify and quantify these phenolic compounds. Coumarin shows absorption bands at 274 and 311 nm, which have been attributed to the benzene and pyrone rings, respectively. Methyl substitution at C-5, C-7, or C-8 leads to a bathochromic shift of the 274 nm maximum but leaves the 311 nm maximum practically unchanged [68]. 7-Oxygenated coumarins show strong absorption bands at about 217 and 315-330 run with weak peaks or shoulders at 240-250 nm. Linear furanocoumarins show four zones of absorption at 205-225, 240-255, 260-270, and 298-316 nm. Angular furanocoumarins can readily be distinguished from linear forms since the maxima at 242-245 and 260-270 are absent [69]. 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

Oxidations of polyaromatics over Ti02 have been studied, for example, naphthalene oxidation to various oxygenated species [235] and phenanthrene to obtain coumarin derivatives as important compounds in the production of pharmaceuticals and dyes [236]. 

Reviews of saturated oxygen heterocycles <00JCS(P1)1291>, routes to 2,2-dimethyl-2H-[l]benzopyrans <00H(53)1193> and pyranonaphthoquinone antibiotics <00T1937>, HIV-1 active Calophyllum coumarins <00H(53)453> and of the application of (3-halovinylaldehydes in heterocyclic synthesis <00H(53)941> have appeared. 

Borisov SM, Klimant I (2007) Ultrabright oxygen optodes based on cyclometalated Iridium (III) coumarin complexes. Anal Chem 79 7501-7509... 

Because of its importance, the application of planar chromatography for the analysis of various secondary metabolites in plants such as heterocyclic oxygen compounds (coumarins, flavonoids, anthocyanins, etc.) has been reviewed many times [143,144],... 

Scheme 7.21 P450 oxygenation of amino acids in coumarin biosynthesis.

The Michael addition of nucleophiles to coumarins catalyzed by solid bases provides an interesting approach to the synthesis of 4-substituted 3,4-dihydrocumarins, because with the conventional Michael catalysts the alkaline hydrolysis of the 8-lactone predominates (Scheme 44). Results were obtained when the Michael addition of diethyl malonate to coumarin was catalyzed by the activated Ba(OH)2 292). An unusual 1,2-addition-elimination process at the C = 0 bond was observed. The mechanism of this reaction was explained on the basis of the microcrystalline structure of the catalyst. It was suggested that the rigid coumarin molecule interacts with the Ba ions through the lone-pair electrons of both oxygen atoms of the... 

Resonance within the unsaturated lactone unit of coumarin gives a strong hint as to its likely reactivity. Thus, the oxygen atom of the carbonyl group receives electron density both via the enone chromophore and from the internal resonance of the lactone group (Scheme 5.2). 

Other less oxophilic electrophiles give C-6 substituted coumarins, but it is unclear whether the substrate for such reactions is the free coumarin or a cation formed by protonation or bonded by a Lewis acid at the carbonyl oxygen. Some typical reactions are shown in Scheme 5.3. 

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