Optical activity Coumarins

Most of the intermediates in this sequence possess a single asymmetric carbon and are optically active. Nonetheless, a surprising fraction of these oxidization products of diverse structures occur as racemates and several have been found in a partly racemic condition. Lemmich and co-workers have summarized the distribution of oxypeucedanin in umbellifers and noted that ( )-oxypeucedanin (23) has been reported from 20 species and both the (+)- and (-)-forms reported from other species. Both (-1-)- and (-)-marmesin (nodakenetin) (26, R=H) occur in different species of Ptelea (Rutaceae).The optically active coumarin... 

Reduction of coumarin in aqueous methanol, pH 5-6, in the presence of alkaloids yields an increased amount of dihydrocoumarin. Tliis is also the case for reduction of 4-methylcoumarin and now the 4-methyldihydrocoumarin isolated is optically active [137]. Tlie enantiomeric excess and yield of diliydrocompound both depend on the alkaloid used (Table 3.9) and Low concentrations of alkaloid are effective in achieving asynunetric induction. Concentrations of codeine above 4 mM do not further influence either the yield of dihydrocompound or the degree of induction. 

The [2+2] photodimerization of coumarin has been extensively studied and previously reviewed in this series <1984CHEC>. An enantioselective variant of this transformation has recently been described <20050L1501>. Irradiation of coumarin or 6-methylcoumarin in the presence of an optically active host compound provided anti-head-to-head dimers with up to 98% ee (Equation 15). 

Figure 2.3.4 Schematic representations of (a) coumarin, thiocoumarin and cyclohex-2-enone molecules (b) optically active diols employed as hosts.

However, if the electrochemical reduction is perturbed by the presence of an optically active but electrochemically unreactive organic base, e.g., a-phenylethylamine, it is possible to obtain preferential formation of one enantiomer over the other. The basis of this principle lies in the results of work by Gourley, Grimshaw, and Millar, who obtained an optically active 3,4-dihydro-4-methyl coumarin product upon electrochemical reduction of 4-methyl coumarin in the presence of optically active organic bases, such as codeine, brucine, and yohimbine. 

If the organic base is adsorbed on the electrode to a sufficient extent together with the 4-methyl coumarin, an optically active product is obtained as well as the dimer. The dimerization indicates that the reaction proceeds through a radical mechanism ... 

Saigo, K., Sekimoto, K., Yonezawa, N., Ishii, F., and Hasegawa, M., Optically active head-to-head coumarin dimer. A new agent for the determination of enantiomeric excess of amines and alcohols. Bull. Chem. Soc. Jpn., 58,1006,1985. 

The [2-1-2]-cycloaddition reactions in inclusion crystals, or host-guest crystals, have been thoroughly reviewed. Coumarin 61a, thiocoumarin 61b, and cyclohex-2-enone 62 efficiently undergo single-crystal-to-single-crystal enantioselective photodimerization in inclusion complexes with chiral host compounds (R,R)-(-)-fra s 63a, (R,R)- -)-trans 63b, and (-)-64, respectively. The products are (-)-anri-HH 65a (100% EE), +)-anti-HH 65b (100% EE), and -)-syn-trans 66 (48% EE) dimers. An example for enantioselective intramolecular [2+2]-photocycloadditions in inclusion crystals is the reaction of guest 67 in chiral host 68. Irradiation of the powdered inclusion crystals in water suspension affords the optically active photocyclization product 69 with 100% EE in 90% yield. 

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