The enolate anions of chlorophylls a and b as ambident nucleophiles in oxidations with (−)- or (+)-(10-camphorsulfonyl)oxaziridine. Synthesis of 13p>2p>(S/R)-hydroxychlorophylls a and b

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• 作者：Paavo H. Hynninen ; Tuomo S. Lepp&#xe4 ; kases and Markku Mesilaakso
• 关键词：Tetrapyrrole ; Porphyrin ; Chlorophyll ; Oxidation ; Enolate ; Reaction mechanism ; NMR
• 刊名：Tetrahedron
• 出版年：2006
• 期刊代码：107_00404020
• 类别：ch
• 出版时间：3 April 2006
• 卷：62
• 期：14
• 页码：3412-3422
• 文件大小：246 K

摘要

The enolate anions of chlorophylls (Chl) are ambident nucleophiles that are of considerable organic chemical interest in relation to the theory of electron delocalization (aromaticity) and charge-transfer in large conjugated π-systems, as well as for their chemical reactivity. Under deaerated conditions, the (−)- and (+)-enantiomers of (10-camphorsulfonyl)oxaziridine (CSOAI) are effective oxidants for the enolate anions of Chl a and Chl b, when 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) serves as a base. In this study, the use of these sterically hindered reagents to hydroxylate Chl a and Chl b is described for the first time. The total yield of 13p>2p>(S/R)-HO-Chl a was 71 and 90%for the oxidations of Chl a with (−)-CSOAI and (+)-CSOAI, respectively. Chl b, however, behaved clearly differently from Chl a. The total yield of 13p>2p>(S/R)-HO-Chl b was 40%in the oxidation with (−)-CSOAI and 60%in the reaction with (+)-CSOAI. A competing side-reaction, which resulted in the 15p>2p>-methyl, 17p>3p>-phytyl ester of Mg-15p>1p>(S/R)-unstable rhodin, was found to lower the yields of the desired main products. The formation of the side-products was largely avoided and the yield of 13p>2p>(S/R)-HO-Chl b was improved by increasing the volume of hexane and using phosphate buffer in the first step of the work-up. With (−)-CSOAI, a 94%diastereomeric excess (de) was achieved for 13p>2p>(R)-HO-Chl a, whereas the de for 13p>2p>(R)-HO-Chl b was 66%. With (+)-CSOAI, the de was 10%for 13p>2p>(R)-HO-Chl a and 8%for 13p>2p>(R)-HO-Chl b. The results were interpreted in terms of a nucleophilic reaction mechanism, kinetically controlled by steric hindrance, originating on the one hand in the 17-propionate phytyl ester side-chain, protruding over the isocyclic ring E of the Chl enolate ion, and on the other hand in the bulky camphorsulfonyl unit of CSOAI. Possible reasons for the different results from the Chl b oxidations as compared with those of the Chl a oxidations are discussed. Comparison of the differences in the NMR δ_C-values between 13p>2p>(S)- and 13p>2p>(R)-HO-Chl a as well as those between 13p>2p>(S)- and 13p>2p>(R)-HO-Chl b, indicated that the change of stereochemical configuration at C-13p>2p> induces only slight differences in the δ_C-values. Of special interest are the δ_C-values of C-13p>2p>, which are at ca. 91 ppm for the a- and b-series diastereomers. This carbon is deshielded by ca. 25 ppm relative to the C-13p>2p> of 13p>2p>(R)-Chl a (δ_C