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

Abstract

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 13 2( 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 13 2( 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 15 2-methyl, 17 3-phytyl ester of Mg-15 1( 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 13 2( 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 13 2( R)-HO-Chl a, whereas the de for 13 2( R)-HO-Chl b was 66%. With (+)-CSOAI, the de was 10% for 13 2( R)-HO-Chl a and 8% for 13 2( 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 13 2( S)- and 13 2( R)-HO-Chl a as well as those between 13 2( S)- and 13 2( R)-HO-Chl b, indicated that the change of stereochemical configuration at C-13 2 induces only slight differences in the δ C-values. Of special interest are the δ C-values of C-13 2, 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-13 2 of 13 2( R)-Chl a ( δ C=65.5). Owing to this, 13C NMR spectroscopy is a good method to distinguish the 13 2-hydroxylated chlorophylls from the intact, naturally occurring chlorophylls.