Oxidation of chlorophyll a with selenium dioxide

Abstract

The SeO 2 oxidation of chlorophyll ( Chl ) a in deaerated pyridine solution under argon, produced, after 4 hours of refluxing, work-up and chromatographic purifications on a sucrose column, 132(S/R)-hydroxy- Chl a (20%), 132(S/R), P 4(S/R)-dihydroxy- Chl a (19%) and P 4(S/R)-hydroxy-132-demethoxycarbonyl-132-oxo- Chl a (10%). The major products were characterized by 1 H and 13 C NMR spectra, electronic absorption spectra (UV-vis) and FAB-MS. The C -132 oxidation of the enolizable isocyclic ring was found to be faster than the allylic C - P 4 oxidation of the phytyl chain. Essential features of the mechanisms proposed for the C - P 4 allylic oxidation and the C -132 oxidation of Chl a are the ene-reaction and the [2,3]-sigmatropic rearrangement. In the C -132 oxidation, the ene-reaction affords the selenite ester of the Chl enol as the first intermediate, which is then converted by [2,3]-sigmatropic rearrangement to the hydrolyzable Chl selenoxylate ester intermediate. However, in pyridine, we regard the attack of the nucleophilic oxygen atom of the Chl enol or enolate to the electrophilic Se -atom of SeO 2 as a likely alternative for the ene-reaction of the Chl isocyclic ring. The C - P 4 hydroxylation of the phytyl chain exhibited very little diastereoselectivity, whereas the C -132 hydroxylation reaction showed moderate diastereoselectivity, producing 80% of 132( R )- HO - Chl a ( de = 60%) and 20% of 132( S )- HO - Chl a. Kinetic or thermodynamic control of the [2,3]-sigmatropic rearrangement was considered as a possible factor underlying the diastereoselectivity observed for the C -132 hydroxylation.