Manganese 2,6-disubstituted tetraphenylporphyrins, bearing halogen atoms on the beta-positions, have been used as catalysts for the first described stereoselective epoxidation of thymidine nucleosides. The oxidations were carried out using dimethyldioxirane (DMDO) as the oxygen atom donor. The diastereoisomeric ratio of the final epoxides might be related to the hydrogen-bonding interaction between the OH groups of the sugar moieties and the OCH(3) groups of the catalysts during the approach of the nucleosides to the core of the macrocycles.
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