Synthesis and characterization of oligodeoxynucleotides containing the two 5R and 5S diastereomers of (5′S,6S)-5′,6-cyclo-5,6-dihydrothymidine;: radiation-induced tandem lesions of thymidine

The insertion of the (5'S,5R,6S) and (5'S,5S,6S) diastereomers of 5',6-cyclo-5,6-dihydrothymidine into oligonucleotides is described. Due to the poor reactivity of the 5'-OH group of this modified nucleoside, the preparation of its 3'-phosphoramidite synthon 8 required the use of the non-standard 5'-levulinyl protecting group. It was successfully applied to the solid-phase synthesis of various oligonucleotides ranging from 3 to 22 bases long in combination with conventional dimethoxytrityl mononucleoside phosphoramidite chemistry. Characterization of these modified DNA fragments by enzymic digestions and mass spectroscopic analyses confirmed the designated sequences and compositions.