Synthesis of the TT pyrimidine (6–4) pyrimidone photoproduct–thio analogue phosphoramidite building block

The phosphoramidite building block synthesis of the thio analogue at the 5,6-dihydropyrimidine C5 position of the thymidylyl(3′–5′)thymidine (6–4) photoproduct 1 is presented. This compound was readily obtained from the appropriately protected dinucleotide P-methyl-5′-O-dimethoxytritylthymidilyl(3′→5′)-4-thiothymidine 2 after irradiation at 366 nm, then S-sulfenylmethylation of the thiol function of the resulting (6–4) adduct, and phosphitylation of the 3′-hydroxyl group.