The phosphoramidite building block synthesis of the thio analogue at the 5,6-dihydropyrimidine C5 position of the thymidylyl(3â²â5â²)thymidine (6â4) photoproduct 1 is presented. This compound was readily obtained from the appropriately protected dinucleotide P-methyl-5â²-O-dimethoxytritylthymidilyl(3â²â5â²)-4-thiothymidine 2 after irradiation at 366 nm, then S-sulfenylmethylation of the thiol function of the resulting (6â4) adduct, and phosphitylation of the 3â²-hydroxyl group.