Chemical structure of eukaryotic thymidine modifications and synthesis of the 5-formyl-2′-deoxyuridine phosphoramidite. (A) Structure of 5hmU, 5fU and base J. (B) Synthesis of the 5fU phosphoramidite: (i) HO(CH2)3OH, TiCl4, (EtO)3CH, CH2Cl2, 0 °C, 1 h, 80%; (ii) HF·pyridine, THF, 0° to rt, 1 h, 99%; (iii) DMTrCl, Et3N, pyridine, rt, 4 h, 92%; (iv) ClPN(iPr)2(OCH2CH2CN), Et3N, 0 °C to rt, 3 h, 57%.