The reactivity of the 2'-deoxy-N 4 -(phenoxycarbonyl)cytidine derivatives 3 and 4 with aromatic amines was studied to form new types of urea derivatives (see 5 -10). On the same basis, labeling of 3 and 4 with 5-aminofluorescein ( 14) was achieved to give the conjugates 15 and 17, respectively (Scheme 1). Treatment of 17 with 2-(4-nitrophenyl)ethanol in a Mitsunobu reaction led to double protection of the fluorescein moiety (! 18) and desilylation yielded 19. Dimethoxytritylation (! 20) and subsequent phosphitylations afforded the new building blocks 21 and 22. Synthesis of the fully protected trimer 28 was achieved by condensation of 21 with 23 to 26 which after detritylation (! 27) was coupled with 25 to give 28 (Scheme 2). Deprotection of all blocking groups was performed with 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) in one step to give 29. The synthesis of the decamer 5'd(C Flu CCG GCC CGC)-3' (33) started from 30 which was attached to the solid support and then elongated with 31, 32, and 22 at the 5'-terminal end (C Flu ¼ deprotected phosphate derivative of 22). Hybridization with the complementary oligomer 5'-d(G GGC CGG GCG)-3' (34) showed the influence of the fluorescein label on the stability of the duplex.