A convenient synthesis of S-cyanoethyl-protected 4-thiouridine and its incorporation into oligoribonucleotides

A reliable preparation of S 4 -cyanoethyl-4-thiouridine and its incorporation into oligoribonucleotides is reported. Deprotection of oligoribonucleotides with DBU in acetonitrile followed by methanolic ammonia allows the use of standard N-benzoyl and N-isobutyryl protected phosphoramidites. Cleavage of hammerhead ribozymes using GCGCCGAAACACCGUG[ 4S U]CUCGAGC as the modified substrate and GGCUCGACUGAUGAGGCGC as the ribozyme resulted in a halving of the cleavage rate when compared to the unmodified substrate, which is consistent with the proposal that the A 9 -U 17 base pair plays a key role in the active structure.