The nature and position of the ring substituent of an arenesulfenyl chloride control the regiospecific formation of either a C-5 substituted product, as in compounds 3a-3d, or a N-3 substituted product, as in compounds 2a-2c. Arenesulfenyl groups have been subsequently found to successfully protect the urethane function of the uracil residue as exemplified by the synthesis of 2'-0-methyl uridine and oligoribonucleotide building blocks.