Synthesis of oligonucleotides with sequences identical with or analogous to the 3'-end of 16S ribosomal RNA of Escherichia coli: preparation of A-C-C-U-C-C via the modified phosphotriester method.

A combination of two different methods for the synthesis of oligoribonucleotides, i.e. the two-step phosphotriester method with 2-chlorophenyl phosphate as bifunctional phosphate source and the modified triester method with 2,2,2-trichloroethyl 2-chlorophenyl phosphorochloridate as monofunctional phosphate source, is applied for the synthesis of the fully-protected hexaribonucleotide A-C-C-U-C-C. The two-step method is used for the synthesis of the required dinucleotide monophosphates 9, 10 and 11. Application of the modified triester method for the coupling of the oligonucleotide blocks results in the formation of the fully-protected hexamer 15. Furthermore, attention is paid to 2,4,6-triisopropylbenzenesulphonyl 4-nitroimidazolide as a new condensing agent for the coupling of larger oligonucleotide blocks.