Chemical synthesis of a messenger ribonucleic acid fragment: AUGUUCUUCUUCUUCUUC.

The synthesis via a phosphotriester method of the octadecaribonucleotide AUG(UUC)5 (19) is reported. The octadecanucleotide is meant to serve as a synthetic messenger in a ribosomal protein synthesizing system. A fully protected octadecamer intermediate (18a) was prepared by a block coupling procedure. For the introduction of the desired 3'-5'-internucleotide bonds a 3'-O-(2,2,2-trichloroethyl 2-chlorophenyl phosphate) function was incorporated into the monomeric building blocks which were applied in the synthesis of 18a. Monomeric and oligomeric compounds with a thus protected 3'-O-phosphotriester function can be selectively deblocked to give 3'-O-phosphodiester derivatives suitable for condensation with 5'-hydroxyl (oligo)nucleotides. Conversion of fully protected oligomers to 5'-hydroxyl derivatives, suitable for further coupling at the 5' end, was effected by selective removal of the levulinyl function at the 5' end. The fully protected octadecanucleotide 18a was deblocked with fluoride ions, followed by ammonia and acid to give the required octadecamer 19. The condensing agent 1-(2,4,6-triisopropyl-phenylsulfonyl)-3-nitro-1,2,4-triazole, which was applied to effect the formation of fully protected 3'-5'-internucleotide phosphotriester functions, may give rise to side reactions with the heterocylic bases uracil and guanine. The consequences of these side reactions for the synthesis of octadecamer 19 are reported.