Synthesis of the 3′-terminal half of yeast alanine transfer ribonucleic acid (tRNAAla) by the phosphotriester approach in solution. Part 2

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Abstract
The preparation, by the phosphotriester approach in solution, of the 3′-terminal decaribonucleoside nonaphosphate [UpCpGpUpCpCpApCpCpA (48)], nonadecaribonucleoside octadecaphosphate [ApUpUpCpCpGpGpApCpUpCpGpUpCpCpApCpCpA (49)], and heptatriacontaribonucleoside hexatriacontaphosphate [GpGpApGpApGpGpUpCpUpCpCpGpGpTpψpCpGpApUpUpCpCpGpGpApCpUpCpGpUpCpCpApCpCpA (50)] sequences of yeast alanine transfer ribonucleic acid are described. With the exception of the 3′-terminal adenosine residue which was protected as its 2′,3′-O-methoxymethylene [Mm] derivative, the 2′-hydroxy functions were protected with 4-methoxytetrahydropyran-4-yl [Mthp] groups. The 5′-hydroxy functions of intermediate building blocks were protected with 2-(dibromomethyl)benzoyl [Dbmb (2)], 2-(isopropylthiomethoxymethyl)benzoyl [Ptmt (3)] or 9-phenylxanthen-9-yl [Px (4)] groups. The base residue of the adenosine, cytidine, guanosine, uridine, pseudouridine and 5-methyluridine building blocks were protected as in (5), (6), (7), (8), (9), and (10), respectively. Internucleotide linkages were protected with the 2-chlorophenyl group, and the 2,4-dinitrobenzyl (Dnb) group was used for the temporary protection of 3′-phosphodiester functions. The first phosphorylation step (leading to 3′-phosphodiester intermediates) was carried out by treatment with 2-chlorophenyl bis(1,2,4-triazoyl)phosphate (11) in the presence of 1-methylimidazole in tetrahydrofuran followed by triethylamine and water. 1-(Mesitylene-2-sulphonyl)-3-nitro-1,2,4-triazole [MSNT (15)] was used as the condensing agent in the second phosphorylation step. The final unblocking procedure involved treatment with (i)N1,N1,N3,N3-tetramethylguanidinium E-2-nitrobenzaldehyde oximate in dioxane–acetonitrile–water, (ii) concentrated aqueous ammonia, and (iii) 0.01 M hydrochloric acid.