Ribonucleoside phosphoramidites

The syntheses of six different ribonucleoside phosphoramidites with fluorobenzenes or fluorobenzimidazoles as nucleobase analogues and of one inosine analogue have been described. Lithiation of the required bromoarene with BuLi was followed by its addition to 2,3,5-tri-O-benzyl-D-ribono-1,4-lac-tone to yield an intermediate lactol which was directly dehydroxylated with triethylsilane and BFj-OEti to afford stereoselectively the respective precursors to (57a-f).""... 

Scaringe SA, Francklyn C, Usman N. Chemical synthesis of biologically active oligoribonucleotides using )8-cyanoethyl protected ribonucleoside phosphoramidites. Nucleic Acids Res 18 5433-5341, 1990. 

Harusawa et synthesised CS-linked Cq- and C2-ribonucleoside phosphoramidites via tetrazole C-nucleosides for probing RNA catalysis. The nitrogen in the tetrazole was protected by methyl-substituted piva-loylojgmiethyl (43) and pivaloylojgmiethyl (44) groups which can easily be removed under basic conditions. 

Stereodefined oligo(ribonucleoside phosphorothioate)s were available only by the enzymatic method [51 -54] or by partially stereocontrolled H-phosphonate procedure [55,56]. However, both procedures could provide only products with RP configuration. Alternatively, compounds containing the single phosphorothioate modification were synthesized by nonstereospecific phosphoramidite or H-phosphonate methods and then separated into diastereomers by tedious HPLC procedure with various degree of success [57-59]. Such constructs became important tools for investigation of the mechanism of action of ri-bozymes. 

An improved procedure for the preparation of isotopically labeled d-[1- C] ribonucleoside 3 -phosphoramidites (34) has been reported. This method, which employed the regioselective 2 -0-silylation of the ribonucleoside via a selective protection and subsequent removal of the 3, 5 -di-terr-butylsilanediyl group, avoided the undesired 3 -0-silyl-2 -0-phosphoramidite nucleosides usually detected with standard procedures. ... 

An interesting acid-labile linker has been designed for a non-standard RNA synthesis with 5 -Fmoc protection [303] and methyl phosphoramidite chemistry by Palom et al. [304], The authors prepared an acetal anchor for the ribonucleoside 2, 3 -diol group (Figure 19.15) by the reaction of 5 -Fmoc-uridine with 4-formyl-phenoxyacetic acid, and attached it to LCAA-CPG (30-40 gmolg"1). The nucleoside can be detached from this support in ca. 70% yield by overnight treatment with dilute aqueous HC1, pH 2, conditions now believed to be too drastic for RNA. 

Ribonucleoside monomers Silylated nucleosides and their phosphoramidite derivatives can be prepared as described in Sections 3.1.2. and 3.1.3., or can be obtained commercially. The following ribophosphoramidites are currently available from Dalton Chemical Laboratories (DCL), Inc. (York University, Toronto, Canada), Peninsula Laboratories, Inc. (California, USA), and Milligen-Biosearch, CA ... 

Phosphate backbone modifications have previously led us to the corresponding phosphorothiate series i.e., S-RNA. In this case, the corresponding monomeric units are commercially available. However, if one considers the sugar-modified RNA series, the situation is much more tedious, as we must design first the modified ribonucleosides (which are generally not commercially available), transform them into the corresponding suitably protected phosphoramidite synthons, and then proceed to the oligonucleotide synthesis. 

The a-RNA synthesis was planned using the solid-phase technique in combination with the phosphoramidite methodology. Such an approach requires the preparation of the corresponding synthons, and, hence, preparation of the a-ribonucleosides. The synthesis of the necessary a-ribonucleosides in the pyrimidine series has been previously described, and their syntheses are straightforward. As shown on Figure 2, a-rU is obtained in an overall yield of 35%, and the corresponding a-rC can be easily reached from the uridine derivative. ... 

Usman N, Ogilvie KK, Jiang MY et al (1987) Automated chemical synthesis of long oligoribonucleotides using 2 -0-silylated ribonucleoside 3 -0-phosphoramidites on a con-trolled-pore glass support synthesis of a 43-nucleotide sequence similar to the 3 -half molecule of an Escherichia coli formylmethionine tRNA. J Am Chem Soc 109 7845-7854... 

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