Novel 4′-Branched Nucleosides

Biocatalytic route to C-3'-azido/-hydroxy-C-4'-spiro-oxetanoribonucleosides

The lipase, Novozyme(®)-435, exclusively deacetylates the 5-O-acetyl over 4-C-acetyloxymethyl group of almost identical reactivity in 5-O-acetyl-4-C-acetyloxymethyl-3-azido-3-deoxy-1,2-O-isopropylidene-α-D-ribofuranose that led to the development of first and efficient synthesis of 3'-azido-/3'-amino-C-4'-spiro-oxetanoribonucleosides T, U, C and A in 20-24% overall yields. The X-ray study on the compound obtained by tosylation of lipase-mediated monodeacetylated product unambiguously confirmed the point of diastereoselective monodeacetylation on diacetoxy-azido-ribofuranose derivative. The capability of Novozyme(®)-435 for selective deacylation of 5-O-acetyl group in 5-O-acetyl-4-C-acetyloxymethyl-3-O-benzyl-1,2-O-isopropylidene-α-D-ribofuranose recently discovered by us has been successfully used for the synthesis of C-4'-spiro-oxetanoribonucleosides A and C in good yields. These results clearly indicate that the broader substrate specificity and highly selective capability of Novozyme(®)-435 for carrying out acetylation/deacetylation reactions can be utilized for the development of environment friendly selective methodologies in organic synthesis.

Synthesis and properties of 3'-amino-2',4'-BNA, a bridged nucleic acid with a N3'-->P5' phosphoramidate linkage

The synthesis and properties of a bridged nucleic acid analogue containing a N3'-->P5' phosphoramidate linkage, 3'-amino-2',4'-BNA, is described. A heterodimer containing a 3'-amino-2',4'-BNA thymine monomer, and thymine and methylcytosine monomers of 3'-amino-2',4'-BNA and their 5'-phosphoramidites, were synthesized efficiently. The dimer and monomers were incorporated into oligonucleotides by conventional 3'-->5' assembly, and 5'-->3' reverse assembly phosphoramidite protocols, respectively. Compared to a natural DNA oligonucleotide, modified oligonucleotides containing the 3'-amino-2',4'-BNA residue formed highly stable duplexes and triplexes with single-stranded DNA (ssDNA), single-stranded RNA (ssRNA), and double-stranded DNA (dsDNA) targets, with the average increase in melting temperature (T(m)) against ssDNA, ssRNA and dsDNA being +2.7 to +4.0 degrees C, +5.0 to +7.0 degrees C, and +5.0 to +11.0 degrees C, respectively. These increases are comparable to those observed for 2',4'-BNA-modified oligonucleotides. In addition, an oligonucleotide modified with a single 3'-amino-2',4'-BNA thymine residue showed extraordinarily high resistance to nuclease degradation, much higher than that of 2',4'-BNA and substantially higher even than that of 3'-amino-DNA and phosphorothioate oligonucleotides. The above properties indicate that 3'-amino-2',4'-BNA has significant potential for antisense and antigene applications.

Synthesis and Biological Evaluation of Some 4'-C-(Hydroxymethyl)-α- and -β-D-Arabinofuranosyl Pyrimidine and Adenine Nucleosides

A series of 4'-C-(hydroxymethyl) analogs of pyrimidine and purine nucleosides have been prepared utilizing standard methodologies, and the α and β anomers were separated. These analogs are part of our continuing efforts to identify new anticancer drugs as well as to explore the substrate specificities of these analogs with the initial activating enzymes in the metabolic pathway leading to nucleoside triphosphates. Although not cytotoxic to CCRF-CEM cells (an acute lymphoblastic leukemia of T-cell origin), many of these compounds were utilized as substrates for the various human nucleoside kinases, including deoxycytidine kinase, thymidine kinase 1, and thymidine kinase 2. Because the 4'-C-(hydroxymethyl) analog of arabinofuranosyl cytosine was identified as a good substrate with deoxycytidine kinase, its metabolism in CEM cells was evaluated. These results indicated that nucleosides with this modification could be activated in human cells without cytotoxicity, which suggested that they should be examined for antiviral activity.

A simplified and efficient route to 2'-O, 4'-C-methylene-linked bicyclic ribonucleosides (locked nucleic acid)

A novel efficient method for the synthesis of locked nucleic acid (LNA) monomers is described. The LNA 5',3'-diols containing thymine, 4-N-acetyl- and 4-N-benzoylcytosine, 6-N-benzoyladenine, and 2-N-isobutyrylguanine as nucleobases were prepared via convergent syntheses. The method is based on the use of the common sugar intermediate 1,2-di-O-acetyl-3-O-benzyl-4-C-methanesulfonoxymethyl-5-O-methanesulfonyl-D-erythro-pentofuranose (8) that easily can be prepared from D-glucose in multigram scale. Four different nucleobases were stereoselectively coupled to 8 using a modified Vorbrüggen procedure to give the corresponding 4'-C-branched nucleoside derivatives. Subsequent ring closing furnished the protected LNA nucleosides. The 5'-O-mesyl groups were efficiently displaced by nucleophilic substitution using sodium benzoate. Saponification of the 5'-benzoates followed by catalytic removal of the 3'-O-benzyl groups afforded the free LNA diols. The exocyclic amino groups of adenosine and cytidine were selectively acylated to give 4-N-acetyl- or 4-N-benzoyl-LNA-C and 6-N-benzoyl-LNA-A. The isobutyryl group of guanine was retained during the preparation of 2-N-isobutyryl-LNA-G. The LNA-T diol and base-protected LNA diols can be directly converted into LNA-phosphoramidites for automated chemical synthesis of LNA containing oligonucleotides.