Synthesis of spiro ɛ,ζ-D-CNA in xylo configuration featuring noncanonical δ/ɛ/ζ torsion angle combination

Synthesis and structural analysis of dinucleotides containing 2',3'-trans-bridged nucleic acids with trans-5,6- or 5,7-fused ring skeleton

Artificial nucleic acids in which the conformation of the sugar or phosphate backbone of the oligonucleotide is appropriately fixed can form stable duplexes. In this study, we designed dinucleotides containing 2',3'-trans-bridged nucleic acids (2',3'-trans-BNAs) based on the idea that the sugar conformation and torsions angles δ, ε, ζ, α, and β of the backbone can be controlled by a 5,6- or 5,7-membered trans-fused ring skeleton cyclized between the 2'- and 3'-positions of the sugar moiety. Given that the construction of trans-5,6-fused ring skeletons is synthetically challenging, the synthesis was optimized and a detailed structural analysis of these new bridged 2',3'-trans-BNA systems was conducted. The 2',3'-trans-BNAs could be synthesized from a commercially available D-glucose derivative with the key intramolecular gold-catalyzed cyclization reaction achieved using a cyclization precursor bearing an intramolecular hydroxy group and an internal alkyne. Structural analysis of the 2',3'-trans-BNAs showed an N-type sugar conformation for all the derivatives, which is similar to that in RNA-duplex, and the ζ and α torsion angles for the 2',3'-trans-BNAs were a characteristic feature of the compounds that differ from the corresponding angles of the natural duplexes.

Synthesis, Duplex-Forming Ability, and Enzymatic Stability of Oligonucleotides Modified with Amide-Linked Dinucleotides Containing a 3',4'-Tetrahydropyran-Bridged Nucleic Acid

Replacement of a phosphodiester linkage with an amide linkage can improve the binding affinity of oligonucleotides to complementary RNA and their stability toward nucleases. In addition, restricting the conformation of the sugar moiety and the phosphate backbone in oligonucleotides effectively improves duplex stability. In this study, we designed amide-linked dinucleotides containing a 3',4'-tetrahydropyran-bridged nucleic acid (3',4'-tpBNA) with a constrained sugar conformation as well as a torsion angle ε. Phosphoramidites of the designed dinucleotides were synthesized and incorporated into oligonucleotides. Conformational analysis of the synthesized dinucleotides showed that the sugar conformation of the S-isomer of the amide-linked dinucleotide containing 3',4'-tpBNA was N-type, which has the same conformation as that of the RNA duplex, while that of another R-isomer was S-type. Tm analysis indicated that the oligonucleotides containing the synthesized S-isomer showed RNA-selective hybridizing ability, although their duplex-forming ability was slightly inferior to that of natural oligonucleotides. Interestingly, the stability of the oligonucleotides toward endonucleases was significantly improved by modification with the two types of amide-linked dinucleotides developed in this study.

Convertible and Constrained Nucleotides: The 2'-Deoxyribose 5'-C-Functionalization Approach, a French Touch

Many strategies have been developed to modulate the biological or biotechnical properties of oligonucleotides by introducing new chemical functionalities or by enhancing their affinity and specificity while restricting their conformational space. Among them, we review our approach consisting of modifications of the 5'-C-position of the nucleoside sugar. This allows the introduction of an additional chemical handle at any position on the nucleotide chain without disturbing the Watson-Crick base-pairing. We show that 5'-C bromo or propargyl convertible nucleotides (CvN) are accessible in pure diastereoisomeric form, either for nucleophilic displacement or for CuAAC conjugation. Alternatively, the 5'-carbon can be connected in a stereo-controlled manner to the phosphate moiety of the nucleotide chain to generate conformationally constrained nucleotides (CNA). These allow the precise control of the sugar/phosphate backbone torsional angles. The consequent modulation of the nucleic acid shape induces outstanding stabilization properties of duplex or hairpin structures in accordance with the preorganization concept. Some biological applications of these distorted oligonucleotides are also described. Effectively, the convertible and the constrained approaches have been merged to create constrained and convertible nucleotides (C2NA) providing unique tools to functionalize and stabilize nucleic acids.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

Stabilization of hairpins and bulged secondary structures of nucleic acids by single incorporation of α,β-D-CNA featuring a gauche(+) alpha torsional angle

A constrained dinucleotide unit featuring a gauche(+) alpha torsional angle configuration was used to stabilize DNA hairpin or bulged structures.

Dioxaphosphorinane-constrained nucleic Acid dinucleotides as tools for structural tuning of nucleic acids

We describe a rational approach devoted to modulate the sugar-phosphate backbone geometry of nucleic acids. Constraints were generated by connecting one oxygen of the phosphate group to a carbon of the sugar moiety. The so-called dioxaphosphorinane rings were introduced at key positions along the sugar-phosphate backbone allowing the control of the six-torsion angles α to ζ defining the polymer structure. The syntheses of all the members of the D-CNA family are described, and we emphasize the effect on secondary structure stabilization of a couple of diastereoisomers of α,β-D-CNA exhibiting wether B-type canonical values or not.

Double sugar and phosphate backbone-constrained nucleotides: synthesis, structure, stability, and their incorporation into oligodeoxynucleotides

Two diastereomerically pure carba-LNA dioxaphosphorinane nucleotides [(S(p))- or (R(p))-D(2)-CNA], simultaneously conformationally locked at the sugar and the phosphate backbone, have been designed and synthesized. Structural studies by NMR as well as by ab initio calculations showed that in (S(p))- and (R(p))-D(2)-CNA the following occur: (i) the sugar is locked in extreme North-type conformation with P = 11 degrees and Phi(m) = 54 degrees ; (ii) the six-membered 1,3,2-dioxaphosphorinane ring adopts a half-chair conformation; (iii) the fixed phosphate backbone delta, epsilon, and zeta torsions were found to be delta [gauch(+)], epsilon (cis), zeta [anticlinal(+)] for (S(p))-D(2)-CNA, and delta [gauche(+)], epsilon (cis), zeta [anticlinal(-)] for (R(p))-D(2)-CNA. It has been found that F(-) ion can catalyze the isomerization of pure (S(p))-D(2)-CNA or (R(p))-D(2)-CNA to give an equilibrium mixture (K = 1.94). It turned out that at equilibrium concentration the (S(p))-D(2)-CNA isomer is preferred over the (R(p))-D(2)-CNA isomer by 0.39 kcal/mol. The chemical reactivity of the six-membered dioxaphosphorinane ring in D(2)-CNA was found to be dependent on the internucleotidic phosphate stereochemistry. Thus, both (S(p))- and (R(p))-D(2)-CNA dimers (17a and 17b) were very labile toward nucleophile attack in concentrated aqueous ammonia [t(1/2) = 12 and 6 min, respectively] to give carba-LNA-6',5'-phosphodiester (21) approximately 70-90%, carba-LNA-3',5'-phosphodiester (22) approximately 10%, and carba-LNA-6',3'-phosphodiester (23) <10%. In contrast, the (S(p))-D(2)-CNA was about 2 times more stable than (R(p))-D(2)-CNA under hydrazine hydrate/pyridine/AcOH (pH = 5.6) [t(1/2) = 178 and 99 h, respectively], which was exploited in the deprotection of pure (S(p))-D(2)-CNA-incorporated antisense oligodeoxynucleotides (AON). Thus, after removal of the solid supports from the (S(p))-D(2)-CNA-modified AONs by BDU/MeCN, they were treated with hydrazine hydrate in pyridine/AcOH to give pure AONs in 35-40% yield, which was unequivocally characterized by MALDI-TOF to show that they have an intact six-membered dioxaphosphorinane ring. The effect of pure (S(p))-D(2)-CNA modification in the AONs was estimated by complexing to the complementary RNA and DNA strands by the thermal denaturation studies. This showed that this cyclic phosphotriester modification destabilizes the AON/DNA and AON/RNA duplex by about -6 to -9 degrees C/modification. Treatment of (S(p))-D(2)-CNA-modified AON with concentrated aqueous ammonia gave carba-LNA-6',5'-phosphodiester modified AON ( approximately 80%) plus a small amount of carba-LNA-3',5'-phosphodiester-modified AON ( approximately 20%). It is noteworthy that Carba-LNA-3',5'-phosphodiester modification stabilized the AON/RNA duplex by +4 degrees C/modification (J. Org. Chem. 2009, 74, 118), whereas carba-LNA-6', 5'-phosphodiester modification destabilizes both AON/RNA and AON/DNA significantly (by -10 to -19 degrees C/modification), which, as shown in our comparative CD studies, that the cyclic phosphotriester modified AONs as well as carba-LNA-6',5'-phosphodiester modified AONs are much more weakly stacked than carba-LNA-3',5'-phosphodiester-modified AONs.

Synthesis of bicyclic 2'-deoxynucleosides with alpha-L-ribo- and beta-D-xylo-configurations and restricted S- and N-type conformations

Two bicyclic 2'-deoxynucleoside analogues are synthesized in 12 steps each from thymidine. With a six-membered ring fused to the C3'-C4' bond and an alpha-L-ribo- and a beta-D-xylo-configuration, these are conformationally restricted in an S- and an N-type conformation, respectively. The constitutions were proven by X-ray crystallography. The beta-D-xylo-configured analogue is successfully converted to a 3'-phosphoramidite and incorporated into oligodeoxynucleotides, which are found to hybridize to DNA and RNA complements with decreased affinity.

Alpha,beta-D-CNA induced rigidity within oligonucleotides

Introduction of alpha,beta-D-CNA featuring canonical values of the torsional angles alpha and beta within oligonucleotides leads to an overall stabilization and improved rigidity of the duplex DNA as demonstrated by UV experiments, circular dichroism and corroborated by molecular dynamics simulations.