## 3 Unlocked Nucleic Acids (UNA)  
In 1995, [PERSON] and collaborators described the synthesis of DNA oligonucleotides with 1-3 acyclic unlocked thymidine nucleotides (7, Chart 2, B=Thy, a dinucleotide is shown), that have an OH group at the 2\({}^{\prime}\) position, yet lack a covalent bond between the 2\({}^{\prime}\) and 3\({}^{\prime}\) carbon atoms [6]. The synthesis of the relevant UNA-T nucleoside (Scheme 2) was based on the oxidative cleavage of the vicinal diol in 5\({}^{\prime}\)-O-DMT-protected 2\({}^{\prime}\)-hydroxymethylproline (8, B=Thy), followed by a reduction of the resultant dialdehyde 9 with sodium borohydride. The unlocked nucleoside **10** (obtained in 88% yield) was then benzoylated, but the desired 2\({}^{\prime}\)-O-Bz compound (**10a**, isolated in 38% yield) was accompanied by the 3\({}^{\prime}\)-O-Bz regioni-somer (17%). After routine phosphorylation of **10a**, the required phosphoramidite monomer **11** (B=Thy) was obtained in good yield and used in the automated solid-phase synthesis of the above-mentioned oligomers. In 2009, the synthesis of other UNA-nucleosides **10** was published and good yields were reported (B=A\({}^{\text{Bz}}\): 73%, C\({}^{\text{AC}}\): 64%, G\({}^{\text{BiB}}\): 63%, U: 79%) [7]. The benzoylated derivatives **10a** were obtained in 63-79% yields. The synthetic route to UNA was improved by the synthesis of 2\({}^{\prime}\)-O-TBDMS monomers [8]. A biocatalytic route for the regioselective preparation of the benzoyl derivative should also be mentioned [9]. In this approach, for the selective benzoylation of 2\({}^{\prime}\)-hydroxyl function in 5\({}^{\prime}\)-O-DMT-2\({}^{\prime}\)3\({}^{\prime}\)-_seco_-uridine (**10**, B=Ura) with vinyl benzoate, five lipases, i.e., _Candida Antarctica_ lipase-B, _Thermomyces lanuginosus_ lipase, _Amano_ PS lipase, _Candida rugose_ lipase and porcine pancreatic lipase, were tested in five organic solvents. _Thermomyces lanuginosus_ lipase was found to induce selective and efficient benzylation in toluene, yielding the desired product **10a** (in 92% yield), which was then successfully converted to the corresponding phosphoramidite monomer **11** (B=Ura). These monomers were used in the automated solid phase synthesis of chimeric DNA-UNA and RNA-UNA oligomers. Further studies showed that the UNA units confer them remarkably high stability in serum [7; 10]. A single UNA nucleotide present in a DNA strand decreases the Tm value for the corresponding (UNA-DNA)/DNA duplex by 10 \({}^{\circ}\)C [6]. The thermal stability of RNA/DNA and RNA/RNA duplexes containing one or two UNA nucleotides, as well as the effects of mismatches, were investigated using a set of 8 RNA 21-mers of a general sequence 5\({}^{\prime}\)-UGCACUGUAUGUCUGUACCAU-3\({}^{\prime}\) and 5\({}^{\prime}\)-ACUUUGGCGCAUUUACGUCGC-3\({}^{\prime}\)[7]. It was found that for duplexes formed with DNA or RNA templates, a single UNA unit in a central position of the RNA strands lowered Tm values by 5-8 \({}^{\circ}\)C. When the mismatches were positioned directly opposite a UNA monomer, only little discrimination was observed. Additionally, two UNA monomers can be used to decrease or increase mismatch discrimination depending on their position in the oligomers. This phenomenon was used to detect a breast cancer oncology microRNA-RNA (miR-10b) and distinguish it from two other miRNAs (miR-10a and miR10c) that differed from miR10b by one and two nucleotides, respectively [11]. A fluorescently labeled 5\({}^{\prime}\)-(FAM)-CACAAATTCGGTTCTACAGGTA-3\({}^{\prime}\) with 2 UNA units (underlined) used in combination with water-soluble nano-graphene oxide proved to be 50 times more effective than the fully complementary DNA probe. The CD spectra indicate that a UNA nucleotide can be considered structurally as an RNA mimic, since the incorporation of one or a few UNA monomers into the RNA/RNA duplexes does not change the overall structure compared to that of the unmodified duplex, which is important for biochemical applications [12].