Sugar-modified nucleosides are prime synthetic targets in anticancer and antiviral drug development. Radical mediated thiol-ene coupling was applied for the first time on nucleoside enofuranoside derivatives to produce a broad range of thio-substituted d-ribo, -arabino, -xylo and l-lyxo configured pyrimidine nucleosides. In contrast to the analogous reactions of simple sugar exomethylenes, surprisingly, hydrothiolation of nucleoside alkenes under the standard conditions of various initiation methods showed low to moderate yields and very low stereoselectivity. Optimizing the reaction conditions, we have found that cooling the reaction mixture has a significant beneficial effect on both the conversion and the stereoselectivity, and UV-light initiated hydrothiolation of C2'-, C3'- and C4'-exomethylene derivatives of nucleosides at -80 °C proceeded in good to high yields, and, in most cases, in excellent diastereoselectivity. Beyond the temperature, the solvent, the protecting groups on nucleosides and, in some cases, the configuration of the thiols also affected the stereochemical outcome of the additions. The anomalous l-lyxo diastereoselectivity observed upon the addition of 1-thio-β-d-gluco- and galactopyranose derivatives onto C4',5'-unsaturated uridines is attributed to steric mismatch between the d-ribo C4'-radical intermediates and the β-configured 1-thiosugars.
The photoinitiated thiol-ene coupling reactions of 2-substituted glycals were studied as a generally applicable strategy for stereoselective 1,2-cis-α-thioconjugation. Although all glycals reacted with full α-selectivity, the efficacy of the reactions varied in a broad range depending on their configuration and glycals bearing axial acetoxy substituents reacted with very low efficacy at room temperature. The study revealed that the reaction progress could be promoted by cooling and inhibited by heating. At -80 °C, the equilibrium of the rapidly reversible addition of the thiyl radical to alkenes is shifted almost completely toward products, leading to efficient addition reactions. By exploiting this unique temperature effect a series of α-thio-l-fucosides, -d-galactosides, and d-GlcNAc derivatives were prepared with high efficacy and complete stereoselectivity.
Radical‐mediated hydrothiolation of enoses was studied as a stereoselective method for synthesizing challenging oligosaccharide structures. Three types of glycals were reacted with various thiols using UV irradiation at the temperature range of rt to −120 °C, producing 39 thioglycosides up to tetrasaccharide. Cooling always proved to be beneficial to the efficacy, −80 °C being the optimal temperature in most cases. The different conformational preferences of the intermediate carbon‐centered radicals were crucial in the stereoselectivity of the reactions. More information can be found in the Full Paper by A. Borbás et al. on page 14555.
Introduction of a sulfonatomethyl moiety into the primary position of thioglycosides by nucleophilic displacement of the corresponding 6‐O‐triflate is described. The 1→6 migration of the anomeric group, which inevitably occurs through a bicyclic sulfonium ion intermediate, from conformationally flexible β‐thioglycosides was prevented by using an α‐thioglycoside or conformationally locked β‐thioglycoside as the starting material. The thioglycoside 6‐sulfonic acids showed excellent α‐selectivity during synthesis of uronic acid containing heparinoid trisaccharides.
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