RESUMEN
Liquid SO2 is a polar solvent that dissolves both covalent and ionic compounds. Sulfur dioxide possesses also Lewis acid properties, including the ability to covalently bind Lewis basic fluoride ions in a relatively stable fluorosulfite anion (FSO2 -). Herein we report the application of liquid SO2 as a promoting solvent for glycosylation with glycosyl fluorides without any external additive. By using various temperature regimes, the method is applied for both armed and disarmed glucose and mannose-derived glycosyl fluorides in moderate to excellent yields. A series of pivaloyl-protected O- and S-mannosides, as well as one example of a C-mannoside, are synthesized to demonstrate the scope of the glycosyl acceptors. The formation of the fluorosulfite species during the glycosylation with glycosyl fluorides in liquid SO2 is proved by 19F NMR spectroscopy. A sulfur dioxide-assisted glycosylation mechanism that proceeds via solvent separated ion pairs is proposed, whereas the observed α,ß-selectivity is substrate-controlled and depends on the thermodynamic equilibrium.
RESUMEN
Ruthenium(II) complexes catalyze the insertion of sulfur dioxide into (het)aryl and alkenyl boronic acids. The transmetalation-sulfination process proceeds with DABSO in the presence of 5 mol % RuCl2(PPh3)3 in methanol at 100 °C. The intermediate sulfinate salt can be quenched with various electrophiles such as alkyl halides, epoxides, Michael acceptors, and λ3-iodanes in moderate to good yields. The reported sulfone synthesis can be performed either as a direct one-pot or one-pot two-step procedure depending on the reactivity of the electrophile.