Glycosides metal catalysis

Sulphonic esters have been obtained from the sulphonyl chlorides in high yields under mild conditions for a range of alcohols and phenols [e.g. 18, 19]. Of particular value is the protection of glycosides possessing a free hydroxyl group and hydroxy-steroids, which are tosylated readily under phase-transfer conditions [20-22]. Alkyl sulphinites have been obtained in a similar manner [23]. Alternatively, preformed tetra-rt-butylammonium sulphonates or their alkali metal salts have also been alkylated with haloalkanes or alkyl fluorosulphonates [24,25]. In contrast with more classical procedures, tosylation of alcohols, which are susceptible to E/Z-isomerism, e.g. Z-alk-2-en-l-ols, occurs with retention of their stereochemistry under phase-transfer catalysis [26]. 

Reviews (a) K. Suzuki and T. Matsumoto, CpjMClj-AgX (M = Zr, Hf) A reagent for glycosidation, 7. Synth. Chem. Soc. Jpn. 48 1026 (1990) (b) K. Suzuki and T. Matsumoto, New methods in glycoside synthesis by using early transition metal-derived reagent, J. Synth. Chem. Soc. Jpn. 51 718 (1993) (c) K. Suzuki, Novel Lewis acid catalysis in organic synthesis. Pure Appl. Chem. 66 1557 (1994). 

Electrophilic catalysis of the departure of halogens in the century-old Koenigs-Knorr reaction is implicit in the use of heavy metal bases such as silver oxide and mercuric cyanide, but the first demonstration of electrophilic catalysis in water (in the hydrolysis of the p-glucoside of 8-hydroxyquinoline by first-row transition metals (Cu Np > C")) was by Clark and Hay in 1973. The observations were expanded to the more conveniently followed (because more labile) benzaldehyde methylacetals or tetrahydropyranyl derivatives of 8-hydroxyquinoline, whose hydrolysis is now known to give solvent-equilibrated oxocarbenium ions (Figure 3.19). Surprisingly, however, the observation of electrophilic catalysis of glycoside hydrolysis itself was not picked up by paper... 

Acid catalysis requires a contribution by the Mg ion, which is thought to induce a conformational change that brings the acidic group into a catalytically competent position. Removal of Mg has little effect on the hydrolysis of pyridin-ium salts but lowers the rate of hydrolysis of O-glycosides. The metal ion, as revealed by experiments with Mn " as an NMR probe, is not coordinated to the sugar. It is positioned at a distance of 9 A from the methyl protons of the Me-S-galactoside bound in the active site (33). 

---