Cyclizations C-glycoside synthesis

Craig, D, Munasinghem, V R N, Stereoselective template-directed C-glycosidation. Synthesis of bicyclic keto oxetanes via intramolecular cyclization reactions of (2-pyridylthio)glycosidic silyl enol ethers, J. Chem. Soc., Chem. Commun., 901-903, 1993. 

Type-Ill cyclizations are initiated by conversion of an acetal, ketal or enol ether into an oxonium ion. Such an activated carbonyl group attacks an olefin intramolecularly in an exo reaction. Mono- and trialkylated double bonds form new C-C bonds regioselectively at the least hindered terminus according to Markovnikov s rule, and in this case endo cyclization products can be produced25. It is worth noting that acetals are also used as initiators in cation olefin cyclizations (see Section 1.5.5.3.)26. Use of activated acetals in Lewis acid catalyzed C-glycoside synthesis is also known 12c 12e. 

Our strategy for the Smij-promoted synthesis of l,2-cis-C-glycosides is illustrated in Figure 3. It was assumed that one-electron transfer to the LUMO of the aryl sulfone group in A would result in concomitant cleavage of the C1 -S bond liberating an anomeric radical intermediate, as previously observed in the reductive lithiation of glycosyl phenyl sulfones." If 5-exo cyclization onto the silicon-tethered alkene/alkyne is substantially favored over a second electron transfer from Smij, then the exocyclic carbon radical B would ultimately be formed. On the other hand, reduction of the Cl-radical by... 

Aside from the type III cyclizations described above, acetals have seen limited use in intermolecular Prins reactionsand extensive use as initiators for cation-alkene cyclizations.Only limited success has been achieved in Lewis acid catalyzed addition of acetals to alkenes. Better success has been achieved in the synthesis of C-glycosides by Lewis acid catalyzed addition of glycosyl acetates or glycals to alkenes. Johnson has extensively developed the use of acetals as initiators for cation-alkene cyclizations. Recent studies have shown that excellent asymmetric induction can be obtained using chiral acetals derived from optically active 2,3-butanediol or 2,4-pentanediol. - ... 

Tether-directed Radical Cyclization Approaches to the Synthesis of C-Glycosides... 

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