Diastereoselective glycosidation

Diastereoselective glycosidation of a sulfoxide at the anomeric center as the gly-cosyl acceptor. The sulfoxide activation is achieved using Tf20. 

These results confirm the strong influence of hydrogen-bonds on the result of glycosidation and characterize these diazirines as subtle, reactivity-based tools for the investigation of intra- and intermolecular hydrogen-bonds and their influence on regio- and diastereoselective glycosidation. 

A recently developed application of the Ramberg-Backlund reaction is the synthesis of C-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the Ramberg-Backlund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside.95 Entries 3 and 4 in Scheme 10.6 are examples. The vinyl ether group can also be transformed in other ways. In the synthesis of partial structures of the antibiotic altromycin, the vinyl ether product was subjected to diastereoselective hydroboration. 

Recent developments of aldol-type reactions with titanium enolates include the a- and /3-C-glycosidation of glycals73 and the diastereoselective addition to 2-acetoxytetrahydrofurans.74 Mukaiyama and co-workers have developed a one-pot procedure for the preparation of unsymmetrical double aldols.75... 

Maddaford reported the diastereoselective synthesis of C-glycosides 29 using conjugate addition catalyzed by cationic rhodium catalysts such as [Rh(COD)2]BF4 (Eq. 1) [24]. Addition of phosphine hgands to the reaction system inhibited the conjugate addition. It is likely that the enone 28 derived from the pyranose is less reactive toward the conjugate addition. 

An equally intriguing report concerned the rhodium(I)-catalyzed hydroformylation of C-glycoside alkenes 12 (Scheme 5.5) [5]. As depicted, aldehydes 13 were formed with excellent diastereoselectivity. It was proposed that the diastereoselectivity is due to a strong conformational bias favoring the depicted alkene conformation. 

Conjugate additions of various organocopper reagents to the enones derived from glycals, e.g. 2-(diethoxyphosphoryl)hex-l-en-3-uloses, has been described. The reactions proved to be rapid, clean, and diastereoselective, giving rise to the formation of 3-oxo-2-phosphono-o -C-glycosides or the corresponding enol acetates.215... 

C-Glycosides. Axial C-glycopyranosides arc obtained exclusively by photolysis of a mixture of an ot-D-glycopyranosyl bromide, acrylonitrile, and tributyltin hydride. The diastereoselectivity may involve an anomcric effect or rapid trapping of the initial radical before inversion can occur. 

Generally, the products derived from the open-chain form are insignificant in the reaction mixture and stereocontrol between the anomeric forms can be achieved. It was observed that the anomeric oxygen in the equatorial alkoxide 276 has enhanced nucleophilicity compared with the axial [284]. Often referred to as the kinetic anomeric effect, this phenomenon can govern the diastereoselective formations of the equatorial glycosides [579]. The anomeric effect, however, favors the formation of the thermodynamically more stable axial glycosides and chelation control can also play a role in determining anomeric stereoselectivity. 

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