Reductive Samariation

Anomeric 2-pyridylsulfones, such as 48, were introduced by Skrydstrup and Beau [18] and have a LUMO energy level low enough that reductive samariation is possible in THF alone (no HMPA needed). Under Barbier conditions, good yields of C-glycosides 50-53 are available with aldehydes and ketones (Scheme 13) [ 19]. The low selectivity associated with 50-53 is again a consequence... 

Nonstereoselective reductive samariations on 2-deoxy glycosides, though of limited synthetic value, are nevertheless of mechanistic interest. However, this type of process does have utility in the preparation of C(2)-oxygenated or C (2)-aminated C-glycosides which is described in Sect. 2.2.3. 

This samarium-based method is also a useful route for a rapid assembly of C-disaccharides. Reductive samariation of sulfone 158 in lie presence of... 

The enolate reactivity associated with this approach to C-glycoside synthesis was, however, first developed with octulosonic acid derivatives, such as 283 [114], 284 [115], and 285 [116], and a series of examples involving aldehyde and halide-based electrophiles are shown in Scheme 74. Related studies involving stereoselective protonation of this class of exocyclic enolate have also been described [13] and Scheme 75 illustrates this with an example of reductive samariation (using ethane-1,2-diol, but no HMPA) of an anomeric acetate 286 [117]. 

The production of a kinetic a-organosamarium(III) species 145 by reductive samariation of the corresponding anomeric radical 144 explains the stereospecific formation of a-C-mannosyl compounds (O Scheme 29). 

This Sml2-induced C-glycosylation was extended to the s)mthesis of C-linked glycosides of 2-acetamido-2-deoxy sugars [88,89,90]. Reductive samariation of the 2-pyridyl sulfone of A-acetylglucosamine 184 in the presence of aldehyde 167 leads selectively to the a-C-gly-cosyl dimer 185 [90] (O Scheme 38). 

In an even more simple approach, the reductive samariation of the readily available anomeric acetates 213 is possible in the absence of any additive that increases the reducing power of the samarium salt [100] (O Scheme 44). 

---