Glycosylation carbanions

C-glycosides Vasella has reviewed his work on glycosylidene carbenes which is of relevance to the subject, and the use of glycosyl carbanions in C-glycoside synthesis has also been surveyed. ... 

A sulfoxide derived from phenyl 3,4-0-isopropylidene-l-thio-P-L-fucoside, on treatment with MeLi followed by fcrt-BuLi, gave the glycosyl carbanion which added to iso-butanal to give p-C-glycosides as an epimeric pair, and the a-thioglycoside led to the a-C-glycosides. ... 

Vasella has reviewed the chemistry of glycosyl carbanions, carbocations, radicals and carbenes and related work covering his own important contributions and those of others. Specific areas to have been reviewed are carba-sugars (with ring oxygen atoms replaced by carbon), the enediyne anticancer antibiotics notably calicheamycin and esperamycin (Nicolaou s synthetic work on the disaccharide component especially). ... 

Nitro-sugar chemistry reponed in the period from 1970 to 1988 has been reviewed (162 refs), and the preparation and chemistry of nitroaldoses and nitroglycals have been included in a review on new chemistiy leading to glycosyl carbanions, oxonium ions, radicals, radical anions and carbenes. ... 

Glycosyl-linkages were determined by GC-EIMS of the partially methylated alditol acetates. RG-II samples (2 mg) were methylated using sodium methyl sulfmyl carbanion and methyl iodide in dimethyl sulfoxide [24] followed by reduction of the uronosyl groups with lithium triethylborodeuteride (Superdeuteride , Aldrich) [23,25]. Methylated and carboxyl-reduced samples were then submitted to acid hydrolysis, NaBIlt reduction and acetylation, partially methylated alditol acetates being analysed by EIMS on two fused-silica capillary columns (DB-1 and DB-225) [20]. 

Boron trichloride appears to form with methyl /3-D-ribofuranoside a complex which, after treatment with appropriate nucleophiles, such as sodium methoxide, or the carbanion derived from diethyl 2-(ethox-ycarbonylmethyl)malonate, gives the corresponding a-glycosyl compound.147... 

Anomeric halides follow the typical reactivity order F < Cl < Br < I for nucleophilic substitutions. They have been used in stereoselective O-glycosylation, nucleophilic displacement, and carbanion as well as in radical reactions. 

Reaction of Glycosyl Halides with Organometallics and Carbanions... 

Stabilized carbanions, such as malonyl ceubanions, which have a soft character, react with glycosyl halides affording the corresponding C-glycosyl malonates. Once more, the presence of a participating group at C-2 directs the attack of the nucleophile from the opposite side. An example of the application... 

The synthesis of C-glycosyl compounds, commonly known as C-glycosides, in ionic reactions relies on the electrophilicity of the anomeric center and, therefore, involves the attack of an appropriate C-nucleophile. An umpolung method has been developed, and is described in the previous chapter. But instead of going from a carbocation to a carbanion, one can also consider homolytic or radical reactions to reverse the philicity (Scheme 1). 

Hanessian, S, Pemet, A G, Carbanions in carbohydrate chemistry. Synthesis of C-glycosyl malonates. Can. J. Chem., 52, 1266-1279, 1974. 

Many other syntheses incorporate such nucleophilic additions to lactones with subsequent reduction to C-glycosyl compounds because of the facile reduction of the tertiary anomeric hydroxyl. Condensation of acetylenic carbanions with lactones, followed by reduction, has been used as a starting point to construct the anthraquinone ring system of vineomycinone B2 [225]. 

---