C-glycosidic product

Often, ethers are constructed by alkoxide displacement of a stereodefined leaving group. Hirakazu Arimoto of Nagoya University has found (Chem. Commun. 2004, 1220) that it is possible to effect diastereocontrolled construction of cyclic benzylic ethers such as 2 by oxidation of 1 to the o-quinone methide. The more stable equatorial C-glycoside product 2 is formed in near quantitative yield. 

The stereochemistry of the C-glycosidic products, obtained with the above-mentioned procedure, depends on the structure of the starting sugar and on the experimental conditions. In general, base catalyzed cyclizations afford a mixture of anomers, which on prolonged reaction time is converted into the thermodynamic product. For example, in the case of 4,6-O-isopropylidene-D-glucose,... 

In contrast, the Barbier reactions of 2-acetamido-2-deoxy sugars with ketones showed a preference for the formation of 1,2-m-C-glycoside products (Scheme 7.39). This was attributed to the complexation of the C2-acetamido group to the glycosyl samarium, thereby reducing the configurational lability of the organometallic species.78,80,81... 

Next, the scope of this C-glycosylation reaction was investigated. The NeuSAc chloride was found to serve as a donor in samarium mediated C-glycosylation, unfortunately, purification of C-glycoside from excess chloride donor is often problematic, making the NeuSAc phenyl sulfone the donor of choice. A variety of acceptors were also evaluated including alkenes, epoxides, vinyl esters, aldehydes and ketones. Only the aldehydes and ketones afforded the desired C-glycoside products. 

Acceptor C-Glycoside product Yield C-fil coside (a fi) Yield O-glycoxide ([Pg.86]

All of the above-mentioned reactions using glycals derived from hexopyranoses gave stere-oelectronically controlled a-C-glycoside products. On the other hand, it is difficult to synthesize the corresponding -C-glycoside by using the same method. Recently, reaction of... 

Beau and Sinay have developed an efficient synthesis of 2-deoxy-jS-glycosides based on anomeric phenylsulfone alkylation followed by in situ reductive lithiation with lithium naphthalenide (LN). The -C-glycoside product is established after protic quench of the intermediate axial alkyllithium (Scheme 12) [20]. 

Complex ketose derivatives can be formed by carbanion attack at the carbonyl group of aldonolactones, deoxygenation at the anomeric centre thus giving C-glycosidic products. In this way the isomeric alkenes (46) and (4 ) were made by use of an acetylinic nucleophile... 

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