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1 2002

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Synteza glikozydów, disacharydów i pseudodisacharydów w reakcjach z udziałem związków palladu

Kryczka B., Porwański S.

Wiadomości Chemiczne

2002

[Z] 56, 7-8

627-660

Carbohydrates are found in nature in variety of forms, in chemical association with a vast number of compounds, including other sugars, and in materials which perform a range of functions varying from structural to specifically functional in many biochemical ways. The attachment of monosaccharide units to other molecules proceeds usually through the anomeric centre by O-, N-, S- or C-glycosidic bond. Most synthetically useful glycosylation reaction is nucleophilic displacement at the anomeric centre. It occurs through glycosyl cations, usually from activated leaving groups in suitable glycosyl derivatives, most frequently halides and imidate esters. There are so many variables involved in glycoside and disaccharide synthesis that each target compound requires a particular strategy. That is why so many procedures have been developed to provide the regio- and stereocontrol in the glycosylation step. However, these methodes have been known for many years-and still modified-the selective synthesis of glycosides still determines the main challenge in the chemistry of carbohydrates. The aim of this paper is to present the role of palladium-catalyzed methodology in glycosylation reactions. The Heck reaction has been applied first by Daves and Czernecki to C-glycosidic synthesis. Reaction of glycals with various reagents offering nucleophilic carbon centres in the presence palladium-catalyst, affords important means of acces to 2,3-unsaturated C-glycosides. The popularity of this methodology started to flourish when it was found that it was able to control the selectivity by using certain reaction procedures to give fairly predictible results. Allylic carbonates are well-known compounds that undergo a variety of palladium-catalyzed reactions. This methodology has been applied to synthesis of many alkenyl glycosides. The reaction proceeds under neutral and very mild conditions and usually gives only one anomer. The same methodology was extended to the preparation of C-, N-, and S-glycosides as well as unsaturated di- and trisaccharides (Scheme 31), [86, 93]. These products are obtained in good yields by alkylation of ethyl a-O-D2-glycosides, having a leaving group at C-4, with C-, N-, S-nucleophiles, with various carbohydrates or with thiocarbohydrates. The reaction is regio- and stereoselective for the a-erytro enoside, and only stereoselective for a-threo enoside.