Glycals syntheses from

The synthetic value of this reaction has been enhanced by the development of a wide range of methodologies for glycal synthesis from 0-acylatcd glycosyl halides1 or from noncarbohydrate precursors. By use of the Lewis acid-catalyzed diene-aldehyde cyclocondensation reaction, glycals of natural and non-natural sugars can be obtained (Scheme 4).31... 

Synthesis from carbohydrate derived 2-deoxy lactones. Routes to C-l glycals from 2-deoxy sugar pyranolactones have been described. The general strategy is outlined in Scheme 9. 

Synthesis from exo-glycals. Several routes to C-l glycals from 1-exo-methylene pyranoses and furanoses have been described recently. 

The reactions involved in the glycal synthesis have been studied in considerable detail, especially in the conversion of arabinose into 2-des-oxyribose. For this particular conversion the overall yield has been doubled by recently introduced improvements but it is still very low.112 The reduction of the acetobromoaldose to the acetylated glycal by zinc dust and acetic acid was found to proceed in better yield if a few drops of chloroplatinic acid were added at intervals to maintain a vigorous reaction.112 120 127 Moreover, the reaction could then be conducted at lower temperatures (i. e., — 5° to —10°). This was particularly the case in the pentose series, and the simultaneous formation of the pentose tetraacetate by replacement of the bromo group by an acetyl residue, was much reduced.60 112 Hughes148 demonstrated that the maximum yield of triacetylglucal from acetobromoglucose was obtained when the addition of zinc dust and catalytic amounts of chloroplatinic acid was spread over several hours and the reaction mixture was maintained at 0°. 

J. K. Hale, S. Manaviazar, and R. Tupprasoot, The synthesis and reactions of some novel pyranoid 5,6-glycals derived from D-fructose, Tetrahedron, 53 (1997) 16365-16376. 

Other complementary methodologies include the preparation of substituted cyclopropanes from glycals using rhodium acetate carbenoid additions [65,66]. Additionally, acid catalyzed cyclopropane opening reactions in alcoholic solutions afford the 2-C-branched-glycosides. These combined reactions were used to prepare a key intermediate in marine diterpene norrisolide synthesis from D-mannose [67]. 

Glycal formation from acylated glycosyl bromides on treatment with zinc dust under aprotic conditions is favored by the addition of l-methylimidazole. A similar process based on the traditional procedure has been incorporated into a method for the hydrolysis of y,S-unsaturated anilides and the synthesis of chiral allylic alcohols. ... 

An easy and efficient one pot synthesis of peracetylated glycals from unprotected sugars has been described. In this method the sugar was sequentially treated with acetic anhydride and catalytic hydrobromic acid-acetic acid (to effect per-acetylation), more hydrobromic acid-acetic acid (to form the glycosyl bromide), sodium acetate (to neutralize excess hydrobromic acid) and finally a buffered mixture of sodium acetate, acetic acid, zinc-copper sulfate and water (to effect a reductive-elimination). The method was applied to the syntheses of glycals derived from D-glucose, D-galactose, L-rhamnose, L-arabinose, maltose, lactose and maltotriose. ... 

Kawakami H, Ebata T, Koseki K, Okano K, Matsushita H, Ebata T, Koseki K, Okano K, Matsumoto K, Matsushita H (1993) Nucleoside synthesis from furanoid glycals. Heterocycles 36 665-669... 

Preparation (4, 4S). D-Ribose may be synthesized from D-arabinose by alkaline isomerization, by the glycal synthesis, or through the pyridine-catalyzed epimerization of D-arabonic acid followed by reduction. The sugar also has been prepared by the oxidative degradation of calcium D-altronate (44) and by the nitromethane synthesis from D-erythrose (4 ). 

Attempts to apply an analogous approach to the synthesis of Kdo disaccharides were made by Sinay by reaction of an exo-glycal obtained from 2,3,5,6-di-O-iso-propylidene-D-mannose with iodine and potassium t-butoxide, but after a positive result with methanol, sugar acceptors did not give the expected disaccharides [8]. 

Easily prepared from the appropriate monosaccharide, a glycal is an unsatu-rated sugar with a C1-C2 double bond. To ready it for use in potysaccharide synthesis, the primary -OH group of the glycal is first protected at its primary -OH group by formation of a silvl ether (Section 17.8) and at its two adjacent secondary - OH groups by formation of a cyclic carbonate ester. Then, the protected glycal is epoxidized. 

Korytnyk and coworkers - " were the first to apply Xep2 (Refs. 321 -323) in the synthesis of 1,2-difluorides from glycals. This reagent, although expensive, is solid and can be conveniently used in the laboratory. Treat-... 

A. Agarwal, S. Rani, and Y. D. Vankar, Protic acid (HC104 supported on silica gel)-mediated synthesis of 2, 3-unsaturated-O-glucosides and a chiral furan diol from 2,3-glycals, J. Org. Chem., 69 (2004) 6137-6140. 

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