Acetylated glycals

AUylic systems are interesting substrates for the investigation of new methods of C-C bond formation. Some of these methods have been adapted to the carbohydrate field. Enopyrano-sides are often crystalline compounds readily available, for example, using Ferrier rearrangement of tri-0-acetyl glycals with alcohols or its variants with carbon nucleophiles. 

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°. 

The Synthesis of 2-Amino-2-deoxysugars from Acetylated Glycals, R. U. Lemieux and T. L. Nagabhushan, Tetrahedron Lett., No. 26 (1965) 2143-2147. 

The Reactions of Nitrosyl Chloride and Dinitrogen Tetraoxide with Acetylated Glycals. Acetylated 2-Deoxy-2-nitroso-a-D-hexopyranosyl Chlorides and Nitrates and Acetylated 2-Nitroglycals, R. U. Lemieux, T. L. Nagabhushan, and I. K. O Neill, Can. J. Chem., 46 (1968) 413-418. 

M. Fuertes, G. Garcia-Munoz, F. G. de las Heras, R. Madronero, M. Stud, and M. Rico, Reactions of 6-chloropurine and benzotriazole with acetylated glycals, Tetrahedron, 28 (1972) 4099 1112. 

R. D. Guthrie and R. W. Irvine, Allylic substitutions in tri-O-acetyl-glycals and related compounds, Carbohydr. Res., 82 (1980) 207-224. 

Isomerization to Pseudoglycals.—By boiling with water, an acetyl group is removed from the (n) acetyl glycals, and isomerization occurs to produce (n — l)acetyl pseudoglycals (2,3-unsaturated didesoxyaldoses). 

Korytnyk and coworkers have shown that reactions of xenon difluoride with acetylated glycals in the presence of boron trifluoride are a convenient route for the synthesis of 1,2-difluoro sugars, while the addition of fluorine to the double bond occurred predominantly from the less hindered side and cis adducts were mainly formed75, 76. Stereoselective transformation of tri-O-acetyl-D-galactal with xenon difluoride at -20 to +5 °C, without a catalyst in CFC13, after hydrolysis, afforded 2-deoxy-2-fluoro-D-galactose in 63% yield77. 

Deoxy-2-iodoglycosyl Phosphoramidates from Acetylated Glycals General Procedure64 ... 

Orsini, F, Pelizzoni, F, Synthesis of C-glycosyl compounds the reaction of acetylated glycals with fert-hutoxycarhonylmethylzinc bromide, Carbohydr. Res., 243, 183-189, 1993. 

Herscovici, J, Muleka, K, Antonakis, K, Olefin addition to acetylated glycals. A new route to C-glycosides, Tetrahedron Lett., 25, 5653-5656, 1984. 

Dawe, R D, Fraser-Reid, B, a-C-glycopyranosides from Lewis acid catalysed condensations of acetylated glycals and enol silanes, J. Chem. Soc., Chem. Commun., 1180-1181, 1981. 

The classical Perrier rearrangements of acetyl glycals lead to the introduction of a nucleophilic group at C-1 (preferentially a-anomers) and the migration of the double bond to C-2 in the presence of acid catalysts (O Scheme 20) [24]. In some particular conditions (high temperature or stronger acid), 3-substituted isomeric products can be formed (O Scheme 20) [25]. 

Research in the laboratory of H.M.I. Osborn showed that the use of cyclohexene derivatives as nucleophiles in the Lewis acid-mediated Type I carbon-Ferrier reaction of 3-0-acetylated glycals can be used to prepare unsaturated 3-linked C-disaccharides. The incorporation of the alkene took place with one equivalent of glucal in the presence of boron-trifluoride etherate in 33% yield. The desired C-disaccharide was obtained by selective hydrogenation of the exocyclic double bond in the presence of an endocyclic one. 

For the addition of these reagents, which were referred to in the previous article,1 to acetylated glycals, details have now been pub-... 

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