Carbohydrate derivatives, formation

In general, chiral propanoates providing simple diastereoselectivity (in favor of yyn-aldols), combined with a reasonable degree of auxiliary-induced stereoselectivity, are rare. Numerous terpenoid- and carbohydrate-derived propionates do not display satisfactory syn selectivity60. Similarly, the titanium(IV) chloride promoted aldol addition of the following JV-metbylephe-drine derived silylketene acetal leads to the formation of the. mi-adduct in the moderate diastereomeric ratio of 78 22 (syn-adduct sum of the other stereoisomers)61. 

Reactions of alkenes such as 120 with a-chloronitrosoalkanes of type 119 proceed under very mild conditions and result in the formation of nitrones 121 that can be easily hydrolyzed into hydroxylamines 122 (equation 86) . Chiral carbohydrate-derived a-chloronitrosoalkenes 123 possess enhanced reactivity and produce good stereoselectivity in reaction with prochiral alkenes such as 124 (equation 87) . ... 

Alkane production from sugars by aqueous phase dehydration/hydrogenation reactions has the advantage that most of the alkane fraction is spontaneously separated from the aqueous phase. Unfortunately, the major compound produced by this process is hexane, which has a low value as a gasoline additive due to its relatively high volatility. This limitation has been partially overcome by promoting a base-catalyzed aldol condensation step which links carbohydrate-derived units via formation C-C bonds to form heavier alkanes ranging from C to C15 [151]. 

The conversion of carbohydrate derivatives into functionalized cyclohexanes and cyclopentanes has recently been reviewed [95]. The key step is the formation of carbon-carbon bonds, and different approaches have been used for this purpose. Radical reactions have in the last decade been recognized as valuable in this context [96] since the regio- and stereocontrol may frequently be predictable [97]. 

Despite the many advantages of converting free sugars into their O-trimethylsilyl derivatives, the formation of anomeric and isomeric derivatives (see Section IV, p. 38) may constitute a problem, especially with complex mixtures. Much effort has, therefore, been expended in seeking carbohydrate derivatives, suitable for analysis, in which the anomeric center has been eliminated. This may conveniently be accomplished by oxidation to the acid or lactone, by... 

It is often possible to predict the reactivity of a chlorosulfonyloxy group by a consideration of the steric and polar factors affecting the formation of the transition state,27-28 as indicated in Section 11,1 (see p. 227) for nucleophilic-replacement reactions of sulfonic esters of carbohydrate derivatives. Thus, it has been found that the presence of a vicinal, axial substituent or of a (3-trans-axial substituent on a pyranoid ring inhibits replacement of a chlorosulfonyloxy group also, a chlorosulfate group at C-2 has been observed to be deactivated to nucleophilic substitution by chloride ion. 

Moreover, triethylamine tris(hydrogen fluoride) has been used to open epoxides in activated carbohydrate derivatives, e.g. formation of 18.08... 

The importance of the 3-O-benzoyl group for the formation of nitrogenated migration-products (see also, Section VI, p. 110) is clearly demonstrated in the ammonolysis of l,2,6,2, 3, 4, 6 -hepta-0-benzoyl-/3-maltose, 2 which lacks that group in the residue wherein acyl migrations can occur. Here, a 40% yield of 6-O-benzoylmaltose (and a 56% yield of maltose) was obtained, whereas nitrogenated carbohydrate derivatives were neither isolated nor detected chro-matographically. 

Further experiments disproved Wohl s theories1-3 4 as to a direct condensation mechanism. By degradation of 2,3,4,5-tetra-O-acetyl-L-arabinononitrile82 (5) in the presence of propionamide or benzamide, l,l-bis(acetamido)-l-deoxy-L-erythritol (6) was obtained in yields comparable to those obtained when the reaction was conducted in the absence of these extraneous amides. Also, an excess of acetamide in the medium did not increase the formation of nitrogenated carbohydrate derivatives. 

Cyclobntane. A few examples of cyclobutane derivatives have been described in the carbohydrate series. Formation of this type of ring involves a 2+2 cycloaddition. Relevant examples of cycloaddition of dichloroketene on glucals, explored by Redlich [195] and Lallemand [196,197], and significant transformations of the four-membered ring, such as ketone 165a, are given in Scheme 56. 

This section is concerned with isolation of carbohydrates, methods used to degrade carbohydrates to their primary structural units, derivative formation and GC of the volatile derivatives. 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

Mass spectra have been measured for ketose acetates only,16 and the most characteristic feature of these mass spectra is the formation of a new series of fragments having no analogy in the mass spectra of other types of carbohydrate derivatives. 

Only syntheses which involve the formation of new glycosidic linkages will be considered in this article. This restriction excludes many interesting examples of copolymerization in which only one of the monomers is a carbohydrate (or carbohydrate derivative), the polymerization of carbohydrate derivatives which contain a polymerizable group4 (such as acrylate), and the polymerization of sugar lactones.1 Many of these topics have already been discussed in reviews.1-8 Also outside the scope of this article is the chemical modification of naturally occurring polysaccharides thus, we have excluded the industrially important process of dextrini-zation,10 except as it may pertain to acid condensation processes. The radiation-catalyzed polymerization and modification of carbohydrate poly-... 

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