Alditols complexes

Some of the multibranched complex glycans of hen ovomucoid have been analyzed by f.a.b.-m.s. together with e.i.-m.s. These glycans exhibit exceptional structural features, because their mannotriose core can be highly substituted, as shown in 25. All samples gave abundant [M+Na]" ions when analyzed as their permethylated alditols, and this permitted the... 

In addition to these physical studies at the Bureau, Tipson was able to return to his synthetic interests, both alone and in collaboration with other staff members. He was especially pleased to prepare D-talose in crystalline form, an accomplishment that had eluded Emil Fischer. Pursuing his longstanding interest in the reaction of sulfonic esters with iodide and following an earlier observation that the tetratosyl ester of erythritol is converted into butadiene by the action of sodium iodide and zinc, he demonstrated (with A. Cohen) that nonterminal unsaturation may be conveniently introduced into alditol derivatives by reaction of contiguous secondary sulfonates with sodium iodide and zinc dust in boiling A.A-dimethylformamide. This Tipson-Cohen reaction subsequently proved of great utility in other hands for the conversion of more complex carbohydrate structures into vicinal dideoxy derivatives. 

The redox and complexation chemistry of alditol/Cr(VI) systems64 and the my<9-inositol/Cr(VI) system65 has been reported. In the first case, when an excess of the alditol over Cr(VI) is used, the secondary OH groups are inert to oxidation, and alditols are selectively oxidized at the primary OH group to yield the aldonic acid as the only oxidation product. The corresponding reaction involves a Cr(VI) —> Cr(V) —> Cr(III) reduction pathway, and the relative rate of each step depends on [H+] at... 

Polyhydroxy compounds are known to form borate complexes that can interfere with the subsequent acetylation step. In essence, two methods of acetylation are employed. In one, the conditions are sufficiently vigorous that borate complexes are cleaved during the acetylation, and, in the second, the complexes are decomposed prior to acetylation. In one example of the former method, the crude, dry alditols were refluxed for 4 hours with a mixture containing equal amounts of acetic anhydride and pyridine ( 1 ml/100 mg), and the cooled solution was injected directly into the gas chromatograph.217 The effect of borate on the rate of acetylation of glucitol by this... 

Although many analyses are performed on alditol acetates (see Section VII, p. 56), in order to avoid the formation of multiple peaks, such a reduction is not practical when the mixture contains ketoses, notably fructose. Such analyses are mainly encountered with medical samples and in the examination of sugars occurring free in Nature. Furthermore, the peak-area ratios may be used as a means of identification, to check on the completeness of trimethylsilylation,67,89 and, despite the complex chromatograms obtained from trimethyl-silyl derivatives, they have the merit of being rapidly formed.89 For all of these reasons, improvements in the separation of monosaccharides as their trimethylsilyl derivatives continue to be of considerable importance. 

The fact that each monosaccharide may give more than one peak owing to the formation of anomeric derivatives has led to a search for means to eliminate this complication. The anomeric center may be removed either by conversion into the oxime5 or the nitrile,394,3943 by oxidation followed by formation of the lactone (see Section IX, p. 71), or by reduction to the alditol. The last method is simpler than oxidation, and the separation of alditols and of aldononitriles will be discussed here additional examples are given in Table V (see p. 119). The early work on the separation of alditols has been discussed by Bishop.4 The necessity of decomposing borate complexes... 

Despite the results obtained by Jones and coworkers,409 interest in the separation of carbohydrate acetates lapsed for several years. This was partly due to the complexity of the liquid phase used by these investigators, and partly because the acetates of certain common alditols could not be separated on the then-available column-packings. In addition, it was at just about this time that Sweeley and coworkers5 introduced the use of trimethylsilylation, and demonstrated that a wide range of carbohydrates and their derivatives could be separated by this technique. 

Blake and Richards26 carefully examined all of the steps involved in the analysis of complex polysaccharides, and preferred to use the alditol acetates in studies on the classification,428 fractionation,427 and molecular aggregation428 of hemicelluloses and speargrass xylan.420,430... 

Diols and polyols can participate in equilibria with boric acid in aqueous solution. The stability of polyolborates is determined by the number of OH groups in cis positions. Complexes with polyols are more stable than with diols, and 1,2-diol complexes are more stable than their 1,3-diol counterparts (Table 10) since the resulting five-membered chelate ring is unstrained.75120 In the case of 1,3,5-triols stable cage-like structures (5) and (6) are favored. Open-chain or five-membered cyclic polyols form more stable chelate complexes than their six-membered counterparts.120 Thus, chelates from alditols and ketohexoses are more stable than the corresponding aldose chelates (Table 10). Many polyols allow quantitative titrimetric determination of boric acid. Of these, mannitol remains the most widely used reagent on the basis of availability, cost and ease of handling.75... 

R Alditol R Intermediate/ transition complex Free radical Mn+... 

Decarbonylation of aldoses.2 Although this rhodium complex has been known since 1968 to effect decarbonylation of aldehydes, it has been used for decarbonylation of sugars only recently, probably for lack of a compatible solvent. Actually, this reaction when carried out in N-methyl-2-pyrrolidinone (NMP) at 110-130° is extremely useful in the case of simple aldoses, which are converted to the lower alditol with formation of carbonylchlorobis(triphenylphosphine)rhodium(I). The yields are 75-95%. This method of degradation has the further advantage that protecting groups are not necessary. Deoxyaldoses, particularly 2-deoxyaldoses, are decar-bonylated in 75-99% yield. A disadvantage of this reaction is that a full equivalent of the complex is required. 

Reduction of sugars into alditols is effected by treatment with sodium borohydride, and acetylation by treatment with acetic anhydride containing sulphuric acid (2%) at 80°C for 15 h [436] or for 4 h by refluxing with a mixture of acetic anhydride and pyridine (1 1) [437]. The excess of reducing agent is usually decomposed prior to the acylation by treatment with an acid. The boric acid so produced should be removed, as it forms a complex with alditols and retards the acylation. Polar stationary phases (e.g., Carbowax 20M), on which strong sorption and decomposition of the derivatives occur, are not very suitable for the GC separation of acetates. Carbowax 20M modified with terephthalic acid and XE-60 provides good results, but some derivatives do not separate. 

From these findings with triols, it follows that, apart from the expectation that formation of five- and six-membered rings would be favored (see, however, the exceptional compound 19), no general conclusions can be drawn regarding the structures of boronates derived from more-complex polyhydric alcohols. In Table V, alditol boronates are listed with structures when these can be concluded either from the method of synthesis, from physical studies, or by deduction (as with the 1,2 5,6-diesters formed from 3,4-di-O-substi-tuted mannitols). 

Other than glycerol, the alditols (see Table II) all form strong complexes, so the technique is usually suitable for separations of free sugars from their reduction products.86 Removal of oxygen atoms from these derivatives, as expected, lessens their association with the acid. 

From the known, differential complexing between boronic acids and polyhydroxy compounds, it follows that carbohydrate mixtures may be separated by column-chromatographic methods that exploit the differences. Nucleoside and nucleotide boronates have been separated on columns of anion-exchange resins,90 and sugars and alditols have been shown to be differentially retained on such resins in the sulfonated phenylboronic acid form,64 but perhaps the best uses of column chromatography in this connection have incorporated the resolving powers of insoluble polymers to which boronic acid groups have been covalently bonded. Such insoluble forms of boronates have been synthesized either by substitution of polysaccharide derivatives, or by polymerization of suitable arylboronic acids. 

---