Glycosylamines formation

Polymers developed specifically for HPLC include the macroporous crosslinked vinylpyridinium type, which has proved very effective in HILIC of sugars in aqueous acetonitrile. These polymers can be used at column temperatures up to 70°C, under which conditions, with the resin in the phosphate or sulfate form, baseline resolution of mixtures of the common monosaccharides and some disaccharides, such as maltose and lactose, can be achieved. Stationary phases in which a polymer replaces silica as the support for the aminopropyl bonded phase much used in HILIC of carbohydrates have also been successfully applied in separations of mono- and disaccharides in this case precolumn derivatization with 4-amino-benzoic acid ethyl ether (ABEE) has been recommended to overcome the problem of glycosylamine formation that occurs with imderivatized sugars. 

The mechanism of glycosylamine formation has not been established experimentally. However, a glycosylamine is probably formed by nucleophilic substitution of the hemiacetal hydroxyl group by an amine. The reaction may also occur through the open-chain form, with subsequent ring closure. Quite weak amines and amides require acid catalysis the function of the acid catalyst is probably to convert the sugar to its conjugate acid form, which is sufficiently reactive for the weak amine to effect condensa-... 

The initiating reaction between aldoses and amines, or amino acids, appears to involve a reversible formation of an N-substituted aldosyl-amine (75) see Scheme 14. Without an acidic catalyst, hexoses form the aldosylamine condensation-product in 80-90% yield. An acidic catalyst raises the reaction rate and yet, too much acid rapidly promotes the formation of 1-amino-l-deoxy-2-ketoses. Amino acids act in an autocat-alytic manner, and the condensation proceeds even in the absence of additional acid. A considerable number of glycosylamines have been prepared by heating the saccharides and an amine in anhydrous ethanol in the presence of an acidic catalyst. N.m.r. spectroscopy has been used to show that primary amines condense with D-ribose to give D-ribopyrano-sylamines. ... 

However, the azide-based protocol often has problems with the formation of byproducts under both azide formation and subsequent reduction. 388 Therefore, mainly aminolysis is used. Glycosylamines are prepared from GlcNAc, GlcNAc2, Glc, maltotriose, mal-toheptaose, and di- and triantenneary complex glycans, isolated from asialofetuin by preparative hydrazinolysis. A modification of the method by Likhosherstov is also employed. 379 ... 

There are two theories as to the mechanism of the formation of imidazole and pyrazine compounds, namely, (a) the prior formation of glycosylamines and aminodeoxy sugars, and (b) the fragmentation mechanism. 

Figure 2.29 Formation of glycosylamine, the initial step in Maillard browning.

Ellis and Honeyman80 have raised strong doubts as to the validity of the structural determinations that have been applied to glycosylamines these doubts are based on the fact that such compounds are readily isomerized, as evidenced by their mutarotation. The formation of a trityl ether cannot be considered valid evidence for the presence of a primary hydroxyl group in the compound tritylated, and periodate oxidation, which is often excessive, may lead to faulty conclusions. 

Results of the many investigations into the mechanism of the Maillard reaction support one of two main theories. The first assumes the formation of glycosylamines which undergo the Amadori (or, for ketoses, the Heyns) rearrangement. The 1-amino-1-deoxyketose derivative (or 2-amino-2-de-... 

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