1 - Amino-1 -deoxy-2-ketose

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. ... 

The generic term monosaccharide (as opposed to oligosaccharide or polysaccharide) denotes a single unit, without glycosidic connection to other such units. It includes aldoses, dialdoses, aldoketoses, ketoses and diketoses, as well as deoxy sugars and amino sugars, and their derivatives, provided that the parent compound has a (potential) carbonyl gTOup. 

Tables V-LVII detail H and F shift and coupling information, and Tables LVIII to LXXI incorporate the C-n.m.r. data. The data within this compilation are arranged according to the following outline hexoses prior to pentoses, followed by anhydro sugars, sugar acids and lactones, amino sugars (and their synthetic, A -containing precursors), mono-, di-, and tri-deoxy sugars, branched derivatives, ketoses, polyfluorinated monosaccharides, and, finally, difluorinated amino sugars. Within this compilation, and even within each table, pyranoid derivatives are listed prior to their furanoid counterparts, hexoses prior to pentoses, functionalized prior to deoxy compounds the arrangement within each sub-table is made alphabetically.

The next step of the Maillard sequence involves the conversion of the -substituted aldosylamine (74 75) into a 1-amino-l-deoxy-2-ketose... 

The Maillard condensation is one of the most extensively studied reactions within the field of degradation chemistry, particularly in the area of food and nutritional science. Louis Mallard reported in 1912 that some amines react with reducing carbohydrates to produce brown pigments. The condensation typically yields a simple glycosylamine, which then readily undergoes the Amadori rearrangement to produce 1 -amino-1 -deoxy-2-ketoses [95]. Reducing carbohydrates... 

Another class of pyrrole derivative may be obtained by the interaction of l-amino-l-deoxy-2-ketoses or 2-amino-2-deoxyaldoses with a jQ-dicarbonyl compound. Unlike the previous type (which is N-substituted), these pyrrole derivatives have a tetrahydroxybutyl group in the a- or /8-position with respect to the nitrogen atom of the ring, in addition to other groups arising from the dicarbonyl compound used in the condensation. The formation and reactions of this type of pyrrole derivative have been discussed in detail in two articles in this series48,49 they will, therefore, only be treated briefly. 1-Amino-l-deoxy-D-fructose (53) reacts with 2,4-pentanedione to give50 pyrrole derivative 54a similar pyrroles were obtained with ethyl acetoace-tate,50,51 which yields 54b. 

Another example of the observation that the side chain attached to the anomeric carbon atom in ketoses affects the proportion of one furanose form [p. 29] is afforded by the series 1-deoxy-D-fructose JV-substituted 1-amino-... 

Amadori compounds (N-substituted-l-amino-l-deoxy-2-ketoses) are potential precursors to the formation of many of these heterocyclic volatile products. The secondary nitrogen in most Amadori compounds is weakly basic and is therefore a likely site for rapid nitrosation reactions via normal reactions with nitrous acid, under mildly acidic conditions. However, purified Amadori compounds are usually obtained only after tedious isolation procedures are invoked to separate them from the complex mixtures of typical Maillard browning systems. Takeoka et al. ( 5) reported high performance liquid chromatographic (HPLC) procedures to separate Amadori compounds in highly purified form on a wide variety of columns, both of hydrophilic and hydrophobic nature. They were able to thus demonstrate that reaction products could be followed for kinetic measurements as well as to ensure purity of isolated products. 

Another possible mechanism7 for the transformation of sugars with amino acids into colored products is through the Amadori rearrangement,44 which is the isomerization of an aldosylamine to a ketose derivative, for example, of a D-glucosylamine derivative to a derivative of 1-amino-l-deoxy-D-fruc-tose. Such a conversion has been shown to occur when an amino acid reacts... 

Hodge7 has advanced several possible routes for the conversion of the enol form of the 1-amino-l-deoxy-2-ketose into melanoidin, and the evidence to support these mechanisms is considerable. Thus, the enol may be converted into the Schiff base of a furaldehyde, or to a reductone by loss of water. It may also be broken down into smaller fragments (for example, hydroxy-2-propanone or pyruvaldehyde), which react further with amino compounds. The enol may also react with an a-amino acid and be converted to an aldehyde by a Strecker degradation. The compounds thus formed from... 

Amino-1-deoxy-2-ketose or 1-Amino-2-deoxy-2-aldoses -3H2O... 

Monosaccharides may possess functionalities other than hydroxyls. Amino sugars are aldoses or ketoses which have a hydroxyl group replaced by an amino functionality. 2-Amino-2-deoxy-glucose is one of the most abundant amino sugars it is a constituent of the polysaccharide chitin. It also appears in mammalian glycoproteins, linking the sugar chain to the protein. Monosaccharides may also be substituted with sulfates and phosphates. Furthermore, deoxy functions can often be present, and important examples of this class of monosaccharides are L-fucose and L-rhamnose. 

Scheme 2.2 Amadori rearrangement leading to the Amadori compound, the N-substituted 1 -amino-2-deoxy-2-ketose...

Compared with the /V-substituted glycosylamines, the l-amino-l-deoxy-2-ketoses are more stable to moist acid atmospheres, but are still heat-labile and decompose rapidly in mild alkali. They exert greater reducing power, although less than reduc-tones. They brown more easily with amino acids. Acid hydrolysis gives much compared with little HMF, but no hexose is recovered, in keeping with the reaction s being irreversible (however, see below). 

Epimerization at C-2 of a sugar nucleotide has been described. An enzyme from Escherichia coli catalyzes the epimerization of UDP-2-acetamido-2-deoxy-D-glucose to UDP-2-acetamido-2-deoxy-D-mannose.59 Such an epimerization, together with the 3,5-epimerase reaction of a 4-ketose and stereospecific reduction at C-4, could lead to inversion at all of the chiral centers of, for instance, 2-amino-2-deoxy-D-glucose. 

The reaction to be discussed here is the isomerization of an aldosylamine to a l-amino-l-deoxy-2-ketose. This rearrangement was named after... 

---