Ketoses 1-amino-1-deoxy-, formation

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. ... 

Concerning their reaction behavior with amines, ketoses react in the opposite manner. The reaction between aromatic amines and ketoses gives the ketosyl-amine in low yields and the rearrangement products in poor yields. N-Alkyl-ketosylamines, on the other hand, undergo the rearrangement to the corresponding 2-deoxy-2-aminoaldoses very easily. Ammonia yields 2-aminoal-doses. Formation of epimers at C-2 may pose a problem in terms of purification and, consequently, yields. 

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. 

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. 

The open-chain forms of glycosylamines (see Section 1.2.3) are Schiff bases and are labilised towards deprotonation of an adjacent carbon in much the same way as the open-chain forms of the sugars themselves. Reprotonation of the first-formed enamine on the carbon bearing the nitrogen atom will give an a-aminocarbonyl compound. The formation of a l-deoxy-l-amino-2-ketoses... 

In the Lobry de Bruyn-Alberda van Ekenstein transformation (reviewed by Speck ) of a ketose to the epimeric aldoses, formation of the 3-deoxy-uloses is normally considered to be a side reaction, and both reactions are considered to proceed through a common intermediate, the 1,2-enediol of the sugar. Under the conditions used for preparation of the 3-deoxy-hexos-uloses from the diketose-(amino acids), the former were, however, the main products and the epimeric aldoses only minor products. Furthermore, under these conditions, both reactions were irreversible and the products so stable that the amounts of the two t3rpes of compound were a measure of their rates of formation. The rapid rate of decomposition of the diketose-(amino acids) was probably due to their ready enolization, even in the absence of strong alkali or acid, to give the 1,2-enolammonium compound... 

Glycosyl isothiocyanates have also been allowed to react with unprotected 2-amino-2-deoxyaldoses and 1-amino-1-deoxy-2-ketoses.68 This reaction leads to the formation of heterocyclic derivatives resulting from cy-clization involving the carbonyl group of the amino sugar moiety following the mechanistic pathway already discussed for similar condensation reactions with alkyl and aryl isothiocaynates. 

The reaction products of the Maillard reaction, such as l-amino-l-deoxy-2-ketose (Amadori product) or 2-amino-2-deoxyaldose (Heyns product), do not contribute to flavor directly but they are important precursors of flavor compounds [48]. These thermally unstable compounds undergo dehydration and deamination reactions to give numerous rearrangement and degradation products. The thermal degradation of such intermediates is responsible for the formation of volatile compounds that impart the characteristic burnt odor and flavor to various food products. For example, at temperatures above 100 C, enolization products (such as l-amino-2,3-enediol and 3-deoxyosone) yield, upon further dehydration, furfural from a pentose and 5-hydroxy methylfurfural and 5-meth-ylfurfural from a hexose [2]. 

Hydrogenation of the ketose derivative (II) produces l-deoxy-l-(aryl-amino) sugar alcohols. Since a new asymmetric center is produced, two isomeric alcohols may be formed, but the yield of the two possible isomers is influenced greatly by the acidity of the medium employed for the hydrogenation (74). In acid solution, catalytic reduction of 1-deoxy-l-p-toluino-fructose (IV) takes place only in the aromatic ring (V) but in alkaline or neutral solution, it takes place with the formation of 1-deoxy-l-p-toluino-mannitol (p-tolyl-D-mannamine) (VI) ... 

FIG. 3 Formation of l-methyl-l-amino-l-deoxy-2-ketose (XIV) from imine (XII) in the Amadori rearrangement. (From Ref. 12.)... 

Ketoses also react with both aliphatic and aromatic amines, and the derived ketosylamines (XXXIII) can also rearrange in a reverse of the Amadori rearrangement with the formation of a 2-amino-2-deoxy-aldose (XXXIV). 

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