Ketoses Heyns rearrangement

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-... 

The first step of the reaction is the condensation of amino acids to carbon atom 1 of aldoses (or C-2 of ketoses) and the rearrangement to the keto (aldo)-sugar (Amadori or Heyns-rearrangement). 

When the carbonyl compound is a ketose (e.g. fructose) rather than an aldose, the fructosylamine precursor (an N-alkyl fructoside) undergoes a Heyns rearrangement to form a 2-alkylamino-2-deoxy-D-glucose (15). These Heyns compounds are precursors of the browning phenonema. 

Ketoses undergo a similar series of reactions, leading to 2-amino-2-deoxyaldoses (Heyns rearrangement). However, browning reactions of fructose differ from those of glucose, e.g., loss of amino acid or of free amino groups (casein) is much lower.28... 

The formation of 2-arylamino-2-deoxyaldoses in the reaction of ketoses with anilines was described as the retro-Amadori reaction (called also the Heyns rearrangement)145. 

Aldoses undergo the Amadori rearrangement and subsequently turn into caramels, the natural brown food colorants, and/or heteroaromatic compounds — derivatives of pyrrole, imidazole, and pyrazine. Ketoses react similarly into ketosylamino acids or ketosylamines, which, in the first step, undergo the Heyns rearrangement (5.17-5.23). These rearrangements are the first steps of either thermal or enzymatic (the Maillard reaction) reactions resulting in the browning of food and the aroma of roasted, baked, or fried foodstuffs. 

The Heyns rearrangement follows, in principle, the same pattern but employs a-hydroxy ketones as starting materials. Applied to free sugars, this reaction starts from a ketose such as D-fructose (3) and proceeds via a glycosylamine to the corresponding 2-amino-2-deoxyaldoses 4 and 5 (Scheme 2). Due to the nature of the intermediate, both epimers at position C-2 can be formed. 

Ketoses may also enter into the reaction, and then the formation of glycosylamine is followed by Heyns rearrangement (Section 4.7.5.1). The reaction could also include not only carbonyl compounds derived from the degradation of sugars, but also Strecker aldehydes arising from amino acids and reactive aldehydes and other compounds produced as secondary decomposition products of fatty acid hydroperoxides in hpid peroxidation. 

As indicated previously, primary and secondary amines can also react with carbonyl compounds to form a mixture of compounds containing small molecules and polymers. The small molecule compounds obtained from an aldose and an amine have the common name Amadori products because the Amadori rearrangement is involved in their formation. The compounds generated from ketoses and amines are known as Heyns products (although the differentiation Amadori/Heyns is not always considered). The mechanism for the reaction of primary amines with a reducing sugar can be formulated as follows ... 

In the first step an aldose sugar reacts with an amino compound to form the Schiff base 1 which undergoes further reactions and rearranges into an aminoketose 2, the so-called Amadori compound [23-25]. Fig. 3.19 shows the reaction scheme with glucose as the reducing sugar. Heyns and co-workers [26] found aminoaldoses as first intermediates in a similar mechanism when ketose sugars were used. 

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]. 

As was mentioned earlier, more than 3,500 volatile compounds have been identified as Maillard products. This number documents the large number of pathways potentially active in this reaction. A simplified view of this process begins with the reaction between amino acids and reducing sugars to form imines (Figure 5.2). These imines rearrange to form unstable Amadori (aldose precursor) or Heyns (ketose precursor)... 

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