2- Amino-2-deoxyaldoses, formation

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. 

Amino-2-deoxyaldohexoses, 1-amino-l-deoxyketohexoses, and their V-alkyl and JV-aryl derivatives react with /3-dicarbonyl compounds, giving substituted (tetrahydroxybutyl)pyrroles. It is, most probably, a general reaction of 2-amino-2-deoxyaldoses and 1-amino-l-deoxyketoses which parallels the formation of (polyhydroxyalkyl)furans from nonnitrogenous monosaccharides. In the reactions with amino sugars in alkaline media, simpler pyrrole compounds lacking the tetrahydroxybutyl chain are also obtained. 

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]. 

The importance of Strecker degradation is mainly in the formation of reactive and often sensory active aldehydes and ammonia (free or bound in reactive a-amino carbonyl compounds). Strecker aldehydes, together with a-amino carbonyl compounds, contribute significantly to the formation of many heterocyclic compounds (which are important flavour-active components of processed foods) and the brown pigments melanoidins. Aldoses simultaneously produce the corresponding 2-deoxyaldoses or 1-amino-1-deoxyalditols and 2-oxoacids. 

---