Dicarbonyl carbohydrates

The death on October 8, 2009 of Antonio Gomez Sanchez is noted with regret. He was one of the successors of the Seville carbohydrate school built up by Francisco Garcia Gonzalez, with whom he coauthored in Volume 20 a chapter on the reaction of amino sugars with 1,3-dicarbonyl compounds, a subject that was a major theme of Gomez Sanchez s research. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

The dehydration reactions initiated by eliminating a hydroxyl group from an enediol are discussed in the present article. The products (usually dicarbonyl compounds) of these eliminations are normally transient intermediates, and undergo further reaction. The final products formed are determined by the carbohydrate reacting, the conditions of reaction, and the character of the medium. Except for a Section on analytical methods (see p. 218), the subject matter is restricted to aqueous acids and bases. The presence of compounds other than the carbohydrate under study has only been considered where it has helped to elucidate the mechanism involved. The approach here is critical and interpretative, with emphasis on mechanism. An attempt has been made to demonstrate how similar reactions can logically lead to the various products from different carbohydrates a number of speculative mechanisms are proposed. It is hoped that this treatment will emphasize the broad functions of these reactions, an importance that is not fully recognized. No claim is made for a complete coverage of the literature instead, discussion of results in the articles that best illustrate the principles involved has been included. 

Tributyltin hydride, 316 Tributyltinlithium, 319 Trichloroacetonitrile, 321 Other carbohydrates (Diethylamino)sulfur trifluoride, 110 Triethyloxonium tetrafluoroborate, 44 Carbonates (see also Enol carbonates) Carbon dioxide, 65 Di-/-butyl dicarbonate, 94 Carboxylic acids (see also Dicarbonyl compounds, Unsaturated carbonyl compounds)... 

The imidazole part of the IQ-compounds suggests creatinine as a common precursor. The remaining parts of the IQ-compounds could arise from Maillard reaction products, e.g., 2-methylpyri-dine or 2,5-dimethylpyrazine. These two compounds could be formed through Strecker degradation. In Maillard reactions, this is induced by a-dicarbonyl compounds derived from carbohydrates, which are thereby converted to pyrroles, pyridines, pyrazines, etc. (8). 

Heating of carbohydrates produces a number of aromatic compounds Including aldehydes, ketones, and dicarbonyls as well as oxygen containing heterocyclic compounds such as furans, dihydrofuranones, and pyrans through caramelizatlon and dehydration reactions. 

Peptides formed enzymically or by mineral acid hydrolysis or thermal degradation of higher molecular veight protein can also serve as flavor precursors in thermally induced reactions. The reactivity of peptides is evidenced by their behavior during pyrolysis/GC (9), heating in air (10), reactions vith mono- (11), and dicarbonyl (12, 13) compounds and reactions in hot acetic acid (1A). The types of reactions observed for peptides include side-chain thermolysis, fragmentation into amino acids, DKP formation and Halliard reaction vith ambient carbohydrates. 

J. Hirsch, V. V. Mossine, and M. S. Feather, Detection of some dicarbonyl intermediates arising from the degradation of Amadori compounds (the Maillard reaction), Carbohydr. Res., 1995, 273, 171-177. 

Coffee flavors form during roasting from dicarbonyl compounds which derive from carbohydrates. The thermal degradation of hexoses is thought to be the preciu ors of fin ones like HDMF. The presence of alkylpyrazines affords the characteristic roast notes. These pyrazines are formed through Strecker degradation and the condensation of the resulting Strecker products (60). 

---