Dicarbonyl derivatives, of carbohydrates

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 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). 

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). 

The use of lead tetraacetate for adding acetoxy groups to a double bond, as here used for preparing the pentaacetate (76), has been widely applied in synthetic organic chemistry, but there have been only a few applications to carbohydrates (see Perlin s review in this Series, for other examples). The conversion of (76) to the dicarbonyl derivative (77) (in 65% yield and in the crystalline state) was made by autohydrolysis in water at room temperature for 3 hr. The structure was deduced from elemental analysis, acetyl analysis, and the absence of methoxyl groups, and from its mode of synthesis. The acetyl determination could be made by direct titration with... 

The dicarbonyl compoimd 51 was oxidized to the anhydride 52, which subsequently reacted with primary or secondary amines to form a-amino acids, a-amino amides and dipeptides 53 (Scheme 14) [48]. 3-Hydroxy j8-lactams obtained from imines derived from carbohydrates [49,50] or prepared via the Sharpless AD reaction [51-53] were directly oxidized to anhydrides by treatment with NaOCl and TEMPO. Anhydrides 54-56 were used for the synthesis of compounds related to the family of polyoxins represented by 57 (Scheme 15) [49-53]. 

Periodic acid does not oxidize mi/o-inositol according to the classical pattern. The consumption of 6 moles of oxidant with the formation of 6 moles of formic acid would be expected. Instead, a complex reaction ensues in which there is an overconsumption of oxidant and only about 4 moles of acid are produced 104). A mechanism has been advanced to account for these results 104) Similar results have been obtained with D-inositol and pinitol 106). Structural studies with periodic acid are of less application here than in other branches of carbohydrate chemistry, although useful results have been obtained Slh, 106a) by taking into account the overoxidation caused by the jS-dicarbonyl anomaly. Certain substituted derivatives, 1,3-di-O-methyl-mi/o-inositol (dambonitol) 41 o) and isopropylidene-inositols (7), for example, appear to be oxidized in the classical manner. 

The discussion of this somewhat heterogeneous collection of free and substituted dicarbonyl carbohydrates will be divided into (a) dialdehyde compounds, (b) keto-aldehyde compoimds having free or substituted carbonyl groups, and (c) the diketo compounds. Each Section (except the last one, containing only a few members) will be divided into (a) an introduction consisting of a general discussion concerning formation and preparation, (b) a description of individual compounds, and (c) a discussion of properties and reactions. A short discussion on oxidized polysaccharides is included before the Tables of dicarbonyl carbohydrates and their derivatives. 

A -Acyl substitution of 2-amino-2-deoxyaldohexoses prevents their reaction with /3-dicarbonyl compounds from giving pyrrole derivatives. Instead, the products of the aldol reaction of the aldehydo form of the sugar with the active methylene group of the /3-dicarbonyl compound are formed. These substances have the structural features of higher-carbon, branched-chain carbohydrates. 

Dehydration of the carbohydrate portion in fiuctosamines probably proceeds through a series of enolic intermediates (Scheme 31), whieh eventually tautomerize into a-dicarbonyl stractures such as 60, 62, and 63, or cycUze into 2-furoylmethyl-amines (61) and many other heterocyclic derivatives. One of the major products of D-fructosamine degradation that occurs through the 1,2-enoUzation pathway is 3-deoxy-D-e 7 /jro-hexos-2-ulose (60), which is regarded as one of the most important intermediates in the Maillard reaction both in foods and in vivo ... 

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