Carboxylic acids, condensation with sugar

Treatment of hydrolyzable tannins with acid or alkali and, in some cases, hydrolytic enzymes splits them into sugars and phenolic carboxylic acids. Condensed tannins do not break down readily in this manner nor do they contain sugar moieties as a part of their structure (Haslam, 1979). However, gallic acid units are sometimes associated with condensed tannins (Lewis and Yamamoto, 1989). 

The condensation of amino sugars with /3-keto esters in an alkaline medium results in considerable degradation of the (tetrahydroxybu-tyl)pyrrole produced, giving rise to small yields of 2-methylpyr-role,57,58 which seems to be the main chromophore in the Elson-Morgan reaction. Under these conditions, 3-amino-3-deoxyhexoses yield 2-methylpyrrole-4-carboxylic acid,59 which is, in part, responsible for the coloration produced with p-(dimethylamino)benzaldehyde. 

Condensation of L-cysteine with carbonyl compounds has been widely employed in the preparation of chiral thiazolidine 4-carboxylic acids <87JMC1891>. a-Hydroxyketenes <87JHC1629> or sugar-derived aldehydes <89CAR(187)223> have been used to form 2-(polyhydroxyalkyl)thiazohdines (Equation (73)) <94CAR(262)147>. This reaction has been also employed in peptide synthesis <94JA4149>. 

The enzyme-catalyzed aldol condensation of ca-functionalized C5- and 06-aldehydes with Dhap (dihydroxacetone phosphate) has been used to synthesize unusual Cg- and C9-sugars. An example is given in Scheme 4. Coupling of carbohydrate carboxylic acids such as 21 and 23 with alkanoic acids by mixed... 

Methylthiazolidine-4-carboxylic acid, a condensation product of cysteine and acetaldehyde, occurs even in human blood as a consequence of ethanol consumption. Serine and threonine analogously produce C-2 substituted (2J S,4S)-oxazolidine-4-carboxylic acids (2-124). Heterocyclic products, C-2 substituted (2J S,4S)-pyrimidine-4-carboxylic acids, are also produced in the reaction of aldehydes with asparagine (2-125). Phenylalanine yields C-1 substituted (lJ S,3S)-tetrahydroisoquinoline-3-carboxylic acids (2-126) and analogous products arise from tyrosine. Tryptophan reacts with aldehydes under the formation of 9H-pyrido[3,4-b]indole (also known as -carboline or norharmane) derivatives, (lJ S,3S)-l,2,3,4-tetrahydro-fi-carboline-3-carboxylic acids (2-127, R = H or alkyl or residues of other aldehydes and sugars), the reaction of tryptamine yields the corresponding (lRS)-l,2,3,4-tetrahydro-P-carbolines. 

Pyrazines are formed from transamination reactions, in addition to carbon dioxide and formaldehyde. A requirement is that the carbonyl compound contains a dione and the amino group is alpha to the carboxyl group (16). If the hydrogen on the ct-carbon oI the amino acid is substituted, a ketone is produced. Newell (17) initially proposed a pyrazine formation mechanism between sugar and amino acid precursors. (See Figure 3). The Schiff base cation is formed by addition of the amino acid to the anomeric portion of the aldo-hexose, with subsequent losses of vater and a hydroxyl ion. Decarboxylation forms an imine which can hydrolyze to an aldehyde and a dienamine. Enolization yields a ketoamine, vhich dissociates to amino acetone and glyceraldehyde. 2,5-Dimethylpyrazine is formed by the condensation of the tvo molecules of amino acetone. 

Adipic Acid—Of the di-carboxy acids which contain more than three carbon groups between the two carboxyls we need only mention two. Adipic acid, like glutaric acid, is found in the juice of the sugar beet. Its systematic name is 1-6 hexan-di-oic acid and its formula is, HOOC CH2—CH2—CH2—CH2—COOH. It may be synthesized by the same general methods as those described in connection with glutaric acid. The constitution of adipic acid has been proven by the following synthesis from /8-iodo propicnic acid, in which two molecules of the acid are condensed by means of silver. 

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