Aldonic acids esterification

Aldono-1,5-lactones and free aldonic acids react with alcohols in the presence of hydrogen chloride to give the corresponding alkyl aldonates (84). The reaction is slower with 1,4-lactones. Because esterification takes place very slowly in the absence of an acidic catalyst, aldonic acids and their lactones may be recrystallized from boiling alcohols without appreciable esterification (85). However, in some instances, alkyl esters are formed under these conditions. For example, essentially pure ethyl L-mannonate was isolated (6.4% yield) from the mother liquors of crystallization L-man-nono-1,4-lactone, obtained by Kiliani synthesis from L-arabinose (86). Similarly, repeated recrystallization from ethanol of crude 2,3,4,6-tetra-O-acetyl-D-glucono- 1,5-lactone afforded the corresponding ethyl gluconate derivative (87). 

Under dehydrating conditions, it is probable that, in addition to lactonization, intermolecular esterification also occurs, yielding esters of aldonic acids and polymers. 

Aldonic acids are in equilibrium with their lactones, formed by reversible esterification of the carboxyl group with a hydroxyl in 4 or 5 position (gamma and delta lactones) (e.g. delta-gluconolactone Fig. 7.6). 

In addition to lactone formation, it is probable that extramolecular esterification may take place with the formation of aldonic esters of aldonic acids (e.g., gluconic acid gluconate) and chain polymerization also may occur. In such systems, the concentration of water present would be expected to exert a profound influence on the composition of the equilibrium solution. Lactic acid forms external esters (lactides), but this type of condensation through carboxyls and a-hydroxyls has not been observed for hexonic and pentonic acids. 

Esters of aldonic acids are prepared from 6-lactones, slowly from 7-lac-tones, by reaction with alcohols in the presence of hydrogen chloride or of the free aldonic acid (26). The acids may be recrystallized from boiling methanol without much esterification taking place (27). At the melting point, ethyl mannonate is converted to the 7-lactone with the loss of ethyl alcohol. 

Redox glycosylation via thionoester intermediates. Barrett [112, 113] introduced the concept of redox glycosylation , where the usual glycosylation reaction is replaced by the esterification of an aldonic acid 164, followed by reduction to a reactive intermediate (enol ether from an ester or 0,S-ace al 166 from the thionoester... 

Pollock and Jermany (1968), using the asymmetric alcohols, butan-2-ol, 3-methylbutan-2-ol and 3,3-dimethyl butan-2-ol as the esterification reagents for each of a number of acids including lactic, glyceric, malic, mandelic and tartaric acids, demonstrated the separation of the resulting diastereoisomers after acetylation on a Carbowax 20M column. Their study, however, was directed principally at resolving various racemic carbohydrate diastereoisomers by conversion to aldonic acids prior to derivatization and gas chromatography. 

The acid-catalyzed acetalation of aldonolactones with alkyl acetals of aldehydes or ketones takes place, in some instances, with esterification of the lactone group to give acetal derivatives of alkyl aldonates (11,22). 

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