Tagatose acetal

Emil Fischer first described the condensation of D-fructose with acetone in 1895, and most of the early work on cyclic acetals of ketoses was performed with D-fructose. In 1934, Reichstein and Griissner published their classic synthesis of L-ascorbic acid (vitamin C), in which L-sorbose was converted into 2,3 4,6-di-0-isopropylidene-a-L-sorbofuranose or other di-alkylidene acetals. The emphasis of research activity then shifted to L-sorbose, and to the elucidation of an optimal procedure for preparing such diacetals. At about the same time, Levene and Tipson used isopropylidene acetals as derivatives for the purification of L-cri/thro-pentulose (as the di-isopropylidene acetal) and D-thrco-pentulose (as the monoisopropyli-dene acetal). Soon thereafter, Reichstein and coworkers used diisopropylidene acetals of D- and L-psicose, and D-tagatose, to purify the respective sugars. 

Miethchen and co-workers have published a number of papers concerning the use of chloral and dicyclohexylcarbodiimide (DCC) as a means to invert configuration of chiral centres in carbohydrates. These reagents were found to react with his-vicinal triols with a cis, trans sequence of hydroxyl groups and this resulted in the formation of cyclic acetals in which the central atom of the triol had been inverted. For example, the rare monosaccharide o-tagatose has been easily prepared from D-fructose (Scheme 4) [6]. Concerning the mechanism of this reaction, chloral and DCC react with fructose derivative 10 to give intermediates 11 and 12. The latter mentioned intermediate then reacts intra-molecularily in an SN2-type reaction as shown. This furnished product 13 in 59% yield. 

A new route to monosaccharides employing catalysed asymmetric aldol condensation has been illustrated by the preparation of L-fucose from crotonaldehyde and ketene silyl acetal 11 (Scheme 3) in 4 steps and 49% overall yield. The preparations of D-tagatose 1,6-diphosphate and of (3S/4/ )-configurated ketose 1-phosphates by use of aldolases are covered in Chapter 7. 

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