2-Glyculosonates

A number of papers describe the preparation of glyculosonic acids by bacteria. The genus Pseudomonas is reported to be particularly efficient for the preparation of D-araZh o-2-hexulosonic acid (94), a compound used on the industrial scale for production of isoascorbic acid. Lockwood described the preparation of 94 in good yield by such a strain.279 2-Glyculosonic acids can be degraded to lower aldonic acids by the Ruff procedure. 

Chain-extension reactions constitute a more widely used approach. Thus, the cyanohydrin synthesis followed by base-catalyzed cyclization and /1-elimination to iminolactones, which then undergo stepwise hydrolysis, affords 3-deoxy-2-glyculosonic acids.313 The overall yield of this reaction is low. Paerels314 used this method to prepare the first crystalline members of this group, namely 3-deoxy-D-m //iro-hex-2-ulosopyranosonic acid (2-keto-3-deoxy-D-gluconic acid, KDG, 119), and the L isomer, starting from D-ribose and L-arabinose, respectively. The synthesis of 119 is illustrated in Scheme 10. 

Fourthly, biotransformations have been used for the synthesis of 3-deoxy-2-glyculosonic acids, using whole cells or purified enzymes. For instance, 3-deoxy-D-araZu rao-heptulosonic acid (DAH) and its 7-phosphate (DAHP, 122) have been produced directly from D-glucose by mutants of E. coli JB-5, that lack dehydroquinate synthase, the enzyme that converts DAHP into the cyclic intermediate dehydroquinic acid (DHQ, Scheme 14). Both DAH and DAHP are secreted into the medium. The dephosphorylated product could be generated in vivo by a phosphatase acting on DAHP.312... 

The second approach, the simultaneous enolization and lactonization of 2-glyculosonic acids or esters, is generally the method of choice for the preparation of L-ascorbic acids, provided that the required 2-glyculosonic acid is an accessible starting material. 

Ascorbic acids reduce Fehling solution in the cold, and react with ferric chloride to produce the violet color typical of enolic compounds. They are readily oxidized reversibly to their primary oxidation products, 2,3-glycodiulosono-1,4-lactones (commonly known as dehydroascorbic acids 148, Scheme 16), by such mild oxidizing agents as aqueous iodine. Titration with such oxidants constitutes a quantitative method for distinguishing ascorbic acids from 2-glyculosonic acids. 

The principal synthetic application of the aldos-2-uloses has been as intermediates in the preparation of L-ascorbic acids (see Section II.5.a). Aldos-2-uloses can be oxidized to 2-glyculosonic acids. They are more stable... 

Secondary alcoholic groups are slowly oxidized to ketone groups, and 2-glyculosonic and 5-glyculosonic acids are formed in this way. More-extended oxidation results in the cleavage of carbon-carbon bonds and in the production of chain-shortened acids. 

Chloric acid, in conjunction with catalysts (particularly vanadium pentaoxide), is used for the oxidation of aldonic acids or lactones to the 2-glyculosonic acids. Thus, D-glucono-1,4-lactone (9) and potassium D-galactonate in methanol, in the presence of phosphoric acid and vanadium pentaoxide, are oxidized by chloric acid to methyl D-arabino-2-hexulosonate (10) and methyl D-/yxo-2-hexulosonate, respectively.38 At moderate temperatures in the absence of a catalyst, aldoses, ketoses, and sucrose are inert to the action of chlorates over a several weeks time period 39 bromates in alkaline solution also exert no oxidative action (Scheme 5). 

Schmidt, R R, Frick, W, Haag-Zeino, B, Apparao, S, De-novo synthesis of carbohydrates and related natural products. Part 28. C-arylglycosides and 3-deoxy-2-glyculosonates via inverse type hetero-Diels-Alder reaction, Tetrahedron Lett., 28, 4045 -4048, 1987. 

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