Ketoses, formation synthesis

The synthesis of cellulose by A. xylinum from various polyalcohols14 is accompanied by the formation of carbon dioxide, formic acid, nonvolatile acids, ketoses and sometimes ethanol. The much greater variety of substrates suitable for cellulose synthesis, as compared with the small number for dextran or levan, may account for the widespread natural occurrence of cellulose. 

Zhdanov and Polenov have reported58 a synthesis of the first sugar phosphorane (70), which was shown to react with p-nitro-and o-hy-droxy-benzaldehyde, forming unsaturated ketoses 71 and 72, respectively. Attempted reactions with p-dimethylamino-, p-hydroxy-, and 2,4-dihydroxy-benzaldehyde were unsuccessful. The formation of the dienic ketose 72 is undoubtedly caused by /3-elimination of a methoxyl group during treatment of the reaction mixture with aqueous sodium hydroxide during isolation. 

In DERA reactions, where acetaldehyde is the donor, products are also themselves aldehydes. In certain cases a second aldol reaction will proceed until a product has been formed that can cyclize to a stable hemiacetal.71 For example, when a-substituted aldehydes were used, containing functionality that could not cyclize to a hemiacetal after the first aldol reaction, these products reacted with a second molecule of acetaldehyde to form 2,4-dideoxyhexoses, which then cyclized to a hemiacetal, preventing further reaction. Oxidation of these materials to the corresponding lactone, provided a rapid entry to the mevinic acids and compactins (Scheme 5.43). Similar sequential aldol reactions have been studied, where two enzyme systems have been employed72 (Scheme 5.44). The synthesis of 5-deoxy ketoses with three substitutents in the axial position was accomplished by the application of DERA and RAMA in one-pot (Scheme 5.44). The long reaction time required for the formation of these thermodynamically less stable products, results in some breakdown of the normally observed stereoselectivity of the DERA and FDP aldolases. In a two-pot procedure, DERA and NeuAc aldolase have... 

Other reactions of carbohydrates include those of alcohols, carboxylic acids, and their derivatives. Alkylation of carbohydrate hydroxyl groups leads to ethers. Acylation of their hydroxyl groups produces esters. Alkylation and acylation reactions are sometimes used to protect carbohydrate hydroxyl groups from reaction while a transformation occurs elsewhere. Hydrolysis reactions are involved in converting ester and lactone derivatives of carbohydrates back to their polyhydroxy form. Enolization of aldehydes and ketones leads to epimerization and interconversion of aldoses and ketoses. Addition reactions of aldehydes and ketones are useful, too, such as the addition of ammonia derivatives in osazone formation, and of cyanide in the Kiliani-Fischer synthesis. Hydrolysis of nitriles from the Kiliani-Fischer synthesis leads to carboxylic acids. 

Epimerase. Another rearrangement of the pentose phosphate molecule was recently discovered. This involves an isomerization at carbon 3, to form xylulose-5-phosphate (IV). The enzyme responsible has been named phosphoketopentose epimerase. The equilibrium of the epimerase favors slightly the formation of xylulose. The isomerization of the aldose and ketose forms may be presumed to proceed via an ene-diol, as was considered for the isomerization of hexoses. Such a mechanism may also be involved in the synthesis and utilization of arabinose. A comparable mechanism may be invoked for the inversion at carbon 3. Evidence for... 

The (3S,4R) configured probes were prepared by enzymatic routes based on the stereospecific formation of a C-C bond catalyzed by either TK with Li-HPA as donor or by fructose-6-phosphate aldolase (FSA) Ref. [41]. The advantage of FSA for the synthesis of these probes is that acceptor substrates were commercially available, whereas a-hydroxylated TK acceptor substrates had to be prepared first by chemical routes [40, 41]. In particularly, the recently engineered FSA(A129S) variant that was optimized for dihydroxyacetone (DHA) as the donor substrate was found to be a powerful biocatalyst, leading to D-ketose analogs 16a and 16b with 67% and 77% yields, respectively. TK reactions furnished the same products but with lower yields only (37% and 47% respectively) (Scheme 15.17). 

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