Talose formation

The (—)-anisomycin work is presented in Scheme 35. Its key step centered around the formation of a pyrrolidine ring that possessed all three of the asymmetric centers present in the target this was done by nucleophilic displacement of a 3-tosyloxy function in an appropriately functionalized 6-amino-6-deoxy-p-i.-talose derivative, whose 1,2-diol was later released and oxidatively cleaved with sodium periodate. Grignard coupling, O-acetylation, and catalytic hydrogenation then furnished the desired natural-product target. 

Formation of neither a furanose nor a pyranose form can occur for 2,5-anhydro-D-talose, and the mutarotation reported for an aqueous solution must be due to hydration of the aldehyde group, or other processes,6,75 or both (see p. 20). 

Formation of the identical sugars of the D-series, 6-deoxymannose (rhamnose) and 6-deoxytalose, seems to proceed by a different pathway. According to Winkler and Markowitz (13), GDP-6-deoxy-D-mannose is first converted to GDP-6-deoxy-D-lyxo-4-hexulose. This 4-keto intermediate is the direct precursor for the unspecific enzymatic reduction leading to GDP-6-deoxy-D-mannose and GDP-6-deoxy-D-talose. For a pyridine-nucleotide requiring enzyme, the transformation seems to be unusual because of its lack of stereospecificity. However, closer examination and evaluation of properties of the different 4-keto-intermediate reductases must await availability of more highly purified enzyme preparations. 

For example, it was possible to demonstrate formation of the modified polysaccharides of S. anatum in which a D-galactose residue is replaced by D-fucose, D-gluc-ose, D-talose, or 4-deoxy-D-xylo-hexose residues. 3-Deoxy-D-arabino-hexose or P-glucose residues may substitute for a P-mannose residue. 

Hodosi and Kovac observed that, when free sugars are treated with excess dibutyltin oxide in methanol for extended periods of time at temperatures above 60°C, equilibration of the configuration at C-2 occurs.73 This observation led to the efficient formation of 6-O-trityl-D-talose from 6-0-trityl-D-galactose, but also indicates the need for care in the formation of stannylene acetals from free sugars.73... 

Reeves has suggested that the spontaneous formation of 1,6-anhydro derivatives of idopyianose and altropyranose may be due to the conformational behavior of the aldoses his instability factors show that /3-Didose and /3-D-altrose will exist partly in the conformations favorable for closure of the 1,6-anhydro ring. This reasoning could lead to the further conclusions that D-talose, but probably not n-gulose, should fairly readily afford a 1,6-arihydro derivative of the pyranose form. 

Chemical Synthesis.—Purine nucleoside 5 -monophosphates enriched with or 0 on the phosphate group are conveniently prepared by treating phosphorus pentachloride in dry triethyl phosphate with one equivalent of the appropriately labelled water to give PO]- or P 0]P0Cl3, which is not isolated but mixed with adenosine or guanosine in the same solvent. Work-up of the resulting 5 -phos-phorodichloridate in similarly labelled water permits the formation of pO]-or P 0]AMP (or GMP) in fair yield with good enrichment. The 5 -monophos-phates of the 5 -C-methyl uridines derived from 6-deoxy-D-allose and 6-deoxy-L-talose have been prepared via phosphorylation of the 2, 3 -0,0-ethoxymethyl-idene derivative of the nucleosides (1) with -cyanoethyl phosphate and DCC or TPS-Cl. The same method has been used to phosphorylate iV -benzoylated 2, 3 -0,C -isopropylidene derivatives of various 5 -C-alkyladenosine species (2) and also 4 -allyladenosine (3) as part of a study in which derivatives of AMP... 

A reaction catalyzing the inversions at C-3 and C-5 is postulated to occur in the formation of GDP-L-galactose from GDP-D-man-nose also, the synthesis of 6-deoxy-D-talose from GDP-D-mannose is assumed to involve a reduction at C-6 and inversion at C-4. 

On treatment with molybdic acid, in addition to the primary C-2 epimeric products, the C-4, C-5 threo aldoses also similarly provide the complementary C-3 epimers. However, a proportion of these C-2, C-3 diastereoisomers, especially idoses and to some extent guloses and taloses, are transformed into more stable 2-ketoses, namely to sorboses and tagatoses in the case of hexoses [9] and their 6-deoxy derivatives [6] or to g/uco-glyco-2-uloses and mawwo-glyco-2-uloses in the case of higher aldoses [6,13]. The formation of these 2-ketoses from their parent aldoses is not associated with the molybdate catalysis and is obviously ascribed to the general acidity [41] of the reaction medium. 

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