Polyols acetyl deriv

Methyloxime-acetyl derivatives of sugar alcohols that avoid interference due to monosaccharides and resolve isomeric hexitols have been described. The procedure is as follows 0.5 ml of methoxylamine hydrochloride in pyridine (10 mg ml ) is added to dried samples containing polyols, and then incubated at 70°C for 30 min. The specimens are acetylated by adding 0.5 ml of acetic anhydride for a further 10 min at 70°C. Two drops of methanol are added and solvents evaporated in a water bath at 30°C vm-der a stream of air. The residues are desiccated for at least 1 h and then dissolved in 50 pi of methanol to be chromatographed. The column was filled with 3% XE-60 (80-100 mesh) and the chromatograph operated at a column temperature of 230°C with nitrogen as carrier gas at a flow of 50 ml min Figure 3... 

Analogous derivative 338, lacking the TBDMS protection in the polyol side chain, when treated with DMAD in acetonitrile provides a mixture of unstable products, which after acetylation yields products 339-341. On the other hand, the same compound treated with DMAD in the presence of PTSA in boiling benzene gives a mixture of isomeric products 342 and 343 in a 4 1 ratio (Scheme 50) <2002T1199>. 

Dideoxyhex-2-enono-1,5-lactone derivatives (penten-5-olides) have been prepared (255-258) and employed as starting compounds in synthesis. Thus, Michael addition of benzylhydroxylamine to racemic 6-0-acetyl-2,3,4-trideoxy-D,L-g/ycerohex-2-enono-1,5-lactone (267) took place ster-eoselectively to give the unstable benzyloxyamino-2-pyrone 268, which was readily converted into the /Mactam derivative 269, a precursor of thienamy-cin (259). / -Lactams were also obtained (260) by 1,3-dipolar cycloaddition of nitrone 270 to the unsaturated 1,5-lactone 267, followed by hydrogenoly-sis and subsequent cyclization to the /Mactam 271, having a polyol side-chain at the C-3 position. 

It so happens that the great majority of compounds separated as acetates are alditols or other polyhydric compounds, and this Section is therefore concerned with the problems of reduction and acetylation. There is no a priori reason why alditols and other polyols should not be separated as their trimethylsilyl ethers, and such methods are known (see Section VII,l,p. 57 Section XIII, p. 90 Table Va, p. 119 and Table Xlla, p. 151), but experience shows that the resolution of acyclic O-trimethylsilyl derivatives is less satisfactory than that of cyclic compounds. 

The anomeric forms derived from equilibration of aldoses give rise to multiple peaks when trimethylsilylated and gas chromatographed [311]. A method of overcoming this problem, assuming that mutarotation itself is not under study, is to modify the aldose. It can be oxidised and lactonised to the aldonolactone, for example, and characterised as its TMS derivative [322]. Alternatively for the identification of aldoses and alditols, more use may be made in the future of the separations achievable on open tubular columns of the poly-0-acetylaldonic nitriles (18) produced from aldoses and the poly-acetyl esters from alditols [323]. Figure 1.18 shows the separation of 32 assorted polyols and aldoses. 

In the procedure of Laine and Sweeley [405], methoximation and trimethylsilyla-tion are combined to avoid variation in the ratios of a- and j8-anomers of the pyranose and furanose forms. This approach was used to handle successfully 33 sugar derivatives, including aldoses, ketoses, polyols, acidic forms and A -acetylated amino sugars, on a fused silica capillary column [406]. Obviously, the problems of multiple peak formation can be overcome through the use of highly efficient capillary columns. The application of capillary columns in carbohydrate analysis, used probably for the first time by Tesarik [407] in 1972, has now become widespread. 

Acetate derivatives of polyols show certain advantages for GC compared with TMS ethers they are more stable, unaffected by water and other solvents containing free hydroxyl groups, and are freely soluble in the pyridine used for their preparation. Acetate derivatives are generally prepared by the action of acetic anhydride in the presence of a suitable catalyst. Reaction is carried out for 1-4 h at 70°C. Eree hydroxyl groups are converted to the acetyl esters, so that substitution occurs on all carbon atoms of the polyols, and all but one of the aldoses. In the most widely used method, the sample is dried and treated with equal volumes of acetic anhydride and pyridine, which can be substituted with sulfuric acid and sodium acetate. 

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