Carbohydrates, acetals, chromatography methylation

B. A. Dmitriev, L. V. Backinowsky, O. S. Chizhov, B. M. Zolotarev, and N. K. Kochetkov, Gas-liquid chromatography and mass spectrometry of aldononitrile acetates and partially methylated aldononitrile acetates, Carbohydr. Res., 19 (1971) 432 135. 

Gas — liquid chromatography has proved a useful analytical technique in carbohydrate chemistry, particularly for methyl ethers and acetic esters. By forming trimethylsilyl ethers of relatively nonvolatile sulfonates of sugars, gas — liquid chromatography can be performed. 

Other reagents suitable for paper chromatography are also applicable to TLC (Nos. 10,21,134,159,178,234, 255) and some other reagents containing periodic acid. Of these, iodine vapor [18] has the most universal application since it detects a variety of carbohydrate derivatives including the free sugars, partially and fully substituted methyl and benzyl ethers, esters and acetals. Although iodine is less sensitive than suKuric acid, it is non-destructive in the short exposure time required (5—20 min) and can therefore be used to detect components on preparative plates or for quantitative analysis. The adsorbed iodine disappears when the plate is exposed to the air. 

Suitable conditions for chromatography of these derivatives have already been discussed in the sections dealing with acetates, osazones and methyl ethers (section IV, 8—10) and section V contains further examples. A few applications of TLC to the analysis of still other carbohydrate derivatives have been selected here to illustrate the scope and utility of this technique. 

GC/MS has also been widely used to characterize the carbohydrate profile of lignocellulosic materials (5). However, this technique requires that sugar analytes be relatively volatile so manpower-intensive chemical derivatization is needed prior to analysis. Typical volatile derivatives include alditol acetates, silyl derivatives and methylated sugars (5). Compared to all of the chromatography-based techniques discussed above, the NMR method is an attractive alternative because it provides excellent resolution of sugars, including minor components, does not require sample derivatization or neutralization, and is very fast (instrument time 15-30 minutes). 

Benzylidene acetals have found broad application in synthetic carbohydrate chemistry. A new chiral center (PhC H<) is formed during their formation, wherein the bulky phenyl substituent adopts predominantly the thermodynamically more stable equatorial orientation. Our laboratory has previously developed a simple protocol for the preparation of methyl 4,6-0-benzylidene-a-D-glucopyranoside (2) from commercially available methyl a-o-glucopyranoside (1). The purification of the product was achieved by simple precipitation without the need for chromatographic separation. Herein, we describe the application of this procedure to the synthesis of benzylidene acetals of ethyl thioglycosides 3 and 5. All product purification can be achieved by crystallization/precipitation only. Additional quantities and hence higher yields can be achieved by subsequent chromatography of the mother liquor. 

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