Hydroxyl permethylation

Methylation analysis Permethylation of intact oligosaccharides Hydrolysis of glycosidic linkages Reduction to monosaccharide alditols Acetylation of free hydroxyl groups previously involved in link-... 

Permethylation of CD hydroxyl groups markedly changes the chromatographic selectivity in comparison to native CDs. Various CD derivatives are described. The various moieties used to functionalize the CDs greatly affect the enantioselectivity. Since the pioneering work of Armstrong, the topic of separations with cyclodextrins or cyclodextrin derivatives has been well documented (44). 

Urethane-forming reaction between isocyanate and hydroxyl group was utilized by Osakada et al. to prepare polyurethane-cyclodextrin pseudoro-taxanes (Scheme 15) [86]. Polyaddition of a diol and MDI in the presence of permethylated a-cyclodextrin or permethylated (5-cyclo dextrin was carried out in DMF for 20 h at 120 °C to yield the pseudopolyrotaxane. The molar... 

Permethylation. The process of derivatizing all hydroxyl groups (—OH) in an organic molecule to methoxyl groups (—OCH3). 

In another study [243] an alternative to -acylation was presented initial permethylation of the amino group using formaldehyde and the imino group-specific reducing agent, sodium cyanoborohydride (NaBH3CN) (cf. Ref. 239). Subsequent acetylation proved necessary, nonetheless, to derivatize any hydroxyl and sulfhydryl groups present in the amino acid. 

Permethylation of hydroxyl groups is used widely in the analysis of sugar derivatives and usually involves reaction with a strong base, such as sodium hydride, followed by treatment with methyl iodide. Alternative bases can be used and the efficiency of the derivatization varies from compound to compound. Permethylated derivatives are stable and can be purified without breakdown. However, except for experienced investigators, analysis of permethylated plant hormones has received relatively little attention, primarily beeause derivatization is time consuming and complex. Nonetheless, once effective derivatization is achieved, permethylated GA glycosyl ether can be analysed by GC-MS [74] while cytokinins are best analysed as their permethyl derivatives [6]. 

The dimsyl base was seen to be highly effective for the permethylation of P-CD. A moleeular [M+Na] adduct ion was observed at m/z 1451, which corresponds to the molecular mass of P-CD with all of the 21 free hydroxyls replaced by methyl groups (Fig. 2). Except for a single [M-14+Na] at m/z 1437, there was little evidence for partially methylated p-CDs. Moreover, the folly methylated p-CD was also observed as a single band by the TLC analysis (Fig. 2). By contrast, M/LLDI-MS analysis of the NaOH-catalyzed reaction showed a series of ions incrementally 14 mass units less than the folly methylated m/z 1451 molecular ion. This ion series is evidence of a mixture of under-methylated P-CDs. Hence, under-methylated ions are observed at m/z 1437, 1423, 1409, etc. (Fig. 2) where the observed 14 mass unit differentials are due to the mass difference between -H and a -CH3. Furthermore, under-methylated P-CDs formed under the NaOH-catalyzed conditions are also apparent as a less motile ladder when analyzed by TLC (Fig. 2). 

A most useful application of methylation is for the structural analysis of polysaccharides. Permethylation, followed by hydrolysis of the product and identification of the individual methylated monosaccharides, indicates the original positions of the linkage points, as these are denoted by hydroxyl groups in the methylated monosaccharides. Considerable work has been done on the separation and identification of methylated monosaccharides by gas-liquid chromatography and mass spectrometry this topic is treated in detail in this Volume by Lbnngren and Svensson (see p. 41), Clearly, for such structural analyses to be valid, complete methyl-... 

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