Of permethylated glycosides

El and Cl are normally unsuitable for most flavonoid glycosides because of their polarity and thermolability. Preparation of permethylated or trimethylsilylated derivatives may overcome this problem. However, derivatization often produces mixtures of partially derivatized compounds and may involve rearrangements,and usually only weak molecular ion signals is observed when permethylated compounds are studied under El conditions. 

The development of mass spectrometry (MS) procedures for the study of C-glycosylflavo-noids is particularly significant because of the resistance of the C-glycosidic bond to hydrolysis. It is important, from the structural point of view, to determine the molecular mass and to localize the sugar substituents on the aglycone moiety. This has been achieved in the past by electron impact EI-MS of permethylated derivatives. 

The peak at m/e 203 (Bi) is more intense than that at m/e 214 (As). Hence, the regularity observed for permethylated glycosides of simple hexoses (see Section III,2b,ii) does not hold for amino sugars. 

Detailed investigation of the mass spectra of permethylated methyl glycosides of the pentose,1617 27 hexose,13-16 6-deoxyhexose,16 and 2-amino-2-deoxyhexose30 series, using compounds specifically labeled with trideuterio-methyl groupings in all of the positions, served as a basis for elucidating... 

This information together with that from the MS/MS fragmentation spectmm of the whole permethylated oligosaccharide (see Figure 8.48), allows the complete structure of LNF-II to be deducted, except for the anomeric configuration of the glycosidic bonds. 

Methylated alditol units in oligosaccharides are cleaved in a manner similar to that described previously for partially methylated alditol acetates. On the other hand, substituted cyclic carbohydrates undergo a variety of types of fragmentation, the most important of which are summarized in Fig. 4 for simple permethylated glycosides. 

Also related to the glycosidic cleavage, the ions differing by 2 daltons which appear at the expected incremental intervals of 204 daltons, which corresponds to the incremental mass of a permethylated glycosidic unit. These ion pairs are of considerable importance for sequence interpretation and their origin appears clear. However, the exact structures of these ions have not been clarified. 

The mass spectra of permethylated methyl glycosides show ions derived from a number of specific degradation pathways. These fragmentations have been carefully studied by using deuterium-labelling techniques. The nomenclature employed by Kochetkov and Chizhov will be used in the following discussion. The most important of these series of pathways are shown schematically in Scheme 1. 

Peracetylated glycosides are fragmented in a manner analogous to permethylated glycosides. A particular feature is the appearance of the triacetoxonium (2) and diacetoxonium (3) ions. The degradative path-... 

The mass-spectrometric behavior of permethylated disaccharides having (l- 2)-, (l->4)-, and (l->6)-linked hexose residues has been investigated by Kochetkov and coworkers. It was found that the fragmentation of both moieties of the disaccharide follows principles similar to those for permethylated glycosides. The nomenclature used by Chizhov and Kochetkov for the different fragmentation series of permethylated... 

Much of our knowledge on the fragmentation patterns of carbohydrate derivatives arises from studies on methylated monosaccharides, but in structural investigations, such compounds are seldom subjected to mass spectrometry. On the other hand, the molecular weight of a permethylated oligosaccharide is significantly less than that of the per(trimethylsilyl) derivative. For this reason, the mass spectra of disaccharides as -their permethylated alditols have been determined by Chizhov and coworkers, Karkkainen, and Krone and Beckey. Trisaccharides have also been studied by Karkkainen, either as permethylated glycosides or as alditols. ... 

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