Permethylated derivatives

Fig. 11.—Sequence of a 2S-Sugar Residue Glycosphingolipid Isolated from Rabbit Erythrocyte Membranes. (Cleavage points, and the masses of fragment ions of the permethylated derivative, are shown. No fragment-ions were observed above 4000, because of the poor sensitivity at high mass.)...

Fig. 13.—Predicted Sequence of the Smith-Degradation Product of the Glycosphingolipid Shown in Fig. 12. (Predicted Fragment-ions for the permethylated derivative are shown.)...

Storage products derived from complex glycoproteins have been identified in the brains of Springer spaniels suilering from a progressive nervous disorder. F.a.b.-m.s. of the native compounds and their acetylated and permethylated derivatives allowed the assignment of structures 26, 27, and 28. 

Data from f.a.b.-m.s., and also f.d.-m.s., revealed the existence of naturally occurring, large cyclic polysaccharides. The first indication that a molecule may be cyclic comes from its precise molecular-weight determination. Cyclic molecules are 18 mass units less than their linear counterparts. Loss of water may, of course, occur in a number of ways, for example, by dehydration or lactonization, and conclusive evidence for the presence of a cyclic molecule can only be obtained from f.a.b.-m.s. of suitable derivatives, such as the permethyl derivative. Cyclic and dehydrated linear polymers are distinguishable after permethylation, as the cyclic polymer will incorporate one methyl group less than the linear molecule. 

We realize that a more advanced method of calculation might place a larger weight on the role played by the orbitals of the two substituents carried by the Si atoms, and thus produce a larger difference between the parent polysilanes and their permethylated derivatives (cf. Ref. 18), but an inspection of the results suggests that this will not affect our conclusions. Another possible concern has to do with the absence of d orbitals in the INDO/S basis set In a large basis set calculation, they would undoubtedly contribute to some degree to the description of both the a and the lower excited states. They... 

Finally, the presence of the substituents on the nitrogen donors also influences the reactivity of the corresponding complexes significantly. Thus, utilization of the permethylated derivative in place of H2L23 drastically alters the Lewis-acidity of the metal ions, that is the coligands in the [Ni2(L19)(L )]+ compounds become more polarized than in the [Ni2(L23)(L )]+ complexes. In the latter complexes, the Lewis-acidity of the divalent nickel(II) ions is in large part consumed for the polarization of the six secondary NH functions. This explains why only the complexes of the permethylated macrocycle are able to activate and transform small molecules such as H20 or C02 (210,239,241). 

In order to get insight into the preferred orientations of the various radialene systems, we might consider the permethylated derivatives of the parent compounds, since... 

As far as the redox aptitude is concerned, two separate chemically reversible oxidations are evident corresponding to the Nin/Nim and Nini/NiIV electron transfers. The relative potentials are summarized in Table 12, together with those for the permethylated derivative. 

At the other extreme, Hogeveen and Kwant (59) have produced the permethyl derivative of which is isoelectronic with BgHio the... 

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. 

Berchemia zeyheri (Rhamnaceae), a tree native to southern Africa which is prized for its beautiful wood, known as pink ivory or red ivory. The complexity of the phenolic compounds present in heartwood extracts prompted their analysis as permethylated derivatives. Stereochemical features were determined by using both NMR and circular dichroism spectroscopy of the parent compounds and their degradation products. These methods were used successfully to obtain a full stereochemical description of the zeyherin epimers 374 and 375, ° which were first isolated in 1971 but not fully characterized at that time. Subsequent work has led to the discovery of further auronol dimers and novel heterodimers with flavanone or isoflavanone constituents as summarized in Table 16.15. ° ° °... 

The analysis of monosaccharide mixtures as the permethylated derivatives was proposed early in the application of gas-liquid chromatography to carbohydrates, but the method has now been superseded by more convenient procedures.230,231 There are, however, situations in which this method is useful, such as during a structural study of a polysaccharide by the methylation technique. The mixture of partially methylated monosaccharides obtained by methanolysis may then be fully methylated, and the proportions of the various monosaccharides determined. This approach has been used, for example, in studies on a galactomannan392 and on tamarind-kernel polysaccharide.393 Such an analysis also constitutes a useful check to ensure that no significant change in the composition of the polysaccharide occurred during methylation. 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

In the polysaccharide and the glycoprotein series, permethylated derivatives are almost exclusively used for their solvolytic decomposition and for the analysis of the mono- and oligo-saccharides formed, whereas, in the glycosphingolipid series, the investigation of the permethylated derivatives themselves by physicochemical methods has acquired great importance. They proved to be very convenient both for mass-spectrometric analysis and for n.m.r. spectroscopy (see later). 

Along with permethylated derivatives, the permethylated, LiAlH4-reduced derivatives of glycolipids are also used for mass-spectrometric analysis. The amide groups of acetamido sugars and the ceramide part are reduced to alkylamino groups, and, in the case of gangliosides, the ester... 

The reaction of 5,5 -bis(pentaethyldisilanyl)-2,2 -bithiophene was chosen as a model compounds of poly[(tetraethyldisilanylene)bis(2,5-thie-nylene)]. After 77 h irradiation in benzene, they obtained 5-(pentaethyldi-silanyl)-2,2 -bithiophene (23%) and triethylphenylsilane (46%). The corresponding permethyl derivative was found to be photochemically inert. The inactivity of permethyl derivative is similar to the low photosensitivity of the permethyl polymer. 

Figure 15. Field desorption mass spectra recorded at 18 ma for permethylated derivatives of (A) N-stearoyl dihydroglucocerebroside and (B) N-stearoyl dihydrolactocerebroside.

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