Fragmentation monosaccharide acetals

As revealed by the data available, the type of compound closest to the ideal for structural analysis of monosaccharides is the class of dialkyl dithioacetals or their acetates their mass spectra contain a considerable peak due to molecular ion, and their fragmentation patterns are simple enough (due to the absence of a sugar ring) and specific enough to permit determination of the position of substituents on the basis of the position of peaks. Thus, elimination is characteristic of the C-2-substituents, whereas substituents at C-3 tend to be retained, producing a peculiar difference between the mass spectra. However, the mass spectra of dialkyl dithioacetals provide almost no information regarding the stereochemistry of the monosaccharide molecule. 

The use of mass spectrometry in the structural analysis of carbohydrates, first reported in 1958 (114), was developed in detail by Kochetkov and Chizhov (115). They showed that, under electron impact, the acetylated and methyl ether derivatives of monosaccharides provided a wealth of structural information through analysis of typical fragmentation pathways of the initial molecular ion. This has proved of enormous utility in the structural elucidation of polysaccharides and complex oligosaccharides sequential permethylation, hydrolysis, reduction to the alditol, and acetylation, affords mixtures of peracetylated, partially methylated alditol acetates that can be separated and analyzed by use of a gas chromatograph coupled directly to a mass spectrometer (25). The mass spectra of stereoisomers are normally identical, while the gas chromatographic retention times readily permit differentiation of stereoisomers. 

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