Thermal glycosidation

Nishizawa, M, Kan, Y, Shimomoto, W, Yamada, H, a-Selective thermal glycosidation of rhamnosyl and mannosyl chlorides, Tetrahedron Lett., 31, 2431-2434, 1990. 

The depurination of DNA, which happens spontaneously owing to the thermal lability of the purine N-glycosidic bond, occurs at a rate of 5000-10,000/cell/d at 37 °C. Specific enzymes recognize a depurinated site and replace the appropriate purine directly, without interruption of the phosphodiester backbone. 

Many nonvolatile and thermally labile allelochemicals can be well separated by liquid chromatography (LC). Identification of the separated components on-line by mass spectrometry (MS) is of great value. Fused-silica LC columns of 0.22 mm ID packed with small-particle material are used in the described LC/MS system. The shape of the column end allows direct connection to a electron impact ion source of a magnetic sector mass spectrometer. Separations by LC are reported and LC/MS mass spectra are shown for monoterpenes, diterpene acids, phenolic acids and cardiac glycosides. The LC/MS system provides identification capability and high-efficiency chromatography with a universal detector. 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

N-alkylations and rearrangements are important for the synthesis of glycosides. N-3 and N-4 glycoside, e.g., of 5-oxo-TPs, are obtained under different conditions as the kinetically and thermodynamically controlled product, respectively [75BSF(2)2561] therefore a thermal rearrangement from an N-3 to an N-4 glycoside is possible. Table X lists these glycosides included are those derived from thioxo-TPs and mercaptomethyl-TPs. 

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