Fragmentation methyl glycosides

SCHEME 16. A methyl glycoside analogue of a trisaccharide fragment found in E. coli. 

The total synthesis of apoptolidin was accomplished in the laboratory of K.C. Nicolaou. The key C12-C28 vinyl iodide fragment was prepared using the Schwartz hydrozirconation of an internal alkyne followed by trapping of the alkenylzirconium intermediate with iodine (I2). The vinyl iodide was formed as a 6 1 mixture of regioisomers. Under the reaction conditions, the methyl orthoester was converted to the methyl glycoside moiety at C21, which was presumably facilitated by the complexation of Zr with the pyranoside oxygen atom. 

F. Belot, C. Costachel, K. Wright, A. Phalipon, and L. A. Mulard, Synthesis of the methyl glycoside of a branched octasaccharide fragment specific for the Shigella flexneri serotype 2a O-antigen, Tetrahedron Lett., 43 (2002) 8215-8218. 

L. A. Mulard and J. Ughetto-Monfrin, Linear synthesis of the methyl glycosides of tri-, tetra-, and pentasaccharide fragments of the Shigella flexneri serotype 5a O-antigen, J. Carbohydr. Chem., 19 (2000) 503-526. 

Four other oligosaccharides (18-21, Fig. 6) corresponding to fragments of either the linear or the branched hexasaccharide moieties of the arabinan have been synthesized as methyl glycosides [55,74]. These were obtained in a maimer similar to the disaccharide and trisaccharide via glycosylation of methyl glycoside acceptors with thioglycoside donors. 

Fragments of dermatan that sulfate have been synthesized include the methyl glycoside derivatives of the basic monosulfated disaccharide, a hexasaccharide containing three disulfated residues and most recently, a basic disulfated disaccharide. 

Fig. 6. Survey of the selected fragment ions A-H worked out for the following derivatives trimethylsilylated methyl ester or pertrimethylsilylated derivatives of /V,0-acylneuraminic acids or of A/-acyl-0-alkylneuraminic acids, and trimethylsilylated/methylated /V,/V-acyl,methyl-neuraminic acid methyl ester methyl glycosides [ 15].

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. 

The mass spectra of trimethylsilylated hexuloses and their methyl glycosides have been studied by using deuterium-labelling techniques. Trimethylsilyl tetra-0-(trimethylsilyl)-n-fructopyranoside gives an intense peak at mje 437, produced by fission of the C-l-C-2 bond (see Scheme 3). This fragment can rearrange to form mie 204, which contains... 

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