Glycosylation stepwise synthesis

In addition, the fulvestrant could be glycosylated effectively at its 17-OH position with pivaloylated glycosyl trichloroacetimidates, which suppressed the competing transacylation side reaction and led to improved yields of the desired glycosides (Scheme 3.48d) [503]. In this synthesis, the inverse procedure (i.e. addition of a trichloroacetimidate donor to a mixture of an acceptor and a promoter) was found to be superior for glycosylations. Very recently, a stepwise synthesis of branched... 

The proper selection of glycosyl and aglycone syn-thons is essential for the development of a standard procedure for the stepwise synthesis of polysaccharides. Their preparation from the same oligosaccharide precursor seems the most practical. 

For the next stage of the stepwise synthesis, the preparation of the trimer of I, VIII was converted into the glycosyl bromide IX which was condensed with the aglycone synthon VII in a similar manner to give the protected nonasaccharide X (yield 57%). After removal of the protecting groups, the trimer of the repeating unit I, compound XI was obtained. 

A crucial step in the chemical synthesis of glycopeptides is the incorporation of the saccharide into the peptide. Two approaches can be considered to accomplish this the direct glycosylation of a properly protected full-length peptide and the use of a preformed glycosylated amino acid building block for the stepwise synthesis of the peptide backbone (Scheme 1). 

A one-pot technique, combining two or more glycosylation steps based on activation of one donor over another, have also been developed [182,189-191]. This one-pot technique is virtually a variation of a simple stepwise selective activation strategy, which allows further improvement in the efficiency of the synthesis by avoiding the necessity for isolation (and purification) of the intermediates. 

The base-catalyzed condensation of azides with activated methylene compounds is a well-established route to 1/7-triazoles. This is a regiospecific method ideally suited for the synthesis of triazoles with a 5-amino or -hydroxy substituent, and an aryl or carbonyl function at C-4. The general scheme is shown in Scheme 8. The stepwise character of the reaction is best revealed by the anomerism of glycosyl azides during their reaction with... 

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