Sugar derivatives, protected

The Swern conditions were applied to a number of substrates with different functional and protecting groups, such as sugar derivatives, protected amino acid derivatives, tartaric acid derivatives, and optically active synthetic intermediates. Both acid-sensitive (epoxide, acetonide, silyl, NBoc, NCBz) and alkaline-sensitive groups (Ac, Bz, ester, silyl) were found to be completely unaffected. The proposed mechanism begins with tautomerisation of the amide to the hydroxy imine, followed by reaction with the dimethylchlorosulfonium species. The resulting oxysulfonium species 63 collapses when treated with base to regenerate DMSO and produce the nitrile. 

The mono-silylated or free acetamides, which are liberated during silylation with 22 a, can, furthermore, interfere with any subsequent reaction, e.g. with electrophiles. Thus in the one-pot/one-step silylation, Friedel-Crafts catalyzed, nucleoside synthesis starting from protected sugar derivatives and pyrimidine or purine bases, the mono- or bis-silylated amides such as 22 a can compete with less reactive silylated heterocycHc bases for the intermediate electrophilic sugar cation to form protected 1-acetylamino sugars in up to 49% yield [42, 47]. On silylation with trimethylsilylated urea 23 a the Hberated free urea is nearly insoluble in most solvents, for example CH2CI2, and thus rapidly precipitated [43]. 

The Mukaiyama condensation method was also utilized in the synthesis of several antibiotics and related bioactive substances. These syntheses include apramycin, using 4-azido-2,3,6-tri-(P-benzyl-4-deoxy-) -D-glucopyranosyl fluoride (prepared from the corresponding a-D-glucopyranosyl chloride by the AgF method ) avermectin B , using protected sugar derivatives of oleandrosyl fluoride (a-L-Ole-F) and 0-(a-L-01e)-(1 4)-a-L-0le-F (both... 

Raney Nickel was also used as the catalyst for the reduction of nitro sugars to amino sugars by hydrogenation.87 Under these conditions, nitro sugar 127 was reduced to amino sugar derivative 128, without removal of the benzyl protecting group. Compound 128 was finally converted into azepane 129 (Scheme 39).88... 

In related work, Rutjes and co-workers reported a four-step protocol from conveniently protected sugar derivatives to substituted C-l glycals (Scheme 16).77 The key step in their synthetic route included RCM of highly functionalized a-alkoxyacrylates. The potential of these glycal derivatives was illustrated by their conversion to natural products such as DAH (3-deoxy-D-arabino-2-heptulosonic acid) or KDO (3-deoxy-D-manno-2-ulosonic acid). 

Protected sugar derived acids, having no a-hydroxy substituent, have also been used in die mixed Kolbe reaction [106]. The Kolbe reaction is used to graft short functionalised alkyl chains onto poly(acryIic acid) [107],... 

In addition to N-deprotonation, pyrimidinones can also be G-metallated. Lithiation at the 6-position of uracils and related uridines and thymidines can be achieved with butyllithium or LDA provided that the hydroxyl groups of the sugar are protected in the case of the nucleoside derivatives <1993AHC(56)155, 2006JOC8842>. When the... 

To catalyze a protective step in a chemo-enzymatic synthesis of sugar derivatives or of more complex oligosaccharides. 

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