Syntheses of Glycals

The D-allal derivative 1 was formed by treating the unsaturated 1-thiophenyl glycoside 2 first with MCPBA then with diethylamine to effect a [2,3] sigmatropic rearrangement of the intermediate sulfoxide. Product 1 was incorporated into the core trisaccharide unit of esparamicin and the aryl tetrasaccharide unit of calicheamicin.  

6-Tri-rerf-butyldiphenylsilyl-2-deoxy-D-flrafeino-hexonolactone, on sequential reaction with bromomagnesium(trimethylsilylacetylide), phosphorus oxychloride-pyridine then sodium hydroxide, gave glycal derivative 3. The acetylene moiety was further elaborated into an anthracyclinone system representing the core unit found in vineomycinone B.  

A convenient synthesis of L-2-sorbar (2,6-anhydro-3-deoxy-L-rArco-hex-2-enitol) starting from 2,5-anhydro-D-galactitol has been reported. Thus treatment of the latter with 2,2-dimethoxypropane (to produce a mixture of di- and triacetals) then potassium /err-butoxide in DMSO (to effect elimination of acetone) gave glycal 4. Its reaction with methanolic acetic acid yielded L-2-sorbal . In 

Alkyl 4-0-(2-bromoallyl)-2,3-dideoxy-a-D-er7tAro-hex-2-enosyl derivatives 8 (R = OEt, 0 Bu, OC6H4 Bu) imdergo a palladium-mediated Heck-type reaction to give mainly the bicyclic glycal 9 when the base used is triethylamine. The glycal 10 is formed when 8 (R = H) is treated with the base sodium carbonate.  

The reaction of 2,3-0-isopropylidene-5-0-trityl-p-D-ribofuranosyl chloride with the lithioimidazole reagent 11, then TBAF, affords access to glycal 12. Treatment of 2,3,4,6-tetra-O-acetyl-D-glucopyranosylidene 1,1-diazide with triphenylphosphine unexpectedly gave the unusual glycal derivative 13.  

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An easy and efficient one pot synthesis of peracetylated glycals from unprotected sugars has been described. In this method the sugar was sequentially treated with acetic anhydride and catalytic hydrobromic acid-acetic acid (to effect per-acetylation), more hydrobromic acid-acetic acid (to form the glycosyl bromide), sodium acetate (to neutralize excess hydrobromic acid) and finally a buffered mixture of sodium acetate, acetic acid, zinc-copper sulfate and water (to effect a reductive-elimination). The method was applied to the syntheses of glycals derived from D-glucose, D-galactose, L-rhamnose, L-arabinose, maltose, lactose and maltotriose. ... 

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