Glycals, 2-hydroxy synthesis

Seeberger PH, Bilodeau MT, Danishefsky SJ. Synthesis of biologically important oligosaccharides and other glycoconjugates by the glycal assembly method. Aldrichim. Acta. 1997 30 75-92. Gin D. Dehydrative glycosylation with 1-hydroxy donors. J. Carb. Chem. 2002 21 33-52. 

The most common method for the preparation of pyrazoles from other heterocycles is from pyranone-type compounds. Condensation of 2,3-dihydro /7-pyran -ones 787 with various aryl hydrazines in the presence of montmorillonite KSF clay under mild conditions proceeded rapidly to afford enantiomerically pure 5-substituted pyrazoles 788 (Equation 172) <2004TL6033>. Comparable results were obtained when arylhydrazines were reacted with 2-formyl glycals under microwave irradiation <2004TL8587>. Phenylhydrazine and hydrazine were reacted with 3-acetyl-4-hydroxy-6-methyl-2/7-pyran-2-one to afford 4-acetoacetyl-3-methylpyrazolin-5-ones, which were employed in the synthesis of bipyrazoles and pyrazoloisoxazoles <1999JHC1291>. Reaction of 3,3-dialkyl-6-(trifluoromethyl)-2,3-dihy-dro -pyrones with hydrazine hydrate afforded 3-(trifluoromethyl)-5-substituted-pyrazoles <1998RCB1365>. 

In the first total synthesis of elaiophylin (azalomycin B 181), glycosidation of P-hydroxy ketone 178 and glycal 177 was examined (Scheme 26) [111]. NBS-promoted glycosidation [112] followed by debromination with -Bu3SnH-AIBN was first applied to give 179 in only a 30% yield. Then, the Wakamatsu procedure using CSA-MS 4A [113] was found to afford the desired 179 in 80% yield as the sole anomer, which led to 181 via aldol reaction of the (Z)-boron enolate of the ketone 179 with dialdehyde 180. 

C-Glycosides have been of interest as building units in natural product synthesis, and unsaturated sugars have proved to be Important precursors (see also Chapter 12). Glycals and hydroxy-glycals react with several nucleophilic carbon reagents to give 2,3-... 

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