Higher glycosamino acid precursors: C 7 and C 8 aminodialdoses via regio- and stereoselective [3+2] cycloaddition of vinyl trimethylsilane to C-glycosyl nitrones

Abstract

Protected C 7 and C 8 aminodialdoses were prepared stereoselectively from readily available C 5 and C 6 monosaccharide N-benzyl nitrones, by regio- and diastereoselective 1,3-dipolar cycloaddition reactions with vinyl trimethylsilane, followed by acetyl chloride-mediated cleavage of the 5-(trimethylsilyl)isoxazolidine formed. The cycloaddition reaction took place in moderate to good global yields (67–74%); estimation of diastereoselectivities from isolated yields showed total endo preference for the reaction of the d- galacto configured nitrone and high endo preference for the d- ribo analogue, but exo preference for the d- xylo configured substrate. Attack on the re face of the nitrone was predominant in all cases. The absolute configuration of one of the protected 3-(α- d- galacto-pentopyranos-5-yl)isoxazolidine products was assigned by X-ray crystallographic analysis, allowing correlation of the configuration at the new stereogenic centre in the corresponding aminodialdose. For non-crystalline isoxazolidines, configurations were assigned on the basis of NOESY experiments and/or chemical correlation. Combined yields of aminodialdoses coming from isoxazolidines having identical configuration at C(3) sometimes reached high values (up to 90%). These compounds are precursors of higher-chain glycosamino acids.