Higher glycosamino acid precursors: C7 and C8 aminodialdoses via regio- and stereoselective [3+2] cycloaddition of vinyl trimethylsilane to C-glycosyl nitrones
摘要:
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-(alpha-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. (C) 2002 Published by Elsevier Science Ltd.
Higher glycosamino acid precursors: C7 and C8 aminodialdoses via regio- and stereoselective [3+2] cycloaddition of vinyl trimethylsilane to C-glycosyl nitrones
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-(alpha-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. (C) 2002 Published by Elsevier Science Ltd.
Regio- and stereoselective conjugate addition of nitrogen nucleophiles to 2-alkenyl n-methylthiazolium iodides. synthesis of d-3-epi-daunosamine and some lincosamine analogues
on technology of aldehydes is described. Key steps involve the Michael-type addition of a nitrogen nucleophile (benzylamine, trimethylsilylazide, potassium phtalimide) to a 2-alkenyl N-methylthiazolium salt obtained by olefination of the aldehyde with the phosphorane (1), and the thiazole-to-formyl deblocking. The addition of benzylaminine to the thiazolium salt derived from the acetonide of D- glyceraldehyde