The catalytic osmylation of electron-poor allylic ethers and alcohols was studied. In the case of γ-alkoxy E-enoates reaction selectivity was found to range from 2:1 to 8:1 in favor of the arabino (2,3-syn - 3,4-anti) product, regardless of the double bond substitution. Lower (if any) selectivity was found for the Z-isomers. On the contrary, 2-Mthylene-3-hydroxy esters were osmylated with virtually
Improved <i>E</i>-Selectivity in the Wittig Reaction of Stabilized Ylides with α-Alkoxyaldehydes and Sugar Lactols
作者:Christian Harcken、Stephen F. Martin
DOI:10.1021/ol016729+
日期:2001.11.1
[GRAPHICS]The Wittig reactions of alpha -alkoxyaldehydes and sugar lactols with stabilized ylides such as (alkoxycarbonylmethylene)triphenylphosphoranes typically proceed with low E-selectivities. However, we have discovered that the reaction of such aldehydes with (methoxycarbonylmethylene)tributylphosphorane in toluene in the presence of catalytic amounts of benzoic acid proceeds to give the E-alpha,beta -unsaturated esters with high selectivities and in high yields.
The reaction of carbohydrate-derived alkoxyaldehydes with methoxycarbonylmethylenetriphenylphosphorane: stereoselective synthesis of β-unsaturated esters