Out-of-Ring Claisen Rearrangement are Highly Stereoselective in Pyranoses: Routes to gem-Dialkylated Sugars

Article abstract of DOI:10.1021/jo00187a010

The Claisen rearrangement has been evaluated as a means for stereoselective creation of functionalized geminal substituents at C-2 and C-3 of hexopyrano systems.C-2 and C-3 keto sugars react with Wittig reagents to give α,β-unsaturated esters, one geometric isomer being obtained in each case.Reduction of the ester and trans vinylation with ethyl vinyl ether leads to allyl vinyl ethers wich are thermolysed in refluxing benzonitrile.The oxy-Cope rearrengement proceeds with complete stereoselectivity, the folding pattern being always from the β-face of the pyranose ring.Thus, the acetaldehyde moiety ends up axially oriented at C-2 and equatorially oriented at C-3.These stereochemical results are not affected by neighboring oxygen substituents nor by the presence or absence of an anomeric alkoxyl functionality.