Synthesis of oligosaccharides by cycloaddition. Part 5. Cycloaddition on cis- and trans-dienyl ethers of sugars and regioselective removal of ethoxycarbonyl from substituted malonic esters

Abstract

Both cis-, and trans- 3-O-(buta-1,3-dienyl)-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose react with diethyl mesoxalate to give near quantitative yields of 2,2-bisethoxycarbonyl-2,3-dihydro-6H-pyran-(6R,S)-yl ethers (5) and (7), with the S-isomer predominating. Removal of ethoxycarbonyl then gives disaccharide analogues, mainly the derivatives trans-disubstituted at C-1′ and C-5′ of the newly-built unit, i.e. the α-glycosides (9) and (15). Cycloaddition onto the corresponding ethers of benzyl 2-O-allyl-4,6-di-O-benzylidene-α-D-galactopyranoside proceeds in the same way, giving a preponderance of the R-isomer. The faces of the cis- or trans- dienylethers which are the more reactive towards both diethyl mesoxalate and glyoxylic esters show similar disposition of atoms close to the sugar. This new sequence, which avoids the wasting of cis-dienyl ethers, is considered a significant improvement in the synthesis of disaccharides by cycloaddition.