SYNTHESIS OF OLIGOSACCHARIDES BY CYCLO-ADDITION .5. CYCLO-ADDITION ON CIS-DIENYL AND TRANS-DIENYL ETHERS OF SUGARS AND REGIOSELECTIVE REMOVAL OF ETHOXYCARBONYL FROM SUBSTITUTED MALONIC ESTERS

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