Beta-elimination in uronic acids: evidence for an ElcB mechanism

Abstract

Methyl esters of methyl pyranosides of 2,3,4-tri- O-methyl-α- D-mannuronic acid ( 3), -α- D-glucuronic acid ( 2), and -β- D-galacturonic acid ( 1) undergo rapid beta-elimination of the 4-methoxyl group in 0.50, 0.20, and 0.02 M sodium methoxide in methanol at room temperature to give 4,5-unsaturated esters via cis, cis and trans elimination, respectively. The ease of cis elimination in methyl (methyl 2,3,4-tri- O-methyl-α- D-glucopyranosid)uronate ( 2) and methyl (methyl 2,3,4-tri- O-methyl-α- D-mannopyranosid)uronate ( 3) to form the 4,5-unsaturated glycoside is explained by ring flexibility which allows a change in conformation. Only the methyl (methyl 2,3,4-tri- O-methyl-α- D-mannopyranosid)uronate ( 3) yields a 2,3:4,5-di-unsaturated ester ( 7) ( trans elimination), and this occurs in 0.50 M methoxide, in a much slower reaction. The favoring of trans elimination over the cis elimination that would have been needed in the other two compounds in order to form the second double bond is explained by the rigidity of the half-chair conformation of the 4,5-unsaturated esters. It is suggested that both eliminations proceed via an ElcB mechanism. The results of treatment of hydroxypropyl alginate with potassium hydroxide confirmed that its depolymerization is much slower, and more dependent on the concentration of alkali, than that of pectin.