We studied the effect of the CG-substituent on the ring-opening polymerizations of ortho esters using the novel starting ortho ester derivative, 3-O-benzyl-6-deoxy-alpha-(D)-glucopyranose 1,2,4-orthopivalate (1). The ring-opening polymerization of orthopivalate 1 by Ph3CBF4 gave the new stereoregular polysaccharide derivative, 3-O-benzyl-6-deoxy-2-O-pivaloyl-( 1 -->4)-beta-(D)-glucopyranan with [alpha](D)(25) -26.7 degrees and a number-average degree of polymerization ((DP) over bar (n)) of approximately 34. Thus, the electronic, not steric, effect of the substituent at the CG-position was the important factor for forming stereoregular (1 -->4)-beta -pyranan in the ring-opening polymerization of ortho esters.