Anionic ring-opening polymerization of five-membered cyclic carbonates derived from aldohexopyranosides

To investigate the relationship between the steric structures and polymerizability of five-membered cyclic carbonates, the anionic ring-opening polymerizations of methyl 4,6-O-benzylidene-2,3-O-carbonyl-alpha-d-galacto- and mannopyranosides (MBCGa and MBGM, respectively), were examined and compared to the results of the already reported glucopyranoside analog (MBCG). The polymerization of MBCGa proceeded without the elimination of CO2 as in the case of MBCG, while MBCM was not polymerized. To estimate the thermodynamic stability of the carbonate rings of these cyclic carbonates, the model reactions using the corresponding hydroxycarbonates were carried out. The ring-closing reactions of the mannopyranoside-based hydroxycarbonate proceeded to produce cyclic carbonate, while the others did not give cyclic carbonates. This should indicate that the carbonate rings of MBCG and MBCGa were less stable than that of MBCM. The single-crystal X-ray structural analysis indicated that the carbonate rings of MBCG and MBCGa have higher angle strains than that of MBCM. This should suggest that the ring strain of the five-membered cyclic carbonate increases by trans-fusing to the pyranose ring.