Furan 呋喃糖 吡喃糖yan HO- CHz \OH HO Cac OH OH HAC 8: Q OH OH ar-D-Ribofuranose a-D-Ribopyranose OH OH OH HO一cH2 D-Ribose H H HO CnC OHOH OH OH B-D-Ribofuranose B-D-Ribopyranose呋喃糖 吡喃糖