OH OH 酸催化： +R-C=0 R-CH-OH OH OH OH CH2OH HCHO or CH2OH CH3 OH+CH3-C-CH3 HO OH -H20 OH + R C O H H + OH R CH OH R C H O H + R C H OH + R C OH H + 酸催化： or OH HCHO OH CH2OH OH CH2OH OH + CH3 C O 2 CH3 H + C CH3 CH3 HO OH -H2O