Abstract: Knoevenagel condensation of 3,5-dimethoxyphenylacetaldehyde with ethyl hydro-gen malonate in pyridine-piperidine led to the isolation of two products. One is not the ex-pected 4-(3, 5-dimethoxyphenyl)-2-butenoate,but is its isomeric 3-butenoate which could not be equilibrated with its 2-isomer by a base like its parent 4-phenyl-2-and-3-butenoates.The other is 2', 4', 2", 4"-tetramethoxy-2, 3: 6, 7-dibenzo-9-oxabicyclo [3.3.1]nona-2, 6-di-ene which is formed by phenol-aldehyde condensation under the basic condition rather than usual dimerization brought about by Lewis acid catalysis.

Key words: Knoevenagel condensation, 4-Phenylbutenoate rearrangement, 4-(3,5-Dimethoxyphenyl)-3-butenoate, 9-Oxabicyclo[3.3.1]nonane