Active methylene compounds, including cyanoacetic acid (I), ethyl cyanoacetate (II), malonitrile (III), dimethyl malonate (IV), diethyl malonate (V), ethyl acetoacetate (VI), acetylacetone (VII), malonamide (VIII), and nitromethane (IX), exhibited chemoselective reactions with the carbon-nitrogen double bond of 1-(3-(3- phenylallylideneamino)phenyl)ethanone. This resulted in the formation of mono addition-elimination products (Ia-IXa) while preserving the carbon-oxygen double bond, considered to be more reactive, even in the presence of two moles of active methylene compounds. The products (Ia-IXa) were characterized through elemental analysis and spectral studies. Evaluation against two phytopathogenic fungi, Helminthosporium oryzae and Colletotrichum capsici, revealed that 2-cyano-5- phenylpenta-2,4-dienoic acid (Ia), ethyl 2-cyano-5-phenylpenta-2,4-dienoate (IIa), and 2-(3-phenylallylidene)pentane-2,4-dione (VIIa) exhibited ED50 values of 22, 27, and 32 μg/ml, respectively—significantly lower than the ED50 value of Indofil M-45 (46 μg/ml) against H. oryzae.