Vanillin undergoes sequence of reaction forming phosphonium salt through dimethyaminomethyl derivative (Mannich reaction). The synthesis of phosphonium salt can be achieved by sequence of three steps which was condense with series of aliphatic/aromatic acid chlorides by refluxing in toluene in presence of triethylamine (Wittig reaction) as key step resulting 7-methoxy-2-phenyl-l-benzofuran-5-carboxaldehyde. The crude product was purified by using column chromatography and characterized by FTIR, NMR and Mass spectroscopy. Computational study of (4) such as Binding energy, density of state, HOMO, LUMO, charge density and reactivity is done by using density functional theory. Based on the results of MTT assay and MB assay, the test compound 04 is shows toxicity against cancerous lung and breast cancer cell lines.