Herein, we established an environmentally benign green approach for the synthesis of new spirochromeno[2,3-d]pyrimidines. PEG-4000-OSO3H catalysed microwave-assisted multicomponent reaction of sesamol, isatin and barbituric acid enables us to produce spirochromeno[2,3-d]pyrimidines. This approach is notable for its eco-friendly polymer catalyst, rapid metal-free synthesis, easy workup, higher yield (88–97%), no need for column chromatographic purification and outstanding green credential parameters of the process. The use of harmless solvents, microwave process and reusability of reaction media made the present protocol sustainably effective from a green perspective. The molecular structure of 4j was confirmed by Single crystal XRD analysis and their DFT study was carried out. To gain insight into the molecule's geometry and physicochemical properties, quantum chemical density functional theory was employed at B3LYP/6–31G(d, p) basis set. Furthermore, all compounds were examined for antiproliferative activity against six solid human tumour cell lines. Compounds 4j and 4k exhibit excellent potency against T-47D (breast) and WiDr (colon) cell lines as compared to the standard drug cisplatin (CDDP). The most potent compound is 4j, which exhibits GI50 values in the range 5.2–11 µM against all cell lines.