ESR1 and cancer: A novel series of neo-tanshinlactone–chalcone hybrids was synthesized via Horner–Wadsworth–Emmons (HWE) olefination from 4-formyl-2H-benzo[h]chromen-2-one and the corresponding phosphonate, then evaluated by MTT across four human cancer cell lines—MCF-7 (ER-positive), MDA-MB-231 (ER-negative), HeLa (cervical), and Ishikawa (endometrial)—with tamoxifen as the reference.