This paper describes a new method for the preparation of sodium 4-[5-(4-hydroxy-3-methoxyphenyl)-3-oxo-penta-1,4-dienyl]-2-methoxy-phenolate, DM-1, and 3-oxo-penta-1,4-dienyl-bis (2-methoxy-phenolate), DM-2. The aim of this work was to evaluate the antitumor effects of DM-1 in adjuvant chemotherapy for breast cancer treatment. Mice bearing mammary adenocarcinomas (Ehrlich ascites tumors) were treated with paclitaxel alone, DM-1 alone, and paclitaxel + DM-1. Tumor samples were used to perform cytological analysis by the Papanicolaou method and apoptosis analysis by annexin V and phosphorylated caspase 3. The paclitaxel + DM-1 group had decreased tumor areas and tumor volumes, and the frequency of metastasis was significantly reduced. This caused a decrease in cachexia, which is usually caused by the tumor. Furthermore, treatment with paclitaxel + DM-1 and DM-1 alone increased the occurrence of apoptosis up to 40% in tumor cells, which is 35% more than in the group treated with paclitaxel alone. This cell death was mainly caused through phosphorylated caspase 3 (11% increase in paclitaxel + DM-1 compared to the paclitaxel group), as confirmed by reduced malignancy criteria in the ascitic fluid. DM-1 emerges as a potential treatment for breast cancer and may act as an adjuvant in chemotherapy, enhancing antitumor drug activity with reduced side effects.