Colorectal cancer is a major global health challenge, with current treatments limited by toxicity and resistance. Thiazole derivatives, known for their bioactivity, are emerging as promising alternatives. Juglone (5-hydroxy-1,4-naphthoquinone) is a naturally occurring compound with known anticancer properties, and its incorporation into thiopyrano[2,3-d]thiazole scaffolds may enhance their therapeutic potential. This study examined the cytotoxicity of thiopyrano[2,3-d]thiazoles and their effects on apoptosis in colorectal cancer cells. Les-6547 and Les-6557 increased the population of ROS-positive HT-29 cancer cells approximately 10-fold compared with control cells (36.3% and 38.5% vs. 3.8%, respectively), potentially contributing to various downstream effects. Elevated ROS levels were associated with cell cycle arrest, inhibition of DNA biosynthesis, and reduced cell proliferation. A significant shift in the cell cycle distribution was observed, with an increase in S-phase (from 17.3% in the control to 34.7% to 51.3% for Les-6547 and Les-6557, respectively) and G2/M phase (from 24.3% to 39.9% and 28.8%). Additionally, Les-6547 and Les-6557 inhibited DNA biosynthesis in HT-29 cells, with IC values of 2.21 µM and 2.91 µM, respectively. Additionally, ROS generation may initiate the intrinsic apoptotic pathway. and activated both intrinsic and extrinsic apoptotic pathways, demonstrated by notable increases in the activity of caspase 3/7, 8, 9, and 10. This study provides a robust basis for investigating the detailed molecular mechanisms of action and therapeutic potential of Les-6547 and Les-6557.