A series of methyl 2-[3-(3-phenyl-quinoxalin-2-ylsulfanyl)propanamido]alkanoates and their corresponding hydrazides and N-alkyl 3-((3-phenylquinoxalin-2-yl)sulfanyl)propanamides were prepared on the basis of the chemoselective Michael reaction of acrylic acid with the parent substrate 3-phenylquinoxaline-2(1H)-thione. The parent thione was produced by a convenient novel thiation method from the corresponding 3-phenylquinoxalin-2(1H)-one. The chemical structures of the newly synthesized compounds were confirmed by elemental analyses, 1H and 13C NMR. The antiproliferative activity of the synthesized compounds was tested against human HCT-116 and MCF-7 cell lines. Out of 25 screened derivatives, 10 active compounds exhibited IC50's in the range 1.9-7.52 μg/mL on the HCT-116, and 17 active compounds exhibited IC50's in the range 2.3-6.62 μg/mL on the MCF-7 cell lines compared to the reference drug doxorubicin (IC50 3.23 μg/mL). The structure-activity relationship of the tested compounds was studied through their binding affinity to the human thymidylate synthase allosteric site in silico using molecular docking and proved the quinoxaline ring as a suitable scaffold carrying a peptidomimetic side chain in position 3.