Multidrug resistance (MDR) to antitumor agents represents a major obstacle to a successful chemotherapy of cancer. Overexpression of P-glycoprotein (p-gp) seems to be the major factor responsible for MDR. A large number of chemically unrelated compounds are known to interact with p-gp resulting in a decreasing resistance. In our efforts related to structure-activity studies of new potential MDR reversal agents we synthesized a series of compounds that differ in the aromatic core structure, the linker, and the basic moiety. For our search of new aromatic core structures we synthesized novel 2,2-diphenyl-1,3-dioxolane, 2,2- diphenyl-1,3-dioxane, and 4,5-diphenyl-1,3-dioxolane derivatives. A range of lipophilic linker structures and protonable basic moieties were synthesized and investigated to optimize the structure of the potential MDR-modulators. The compounds were tested in vitro using human Caco-2 cells. Both the cytotoxicity of the synthons and their ability to resensitize the cells were determined with a MTT assay. The results show that at low concentration various substances reverse tumor cell MDR. Some of the new structures show better effects than established modulators like trifluoperazine.