Non-peptidic cell-penetrating agents: Synthesis of oligomeric chiral bicyclic guanidinium vectors

Polycationic oligo(chiral bicyclic guanidines) constitute useful non-peptidic penetrating agents for cell uptake and protein surface recognition. We report herein improved and selective procedures for the preparation of oligoguanidinium scaffolds linked through thioether bonds, with similar or different groups and functions at both ends of the chain. Two synthetic strategies were developed to obtain these compounds in relatively good yields from a common thioacetate precursor: generation of a disulfide intermediate or thiolate formation. Thus, tetraguanidinium intermediates 8 and 22 are best synthesized by the disulfide route, whereas hexamer 29, octamer 31, and trimer 37 arise from a combination of both the disulfide and the thioacetate routes. Finally, tetramer 28 can be readily obtained from either strategy.