Synthesis, crystal structure, spectroscopic properties and potential anti-cancerous activities of four unsaturated bis-norcantharimides

•Four unsaturated norcantharimide dimers were synthesized and well characterized.•Single crystal structure studies have been carried out for the first time about cantharidin dimers.•Introduction of various lengths of single bond chains provides high conformational flexibility.•In vitro antitumor evaluations indicate that the antitumor properties of dimers depend on the length of link chains.

Four unsaturated norcantharimide (UNCI) dimers were synthesized and characterized by elemental analysis, ESI-QTOF-MS, FT/IR, UV–Vis, 1H and 13C NMR as well as single crystal X-ray diffraction. In addition, theoretical studies have been investigated to compare with the experimental findings. Introduction of various lengths of single bond link chains provides high conformational flexibility and thus unusual molecular and crystal structures for dimers. Two of the four dimers twist into helicate, but crystallize into centrosymmetric lattice; one adopts approximately centrosymmetric conformer, but packs into non-centrosymmetric polar space group (P21). Moreover, in vitro cytotoxic activities of four UNCI dimers and their corresponding saturated NCI dimers were evaluated. All four UNCI dimers are inactive and one NCI dimer shows modest cytotoxicity. These findings were compared with the relevant results in literature. It is found that the antitumor properties of UNCI/NCI dimers depend mainly on the length of link chains (the longer chain, the higher therapeutic efficacy) and have relationship with the double bond, which requires more experimental support.