Novel R3M (M = Si, Ge) substituted furan and thiophene-derived aldimines: Synthesis, electrochemistry, and biological activity.

New furan and thiophene derivatives of aldimines o -HO-C 6 H 4 N CHC 4 H 4 X(R) (X = O, S; R = H, SiMe 3 , SiEt 3 , GeMe 3 , GeEt 3) were synthesized by condensation of o -aminophenol with the substituted aldehyde precursor. Their structure, electrochemical reduction/oxidation (in CH 3 CN/0.1 M Bu 4 NPF 6), frontier orbital energies, and cytotoxicity have been studied. Their electrochemical redox potentials E p show good correlation with the corresponding orbital energies and the difference E p ox – E p red corresponds well to their orbital hardness. These new compounds have a pronounced cytotoxicity toward cancer cells of human fibrosarcoma HT-1080 and mouse hepatoma MG-22A (IC 50 ≅ 1–8 μg ml−1) that can be modulated by introducing a Me 3 M substituent into the fifth position of the heterocycle (e.g., IC 50 (Me 3 Si)/IC 50 (H) ≥ 50). R 3 M-substitution reduces the orbital hardness of the aldimines studied and facilitates oxidation, promoting their oxidative metabolism. The neighboring group effect in the α-Me 3 Si-substituted thiophene derivative favors S-oxidation, which supposedly makes its metabolic mechanism different compared to R 3 M-substituted furan series (or for M = Ge in the thiophene series). Interestingly, SiMe 3 and GeMe 3 groups in both heterocyclic series (furan and thiophene) cause opposite trends in cytotoxicity, while the silyl group increases it, the germyl group decreases it. [ABSTRACT FROM AUTHOR]