Synthesis,Spatial Structure And EPR Properties Of Trityl Radicals With Chiral Substituents On Side Chain

Objective:（1）A molecule consisting of three aromatic rings connected with a central atom is called a triaryl compound.Because of the existence of three aromatic ring substituents,this kind of molecule has a larger space volume and a closer distance between adjacent atoms,so this kind of compound is not planar structure,but adopts a propeller-like conformation,also known as a"molecular propeller".Triphenylmethyl radicals belong to triaryl compounds.As a paramagnetic probe,trityl radicals are widely used in electron paramagnetic resonance spectroscopy and imaging（EPRI）,blood oxygen determination,dynamic nuclear polarization（DNP）and superoxide anion radical detection.Nevertheless,few studies have been reported on the conformation and configuration of trityl radicals.Like all aromatic compounds,Trityl radicals exhibit propeller conformations,in which three aromatic rings are distorted in the same direction relative to the reference plane,resulting in two conformations,i.e.right-handed helix（P）and left-handed helix（M）,which are enantiomeric.Because of the large volume of triaryl group,the rotation of C-C bond between central carbon and benzene ring is blocked,which makes the transformation between the two enantiomers slow,thus facilitating the enantiomer separation.In addition,studies have shown that the conjugation of chiral substituents with trityl radicals produces non-enantiomeric mixtures with different chemical,biological and physical properties（e.g.different EPR spectra）.Therefore,it is important to study the effects of conformation and configuration on the properties of trityl radicals for their applications.Based on the above research,we designed and synthesized trityl radicals with stable conformation and chiral substitution of side chains.By introducing chiral centers into side chains,we investigated the effects of conformation and configuration on EPR properties of trityl radicals,and provided theoretical and experimental references for trityl radicals to select new functional group modification sites.（2）The detection of reactive oxygen species（ROS）is one of the most important research areas in the scientific research field.There are many methods to detect reactive oxygen species（ROS）.Electron paramagnetic resonance（EPR）is called the"gold standard"for ROS detection because of its specificity,low detection limit and high reliability.The commonly used EPR probes include spin trapping agents,cyclohexanolamine probes and trityl radicals.Spin trapping agents are the most commonly used EPR probes because of their small molecular size,high water solubility and fingerprint detection.However,spin trapping agents also have some shortcomings,such as poor stability with superoxide spin adducts,which limits their application.In this study,a new type of nitrone spin trapping agent was designed and synthesized.This new type of spin trapping agent is expected to improve the short half-life of adduct instability existing in previous spin trapping agents,and thus improve the application of spin trapping agents.Methods:（1）Side chain chiral substituted trityl radicals were synthesized from1,2,4,5-tetra（tert-butyl thio）benzene and ethyl pyruvate in six steps.The absolute configurations of the compounds were determined by 1D NMR,2D NMR and X-single crystal diffraction.The conformational stability of the compounds was evaluated by dynamic high performance liquid chromatography and the thermodynamic parameters involved in enantiomer transformation（ΔH^≠、ΔS^≠、（35）G^≠）were quantitatively evaluated.The free radicals were separated by CSP-HPLC and the circular dichroism of the free radicals with different conformations and configurations was studied.The effect on the properties of EPR.（2）The synthesis of spin trapping agent BMPO and new nitrone spin trapping agent compound 17 was carried out by different synthetic routes from those reported in the literature.The target compound was synthesized from L-proline tert-butyl ester by four steps of reduction and oxidation.Results:（1）To synthesize stable trityl radicals with four chiral pure enantiomers and determine their absolute configurations:7a₁-1（P,S,S）,7a₁-2（M,R,R）;7a₂-1（P,R,R）,7a₂-2（M,S,S）;7b₁-1（P,S,R）,7b₁-2（M,S,R）;7b₂-1（P,R,S）,7b₂-2（M,R,S）;to evaluate the conformational stability of the compounds.It is estimated that side-chain substituted Triphenylmethanol and free radicals exhibit higher stability than non-side-chain substituted Triphenylmethanol and free radicals;The effects of circular dichroism and conformational configurations of free radicals with different conformations and configurations on EPR properties are studied.These new side-chain substituted trityl radicals exhibit almost the same EPR properties,indicating that the role of chiral centers can be neglected.This is the case.（2）Spin trapping agent BMPO and new nitrone spin trapping agent compound 17 were synthesized by reductive oxidation.Conclusions:（1）The efficient synthesis of conformationally stable side chain chiral substitutes for trityl radicals has been achieved.For the first time,trityl radical side chains have been modified to provide theoretical and experimental references for trityl radicals to select new functional group modification sites.By introducing hydroxyl modifiable groups into side chains to construct intramolecular hydrogen bonds,the conformation of trityl radicals is more stable.It provides a new method for obtaining stable chiral helix of trityl radicals;determines the influence of the introduction of chiral centers in side chains on the properties of trityl radicals;the introduction of chiral centers does not affect the EPR properties of trityl radicals,and provides a certain reference value for the subsequent study of trityl radicals;this study will be helpful for the design and synthesis of new trityl radical mother nuclei and trityl radicals.Functionalization,propeller conformation of triphenylmethyl derivatives and biomedical applications of trityl radicals are of great significance.（2）Spin trapping agent BMPO and new nitrone spin trapping agent 17 were synthesized by a synthetic route different from that of reduction oxidation in literature.This part provides another synthetic route for the synthesis of spin trapping agent.The synthesis of new spin trapping agent is expected to improve the instability and short half-life of spin trapping agent adduct,thus further expanding the application of spin trapping agent.