Synthesis And Characterization Of Trityl Radicals With Subsituents On The Side Chain

Objective: In recent years,the fully-substituted trityl radicals have accepted wide attention in the magnetic resonance-related fields.Compared to the most commonly used nitroxide radicals,these trityl radicals have the following advantages: high biostability,narrow electron paramagnetic resonance(EPR)single line signal,long relaxation time(> 10 μs at 25 ℃ under anaerobic conditions),and high detection sensitivity.At present,trityl radicals have been functionalized to measure various physiological parameters such as intracellular or extracellular O2 levels,O2·-,p H andredox state.In addition,they also found applications in the fields of dynamic nuclear polarization(DNP)enhanced nuclear magnetic spectroscopy and imaging and spin labeling.Among the available trityl radicals,CT-03 and its hydrophilic analogue OX063 are mostly used.Owing to its facile synthesis,the development of trityl radicals as paramagnetic probes is mainly based on CT-03.However,the biomedical applications of CT-03 have been limited due to its relatively high hydrophobicity.The derivatization of three carboxylate groups(the only modification site)in CT-03 is non-selective and this modification may change their redox properties and EPR spectra with hyperfine splittings from hydrogen and/or nitrogen atoms.In addition,this derivatization can possibly reduce the water solubility of trityl radicals and shorten their relaxation times.To address these problems,we herein synthesize new trityl radicals with mono(or bis)substituents on the side chain which afford high water solubility and sharp EPR signal,and allow for selective modifications.Methods: Using 1,2,4,5-Tetra-tert-butylthiobenzene,acetone and ethyl pyruvate as starting materials,we synthesized two monomers with mono(or bis)substitution on the side chain through 3-step reactions including ketalization,reduction and hydroxyl protection.In these reactions,solvents,temperature,the addition order and the equivalents of the reactants were varied and their effects on the reactions were investigated.The corresponding triarylmethanol intermediates were prepared using the "1 + 2" strategy from : the mono-substituted monomer and aryl ketone.Due to the intrinsic chirality,the mono-substituted triarylmethanol has eight diastereoisomers which were separated by chiral reagents in order to explore the effect of the configuration of the trityl part on their properties.Finally,trityl radicals with different functional groups on the side chain were obtained by hydrolysis,deprotection and conjugation with functional groups,followed by transformation into the corresponding radicals with acids.In addition,EPR properties of these trityl radicals were also studied.Results: In this work,trityl radicals with mono(or bis)substituents on the side chain were synthesized with good yield.Firstly,five precursors containing different mono-functional groups on the side chain were obtained and well characterized by NMR and HPLC.Two of these precursors were transformed them into the corresponding trityl radicals and effect of the mono-substituents on their EPR properties was studied.In addition,chiral separation of racemic compound 7 was achieved by chiral agents to afford four components which were characterized by NMR,CD and EPR.Secondly,triarylmethanol intermediates with trans-and cis disubstituents on the side chain was also synthesized and the single crystal structure of the dihydroxy-substituted monomer(cis configuration)was investigated.Conclusion:(1)The synthetic route utilized in this work has the advantages of conciseness,good reproducibility and convenient operation under mild conditions.(2)Novel trityl radicals with mono substituents on the side chain were synthesized for the first time which exhibit almost identical EPR properties with CT-03.(3)The configuration of the trityl part exerts an effect on the properties of trityl derivatives as determined by EPR studies on four components of the mono-substituted triarylmethanols.(4)This study sheds light on the design and synthesis of new trityl radicals and their functionalization,and will be very helpful to investigate the configuration of trityl-related molecular propeller.