The Construction Of N-containing Heterocycles Based On The Hydrazones

Heterocycles represent one of the most important classes of biologically active molecules that frequently existed in natural products,pharmaceuticals and functional materials.Therefore,the development of simple and efficient methods to assemble structutally manifold heterocycles is still a hot research field.All the time,hydrazone plays an important role in the field of the rapid synthesis of diverse N-containing heterocycles.However,there are still some shortcomings,such as poor selectivity,harsh reaction conditions,etc.In recent years,visible-light photocatalysis,as a new thinking and method,has been widely applied in carbon-carbon bond,carbon-hetero bond formation and the synthesis of heterocycles.Our research mainly consists of designing reactions for the construction of N-containing heterocycles based on hydrazone and focused on applying the visible-light photocatalyic strategy for the synthesis of such heterocycles..First,visible-light photocatalysis was investigated in radical cycloaddition based on hydrazone.(1)The use of photoredox catalysis for the facile generation of N-centred radicals from N-H bonds was applied to the radical cyclization of a-halohydrazones with?-ketocarbonyls,affording a series of biologically important pyrazoline derivatives.(2)A photocatalytic oxidative cyclization of ?-halohydrazones with tetrahydroisoquinoline was developed,giving[1,2,4]triazino[3,4-a]isoquinolins by intramolecular cyclization.It should be noted that a stochiometric amount of BrCCl3,as external oxidant,was critical for the catalyst turnover in this reaction system.(3)Futhermore,an ambient-light-promoted radical cyclization/perfluoroalkylation cascade of ?,?-unsaturated hydrazones was developed.This protocal was conducted with TMG as base and perfluoroalkylated pyrazolines were obtained up to 96%yields.From the viewpoints of atom-economy synthesis,visible-light photocatalysis has tremendous potential.Through this technique various active reactive species were generated under mild conditions compared with conventional methods,that employ stoichiometric amount of radical initiators.In addition,new carbon-carbon bonds,carbon-nitrogen bonds were formed in this process.Next,iodine(?)-mediated cyclization based on ?,?-unsaturated hydrazones and Oximes was studied.Pyrazoline and isoxazoline compounds are important classes of biologically and pharmacologically active five-membered nitrogen-containing heterocycles,as well as important chemical building blocks in organic and biological synthesis.An iodine(?)-mediated radical cyclization/sulfenylation or selenylation of ?,?-unsaturated hydrazones was developed by employing DBU as base and disulfides/diselenides as the S/Se-sources.Furthermore,this reaction can be successfully extended to the radical cyclization/sulfenylation or selenylation of(?,?-unsaturated oximes.The use of benign PIDA made this reaction free of metal and poisonous ligands,furnishing hetero-contianing pyrazoline and isoxazoline derivatives in high efficiency.Finally,an efficient[4+3]cycloaddition of a-halohydrazones and azomethine imines for the construction of seven-membered N-containing heterocycles was developed.In the presence of base,azoalkenes could be generated in situ from a-halohydrazones and react with imino esters to give the desired triazepine derivatives in the yields up to 82%.This reaction provides a straightforward and mild access to the triazepine derivatives.