| 1,4-Dihydropyridine is an important nitrogen-containing heterocycle contained in a lot of natural products and drug molecules with biological activity,which are valuable and applied widely in the fields of medicine,agriculture and functional materials.Therefore,it is of great significance to develop a simple,efficient,atom-economical and environmental-friendly method for the synthesis of 1,4-dihydropyridine compounds.On the other hand,amino acids can not only be used as ligands of complex and micromolecular organocatalysts for organic reactions,but also can be used as nitrogen sources and molecular structure unit,which provide a broad possibility of reaction transformation for the synthesis of complex structure molecules.Herein,new green methods for the synthesis of 1,4-dihydropyridines from glycine derivatives were developed.The details are as follows.（1）Synthesis of 1,4-dihydropyridines via visible-light photocatalytic cascade cyclization using metal complex Ir（ppy）3.N-arylglycine esters and ethyl acetoacetates were used as starting materials,and Ir（ppy）3,diisopropyl peroxide and toluene were selected as photocatalyst,oxidant and solvent,respectively.The reaction took place under the irradiation from blue LED lamp.Eighteen polysubstituted 1,4-dihydropyridines were synthesized with yields of 41～69%.It was realized that amino acid derivatives were functionalized via a-H oxidaiton by photoredox catalysis,which initiated cascade cyclezation reaction.In contrast of reported thermal catalysis using transition metal complexes,the photoredox catalytic reaction takes place at room temperature.（2）Synthesis of 1,4-dihydropyridines via cascade reaction catalyzed by an organic photosensitizer methylene bule under visble light irradiation.Using cheap metal-free methylene blue as photosensitizer,diisopropyl peroxide or air as oxidant and chlorobenzene as solvent,the cascade cyclization reaction from N-arylglycine esters and ethyl acetoacetates was carried out under the irradiation from an 18 W blue LED lamp.Twenty 1,4-dihydropyridines were synthesized with yields of 22～78%.This developed a method for the synthesis of 1,4-dihydropyridines catalyzed by cheap and readily available organic photosensitizer with no metal addition,avoiding the metal residues in the products.（3）Synthesis of 1,4-dihydropyridines via cascade cyclization of N-arylglycine esters andβ-ketone esters promoted by 1,2-dichloroethane.The method eliminates the need for redox catalysts,and the reaction took place directly in 1,2-dichloroethane solvent using oxygen as the green terminal oxidant.Under standard conditions,22 1,4-dihydropyridines were synthesized.This provides a new green method for the synthesis of1,4-dihydropyridines under mild reaction condition,which is simple,atom-economic and low-cost. |
