Synthesis Of Carbamates And 19-Membered Cyclic Ether Via Cascade Reactions Involving Tetrahydrofuran

Tetrahydrofuran is not only an organic solvent widely used in industry,but is versatile intermediate in organic synthesis.It is of great significance to develop new and atom-economic reactions to convert tetrahydrofuran as raw material into valuable organic chemicals.In this paper,new methods for synthesis of carbamates and 19-membered polyether macrocycles via transition metal-free cascade reactions involving tetrahydrofuran have been developed.Firstly,a base-promoted four-component coupling of tetrahydrofuran,α-diazoesters,carbon dioxide,and amines has been developed for the construction of a wide range of carbamates.In this reaction K₂CO₃was used as promoter and tetrahydrofuran was used as raw material as well as the solvent.The reaction allows simultaneous construction of four new chemical bonds（two C-O bonds,one C-N bond and one C-H bond）in a single step.Readily available raw materials,high atom economy and wide substrate scope are the features of the method.Secondly,an efficient visible light-promoted synthesis of carbamates from carbon dioxide has been developed via photolysis ofα-diazoesters.When using tetrahydrofuran（THF）as the solvent,a four-component coupling reaction ofα-aryldiazoesters,THF,amines and CO₂occurred smoothly,while a three-component reaction betweenα-aryldiazoesters,amines and CO₂ would take place when a mixed Me CN/1,4-dioxane solvent was employed,thus providing rapid and direct access to an array of structurally different organic carbamates in the absence of any catalysts and additives under mild conditions.Thirdly,a novel[4+5+5+5]macrocyclization reaction of arynes and tetrahydrofuran for the synthesis of 19-membered cyclic ethers was developed through an anionic Fries rearrangement.In this reaction potassium fluoride was used as the fluorine source,realizing the formations of four new C-O bond and one new C-S bond in a single step.