Strong Acid-catalyzed Reaction Of Oxetanes With Sulfoxonium Ylides

Oxetanes are a kind of oxygen-containing four-membered heterocyclic compounds,which are important components of many drug molecules.Oxetanes can not only occur ring-opening reactions,in the field of organic synthesis,but also ring expansions as intermediates in organic synthesis to construct heterocyclic compounds.In the field of polymer chemistry,they are also important raw materials for the production of ether polymers.The previously reported ring expansion reactions of oxetanes were mostly electrophilic reactions with diazo compounds under the catalysis of transition metals to form five-membered rings or larger heterocycles,or cycloaddition reactions catalyzed by transition metals.In addition,the reaction of trimethylsulfoxonium iodide and strong base can form dimethylsulfoxonioum methylide intermediate,which has nucleophilicity and can undergo nucleophilic ring expansion reaction with oxetanes.Although tetrahydrofuran products containing substituents were obtained,this reaction only added a methylene group into the original fourmembered oxetane ring,and could not produce compounds with diverse substituents.This dissertation reports a strong protic acid-catalyzed electrophilic ring expansion reaction of sulfoxonium ylides to oxetanes.Unlike the previously reported reaction of oxetanes and diazo compounds,this reaction can produce trans-2,3-disubstituted tetrahydrofuran derivatives stereospecifically by using safer sulfoxonium ylides without the addition of metal catalysts and the protection of inert gas.At first,the reaction of catalytic amount protic acid with partial sulfoxonium ylides forms cationic intermediates.The oxetanes react with the intermediates and then undergo a ring expansion,instead of the protic acid directly activates the oxetanes.The protonated oxetanes undergo a ring opening and further eliminate a proton,competitively generating cinnamyl alcohols as byproducts.The stabilization effect of the aryl ring on the carbocation plays a decisive role in the reaction,and steric hindrance also has a great influence on this reaction.After the attempts of various substrates,we found that electronrich 2-aryloxetanes favor the reaction,but the electron-deficient 2-aryl,2alkyl and 3-substituted oxetanes disfavor the reaction.The reaction is simple in operation,good in stereoselectivity,leading to completely trans ring-expanded products.