Acid And Base-mediated Ring Expansion And Rearrangement Of Epoxides

Epoxides are very important organic synthetic intermediates and the oxirane structures are also very common in natural products and drug molecules.For a long time studies on the synthesis and reactivity of epoxides have been an significant research topic in organic chemistry.Because of the high strain of the special oxygen containing three-membered ring,epoxides are extremely reactive.Substitution reactions,rearrangements,and ring expansion reactions are common reaction types of epoxides,producing a variety of highly active organic compounds such as alcohols,aldehydes,ketones,and other heterocycles.This dissertation,research mainly focuses on the ring expansion and rearrangement reactions of epoxides.First of all,epoxides and diazo compounds were used as materials to prepare 2-acyl-5,6-dihydro-1,4-dioxines which have potential medicinal value via the Cu(Ⅱ)-catalyzed intermolecular ring expansion reaction.The ring opening step occurs on more substituted carbon atom,and has high regioselectivity.The reaction process undergoes an intimate ion-pair mechanism to obtain products which mainly retained the configuration.The reactions were assisted by microwave irradiotion,for which the operation is simple,and the reaction time can be effectively shortened while the efficiency can be improved.The copper(Ⅱ)catalyst is inexpensive and reduces the economic cost of the reaction.The reaction substrate has a wide range of applications.Secondly,DABCO-mediated rearrangement of terminal epoxides to methyl ketones was achieved.Usually,the acid-catalyzed Meinwald rearrangement of terminal epoxides mainly forms aldehydes.A few reports have reported that epoxides could create methyl ketones catalyzed by nucleophilic metal complexes and Lewis acid catalysts,but most of these reactions are only applicable to alkyl-substituted epoxides.Only very few reports have been made about such rearrangement reactions of aryl epoxides usually with aldehydes as by-products.In our research,not only the nucleophilic organic base DABCO-promoted Meinwald rearrangement of mono-substituted oxiranes generates methyl ketones,but also the same conversion is achievedu nder mild conditions by introducing Lewis acid into the system.The two reaction conditions both have good applicability to arylepoxides,alkenylepoxides and alkynylepoxides to form the corresponding methyl ketones with high yield and high selectivity.DABCO-mediated rearrangement can be metal-free,acid-free,regiospecific,and suitable for monosubstituted nonalkylepoxides.Our current strategy can be taken as a complementary method for the acid-catalyzed isomerization of epoxides.Thirdly,in the study of DABCO-mediated monosubstituted epoxides,it was found that DABCO could promote the reaction of 2,2-disubstituted epoxides to lose one methylene group to produce ketones.Which can be regarded as retro-Corey-Chaykovsky reaction.Finally,we investigated the microwave irradition assisted organic base-acid co-catalyzed tandem convertion of styrylepoxides into m-phenyl benzaldehydes.It is an effective method for preparing m-phenylbenzaldehydes by styrylepoxides.The reaction features high yield,short reaction time,and simple operation.For mechanism studies,the tandam reaction involves a series of reactions of Meinwald rearrangement,Micheal addition,aldol condensation,and aromatization.At the same time,we also explained the selectivity of DABCO and triethylamine as different organic bases in the reaction of styrylepoxides.DABCO as a strong nucleophilic amine can directly attacks epoxides at their less sterically hindered carbon atom However,the weak nucleophilic triethylamine cannot make the same transfer.Epoxides are catalyzed by trace acid in the system to form aldehydes and then undergo in the next steps in the tandem reaction.In conclusion,this dissertation focuses on the reactivity of epoxides,mainly to study their expansion and rearrangement reactions.We developed the copper-catalyzed ring-expansion reactions of epoxides and diazo compounds,DABCO-mediated rearrangement of monosubstituted epoxides,and organic acid-base co-catalyzed rearrangement of styryl epoxides.These make some progress in the methodologies and mechanisms related to epoxide compounds.