Protonic Acid Catalyzed Alkynylcyclopropane-cyclopentadiene Rearrangement

Cyclopentadienes are not only widely used Diels-Alder dienes but also important ligands for transition metal cyclopentadienyl complexes with broad applications in catalysis.Although there have been many synthetic routes disclosed in the literature,the synthesis of highly functionalized cyclopentadienes remains challenging for the lack of general synthetic methods and their facile isomerization via 1,5-sigmatropic hydrogen shift.The vinylcyclopropane-cyclopentene rearrangement affords a robust tool for the synthesis of five-membered carbocycles in organic synthesis.Conceivably,a similar rearrangement of alkynylcyclopropanes could lead to the cyclopentadiene derivatives,however,the formation of a highly strained cyclic allene intermediate is quite challenging.Therefore,alkynylcyclopropane-cyclopentadiene rearrangement is underdeveloped.In this thesis,by taking advantage of interactions between the carbonyl and alkynyl groups upon electrophilic activation,ring expansion of alkynyl cyclopropyl ketones is selectively modulated towards the cyclopentadiene derivatives without the formation of the highly strained cyclic allene intermediate.Specifically,through intramolecular addition of the carbonyl groups such as acyl,ester and the spiro amide groups to the alkynes upon electrophilic activation by Tf₂NH,alkynylcyclopropane-cyclopentadiene rearrangements ofα-keto ester and isatin derived alkynylcyclopropanes have been developed,affording a new method for the synthesis of highly functionalized cyclopentadienes.Under heating conditions,the spirooxindole1,1,2,5-tetrasubstituted cyclopentadienes is selectively converted to the planar quinolone-fused cyclopentadienes,on the other hand,reactions with non-spiro 1,1,2,3-tetrasubstituted cyclopentadienes are unselective under the same conditions.Fortunately,they are able to be selectively rearranged to the 1,2,3,4-tetrasubstituted cyclopentadiene derivatives under the catalysis of nickel acetate.