Investigation On Novel Dearomatization Reactions Based On Furanoxonium Ion Intermediates

The increasing use of biomass as a renewable source of fuels and bulk chemicals has been attracting extensive interest due to the restricted supply of fossil resources.Among the products of biomass processing,furan derivatives are important and referred to as biomass molecular platforms.Due to their low aromaticity energy,tremendous effort has been devoted to the development of efficient methods and applications for the dearomatization of furan.Despite extensive efforts,the construction of potentially bioactive molecules by dearomatization of furan with novel,efficient,and straightforward process is highly desirable.Transition-metal-catalyzed carbon–carbon or carbon–heteroatom bonds formation serves as an important processes in organic chemistry.Recently,Br?nsted acids as acid catalysts or dual ligand in the context of metal catalysis have been developed.In this context,we have studied the transition-or Br?nsted acid-catalyzed dearomatization of furan involving a furanoxonium ion access to heterocycles and spirocycles.A series of new results will be presented in the following:(1)Copper catalyzed aerobic oxidative annulation of N-Furfuryl-?-Enaminones for the synthesis of pyrroles:we developed a practical,simple route to pyrroles and indoles starting from furans with molecular oxygen as the terminal oxidant.This reaction has a mild condition,broad substrate scope and high atom efficiency.(2)We achieved regioselective Pd-catalyzed intramolecular direct arylation of the furan rings of N-(2-bromophenyl)-2-furancarboxamides using different reaction conditions to synthesize biologically interesting spirooxindoles and 5H-furo[2,3-c]quinolin-4-ones.We propose three possible pathways for these transformations: Heck-type insertion and electrophilic palladation for the a-arylation to synthesize spirooxindoles,concerted metalation-deprotonation(CMD)for the ?-arylation to synthesize 5H-furo[2,3-c]quinolin-4-ones.(3)As our continuous interest in palladium-catalyzed a-arylation of furans.We report a novel palladium-catalyzed 2,5-alkoxyarylation of furan rings,which provided convenient and diastereospecific access to structurally novel spirooxindoles.This reaction likely involves an intramolecular Heck-type a-arylation of the furan ring to produce a cyclic allylic palladium,and then intramolecular introduction of an alkyloxyl group at the other a-position of the furan ring.(4)As our continuous interest in palladium-catalyzed a-arylation of furans involving a spiro ?-allyl palladium.We developed a palladium-catalyzed Heck-type arylative dearomatization/ring expansion cascade reaction of furfurylcyclobutanols for the construction of dispirooxindoles with high diastereoselective and step economy.Various dispirooxindoles containing two quaternary carbon centers were synthesized in good yields enantioselective when used chiral phosphoramidite as ligand.(5)Br?nsted acid-catalyzed pinacol rearrangement involving a furanoxonium ion.Based on research on br?nsted acid-catalyzed pinacol rearrangement,we used furfuryl alcohols as substrate,Various spiro dienyl ethers were formed smoothly under mild reaction conditions with high yields and enantioselective when using chiral br?nsted acid as catalyst.