Asymmetric Synthesis Of Six-Membered Cyclic Compounds Via N-Heterocyclic Carbene Catalysis

1.Dynamic Kinetic Resolution Enabled by Intramolecular Benzoin Reaction:Synthetic Applications and Mechanistic Insights.The highly enantio-,diastereo-,and regioselective dynamic kinetic resolution of?-ketoesters and 1,3-diketones was achieved via a chiral N-heterocyclic carbene catalyzed intramolecular cross-benzoin reaction.A variety of tetralone derivatives bearing two contiguous stereocenters and multiple functionalities were liberated in moderate to excellent yields and with high levels of stereoselectivity(>95%ee and>20:1 dr in most cases).Moreover,a set of new mechanistic rationale that differs with the currently widely accepted understanding of intramolecular benzoin reactions was established to demonstrate the superior preference of benzoin over aldol transformation:(1)A coexistence of competitive aldol and benzoin reactions was detected,but a retro-aldol-irreversible benzoin process performs a vital role in the generation of predominant benzoin products.(2)The most essential role of an N-electron-withdrawing substituent in triazolium catalysts was revealed to be accelerating the rate of the benzoin transformation,rather than suppressing the aldol process through reducing the inherent basicity of the catalyst.2.Highly efficient enantioselective intermolecular[4+2]annulation reaction enabled by N-Heterocyclic Carbene Catalysis.The highly enantioslective stepwise[4+2]annulation reaction of a-bromo enals with newly developed dienolate precusor a-acyloxybutenones was achieved via N-heterocyclic carbene catalysis.A variety of cyclohexene derivatives bearing four contiguous stereocenters and multiple functionalities were formed in moderate to good yields and with high levels of stereoselectivity(up to 97%ee and>20:1 dr in most cases).In addition,a possible isomerization process occurred in this reaction,leading to the products that are inaccessible via Diels-Alder reactions.