Enantioselective Direct Aldol-type Reaction Of Azlactone Via Protonation Of Vinyl Ethers By A Chiral Bronsted Acid Catalyst

摘要

Chiral Bronsted acids have emerged as efficient enantioselective catalysts for a variety of organic transformations. A critical factor in achieving high stereoselectivities in these transformations is the hydrogen bond formed between the donor site of the acid catalyst and the acceptor (basic) site of the electrophilic component, X-H···Y (heteroatom-hydrogen···heteroatom). In this regard, C-H···X (carbon-hydrogen···heteroatom (X = O or N)) hydrogen bonding interactions have recently been identified as important factors in some stereoselective transformations.rnActivation of vinyl ethers by a Br0nsted acid catalyst is an extensively utilized and fundamental method in synthetic organic chemistry and is used in the protection of alcohols and the formation of carbon-carbon bonds among other processes. In this activation mode, the use of chiral Br0nsted acids, instead of achiral acids, would give rise to ion pairs of a chiral conjugate base and an oxocarbenium ion via protonation of the vinyl ether. In this case, the acidic proton(s) of the oxocarbenium ion would interact with the anionic site(s) of the chiral conjugate base.