Abstract
As a part of our studies aimed at asymmetric catalytic reactions by using an external chiral ligand, we have developed a catalytic asymmetric addition reaction of an arylthiol to α,β-unsaturated esters under the control of an external chiral ligand. The characteristic of our technology is a double activation of a thiol by lithiation and chelate formation with a chiral tridentate amino diether ligand, which simultaneously and effectively controls a stereochemistry of the reaction. One significant feature of an arylthiol is a bulky 2-substitution on aryl group, which enables the formation of a really reactive monomeric thiolate species. s-Cis conformation and capability of electron lone pair-differentiating coordination of a carbonyl oxygen to lithium are structural requirements of the substrates for high enantioselectivity. The enantioselectivity came up to 97% under the cited conditions. Asymmetric protonation of a transient enolate, generated by conjugate addition of a lithium thiolate to an enoate, was also realized. The stereochemistry of the protonation was controlled by the conformation of initially formed transient enolate in a 1,2-asymmetric induction manner. This technology enabled the asymmetric synthesis of (S)-naproxene. Stereoselective tandem C-S and C-C bond-forming reaction was developed as a logical extension by trapping the transient enolate intermediate with an aldehyde as a carbo-electrophile in the presence of phenylthiotrimethylsilane as an equilibrium-shift reagent. This tandem reaction was extended to a stereoselective cyclization of ω-oxo-α,β-unsaturated esters initiated by a lithium thiolate. Stereoselectivity of both tandem inter- and intramolecular reaction is predictable by an allylic strain-controlled conformation model of the enolate, in which an approach of aldehyde takes place anti to C-S bond through coordination of an aldehyde oxygen to lithium. Total synthesis of (−)-neplanocin A was achieved by using the tandem cyclization as a key tool for the direct construction of a five-membered carbocycle where every carbon is functionalized.
