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Org Lett. 2019 Jun 7;21(11):4030-4034. doi: 10.1021/acs.orglett.9b01216. Epub 2019 May 24.

Enantioselective Protonation of Enol Esters with Bifunctional Phosphonium/Thiourea Catalysts.

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1
Department of Chemistry, Graduate School of Science , Kyushu University , Fukuoka , 819-0395 , Japan.

Abstract

Bifunctional phosphonium/thioureas derived from tert-leucine behaved as highly selective catalysts for enantioselective protonation of enol esters, providing α-chiral ketones in yields of up to 99% with high enantioselectivities (up to 98.5:1.5 er). Control experiments clarified that a bulky tert-butyl group and phosphonium and thiourea moieties were necessary to achieve such high stereoselectivity. In addition, mechanistic investigations indicated the catalyst was converted to the corresponding betaine species, which served as a monomolecular catalyst.

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