title: Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with chiral Cu(II) Catalysts: an example of a highly active Lewis acid catalyzed reaction.
abstract: Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with half equivalents of isocyanates in the presence of chiral Cu(II) catalysts was studied. Among a series of catalyst bearing chiral bis(oxazoline) (box) and pyridine(bisoxazoline) ligands, [Cu(tBu-box)]X(2) [X=OSO(2)CF(3) (3 a), SbF(6) (3 b)] showed the highest enantioselectivity in the reaction of pantolactone (1 a). Use of n-C(3)H(7)NCO, a small alkyl isocyanate, in CH(2)Cl(2) solution was important to achieve a high level of enantiomer selection. The tBu-box-Cu(II) catalyst efficiently differentiated two enantiomers of beta-substituted alpha-hydroxy gamma-lactones under the optimized reaction conditions, resulting in a stereoselectivity factor (s=k(fast)/k(slow)) of up to 209. Furthermore, this catalyst is highly active, so that the carbamoylation can be conducted with a substrate-to-catalyst molar ratio of 2000-3000. A catalytic cycle of this reaction is also proposed.
authors.affiliation: Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Japan.
authors.forename: Nobuhito
authors.initials: N
authors.lastname: Kurono
authors.forename: Tadahiro
authors.initials: T
authors.lastname: Kondo
authors.forename: Masanori
authors.initials: M
authors.lastname: Wakabayashi
authors.forename: Hirohito
authors.initials: H
authors.lastname: Ooka
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Inoue
authors.forename: Hiroto
authors.initials: H
authors.lastname: Tachikawa
authors.forename: Takeshi
authors.initials: T
authors.lastname: Ohkuma