Axially chiral cyclohexylidene oxime ethers exhibit unique chirality because of the restricted rotation of C=N. The first catalytic enantioselective synthesis of novel axially chiral cyclohexylidene oximes has been developed by catalytic desymmetrization of 4-substituted cyclohexanones with O-arylhydroxylamines and is catalyzed by a chiral BINOL-derived strontium phosphate with excellent yields and good enantioselectivities. In addition, chiral BINOL-derived phosphoric acid catalyzed dynamic kinetic resolution of α-substituted cyclohexanones has been performed and yields versatile intermediates in high yields and enantioselectivities.
Axially chiral cyclohexylidene oxime ethers exhibit unique chirality because of the restricted rotation of C=N. The first catalytic enantioselective synthesis of novel axially chiral cyclohexylidene oximes has been developed by catalytic desymmetrization of 4-substituted cyclohexanones with O-arylhydroxylamines and is catalyzedb yachiral BINOLderived strontium phosphate with excellent yields and good enantioselectivities.I na ddition, chiral BINOL-derived phosphoric acid catalyzed dynamic kinetic resolution of a-substituted cyclohexanones has been performed and yields versatile intermediates in high yields and enantioselectivities.The development of novel asymmetric reactions has been one of the prime foci of modern synthetic organic chemistry. Substantial advances have been made in asymmetric synthesis of compounds with central chirality by using transition metals and organocatalytic methods.I na ddition, compounds with axial chirality,p lanar chirality,a nd helical chirality have attracted recent attention because of their importance in synthesis and asymmetric catalysis. [1] Axially chiral compounds,a lso known as atropisomers,e xhibit unique chirality because of the non-coplanar arrangement of groups about an imaginary axis.T his arrangement is attributed to the restricted rotation around either as ingle or double bond. [2] Although the first axially chiral compound was observed in 1910, [3] their importance was not realized until recently as ac onsequence of their occurrence in natural products and their application as chiral ligands. [4] Over the last decade, tremendous progress has been made for the synthesis of axially chiral biaryls,allenes,spiranes,and cyclohexylidenes. [5] However,m ethods for catalytic enantioselective synthesis of cyclohexylidene oximes and its analogues are scarce.Oximes and oxime ethers are versatile intermediates and key structural motifs present in several biologically active compounds which exhibit medicinal properties. [6] Recent progress in CÀHa ctivation proved that oxime ethers are efficient directing groups in several synthetic transformations. [7] Chiral cyclohexylidene oximes contain stereogenic axes which arise from the restricted rotation about the C = N bond and high activation energy barrier for nitrogen inversion. [8] To date,only two reports have described the enantioselective resolution of chiral cyclohexylidene oximes.In1990, Toda first reported the successful isolation of optically active oximes by the conventional second-order asymmetric transformation starting from racemic compounds (Scheme 1a) [9] Later, in 1994, Hoshino et al. demonstrated the kinetic resolution of phenylcyclohexanone oxime esters by lipasecatalyzed transesterification [9b] to give optically active oximes and oxime esters (Scheme 1b). Herein, we report the first enantioselective synthesis of chiral cyclohexylidene oxime ethers by desymmetric condensation of 4-phenylcyclohexanone with aryloxyamine catalyzed by ac hiral BINOL phosphate complex (Scheme 1c). We further applied this methodol...
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