High-performance liquid chromatographic enantioseparation of unusual amino acid derivatives with axial chirality on polysaccharide-based chiral stationary phases

The successfulenantioseparationofaxiallychiralaminoacidderivativescontainingacyclohexylidene moiety onan an alyticalandsemipreparativescalewasachievedforthefirsttimebyHPLCusing polysaccharide-based chiral stationary phases. Racemicmethyl N-benzoylamino esters,easilyobtained by methanoly sisof the corresponding 5(4H)-oxazolones, were subject edtochiralHPLCresolutionusing chiral stationaryphasesbasedonimmobilized3,5-dimethylphenylcarbamatederivativesofamylose (Chiralpak® IA column)orcellulose(Chiralpak® IB column). The behaviour of both selectors under dif-ferent elution conditions was evaluated and compared. The amy los ecolumns howed better performance than the cellulose column for allenantiomers tested. These mi preparative resolution of axially chiral amino acidderivatives with different side chains has been achievedona 250mm×20mmIDChiralpak® IA columnusingtheappropriatemixtureof n-hexane/chlorofom/ethanol aseluent by successive injec-tions of asolution of the sample in chloroform. Using this protocol up to 120 m go feachenan tiomer of the correspondingaxiallychiralaminoacidderivativewereobtainedfrom300mgofracemate.[(Sa)-2a, 105 mg;(Ra)-2a, 60mg,[(Sa)-2b, 105mg;(Ra)-2b, 90mg,[(Sa)-2c, 120mg;(Ra)-2c, 100mg]

Synthesis, characterization, properties, and asymmetric catalytic Diels-Alder reactions of chiral-at-metal imino-iridium(III) complexes

The synthesis and characterization of optically active imino complexes (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)IrCl(imine)][SbF6] (imine = Ln = N-(2-pyridylmethylene)−(R)-1-phenylethylamine (L1; 1a,a‘), N-(2-pyridylmethylene)-(R)-1-naphthylethylamine (L2; 2a,a‘), N-(2-quinolylmethylene)−(R)-1-naphthylethylamine (L3; 3a,a‘), N-(6-methyl-2-pyridylmethylene)−(R)-1-naphthylethylamine (L4; 4a,a‘), N-(2-pyridylmethylene)−(R)-1-cyclohexylethylamine (L5; 5a,a‘), N-(2-pyridylmethylene)−(1R,2S,4R)-1-bornylamine) (L6; 6a,a‘)), (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)IrCl(L2)][A] (A = Cl (7a,a‘), BF4 (8a,a‘), PF6 (9a,a‘), (1S)-camphor-10-sulfonate (R*SO3) (10a,a‘)) and (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)(L6)(H2O)][SbF6]2 (11a,a‘)) are reported. The absolute crystal structures of (RIr,RC)-1a, (RIr,RC)-2a, and (RIr,RC)-3a were determined by X-ray analysis. All three complexes show the chiral metal center in a pseudo-octahedral environment, being bonded to an η5-C5Me5 ring, to a terminal chloride, and, in a chelate fashion, to the two nitrogen atoms of the imine ligands. For the chloride compounds 1−6, 1H NMR solution data reveal conformational differences between the crystal and solution structures. At room temperature, in acetone, complexes 1−11 are configurationally stable. At 62 °C, in methanol, the more labile chloro complex 5a epimerizes at Ir with a half-life of 68.0 min (activation parameters, ΔH⧧ = 97.7 ± 13.2 kJ mol-1 and ΔS⧧= −26.3 ± 6.2 J K-1mol-1; equilibrium constant, 5a‘/5a = 4.26 ± 1.65). Dichloromethane/acetone solutions of the solvate complexes [(η5-C5Me5)Ir(imine)S][SbF6]2, prepared in situ by treating complexes 1−6 with equimolar amounts of AgSbF6, are active catalysts for the Diels−Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good exo:endo selectivity (84:16 to 95:5) and moderate enantioselectivities (up to 46%).We thank the Dirección General de Investigación Científica y Técnica for financial support (Grant No. PB96/0845).Peer reviewe