Probing the resolution of 2-phenylpropanoyl chloride using quasi-enantiomeric Evans’ oxazolidinones

The parallel kinetic resolution of 2-phenylpropanoyl chloride using quasi-enantiomeric oxazolidinones is discussed. The levels of diastereoselectivity were found to be dependent on the presence of an additional (quasi)-enantiomeric oxazolidinone and its structural nature. The origin of this stereocontrol was believed to be due to hetero-aggregation between (quasi)-enantiomeric oxazolidinone components.     

Parallel kinetic resolution of racemic oxazolidinones using quasi-enantiomeric active esters

Racemic Evans’ oxazolidinones were efficiently resolved using a combination of quasi-enantiomeric profens. The levels of stereocontrol were high, leading to products with predictable configurations.     

Parallel kinetic resolution of 2-methoxy and 2-phenoxy-substituted carboxylic acids using a combination of quasi-enantiomeric oxazolidinones

2-Methoxy-2-phenyl acetic acid and 2-phenoxy-2-phenylpropionic acid were resolved by the parallel kinetic resolution of their corresponding pentafluorophenyl active ester using a quasi-enantiomeric combination of lithiated oxazolidinones.     

Efficient parallel resolution of an active ester of 2-phenylpropionic acid using quasi-enantiomeric Evans’ oxazolidinones

The parallel kinetic resolution of pentafluorophenyl 2-phenylpropionate using an equimolar combination of quasi-enantiomeric oxazolidinones is discussed. The levels of diastereocontrol were excellent leading to separable quasi-enantiomeric syn-oxazolidinone adducts in good yield.     

Probing the parallel kinetic resolution of 1-phenylethanol using quasi-enantiomeric oxazolidinone adducts

The parallel kinetic resolution of racemic 1-phenylethanol using an equimolar combination of quasi-enantiomeric oxazolidinones is discussed. The levels of diastereoselectivity were high leading to separable quasi-enantiomeric esters in good yield.     

Investigations into the parallel kinetic resolution of acetyl mandelic acid

The resolution of a series of active esters (derived from acetyl mandelic acid) using an equimolar combination of quasi-enantiomeric oxazolidin-2-ones is discussed. The levels of diastereoselectivity and the facial selectivity were found to be dependent on the structural nature of the pro-leaving group.     

Formation of enantiopure 5-substituted oxazolidinones through enzyme-catalysed kinetic resolution of epoxides

Halohydrin dehalogenase from Agrobacterium radiobacter catalyzed the enantioselective ring opening of terminal epoxides with cyanate as a nucleophile, yielding 5-substituted oxazolidinones in high yields and with high enantiopurity (69-98% ee). This is the first example of the biocatalytic conversion of a range of epoxides to the corresponding oxazolidinones.     

Stereochemical consequences of the use of chiral N-phosphoryl oxazolidinones in the attempted kinetic resolution of bromomagnesium alkoxides

A number of chiral N-phosphoryl oxazolidinones have been prepared and evaluated as asymmetric phosphoryl transfer agents with the magnesium alkoxide of 1-phenyl ethanol. The reaction proceeded with little stereoselection, which was shown to be a consequence of the reaction mechanism that occurs with inversion of configuration at phosphorus consistent with in-line attack opposite the leaving group.     

Parallel kinetic resolution of racemic 1-phenylethanol using quasi-enantiomeric combinations of carboxylic acids mediated by N,N′-dicyclohexylcarbodiimide and 3,5-lutidine

The parallel kinetic resolution of racemic 1-phenylethanol using an equimolar combination of quasi-enantiomeric 2-arylpropionic and butanoic acids mediated by a N,N′-dicyclohexylcarbodiimide (DCC)/3,5-lutidine coupling is discussed. The levels of diastereoselectivity were high leading to separable quasi-enantiomeric esters in good yield.     

Dynamic kinetic and kinetic resolution of N-Boc-2-lithiopiperidine

Asymmetric substitution of 2-lithiopiperidines can be achieved by dynamic resolution; the organolithium is formed as a racemic mixture by proton abstraction (or tin-lithium exchange) and is resolved in the presence of a chiral ligand.     

Quasienantiomeric levoglucosenone and isolevoglucosenone allow the parallel kinetic resolution of a racemic nitrone

Cycloadditions of chiral pyrroline N-oxides to levoglucosenone (1) and isolevoglucosenone (2) proceed with a high level of double asymmetric induction. Partial kinetic resolutions (KR) of both enantiomers of a racemic nitrone 3 were achieved, and a parallel kinetic resolution (PKR) experiment allowed the stereoselective divergent synthesis of two quasienantiomeric imino-C-disaccharide precursors.     

A method for the parallel synthesis of multiply substituted oxazolidinones

There are many examples of both naturally occurring and synthetic molecules containing a 2-oxazolidinone ring that have significant biological activity. The oxazolidinone ring potentially has three sites for attachment of diversity elements. A synthesis that can provide for inclusion of diversity elements at all three positions should be a powerful method for the preparation of oxazolidinone libraries. In this paper we present the preparation of a 3 x 3 x 3 array yielding 27 different products with minimal workup, high yields, and, most importantly, high purity. Using an intramolecular acylnitrene-mediated aziridination reaction, we have prepared (triphenylmethoxymethyl)-3-oxa-1-azabicyclo[3.1.0]hexan-2-one as a starting material for library generation. The first substitution involves opening the aziridine ring of the azabicyclo[3.1.0]hexane ring system using a Grignard reagent. The nitrogen of the oxazolidinone is now ready for substitution via alkylation or arylation. Removing the trityl protecting group via esterification under mildly acidic conditions accomplishes the final substitution.     

Determination of absolute configuration using kinetic resolution catalysts

A new method was developed to assign the absolute configuration of molecules using kinetic resolution catalysts. Secondary alcohols were acylated in the presence of Birman's S-HBTM and R-HBTM catalysts, and the fast-reacting catalyst was identified by NMR analysis of the reaction mixture. A mnemonic was developed to assign configuration based on the identity of the fast-reacting catalyst. The method uses only 1-3 mg of alcohol, and it is more convenient than the Mosher method. The kinetic resolution strategy may be extended to other classes of molecules.     

Dynamic kinetic resolution of N-Boc-2-lithiopyrrolidine

Asymmetric substitution of the organolithium derived either from N-Boc-2-tributylstannylpyrrolidine by tin-lithium exchange or from N-Boc-pyrrolidine by deprotonation occurs in the presence of a commercially available chiral diamine ligand with high levels of enantioselectivity by a dynamic kinetic resolution pathway.     

Enantioselective kinetic resolution of 3-phenyl-2-ketones using Baeyer–Villiger monooxygenases

The enantioselective kinetic resolution of two 3-phenyl-2-ketones using four different Baeyer–Villiger monooxygenases (BVMO) expressed recombinantly in Escherichia coli was studied. The highest enantioselectivity (E = 82) was achieved for 3-phenyl-2-butanone using a BVMO originating from Pseudomonas fluorescens. A BVMO from Pseudomonas putida showed an opposite (R)-enantiopreference and E = 12.     

Lipase-catalyzed kinetic resolution of 2-aminocyclopentane- and 2-aminocyclohexanecarboxamides

Candida antarctica lipase B (CAL-B)-catalyzed N-acylation with 2,2,2-trifluoroethyl butanoate in solvent mixtures of tert-butyl methyl ether and tert-amyl alcohol was used to prepare all the enantiomers of cis- and trans-2-aminocyclopentane- and -cyclohexanecarboxamides. An unexpected change in enantiopreference, accompanied by low enantioselectivity, was observed when Pseudomonas cepacia lipase (cis-cyclohexane substrate) or C. antarctica lipase A (cis-cyclopentane and -cyclohexane substrates) replaced CAL-B.     

A reactivity/affinity switch for parallel kinetic resolution: alpha-amino acid quasienantiomers and the resolution of cyclopropene carboxylic acids

A new type of parallel kinetic resolution (PKR) is reported in which quasienantiomers with very similar reactivities give products whose chromatographic properties diverge upon the addition of fluoride. This concept of a reactivity/affinity switch is applied to the PKR of cyclopropene carboxylic acids with all-carbon quaternary centers. This is the first application of alpha-amino acid quasienantiomers in PKR, and it is a complementary approach for acyltransfer systems where the asymmetry is induced by the nucleophile rather than the leaving group. Excellent diastereoselectivities (ranging from 90:10 to 99.5:5) and good yields were obtained for both quasienantiomeric products, and the reactions can be run on significant scale because the separation is trivial. High-level DFT calculations (B3LYP functional with the 6-31+G(d,p) basis set) provided transition-state structures with relative energies that are in accord with the experimental observations.     

Lipase-catalyzed kinetic resolution of 3-butyn-2-ol

Optically active 3-butyn-2-ol, an important building block, was synthesized successfully using lipase-catalyzed kinetic resolution. Starting from the thus obtained 3-butyn-2-ol, several 4-aryl-substituted compounds were synthesized enantioselectively.     

Dynamic kinetic resolution of hemiaminals using a novel DMAP catalyst

We describe the first catalytic dynamic kinetic resolution of hemiaminals mediated by an organocatalyst. A 0.1-1 mol % catalyst loading is effective for the dynamic kinetic resolution of hemiaminals to produce esters up to 88% ee in high yields. A 10 mol % catalyst loading resulted in a decreased selectivity, whereas the selectivity increased at 50 °C. The absolute configuration is assigned on the basis of the empirical Cotton effect rule.     

Dynamic thermodynamic and dynamic kinetic resolution of 2-lithiopyrrolidines

Dynamic resolution has been studied as a method for the asymmetric synthesis of 2-substituted pyrrolidines. Highly enantioselective electrophilic substitutions of racemic 2-lithiopyrrolidines in the presence of a chiral ligand have been achieved. The organolithium compounds were prepared by tin-lithium exchange from the corresponding tributylstannanes and n-butyllithium or by deprotonation of N-(tert-butyloxycarbonyl)pyrrolidine with sec-butyllithium. A range of N-substituents and chiral ligands were investigated for the dynamic resolution. Electrophilic quench of the resolved diastereomeric 2-lithiopyrrolidine-chiral ligand complexes provided the enantiomerically enriched 2-substituted pyrrolidines. With N-alkyl derivatives, the resolution occurs conveniently at (or just below) room temperature and either enantiomer of the product can be formed by appropriate choice of the chiral ligand. The asymmetric induction occurs as a result of a thermodynamic preference for one of the diastereomeric complexes. The minor complex was found to have a faster rate of reaction with the electrophile. The use of N-allylic derivatives provides a means to prepare the N-unsubstituted pyrrolidine products. Best results were obtained with the N-2,3-dimethylbut-2-enyl derivative, and this N-substituent could be cleaved using 1-chloroethyl chloroformate. With N-Boc-2-lithiopyrrolidine, the enantioselectivity arises by a kinetic resolution and high levels of asymmetric induction in the presence of excess n-butyllithium can be obtained. Dynamic kinetic resolution of the N-Boc derivative is limited in the scope of electrophile that can be used.