Kinetic resolution of atropisomeric amides

We have demonstrated the feasibility of the kinetic resolution of atropisomeric amides using the commercially avaliable AD-mix. To our knowledge, this methodology represents the first catalytic kinetic resolution of such compounds. Relative rates of up to 32 have been found for the kinetic resolution processes. We have also determined the barriers to rotation and half-lives of some of these amides. The half-lives range from 7 to 135 h at room temperature.     

Dynamic resolution of atropisomeric amides using proline-derived imidazolines and ephedrine-derived oxazolidines

Condensation of atropisomeric tertiary 2-formyl naphthamides or 2-formyl benzamides with some chiral diamines and amino alcohols leads, via a dynamic resolution process, to single atropisomers of tertiary amides bearing chiral imidazolidines or oxazolidines. Hydrolysis of the new heterocycle competes a dynamic thermodynamic resolution of the starting aldehyde, and rapid reduction allows the isolation of atropisomeric amides bearing 2-hydroxymethyl substituents in enantiomerically enriched form. Evidence that the reactions are under thermodynamic control is presented.     

Kinetic resolution and chemoenzymatic dynamic kinetic resolution of functionalized gamma-hydroxy amides

[reaction: see text] An efficient kinetic resolution of racemic gamma-hydroxy amides 1 was performed via Pseudomas cepacia lipase (PS-C)-catalyzed transesterification. The enzyme PS-C tolerates both variation in the chain length and different functionalities giving good to high enantioselectivity (E values of up to >250). The combination of enzymatic kinetic resolution with a ruthenium-catalyzed racemization led to a dynamic kinetic resolution. The use of 2,4-dimethyl-3-pentanol as a hydrogen source to suppress ketone formation in the dynamic kinetic resolution yields the corresponding acetates in good yield and good to high enantioselectivity (ee's up to 98%). The synthetic utility of this procedure was illustrated by the practical synthesis of the versatile intermediate gamma-lactone (R)-5-methyltetrahydrofuran-2-one.     

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.     

Rapid,room-temperature acylative kinetic resolution of sec-alcohols using atropisomeric 4-aminopyridine/triphenylphosphine catalysis

Two new atropisomeric 4-aminopyridine-based nucleophilic catalysts containing terphenyl ‘blocking groups’ have been prepared and evaluated for kinetic resolution (KR) of aryl alkyl sec-alcohols. One of these biaryls is shown to be the most selective atropisomeric catalyst yet prepared for several sec-alcohols but its low reactivity makes it non-optimal for use at room temperature (rt). Optimisation of the conditions for conducting KRs at rt using a previously described catalyst (containing a phenyl blocking group) at the 1 mol% level indicates that PPh₃ (1 equiv) is beneficial for enantioselectivity and allows KR of (±)-1-(naphthyl)ethanol in less than 30 min with s>15 (i.e., ∼40% recovered alcohol with >95% ee). These conditions constitute a convenient and practical method for rapid KR of sec-alcohols and are anticipated to facilitate a detailed kinetic study of this catalytic manifold by calorimetry.     

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.     

Dynamic kinetic resolution of alpha-hydroxy acid esters

[formula: see text] Enzymatic resolution in combination with ruthenium-catalyzed racemization of the substrate led to dynamic kinetic resolution of alpha-hydroxy esters in good yields and excellent ee's. Studies of different parameters showed that the best results were obtained using Pseudomonas cepacia lipase, ruthenium catalyst 3, and 4-chlorophenyl acetate as acyl donor in cyclohexane.     

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.     

Dynamic kinetic resolution of benzoins by lipase-metal combo catalysis

The synthesis of some noncommercial racemic 1,2-diaryl-2-hydroxyethanones (benzoins) is described, optimizing the previously reported methodologies. In a further step, the kinetic resolution of these substrates is reported, obtaining conversions of around 50% and ee(p) higher than 99% in very short reaction times. As enzymatic catalyst, after screening of several enzymes, the lipase TL (from Pseudomonas stutzeri) was the most efficient, working in an organic solvent with a very low log P value, such as THF. Finally, the dynamic-kinetic resolution of different benzoins using a lipase-ruthenium-catalyzed transesterification in organic solvents is described for the first time, obtaining conversions up to 90% maintaining the excellent enantioselectivity in all cases.     

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.     

Enantiomerically enriched atropisomeric N,N′-diaryl ureas by oxidative kinetic resolution of their 2-sulfanyl derivatives

Atropisomeric N-methyl-N,N′-diaryl ureas may be obtained in enantiomerically enriched form by oxidative kinetic resolution of their sulfide derivatives. The atropisomeric sulfides may be obtained in up to 97:3 er and display high stability to racemisation (half-lives at 25 °C of up to 500 years). Unlike related fully alkylated ureas, the product sulfoxides exhibit relatively weak thermodynamic conformational selectivity.     

Dynamic kinetic resolution (DKR) using immobilized amine nucleophiles

The first example of a dynamic kinetic resolution (DKR) using immobilized amine nucleophiles is described. This approach utilizes a nucleophilic amine attached to a solid phase resin via an organic spacer. The optical purities of the N-substituted α-amino ester products are superior to the solution phase DKR with diastereomeric ratios ranging from 11:1 to 18:1 and chemical yields between 66% and 98%.     

Dynamic kinetic resolution of 2-oxo-3-aryl-succinates by organocatalyzed aldolization

The dynamic kinetic resolution of 2-oxo-3-arylsuccinates was achieved vial-proline-catalyzed addition of acetone in acetonitrile at room temperature, providing the desired adduct in good yield with up to 87 : 13 dr and high ee up to 99%.     

Dynamic kinetic resolution of racemic gamma-aryl-delta-oxoesters. Enantioselective synthesis of 3-arylpiperidines

Cyclodehydration of racemic gamma-aryl-delta-oxoesters with (R)- or (S)-phenylglycinol stereoselectively affords bicyclic delta-lactams, in a process that involves a dynamic kinetic resolution. Subsequent reduction of these lactams leads to enantiopure 3-arylpiperidines. Starting from racemic aldehyde esters, this short sequence has been applied to the synthesis of (R)-3-phenylpiperidine and the antipsychotic drug (-)-3-PPP (an (S)-3-arylpiperidine), whereas starting from racemic ketone esters enantiopure cis-2-alkyl-3-arylpiperidines are prepared.     

Dynamic kinetic resolution of racemic tropic acid ethyl ester and its derivatives

The dynamic kinetic resolution of racemic mixtures of tropic acid ethyl ester under substrate racemizing conditions was studied using lipase PS with a ruthenium catalyst. Isopropenyl acetate was used as an acyl donor, since it was found to be compatible with both catalysts; this resulted in an efficient dynamic kinetic resolution. With this process, a variety of racemic tropic acid ethyl esters were transformed to optically active acetoxy-2-arylpropionic acid ethyl esters with 60-88% yields and 53-92% ee.     

Dynamic kinetic resolution of racemic beta-haloalcohols: direct access to enantioenriched epoxides

The direct chemo-enzymatic DKR of racemic beta-haloalcohols is reported, yielding the corresponding optically active epoxides in a single step. The mutant haloalcohol dehalogenase HheC Cys153Ser Trp249Phe is used for the asymmetric ring closure, whereas racemization of the remaining enantiomer of the haloalcohol is achieved using the new iridacycle 3, one of the most effective racemization catalysts to date for beta-haloalcohols.     

Dynamic kinetic resolution in the baker's yeast mediated reduction of 2-Benzenesulfonylcycloalkanones

2-Benzenesulfonylcyclopentanone 1 and ---cyclohexanone 2 undergo efficient dynamic kinetic resolution on treatment with baker's yeast, under a range of conditions including in organic solvents, to form the corresponding cis-2-benzenesulfonylcycloalkanols 5 and 6 in excellent enantiopurity. Reduction of larger ring derivatives is much less efficient.     

Dynamic kinetic resolution of allylic alcohols mediated by ruthenium- and lipase-based catalysts

An enzyme-metal combo reaction has been developed for the dynamic kinetic resolution of allylic alcohols in which racemic substrates are transformed by a lipase and a ruthenium complex in the presence of an acyl donor to allylic acetates of high optical purity in over 80% yield.     

Dynamic kinetic resolution of secondary alcohols combining enzyme-catalyzed transesterification and zeolite-catalyzed racemization

Hydrophobic zeolite beta containing low concentrations of Zr or Al was found to be a good catalyst for the racemization of 1-phenylethanol. The formation of styrene as a side product could be minimized by reducing the metal concentration in the zeolite beta. Combined with an immobilized lipase from Candida antarctica, the dynamic kinetic resolution of 1-phenylethanol to the (R)-phenylethylester can be achieved with high yield and selectivity. The reaction was best conducted in toluene as solvent at 60 degrees C, with higher temperatures leading to a loss in the enantioselectivity of the formed ester. By using high-molecular-weight acyl-transfer reagents, such as vinyl butyrate or vinyl octanoate, a high enantiomeric excess of the product esters of 92 and 98 %, respectively, could be achieved. This is attributed to a steric effect: the bulky ester is less able to enter the pore space of the zeolite catalyst where the active sites for racemization are localized. Close to 100 % conversion of the alcohol was achieved within 2 h. If the more common acyl donor, isopropenyl acetate, was used, the enantiomeric excess (ee) of the formed ester was only 67 %, and the reaction was considerably slower.