Access to Enantiopure α‐Alkyl‐β‐hydroxy Esters through Dynamic Kinetic Resolutions Employing Purified/Overexpressed Alcohol Dehydrogenases

α‐Alkyl‐β‐hydroxy esters were obtained via dynamic kinetic resolution (DKR) employing purified or crude E. coli overexpressed alcohol dehydrogenases (ADHs). ADH‐A from R. ruber, CPADH from C. parapsilosis and TesADH from T. ethanolicus afforded syn‐(2R,3S) derivatives with very high selectivities for sterically not impeded ketones (‘small‐bulky’ substrates), while ADHs from S. yanoikuyae (SyADH) and Ralstonia sp. (RasADH) could also accept bulkier keto esters (‘bulky‐bulky’ substrates). SyADH also provided preferentially syn‐(2R,3S) isomers and RasADH showed in some cases good selectivity towards the formation of anti‐(2S,3S) derivatives. With anti‐Prelog ADHs such as LBADH from L. brevis or LKADH from L. kefir, syn‐(2S,3R) alcohols were obtained with high conversions and diastereomeric excess in some cases, especially with LBADH. Furthermore, due to the thermodynamically favoured reduction of these substrates, it was possible to employ just a minimal excess of 2‐propanol to obtain the final products with quantitative conversions.     

Enzymes in Organic Chemistry, 11:[1] Hydrolase‐Catalyzed Resolution of α‐ and β‐Hydroxyphosphonates and Synthesis of Chiral,Non‐Racemic β‐Aminophosphonic Acids

The enantioselective acylation of racemic diisopropyl α‐ and β‐hydroxyphosphonates by hydrolases in t‐butyl methyl ether with isopropenyl acetate as acyl donor is limited by the narrow substrate specificity of the enzymes. High enantiomeric excesses (up to 99%) were obtained for the acetates of (S)‐diisopropyl 1‐hydroxy‐(2‐thienyl)methyl‐, 1‐hydroxyethyl‐ and 1‐hydroxyhexylphosphonate and (R)‐diisopropyl 2‐hydroxypropylphosphonate. The hydrolysis of a variety of β‐chloroacetoxyphosphonates by the lipase from Candida cylindracea and protease subtilisin in a biphasic system gives (S)‐β‐hydroxyphosphonates (ee 51–92%) enantioselectively. (S)‐2‐Phenyl‐2‐hydroxyethyl‐ and (S)‐3‐methyl‐2‐hydroxybutylphosphonates (ee 96% and 99%, respectively) were transformed into (R)‐2‐aminophosphonic acids of the same ee.     

Enantioselective Synthesis of Chiral β‐Aryloxy Alcohols by Ruthenium‐Catalyzed Ketone Hydrogenation via Dynamic Kinetic Resolution (DKR)

A highly efficient enantioselective synthesis of chiral β‐aryloxy alcohols by the {RuCl₂[(S)‐SDP][(R,R)‐DPEN]} [(S_a,R,R)‐ 1a ; SDP=7,7′‐bis(diarylphosphino)‐1,1′‐spirobiindane; DPEN=trans‐1,2‐diphenylethylenediamine] complex‐catalyzed asymmetric hydrogenation of racemic α‐aryloxydialkyl ketones via dynamic kinetic resolution (DKR) has been developed. Enantioselectivities of up to 99% ee with good to high cis/anti‐selectivities (up to>99:1) were achieved.     

β‐Aminopeptidase‐Catalyzed Biotransformations of β2‐Dipeptides: Kinetic Resolution and Enzymatic Coupling

We have previously shown that the β‐aminopeptidases BapA from Sphingosinicella xenopeptidilytica and DmpA from Ochrobactrum anthropi can catalyze reactions with non‐natural β³‐peptides and β³‐amino acid amides. Here we report that these exceptional enzymes are also able to utilize synthetic dipeptides with N‐terminal β²‐amino acid residues as substrates under aqueous conditions. The suitability of a β²‐peptide as a substrate for BapA or DmpA was strongly dependent on the size of the C_α substituent of the N‐terminal β²‐amino acid. BapA was shown to convert a diastereomeric mixture of the β²‐peptide H‐β²hPhe‐β²hAla‐OH, but did not act on diastereomerically pure β²,β³‐dipeptides containing an N‐terminal β²‐homoalanine. In contrast, DmpA was only active with the latter dipeptides as substrates. BapA‐catalyzed transformation of the diastereomeric mixture of H‐β²hPhe‐β²hAla‐OH proceeded along two highly S‐enantioselective reaction routes, one leading to substrate hydrolysis and the other to the synthesis of coupling products. The synthetic route predominated even at neutral pH. A rise in pH of three log units shifted the synthesis‐to‐hydrolysis ratio (v_S/v_H) further towards peptide formation. Because the equilibrium of the reaction lies on the side of hydrolysis, prolonged incubation resulted in the cleavage of all peptides that carried an N‐terminal β‐amino acid of S configuration. After completion of the enzymatic reaction, only the S enantiomer of β²‐homophenylalanine was detected (ee>99 % for H‐(S)‐β²‐hPhe‐OH, E>500); this confirmed the high enantioselectivity of the reaction. Our findings suggest interesting new applications of the enzymes BapA and DmpA for the production of enantiopure β²‐amino acids and the enantioselective coupling of N‐terminal β²‐amino acids to peptides.     

Synthesis of Chiral 2‐Hydroxy‐1‐methylpropanoates by Rhodium‐Catalyzed Stereoselective Hydrogenation of α‐(Hydroxymethyl)‐acrylate Derivatives

The synthesis of chiral 3‐hydroxy‐2‐methylpropanoic acid esters (e.g., “Roche ester” 3a ) based on the rhodium‐catalyzed stereoselective hydrogenation of Baylis–Hillman reaction products was investigated. Full conversions and enantioselectivities of up to 99% at a substrate/catalyst ratio of up to 500/1 were achieved by application of bisphospholanes of the catASium M series as ancillary ligands. An interesting kinetic resolution was observed by the diastereoselective hydroxy‐directed hydrogenation of related racemic β‐branched precursors affording mainly anti‐isomers with up to 96%ee.     

Kinetic Resolution of Aliphatic β‐Amino Acid Amides by β‐Aminopeptidases

Access to enantiopure β‐amino acids : β‐Aminopeptidases are hydrolases that possess the unique ability to cleave N‐terminal β‐amino acids from peptides and amides. Hydrolysis of racemic β‐amino acid amides catalyzed by these enzymes displays enantioselectivity with strong preference for substrates with the L ‐configuration, and gives access to various aliphatic β‐amino acids of high enantiopurity.

     

Efficient Lipase‐Catalyzed Kinetic Resolution and Dynamic Kinetic Resolution of β‐Hydroxy Nitriles. Correction of Absolute Configuration and Transformation to Chiral β‐Hydroxy Acids and γ‐Amino Alcohols

Chemoenzymatic dynamic kinetic resolution of β‐hydroxy nitriles 1 has been carried out using Candida antarctica lipase B and a ruthenium catalyst. The use of a hydrogen source to depress ketone formation in the dynamic kinetic resolution yields the corresponding acetates 2 in good yield and high enantioselectivity. It is shown that the ruthenium catalyst and the enzyme can be recycled when used in separate reactions. We also report on the preparation of various enantiomerically pure β‐hydroxy acid derivatives and γ‐amino alcohols from 1 and 2. The latter compounds were also used to establish the correct absolute configuration of 1 and 2.     

Enantiopure N‐Benzyloxycarbonyl‐β2‐amino Acid Allyl Esters from Racemic β‐Lactams by Dynamic Kinetic Resolution using Candida antarctica Lipase B

The dynamic kinetic resolution of α‐substituted racemic β‐lactams by alcoholytic ring‐opening, catalyzed by immobilized lipase B from Candida antarctica is described. With this process, a variety of racemic α‐substituted N‐Cbz‐azetidinones (Cbz=benzyloxycarbonyl) was transformed to the corresponding N‐Cbz‐protected β²‐amino acid allyl esters with high enantioselectivity (up to 99%) and high yields (up to quantitative) at room temperature.

     

Enantioselective Biooxidation of Racemic trans‐Cyclic Vicinal Diols: One‐Pot Synthesis of Both Enantiopure (S,S)‐Cyclic Vicinal Diols and (R)‐α‐Hydroxy Ketones

Highly regio‐ and enantioselective alcohol dehydrogenases BDHA (2,3‐butanediol dehydrogenase from Bacillus subtilis BGSC1A1), CDDHPm (cyclic diol dehydrogenase from Pseudomonas medocina TA5), and CDDHRh (cyclic diol dehydrogenase from Rhodococcus sp. Moj‐3449) were discovered for the oxidation of racemic trans‐cyclic vicinal diols. Recombinant Escherichia coli expressing BDHA was engineered as an efficient whole‐cell biocatalyst for the oxidation of (±)‐1,2‐cyclopentanediol, 1,2‐cyclohexanediol, 1,2‐cycloheptane‐diol, and 1,2‐cyclooctanediol, respectively, to give the corresponding (R)‐α‐hydroxy ketones in >99% ee and (S,S)‐cyclic diols in >99% ee at 50% conversion in one pot. Escherichia coli (BDHA‐LDH) co‐expressing lactate dehydrogenase (LDH) for intracellular regeneration of NAD⁺ catalyzed the regio‐ and enantioselective oxidation of (±)‐1,2‐dihydroxy‐1,2,3,4‐tetrahydronaphthalene to produce the corresponding (R)‐α‐hydroxy ketone in >99% ee and (S,S)‐cyclic diol in 96% ee at 49% conversion. Preparative biotransformations were also demonstrated. Thus, a novel and useful method for the one‐pot synthesis of both vicinal diols and α‐hydroxy ketones in high ee was developed via highly regio‐ and enantioselective oxidations of the racemic vicinal diols.

     

Dynamic Kinetic Resolution of Primary Alcohols with an Unfunctionalized Stereogenic Center in the β‐Position

Primary alcohols with an unfunctionalized stereogenic center in the β‐position undergo an enzyme‐ and metal‐catalyzed dynamic kinetic resolution (DKR). The in situ racemization of the primary alcohol, required for the DKR, takes place via: (i) ruthenium‐catalyzed dehydrogenation of the alcohol, (ii) enolization of the aldehyde formed, and (iii) ruthenium‐catalyzed readdition of hydrogen to the aldehyde. The present method widens the scope of metal‐ and enzyme‐catalyzed DKR, which has so far been limited to α‐chiral alcohol and amine derivatives.     

Transfer Hydrogenation of α‐Branched Ketimines: Enantioselective Synthesis of Cycloalkylamines via Dynamic Kinetic Resolution

The transfer hydrogenation of 2‐substituted bicyclic and monocyclic ketimines using HCO₂H/ Et₃N as the hydrogen source and TsDPEN‐based Ru(II) and Ir(III) catalysts proceeds with dynamic kinetic resolution to afford the corresponding cis‐cycloalkylamines with moderate to excellent levels of diastero‐ and enantioselectivity. A “one‐pot” procedure starting from ketones as starting materials with in situ formation of the reacting imines has also been developed.     

Enantioselective Synthesis of cis‐4‐Formyl‐β‐lactams via Chiral N‐Heterocyclic Carbene‐Catalyzed Kinetic Resolution

An efficient synthesis of optically pure cis‐4‐formyl‐β‐lactams (up to 99% ee) by a chiral NHC‐catalyzed ring expansion reaction has been realized, featuring the ready availability of both the substrate and the catalyst, and the mild reaction conditions. The current method is also suitable for the synthesis of enantioenriched 4‐formyl‐β‐lactams and succinimides containing quaternary carbon centers.     

Kinetic Resolution of α‐Bromoamides: Experimental and Theoretical Investigation of Highly Enantioselective Reactions Catalyzed by Haloalkane Dehalogenases

Haloalkane dehalogenases from five sources were heterologously expressed in Escherichia coli, isolated, and tested for their ability to achieve kinetic resolution of racemic α‐bromoamides, which are important intermediates used in the preparation of bioactive compounds. To explore the substrate scope, fourteen α‐bromoamides, with different Cα‐ and N‐substituents, were synthesized. Catalytic activity towards eight substrates was found, and for five of these compounds the conversion proceeded with a high enantioselectivity (E value >200). In all cases, the (R)‐α‐bromoamide is the preferred substrate. Conversions on a preparative scale with a catalytic amount of enzyme (enzyme:substrate ratio less 1:50 w/w) were all completed within 17–46 h and optically pure α‐bromoamides and α‐hydroxyamides were isolated with good yields (31–50%). Substrate docking followed by molecular dynamics simulations indicated that the high enantioselectivity results from differences in the percentage of the time in which the substrate enantiomers are bound favourably for catalysis. For the preferred (R)‐substrates, the angle between the attacking aspartate oxygen atom of the enzyme, the attacked carbon atom of the substrate, and the displaced halogen atom, is more often in the optimal range (>157°) for reactivity. This can explain the observed enantioselectivity of LinB dehalogenase in a kinetic resolution experiment.     

Hydrogenation of α‐Keto Ethers: Dynamic Kinetic Resolution with a Heterogeneous Modified Catalyst and a Heterogeneous Base

The first successful example of the asymmetric hydrogenation of substituted α‐keto ethers with Cinchona‐modified Pt/Al₂O₃ is reported. In the absence of an additional base, kinetic resolution of the racemic starting material was observed with high diastereoselectivity and ee's up to 98% at conversions of <50%. Addition of KOH gave a strong reaction acceleration but racemic product. Immobilization of OH ^– on solid ion exchangers resulted in the desired dynamic kinetic resolution, and ee's of >80% were obtained at >95% conversion. These effects are rationalized on the basis of a simple kinetic and structural model.