Synthesis of Valuable Chiral Intermediates by Isolated Ketoreductases: Application in the Synthesis of α‐Alkyl‐β‐hydroxy Ketones and 1,3‐Diols

Regio‐ and stereoselective reductions of α‐substituted 1,3‐diketones to the corresponding β‐keto alcohols or 1,3‐diols by using commercially available ketoreductases (KREDs) are described. A number of α‐monoalkyl‐ or dialkyl‐substituted symmetrical as well as non‐symmetrical diketones were reduced in high optical purities and chemical yields, in one or two enzymatic reduction steps. In most cases, two or even three out of the four possible diastereomers of α‐alkyl‐β‐keto alcohols were synthesized by using different enzymes, and in two examples both ketones were reduced to the 1,3‐diol. By replacing the α‐alkyl substituent with the OAc group, 1‐keto‐2,3‐diols, as well as 1,2,3‐triols were synthesized in high optical purities. These enzymatic reactions provide a simple, highly stereoselective and quantitative method for the synthesis of different diastereomers of valuable chiral synthons from non‐chiral, easily accessible 1,3‐diketones.     

α‐ and β‐Stannyl Trifluoromethylbutenoates: Regioselective Preparation and Use in Copper(I)‐Catalyzed Allylation and Propargylation Reactions

The palladium‐free hydrostannylation of ethyl 4,4,4‐trifluorobutynoate 1 with tributyltin hydride at room temperature is highly regio‐ and stereoselective, providing good yields of β‐trifluoromethyl (Z)‐α‐ or (Z)‐β‐stannylacrylates 2 . Vinylstannanes 2 undergo a copper(I)‐catalyzed coupling reactions with allylic or propargylic bromides leading selectively to good yields of the corresponding allylated or propargylated products without allylic or allenic transposition.     

Diastereoselective and Enantioselective Epoxidation of Acyclic β‐Trifluoromethyl‐β,β‐Disubstituted Enones by Hydrogen Peroxide with a Pentafluorinated Quinidine‐Derived Phase‐Transfer Catalyst

An efficient catalytic asymmetric epoxidation of β‐trifluoromethyl‐β,β‐disubstituted unsaturated ketones has been achieved by a pentafluorine‐substituted phase‐transfer catalyst with hydrogen peroxide (30%). Thus, the β‐trifluoromethyl‐α,β‐epoxy ketones with a quaternary carbon centre were obtained in excellent diastereoselectivities (up to 100:1 dr) and excellent enantioselectivities (up to 99.7% ee). Low catalyst loading, recycle of catalyst, environmentally benign oxidant and easy transformation of the epoxides into medicinally important trifluoromethylated intermediate make our protocol much more practical.     

Asymmetric α‐Hydroxylation of β‐Indanone Esters and β‐Indanone Amides Catalyzed by C‐2′ Substituted Cinchona Alkaloid Derivatives

The highly catalytic asymmetric α‐hydroxylation of β‐indanone esters and β‐indanone amides using peroxide as the oxidant was realized with a new C‐2′ substituted Cinchona alkaloid derivatives. The two enantiomers of α‐hydroxy‐β‐indanone esters could be obtained by simply changing the oxidant. This protocol allows a convenient access to the corresponding α‐hydroxy‐β‐indanone esters and α‐hydroxy‐β‐indanone amides with up to 99% yield and 98% ee.

     

Enantioselective Vanadium‐Catalyzed Oxidation of 1,3‐Dithianes from Aldehydes and Ketones using β‐Amino Alcohol Derived Schiff Base Ligands

The asymmetric vanadium‐catalyzed oxidation of 1,3‐dithianes from aldehydes and ketones by β‐amino alcohol‐derived Schiff base ligands with two stereogenic centers was investigated. Using aqueous hydrogen peroxide as the oxidant and the Schiff base 3b as a chiral ligand, a variety of 1,3‐dithianes derived from aldehydes were easily converted into the corresponding mono‐sulfoxides in good yields (81–88%) with excellent enantioselectivities (up to 99% ee). Additionally, 99% ee was obtained for the enantioselective vanadium‐catalyzed oxidation of the 1,3‐dithianes derived from ketones. We found a slight kinetic resolution when using a higher ratio of hydrogen peroxide during the oxidation of the aldehyde‐derived 1,3‐dithianes but not in the ketone‐derived 1,3‐dithianes.     

Multi‐Enzymatic Synthesis of Optically Pure β‐Hydroxy α‐Amino Acids

A novel enzymatic production system of optically pure β‐hydroxy α‐amino acids was developed. Two enzymes were used for the system: an N‐succinyl L ‐amino acid β‐hydroxylase (SadA) belonging to the iron(II)/α‐ketoglutarate‐dependent dioxygenase superfamily and an N‐succinyl L ‐amino acid desuccinylase (LasA). The genes encoding the two enzymes are part of a gene set responsible for the biosynthesis of peptidyl compounds found in the Burkholderia ambifaria AMMD genome. SadA stereoselectively hydroxylated several N‐succinyl aliphatic L ‐amino acids and produced N‐succinyl β‐hydroxy L ‐amino acids, such as N‐succinyl‐L ‐β‐hydroxyvaline, N‐succinyl‐L ‐threonine, (2S,3R)‐N‐succinyl‐L ‐β‐hydroxyisoleucine, and N‐succinyl‐L ‐threo‐β‐hydroxyleucine. LasA catalyzed the desuccinylation of various N‐succinyl‐L ‐amino acids. Surprisingly, LasA is the first amide bond‐forming enzyme belonging to the amidohydrolase superfamily, and has succinylation activity towards the amino group of L ‐leucine. By combining SadA and LasA in a preparative scale production using N‐succinyl‐L ‐leucine as substrate, 2.3 mmol of L ‐threo‐β‐hydroxyleucine were successfully produced with 93% conversion and over 99% of diastereomeric excess. Consequently, the new production system described in this study has advantages in optical purity and reaction efficiency for application in the mass production of several β‐hydroxy α‐amino acids.

     

Specificity of Donor Structures for endo‐β‐N‐Acetylglucosaminidase‐Catalyzed Transglycosylation Reactions

To demonstrate the structural specificity of the glycosyl donor for the transglycosylation reaction by using endo‐β‐N‐acetylglucosaminidase from Mucor hiemalis (endo‐M), a series of tetrasaccharide oxazoline derivatives was synthesized. These derivatives correspond to the core structure of an asparagine‐linked glycoprotein glycan with a β‐mannose unit of a non‐natural‐type monosaccharide, including β‐glucose, β‐galactose, and β‐talose in place of the β‐mannose moiety. The transglycosylation activity of wildtype (WT) endo‐M and two mutants, N175Q and N175A, was examined by using these tetrasaccharide donors with p‐nitrophenyl N‐acetylglucosaminide (GlcNAc‐pNp). The essential configuration of the hydroxy group for the transglycosylation reaction was determined. On the basis of these results, the transglycosylation reaction was investigated by using chemically modified donors, and transglycosylated products were successfully obtained.     

Simultaneous refolding,purification, and immobilization of recombinant Fibrobacter succinogenes 1,3‐1,4‐β‐D‐glucanase on artificial oil bodies

BACKGROUND: 1,3‐1,4‐β‐D‐glucanase (1,3‐1,4‐β‐D‐glucan 4‐glucanohydrolase; EC 3.2.1.73) has been used in a range of industrial processes. As a biocatalyst, it is better to use immobilized enzymes than free enzymes, therefore, the immobilization of 1,3‐1,4‐β‐D‐glucanase was investigated. RESULTS: A 1,3‐1,4‐β‐D‐glucanase gene from Fibrobacter succinogenes was overexpressed in Escherichia coli as a recombinant protein fused to the N terminus of oleosin, a unique structural protein of seed oil bodies. With the reconstitution of the artificial oil bodies (AOBs), refolding, purification, and immobilization of active 1,3‐1,4‐β‐D‐glucanase was accomplished simultaneously. Response surface modeling (RSM), with central composite design (CCD), and regression analysis were successfully applied to determine the optimal temperature and pH conditions of the AOB‐immobilized 1,3‐1,4‐β‐D‐glucanase. The optimal conditions for the highest immobilized 1,3‐1,4‐β‐D‐glucanase activity (7.1 IU mg^?1 of total protein) were observed at 39 °C and pH 8.8. Furthermore, AOB‐immobilized 1,3‐1,4‐β‐D‐glucanase retained more than 70% of its initial activity after 120 min at 39 °C, and it was easily and simply recovered from the surface of the solution by brief centrifugation; it could be reused eight times while retaining more than 80% of its activity. CONCLUSIONS: These results indicate that the AOB‐based system is a comparatively simple and effective method for simultaneous refolding, purification, and immobilization of 1,3‐1,4‐β‐D‐glucanase. Copyright © 2009 Society of Chemical Industry     

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.     

Organocatalytic Asymmetric Aldol Reaction of Ketones with β,γ‐Unsaturated α‐Keto Esters: An Efficient Access to Chiral Tertiary Alcohol Skeletons

This update describes a highly efficient organocatalytic aldol reaction of ketones and β,γ‐unsaturated α‐keto esters for constructing the chiral tertiary alcohol motif. With the application of 9‐amino(9‐deoxy)epi‐Cinchona alkaloid and an acidic additive as catalysts, both acyclic and cyclic ketones react with β,γ‐unsaturated α‐keto esters smoothly to afford aldol adducts in good to excellent yields and asymmetric induction. This protocol offers a new pathway for the construction of adjacent chiral carbon centers and the synthesis of chiral β‐hydroxy carbonyl compounds.     

Highly Efficient Suzuki Coupling Reaction of α‐Chloroalkylidene‐β‐lactones and β‐Lactams with Organoboronic Acids

The activation of C Cl bond of (Z)‐α‐chloroalkylidene‐β‐lactones and (E)‐α‐chloroalkylidene‐β‐lactams via the Suzuki cross‐coupling reaction is reported in this paper. Alkyl, heteroaromatic, substituted phenyl‐ and alkenylboronic acids can be coupled with a wide variety of α‐chloroalkylidene‐β‐lactones and β‐lactams in excellent yields within a short period of time. The cross‐coupling reaction of optically active substrates leads to the optically active compounds without racemization of the corresponding chiral center.     

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.     

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.     

Preparation and Use of Polystyryl‐DABCOF2: An Efficient Recoverable and Reusable Catalyst for β‐Azidation of α,β‐Unsaturated Ketones in Water

Novel solid fluorides were prepared to optimize the β‐azidation of α,β‐unsaturated ketones. The higher loading of these catalysts compared to that of commercially available fluorides has allowed the use of a smaller mass of catalyst helping the mixing of the reaction mixture. Porous polymeric supports have proved to be more efficient in the presence of water as reaction medium. Water has played a crucial role showing a beneficial effect on the reactivity by improving dispersion of the reaction mixture and also by avoiding organic fouling caused by the retention of the reaction mixture within the polymeric matrix. This has facilitated the recovery of the products from the catalyst. The protocol reported has allowed a significant reduction in the organic solvent required for the complete recovery of the pure product whilst leaving the catalyst clean and reusable. E‐factors are in the range of 5.9–10.5 and therefore ca. 3 times smaller than previous procedures operating under solvent‐free conditions. To further improve the efficiency of our approach we have developed a protocol operating in a continuous‐flow manner that has allowed us to achieve an E‐factor of 1.7–1.9, with a reduction of ca. 80% of the corresponding batch conditions. The continuous‐flow protocol has allowed us to minimize the use of trimethylsilyl azide making the recovery and reuse of water and catalyst 5f very efficient and simple. Finally, a novel reduction system using palladium on alumina (5 mol%) and equimolar amount of formic acid has been used in the presence of 1 equivalent of di‐tert‐butyl pyrocarbonate to set a multistep protocol operating in continuous‐flow conditions for the preparation of two representative N‐Boc‐β‐amino ketones starting from the corresponding enones with E‐factors of 3.2 and 2.7, respectively.     

Intermolecular Gold(I)‐Catalyzed Alkyne Carboalkoxylation Reactions for the Multicomponent Assembly of β‐Alkoxy Ketones

A new gold(I)‐catalyzed multicomponent synthesis of β‐alkoxy ketones from aldehydes, alcohols, and alkynes is described. This atom economical synthesis was achieved through the use of the gold complex (SPhos)AuNTf₂ as a catalyst, and allows for the preparation of a diverse array of β‐alkoxy ketone products. Mechanistic studies illustrate that these reactions proceed via gold(I)‐catalyzed hydrolysis of the alkyne to an aryl ketone, which then undergoes an aldol reaction with an oxocarbenium ion generated in situ from the aldehyde and alcohol components.     

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.     

Synthese und Charakterisierung von O2S2 — und N2S2‐Übergangsmetallkomplexen ausgehend von β‐Chlor) β‐trifluormethylvinylaldehyden

Synthesis and Characterization of O₂S₂ — and N₂S₂‐Transition Metal Complexes Starting from β‐Chloro‐β‐trifluoromethyl Vinylaldehydes The syntheses of complexes 4 and 5 with O₂S₂ ‐and N₂S₂ — donor atom sets are described as one‐step procedures. Their structures were confirmed by NMR, IR, UV‐ VIS and MS spectroscopy. One nickel complex 5a was determined by X‐ray structure analysis whereas the Cu^II complexes were studied by EPR spectroscopy.     

Structure and morphology of poly(β‐hydroxybutyrate) synthesized by ring‐opening polymerization of racemic (R,S)‐β‐butyrolactone with distannoxane derivatives

Predominantly syndiotactic poly((R,S)‐β‐hydroxybutyrate) (PHB) was synthesized by ring‐opening polymerization of racemic β‐butyrolactone with distannoxane derivatives as catalysts. We have studied the polymerization of (R,S)‐β‐BL using distannoxane derivatives as catalysts and the effects of polymerization time on crude yield and molecular weight of the polymers obtained. Then, a more detailed study of the characterization of polymers obtained using hydroxy‐ and ethoxy‐distannoxanes was performed. ¹³C NMR spectroscopy resolved stereosequences in synthetic PHB at the diad level for the carbonyl carbon and at the triad level for the methylene carbon. These analyses show that distannoxane catalysts produce preferentially syndiotactic polyesters (syndiotactic diads fraction from 0.56 to 0.61). Triad stereosequence distribution of PHB samples agrees favourably with the Bernoullian statistical model of chain‐end control, where ideally Φ = 4(mm) (rr)/(mr + rm)² = 1 for perfect chain‐end control. Polymer samples synthesized from distannoxane catalysts are composed of two distinct transition endotherm components with peak temperatures of approximately 42 °C and 75 °C. The formation of two melting endotherms may be due to the presence of two different crystalline structures. © 2000 Society of Chemical Industry     

N,N′‐Dioxide‐Magnesium Ditriflate Complex‐Catalyzed Asymmetric α‐Hydroxylation of β‐Keto Esters and β‐Keto Amides

The highly catalytic asymmetric α‐hydroxylation of 1‐tetralone‐derived β‐keto esters and β‐keto amides using tert‐butyl hydroperoxide (TBHP) as the oxidant was realized by a chiral N,N′‐dioxide‐magnesium ditriflate [Mg(OTf)₂] complex. A series of corresponding chiral α‐hydroxy dicarbonyl compounds was obtained in excellent yields (up to 99%) with excellent enantioselectivities (up to 98% ee). The products were easily transformed into useful building blocks and the precursor of daunomycin was achieved in an asymmetric catalytic way for the first time.     

In vitro Characterization of Enzymes Involved in the Synthesis of Nonproteinogenic Residue (2S,3S)‐β‐Methylphenylalanine in Glycopeptide Antibiotic Mannopeptimycin

Mannopeptimycin, a potent drug lead, has superior activity against difficult‐to‐treat multidrug‐resistant Gram‐positive pathogens such as methicillin‐resistant Staphylococcus aureus (MRSA). (2S,3S)‐β‐Methylphenylalanine is a residue in the cyclic hexapeptide core of mannopeptimycin, but the synthesis of this residue is far from clear. We report here on the reaction order and the stereochemical course of reaction in the formation of (2S,3S)‐β‐methylphenylalanine. The reaction is executed by the enzymes MppJ and TyrB, an S‐adenosyl methionine (SAM)‐dependent methyltransferase and an (S)‐aromatic‐amino‐acid aminotransferase, respectively. Phenylpyruvic acid is methylated by MppJ at its benzylic position at the expense of one equivalent of SAM. The resulting β‐methyl phenylpyruvic acid is then converted to (2S,3S)‐β‐methylphenylalanine by TyrB. MppJ was further determined to be regioselective and stereoselective in its catalysis of the formation of (3S)‐β‐methylphenylpyruvic acid. The binding constant (K_D) of MppJ versus SAM is 26 μM . The kinetic constants with respect to k_{cat Ppy} and K_{M Ppy}, and k_{cat SAM} and K_{M SAM} are 0.8 s^?1 and 2.5 mM , and 8.15 s^?1 and 0.014 mM , respectively. These results suggest SAM has higher binding affinity for MppJ than Ppy, and the C? C bond formation in βmPpy might be the rate‐limiting step, as opposed to the C? S bond breakage in SAM.