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.     

Highly Enantioselective Michael Addition of Cyclic 1,3‐Dicarbonyl Compounds to β,γ‐Unsaturated α‐Keto Esters

A highly enantioselective Michael addition of cyclic 1,3‐dicarbonyl compounds to β,γ‐unsaturated α‐keto esters catalyzed by amino acid‐derived thiourea‐tertiary‐amine catalysts is presented. Using 5 mol% of a novel tyrosine‐derived thiourea catalyst, a series of chiral coumarin derivatives were obtained in excellent yields (up to 99%) and with up to 96% ee under very mild conditions within a short reaction 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.     

Ruthenium‐Catalyzed One‐Pot Tandem Isomerization–Transfer Hydrogenation Reactions of γ‐Trifluoromethylated Allylic Alcohols and β‐Trifluoromethylated Enones

The ruthenium–2‐propanol combination was found to transform γ‐trifluoromethylated allylic alcohols and β‐trifluoromethylated enones into the corresponding saturated alcohols in excellent yields via a one‐pot tandem process involving isomerization and transfer hydrogenation(s). High stereospecificity was demonstrated and evidence for two mechanistic pathways is provided. The method was applied to a rapid synthesis of trifluoromethylated citronellol.     

Stereo‐Controlled Asymmetric Bioreduction of α,β‐Dehydroamino Acid Derivatives

α,β‐Dehydroamino acid derivatives proved to be a novel substrate class for ene‐reductases from the ‘old yellow enzyme’ (OYE) family. Whereas N‐acylamino substituents were tolerated in the α‐position, β‐analogues were generally unreactive. For aspartic acid derivatives, the stereochemical outcome of the bioreduction using OYE3 could be controlled by variation of the N‐acyl protective group to furnish the corresponding (S)‐ or (R)‐amino acid derivatives. This switch of stereopreference was explained by a change in the substrate binding, by exchange of the activating ester group, which was proven by ²H‐labelling experiments.     

Axial Chirality Control by 2,4‐Pentanediol for the Alternative Synthesis of C3*‐TunePhos Chiral Diphosphine Ligands and Their Applications in Highly Enantioselective Ruthenium‐Catalyzed Hydrogenation of β‐Keto Esters

A highly efficient strategy for the synthesis of a series of C₃*‐TunePhos chiral diphosphine ligands was well established with several remarkable features. The synthetic utility of these ligands was explored for the ruthenium‐catalyzed asymmetric hydrogenation of β‐keto esters. Up to 99% ee values were achieved for the enantioselective synthesis of β‐hydroxy acid derivatives, which are very important chiral building blocks for the synthesis of a variety of natural products and biologically active molecules.     

Enzymatic Synthesis of Enantiopure α‐ and β‐Amino Acids by Phenylalanine Aminomutase‐Catalysed Amination of Cinnamic Acid Derivatives

The phenylalanine aminomutase (PAM) from Taxus chinensis catalyses the conversion of α‐phenylalanine to β‐phenylalanine, an important step in the biosynthesis of the N‐benzoyl phenylisoserinoyl side‐chain of the anticancer drug taxol. Mechanistic studies on PAM have suggested that (E)‐cinnamic acid is an intermediate in the mutase reaction and that it can be released from the enzyme's active site. Here we describe a novel synthetic strategy that is based on the finding that ring‐substituted (E)‐cinnamic acids can serve as a substrate in PAM‐catalysed ammonia addition reactions for the biocatalytic production of several important β‐amino acids. The enzyme has a broad substrate range and a high enantioselectivity with cinnamic acid derivatives; this allows the synthesis of several non‐natural aromatic α‐ and β‐amino acids in excellent enantiomeric excess (ee >99 %). The internal 5‐methylene‐3,5‐dihydroimidazol‐4‐one (MIO) cofactor is essential for the PAM‐catalysed amination reactions. The regioselectivity of amination reactions was influenced by the nature of the ring substituent.     

Ruthenium‐Catalyzed Asymmetric Hydrogenation of β‐Keto‐ enamines: An Efficient Approach to Chiral γ‐Amino Alcohols

A highly efficient and enantioselective hydrogenation of unprotected β‐ketoenamines catalyzed with ruthenium(II) dichloro{(S)‐(−)‐2,2′‐bis[di(3,5‐xylyl)phosphino]‐1,1′‐binaphthyl}[(2S)‐(+)‐1,1‐bis(4‐methoxyphenyl)‐3‐methyl‐1,2‐butanediamine] {Ru[(S)‐xylbinap][(S)‐daipen]Cl₂} has been successfully developed. This methodology provides a straightforward access to free γ‐secondary amino alcohols, which are key building blocks for a variety of pharmaceuticals and natural products, with high yields (>99%) and excellent enantioselectivities (up to 99% ee) in all cases.     

Amine‐Catalyzed Asymmetric Epoxidation of α,β‐Unsaturated Aldehydes

The direct organocatalytic enantioselective epoxidation of α,β‐unsaturated aldehydes with different peroxides is presented. Proline, chiral pyrrolidine derivatives, and amino acid‐derived imidazolidinones catalyze the asymmetric epoxidation of α,β‐unsaturated aldehydes. In particular, protected commercially available α,α‐diphenyl‐ and α,α‐di(β‐naphthyl)‐2‐prolinols catalyze the asymmetric epoxidation reactions of α,β‐unsaturated aldehydes with high diastereo‐ and enantioselectivities to furnish the corresponding 2‐epoxy aldehydes in high yield with up to 97:3 dr and 98 % ee. The use of non‐toxic catalysts, water and hydrogen peroxide, urea hydroperoxide or sodium percarbonate as the oxygen sources could make this reaction environmentally benign. In addition, one‐pot direct organocatalytic asymmetric tandem epoxidation‐Wittig reactions are described. The reactions were highly diastereo‐ and enantioselective and provide a rapid access to 2,4‐diepoxy aldehydes. Moreover, a highly stereoselective one‐pot organocatalytic asymmetric cascade epoxidation‐Mannich reaction, which proceeds via the combination of iminium and enamine activation, is presented. The mechanism and stereochemistry of the amino acid‐ and chiral pyrrolidine‐catalyzed direct asymmetric epoxidation of α,β‐unsaturated aldehydes are also discussed.     

Ruthenium(II)‐Catalyzed Regioselective Synthesis of Allyl Ketones from Alkynes and their Silver(I)‐Catalyzed Hydroarylation into γ‐Functionalized Ketones

The regioselective synthesis of β,γ‐unsaturated ketones from terminal alkynes is achieved by cooperative action of tris(acetonitrile)pentamethylcyclopentadieneruthenium hexafluorophosphate [Cp*Ru(NCMe)₃⁺ PF₆⁻] and para‐toluenesulfonic acid catalysts. These allyl ketones undergo direct regioselective hydroarylation/Friedel–Crafts reaction to introduce an electron‐rich aryl group at the γ‐position in the presence of ligand‐free silver triflate (AgOTf) catalyst. Both catalytic reactions take place with atom economy and provide an alternative to the synthesis of a variety of allyl ketones and γ‐arylated ketones.     

Catalytic Hydrogenation of Chiral α‐Amino and α‐Hydroxy Esters at Room Temperature with Nishimura Catalyst without Racemization

The hydrogenation of carboxylic acid derivatives at room temperature was investigated. With a mixed Rh/Pt oxide (Nishimura catalyst), low to medium activity was observed for various α‐amino and α‐hydroxy esters. At 100 bar hydrogen pressure and 10% catalysts loading, high yields of the desired amino alcohols and diols were obtained without racemization. The most suitable α‐substituents were NH₂, NHR, and OH, whereas β‐NH₂ were less effective. Usually, aromatic rings were also hydrogenated, but with the free bases of amino acids as substrates, some selectivity was observed. No reaction was found for α‐NR₂, α‐OR, and unfunctionalized esters; acids and amides were also not reduced under these conditions. A working hypothesis for the mode of action of the catalyst is presented.     

Titanium‐Mediated Direct Carbon‐Carbon Double Bond Formation to α‐Trifluoromethyl Acids: A New Contribution to the Knoevenagel Reaction and a High‐Yielding and Stereoselective Synthesis of α‐Trifluoromethylacrylic Acids

An efficient strategy for a high‐yielding and stereoselective synthesis of α‐trifluoromethyl unsaturated carboxylic acids directly from the reactions of 3,3,3‐trifluoropropanoic acid (CF₃CH₂COOH) with various aryl aldehydes in the presence of titanium tetrachloride (TiCl₄) is reported here for the first time, which is a valuable expansion for the classical Knoevenagel reaction. Because these compounds may have potential applications in organic electronics and can be easily converted to the corresponding fluorinated alcohols and amino acids with excellent bioactivity, this route should be a good choice for the preparation of α‐trifluoromethyl‐containing derivatives.     

Ruthenium‐Catalyzed O‐Allylation of Phenols from Allylic Chlorides via Cationic [Cp*(η3‐allyl)(MeCN)RuX][PF6] Complexes

The [Cp*(MeCN)₃Ru(II)][PF₆] complex is an efficient catalyst precursor for the O‐allylation of phenols with allylic chlorides in the presence of K₂CO₃ under mild conditions. This ruthenium precursor affords branched allyl aryl ethers according to a regioselective reaction, which contrasts with the uncatalyzed nucleophilic substitution from the same substrates. Stable (η³‐allyl)Ru(IV) cationic complexes resulting from the reaction of [Cp*(MeCN)₃Ru][PF₆] with allylic halides were identified as intermediate catalytic species. An X‐ray structure determination of the complex [Cp*(MeCHCHCH₂)(MeCN)RuBr][PF₆] disclosed an (endo‐trans‐MeCHCHCH₂) allylic ligand. The structural information obtained from the study of Cp*(allyl)Ru(IV) complexes indicated that electronic effects at the coordinated allylic ligand likely account for the better regioselectivity obtained from cinnamyl chloride as compared to aliphatic allylic chlorides.     

Synthesis of poly(β‐pinene)‐g‐polystyrene from allylic brominated poly(β‐pinene)

Poly(β‐pinene) was brominated by N‐bromosuccinimide on the allylic carbons. Then the brominated product was activated by AlEt₂Cl to initiate the polymerization of styrene to give a β‐pinene/styrene graft copolymer. AlEt₂Cl was selected because it alone could not initiate the polymerization of styrene. The obtained graft copolymer was characterized by GPC, ¹H‐NMR, and DSC measurements, respectively. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 599–603, 2000     

Rhodium‐Catalyzed Asymmetric Hydroformylation of 1,1‐Disubstituted Allylphthalimides: A Catalytic Route to β3‐Amino Acids

The high enantioselective rhodium‐catalyzed hydroformylation of 1,1‐disubstituted allylphthalimides has been developed. By employing chiral ligand 1,2‐bis[(2S,5S)‐2,5‐diphenylphospholano]ethane [(S,S)‐Ph‐BPE], a series of β³‐aminoaldehydes can be prepared with up to 95% enantioselectivity. This asymmetric procedure provides an efficient alternative route to prepare chiral β³‐amino acids and alcohols.     

N‐alkylation of chitosan by β‐halopropionic acids in the presence of various acceptors

N‐carboxyethylation of chitosan by β‐halopropionic acids in the presence of various proton and halogen ion acceptors was investigated. It has been observed that carboxyethylation of chitosan in aqueous medium is accompanied by the by‐processes of hydrolysis and dehydrohalogenation of the β‐halopropionic acids yielding β‐hydroxypropionic acid, bis(2‐carboxyethyl) ether, and acrylic acid. Degree of carboxyethyl substitution (DS) of chitosan and the relative rates of the by‐processes varied significantly depending on the conditions used and nature of the proton or halogen ion acceptor. At carboxyethylation of chitosan with the alkaline β‐bromopropionates, the DS increased in the order Cs⁺ < Rb⁺ < K⁺ ～ Na⁺ < Li⁺. For alkaline earth salts BrCH₂CH₂COOM_0.5 (M = Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), the highest DS was obtained with strontium and barium salts, which could be subsequently removed from the reaction mixture by precipitation as sulfates. Among the organic bases applied (tetrabutylammonium hydroxide, triethylamine, trimethylamine, pyridine, 4‐N,N‐dimethylaminopyridine, 2,6‐lutidine, and 1,5‐diazabicyclo[4.3.0] non‐5‐ene), the highest DS was obtained using a moderately strong base triethylamine. For the halogen acceptors (Pb²⁺, Ag⁺, Tl⁺), the stoichiometrically highest DS was achieved in a system comprising iodopropionic acid plus Tl⁺ and a comparable conversion rate was obtained using also a combination of chloropropionic acid and Ag⁺. A novel alternative preparative approach—gel‐state synthesis—was suggested that provides for the highest DS at the optimum reaction conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

A Convenient Synthesis of 2,2′,6,6′‐Tetramethoxy‐ 4,4′‐bis(dicyclohexylphosphino)‐3,3′‐bipyridine (Cy‐P‐Phos): Application in Rh‐Catalyzed Asymmetric Hydrogenation of α‐(Acylamino)acrylates

The first example of the synthesis of an axially chiral bis(aryldicyclohexylphosphine) dioxide via catalytic hydrogenation of the optically resolved parent bis(aryldiphenylphosphine) dioxide was reported. The procedure for the synthesis of Cy‐P‐Phos ( 4d ) has thus successfully avoided the need for an otherwise lengthy synthetic route owing to the π‐excessive nature of one of the aryl groups in the latter. The use of Cy‐P‐Phos in the Rh(I)‐catalyzed asymmetric hydrogenation of the derivatives of methyl (Z)‐2‐acetamidocinnamate gave significantly higher rates of reaction as compared to the use of the previously reported optimal ligand Xyl‐P‐Phos ( 4c ) whilst the level of enantioselectivity was essentially maintained.     

Acetylcholine Promotes Binding of α‐Conotoxin MII at α3β2 Nicotinic Acetylcholine Receptors

α‐Conotoxin MII (α‐CTxMII) is a 16‐residue peptide with the sequence GCCSNPVCHLEHSNLC, containing Cys2–Cys8 and Cys3–Cys16 disulfide bonds. This peptide, isolated from the venom of the marine cone snail Conus magus, is a potent and selective antagonist of neuronal nicotinic acetylcholine receptors (nAChRs). To evaluate the impact of channel–ligand interactions on ligand‐binding affinity, homology models of the heteropentameric α₃β₂‐nAChR were constructed. The models were created in MODELLER with the aid of experimentally characterized structures of the Torpedo marmorata‐nAChR (Tm‐nAChR, PDB ID: 2BG9) and the Aplysia californica‐acetylcholine binding protein (Ac‐AChBP, PDB ID: 2BR8) as templates for the α₃‐ and β₂‐subunit isoforms derived from rat neuronal nAChR primary amino acid sequences. Molecular docking calculations were performed with AutoDock to evaluate interactions of the heteropentameric nAChR homology models with the ligands acetylcholine (ACh) and α‐CTxMII. The nAChR homology models described here bind ACh with binding energies commensurate with those of previously reported systems, and identify critical interactions that facilitate both ACh and α‐CTxMII ligand binding. The docking calculations revealed an increased binding affinity of the α₃β₂‐nAChR for α‐CTxMII with ACh bound to the receptor, and this was confirmed through two‐electrode voltage clamp experiments on oocytes from Xenopus laevis. These findings provide insights into the inhibition and mechanism of electrostatically driven antagonist properties of the α‐CTxMIIs on nAChRs.     

Iridium‐Catalyzed Asymmetric Allylic Etherification and Ring‐Closing Metathesis Reaction for Enantioselective Synthesis of Chromene and 2,5‐Dihydrobenzo[b]oxepine Derivatives

Iridium‐catalyzed asymmetric etherifications of allylic carbonates with 2‐vinylphenols and 2‐allylphenols were realized. With a catalyst generated from 2 mol% of [Ir(cod)Cl]₂ (cod=cycloocta‐1,5‐diene) and 4 mol% of the phosphoramidite ligand L2 , the etherification products were obtained in excellent ees and then subjected to the ring‐closing metathesis reaction providing an efficient synthesis of enantioenriched 2H‐chromene and 2,5‐dihydrobenzo[b]oxepine derivatives.