Advanced procedure for the enzymatic ring opening of unsaturated alicyclic β-lactams

Enantiopure β-amino acids 1a–4a and β-lactams 1b–4b were prepared simultaneously through the lipolase-catalysed enantioselective ring opening of unsaturated racemic β-lactams (±)-1-(±)-4. High enantioselectivities (E>200) were observed when the reactions were performed with 1 equiv of water in iPr₂O at 70 °C. The resolved (1R,2S)-amino acids (yield⩾45%) and (1S,5R)-, (1S,6R)- and (1S,8R)-lactams (yield⩾47%) could be easily separated. The ring opening of lactam enantiomers 1b–4b with 18% HCl afforded the corresponding β-amino acid hydrochlorides 1c·HCl–4c·HCl (ee >95%).     

Highly enantioselective enzymatic resolution of aromatic β-amino acid amides with Pd-catalyzed racemization

The kinetic resolution of an aromatic β-amino acid amide 3a–d via N-acylation was explored with two lipases, Candida antarctica lipase A (CALA) and Pseudomonas stutzeri lipase (PSL). The PSL-catalyzed resolution proceeded with excellent enantioselectivity (E = >400) to give both acylated products and unreacted substrates in enantiopure forms. Three additional aromatic β-amino acid amides 3b–d were also resolved by PSL with a high level of enantioselectivity (E = >200). The PSL-catalyzed resolution of 3a was coupled with a Pd-catalyzed racemization to obtain enantiopure N-acylated product (R)-4a (>99% ee) in high yield (90%).     

Synthesis of novel β-amino alcohols and their application in the catalytic asymmetric sulfoxidation of sulfides

Novel chiral norephedrine-based β-amino alcohol ligands containing a thiophene ring were prepared from norephedrine and substituted furan carbaldehydes (methyl- or ethyl-substituted) and used in combination with VO(acac)₂ for the asymmetric oxidation of aryl methyl sulfides using H₂O₂ as an oxidant. Amino alcohol derived Schiff bases 4,5a–b gave higher enantiomeric excesses than amino alcohol-derived reduced Schiff based ligands 6,7a–b. Of these chiral ligands, (1S,2R)-5b and (1S,2R)-7b gave high yields (90%) with moderate to high enantioselectivities (78%, 96% ee, respectively). The oxidation of other aryl methyl sulfides with (1S,2R)-5b and (1S,2R)-7b as ligands afforded the corresponding sulfoxides in 60–89% yields and with 92–99% ee.     

Synthesis of chiral ligands containing the N-(S)-α-phenylethyl group and their evaluation as activators in the enantioselective addition of Et2Zn to benzaldehyde

The synthesis of chiral ligands 4–18 derived from N-[(S)-α-phenylethyl]-trans-β-aminocyclohexanols (S,S,S)-1a and (R,R,S)-2 is described. Addition of diethylzinc to benzaldehyde catalyzed by ligands 4–18 (6 mol %) proceeds in fair to good yield (45–86%), and low to good enantioselectivities (1–76% ee). Highest enantioselectivities were induced by ligands (S,S,S)-4 and (S,S,S,S,R,R)-18 (76% and 68% ee, respectively). The configuration of the major enantiomer of carbinol 3 is (R) in both cases.     

Enzymatic resolution of 5-hydroxy- and 8-hydroxy-2-tetralols using immobilized lipases

(R)-2-Tetralol (R)-2a, (R)-5-hydroxy-2-tetralol (R)-2b and (R)-8-hydroxy-2-tetralol (R)-2c, which are key intermediates in the synthesis of pharmacologically active 2-aminotetralins 3, were prepared in moderate to very high enantiomeric excess (up to 99% ee) by enzymatic resolution of the corresponding racemic butyrates rac-1a, rac-1b and rac-1c, respectively, using lipases immobilized on octyl agarose. This methodology is an alternative to the microbial reduction of 2-tetralones.     

Transformation of racemic ethyl 3-hydroxybutanoate into the (R)-enantiomer exploiting lipase catalysis and inversion of configuration

Racemic ethyl 3-hydroxybutanoate rac-1 was transformed into ethyl (R)-acetoxybutanoate (ee = 92%) with 85–90% chemical yields using enantioselective acylation with isopropenyl acetate in the presence of Candida antarctica lipase B (CAL-B, Novozym 435) under solvent-free conditions, followed by mesylation of the unreacted (S)-alcohol in the reaction mixture and inversion of configuration with cesium acetate in DMF in one pot. When the (R)-acetoxybutanoate was subjected to alcoholysis with ethanol and CAL-B, enantiopure (R)-1 (ee >99%) was produced.     

Sharpless asymmetric dihydroxylation as the key step in the enantioselective synthesis of spirocyclic σ1 receptor ligands

The enantioselective synthesis of fluorinated spirocyclic σ₁ ligands involved three key steps: (1) the Sharpless asymmetric dihydroxylation of 2-bromostyrene 5 provided enantiomerically pure diols (R)-6 and (S)-6 establishing the stereogenic center; (2) the intramolecular opening of the oxirane ring of (R)-11 and (S)-11, which occurred with excellent regioselectivity and complete inversion of configuration giving access to enantiomerically pure alcohols (S)-7a and (R)-7a; (3) the treatment of alcohols (S)-7b and (R)-7b with DAST, which led to the fluoromethyl derivatives (S)-1 and (R)-1 without racemization. X-ray crystal structure analysis of the tosylate (R)-13 confirmed the absolute configuration of the spirocyclic compounds as well as the enantioselectivity during the Sharpless asymmetric dihydroxylation of 5. The (S)-configured fluoromethyl derivative (S)-1 revealed a high σ₁ affinity (K_i = 1.8 nM), high eudismic ratio (factor 8) and high selectivity over the σ₂ subtype (667-fold).     

An efficient synthesis of (7S,10R)-2-bromo-5,6,7,8,9,10-hexahydro-7,10-epiminocyclohepta[b]indole: application in the preparation and structural confirmation of a potent 5-HT6 antagonist

(7S,10R)-5-Methyl-2-((3-(trifluoromethyl)phenyl)sulfonyl)-5,6,7,8,9,10-hexahydro-7,10-epiminocyclohepta[b]indole 1a is a potent 5-HT₆ antagonist (h5-HT₆ K_i = 1.5 nM) which is derived from an epiminocyclohept[b]indole scaffold. In order to synthesize 1a on a multi-gram scale to support advanced biological testing, an efficient chiral resolution of the intermediate tert-butyl 2-bromo-5,6,7,8,9,10-hexahydro-7,10-epiminocyclohepta[b]indole-11-carboxylate 2 was developed. After derivatizing 2 with (1R)-(?)-menthyl chloroformate it was found that a single diastereomer 7a could be isolated by selective precipitation from n-hexane. The absolute stereochemistry of 7a was determined by X-ray crystallography and the structure was confirmed as (7S,10R)-tert-butyl 2-bromo-5,6,7,8,9,10-hexahydro-7,10-epiminocyclohepta[b]indole-11-carboxylate. Removal of the chiral auxiliary under basic conditions afforded intermediate 2a in >99% enantiomeric purity and with 80% yield based on recovery from the racemic compound 2. Intermediate 2a was used successfully to synthesize 5-HT₆ antagonist 1a on a multi-gram scale.     

Chemoenzymatic synthesis of glycosylated enantiomerically pure 4-pentene 1,2- and 1,3-diol derivatives

1-Dimethylthexylsiloxy-2-chloroacetoxy-4-pentene 2 and 1-dimethylthexylsiloxy-3-chloroacetoxy-4-pentene 3 were saponified with Pseudomonas lipase to give (R)-1-dimethylthexylsiloxy-4-pentene-2-ol (ee=99%) and (S)-2 (ee=99%) and (S)-1-dimethylthexylsiloxy-4-pentene-3-ol (ee=99%) and (R)-3 (ee=98%), respectively. All enantiomers were chemically transformed into the corresponding enatiomerically pure 2-benzoyloxy-4-pentene-1-ols 8 and 3-benzoyloxy-4-pentene-1-ols 14, respectively. Mannosylation of (R)-8 and (S)-14 with 2,3,4,6-tetra-O-benzoyl-a-d-mannopyranosyl trichloroacetimidate afforded the corresponding mannopyranosides.     

Ring-expanded chiral rhombamine macrocycles for efficient NMR enantiodiscrimination of carboxylic acid derivatives

Novel 46-membered chiral rhombamine macrocycles (R,R,R,R)-8a and 8b were synthesized by [2+2] cyclocondensation reactions of (R,R)-1,2-diaminocyclohexane with the corresponding dialdehydes and subsequent reduction with NaBH₄. The X-ray crystal structure of 1:4 dioxane complex with (R,R,R,R)-8a indicated a rhombus conformation of the chiral macrocycle. Compounds (R,R,R,R)-8a and 8b were tested as chiral shift reagents for a wide range of α-substituted carboxylic acids and amino acid derivatives. Enantiodiscrimination of ¹H NMR signals was observed with ΔΔδ values of up to 0.214 ppm.     

Enzymatic reactions for the preparation of homocalycotomine enantiomers

Homocalycotomine enantiomers (R)-4 and (S)-4 were prepared by the Candida antarctica lipase B (CAL-B)-catalysed asymmetric O-acylation of N-Boc-protected 2-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-1-yl)ethanol (±)-1. The preliminary small-scale experiments were performed either in a continuous-flow system or as batch reactions, while the preparative-scale resolution was carried out in two steps with vinyl acetate as the acyl donor in the presence of Et₃N and Na₂SO₄ in toluene at 3 °C, as a batch reaction. Treatment of the resulting amino alcohol (S)-1 and amino ester (R)-3 (ee ?94%) with 18% HCl, and then with 5 M NaOH, furnished the desired (R)-4 and (S)-4 without a decrease in the enantiomeric excess (ee ?94%).     

Enantioselective 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds catalyzed by rhodium(I)-chiral phosphoramidite complexes

A chiral bidentate phosphoramidite (5a) was synthesized from Shibasaki’s linked-(R)-BINOL and P(NMe₂)₃ as a new ligand for rhodium(I)-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds. The effects of 5a and Feringa’s monodentate phosphoramidite (4, R¹, R² = Et) on the yields and enantioselectivities were fully investigated. The reaction was significantly accelerated in the presence of a base such as KOH and Et₃N, allowing the reaction to be completed at the lower temperatures than 50 °C. The addition to cyclic enones such as 2-cyclopentenone, 2-cyclohexenone and 2-cycloheptenone at 50 °C in the presence of an [Rh(coe)₂Cl]₂-4 (R¹, R² = Et) complex resulted in enantioselectivities up to 98%, though it was less effective for acyclic enones (0–70% ee). On the other hand, a complex between [Rh(nbd)₂]BF₄ and 5a completed the addition to cyclic enones within 2 h at room temperature in the presence of Et₃N with 86–99% yields and 96–99.8% ee. This catalyst was also effective for acyclic enones, resulting in 62–98% yields and 66–94% ee. The 1,4-additions of arylboronic acids to unsaturated lactones and acyclic esters with rhodium(I)-phosphoramidites complexes were also investigated.     

An efficient Amano PS-catalyzed chemo-, regio- and enantioselective hydrolysis of (±)-2,3-di-O-acetyl-2-C-methyl-d-erythrono-1,4-lactone: a facile preparation of bioactive natural products (−)-saccharinic acid lactone and potassium (2R,3R)-2,3,4-trihydroxy-2-methylbutanoate

Saccharinic acid lactone (−)-1a is a suitable building block for the synthesis of many bioactive natural products. Amano PS-induced chemo-, regio- and enantioselective hydrolysis of diacetyl lactone (±)-3 has been carried out to obtain (−)-1a in 46% yield with 99% ee and diacetyl lactone (+)-3 in 49% yield with 99% ee. The Amano PS-catalyzed enantioselective acylation of (±)-1a with vinyl acetate as an acyl donor was relatively less efficient and furnished (−)-7 in 31% yield with 99% ee and (+)-1a in 63% yield. The conversion of (−)-1a to leaf-closing substance 2a and an attempted approach to naturally occurring compounds 1b and 2b have been also described.     

Approaches to (R)- and (S)-1′-(1-aminoethyl)ferrocene-1-carboxylic acid derivatives

Lipase-catalyzed esterification of (±)-methyl 1′-(1-hydroxyethyl)ferrocene-1-carboxylate 4 afforded its (R)-acetate (−)-5 (ee = 99%) and (S)-(+)-4 (ee = 90%). Stereoretentive azidation/amination/acetylation of (R)-(−)-5 gave (R)-(+)-methyl 1′-(1-acetamidoethyl)ferrocene-1-carboxylate (R)-3 (ee = 98%). In a similar manner (S)-(+)-4 was converted into (S)-(−)-3 (ee = 84%). Both enantiomers of 3 were obtained in high chemical yields without a loss of enantiomeric purity. The title compounds can be coupled with natural amino acids and peptides on both C- and N-termini.     

Lipase-catalyzed kinetic and dynamic kinetic resolution of 1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid

A dynamic kinetic resolution method for the preparation of enantiopure 1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid (R)-2 was developed involving the CAL-B-catalyzed enantioselective hydrolysis of the corresponding ethyl ester (±)-1 in toluene/acetonitrile (4:1) containing 1 equiv of added water and 0.25 equiv of dipropylamine. This method allowed the preparation of (R)-2 (ee = 96%) with 80% isolated yield. The kinetic resolution of (±)-1 in diisopropyl ether at 3 °C afforded both enantiomers with ee ⩾92%.     

Asymmetric hydrogenation reactions with Rh and Ru complexes bearing phosphine–phosphites with an oxymethylene backbone

Phosphine–phosphites 3a and 3b, derived from diphenylhydroxymethyl phosphine have been prepared. From these ligands [Rh(COD)(3a)]BF₄ 5a and RuCl₂(3b)[(S,S)-DPEN] 6b (DPEN = 1,2-diphenylethylenediamine) were synthesized and their structure determined by X-ray diffraction. Ligands 3 are characterized by a small bite angle of 83°. In addition, 5a led to an active catalyst for the hydrogenation of olefins, giving enantioselectivities of up to 96% ee. Likewise, compound 6b showed good activity and enantioselectivity in the hydrogenation of N-1-phenyl ethylidene aniline and a completed reaction at S/C = 500 in 24 h with 83% ee.     

Continuous-flow enzymatic resolution strategy for the acylation of amino alcohols with a remote stereogenic centre: synthesis of calycotomine enantiomers

Both enantiomers of calycotomine (R)-5 and (S)-5 were prepared through the CAL-B-catalysed asymmetric O-acylation of N-Boc-protected (6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-1-yl)methanol [(±)-3)]. The optimum conditions for the enzymatic resolution were determined under continuous-flow conditions, while the preparative-scale resolution of (±)-3 was performed as a batch reaction with high enantioselectivity (E >200). The resulting amino alcohol (S)-3 and amino ester (R)-4, obtained with high enantiomeric excess (ee = 99%), were transformed into the desired calycotomine (S)-5 and (R)-5 (ee = 99%). A systematic study was carried out in a continuous-flow system on the O-acylation of tetrahydroisoquinoline amino alcohol homologues (±)-1 to (±)-3 containing a remote stereogenic centre.     

Biotransformation of 3-azidomethyl-4-phenyl-3-buten-2-one and analogs by Saccharomyces cerevisiae: New evidence for an SN2′ mechanism

(Z)-3-XCH₂-4-(C₆H₅)-3-buten-2-one enones (X = SCN, N₃, SO₂Me, OC₆H₅) were synthesized and submitted to biotransformations using whole Saccharomyces cerevisiae cells. The enone (X = SCN) produced (R)-4-(phenyl)-3-methylbutan-2-one (R)-6 with 93% ee and enones (X = N₃, SO₂Me, OC₆H₅) yielded a mixture of (R)-6 and the corresponding CC bond reduction products. Biotransformation with enone (X = N₃) mediated by Saccharomyces cerevisiae resulted in two products via two different routes: (i) the ketone (R)-4-azido-3-benzylbutan-2-one in 28% yield and with >99% ee by CC bond reduction; (ii) ketone (R)-6 in 51% yield and with 95% ee via cascade reactions beginning with azido group displacement by the formal hydride from flavin mononucleotide in an S_N2′ type reaction followed by reduction of the newly formed CC bond.     

Second-generation artificial hydrogenases based on the biotin–avidin technology: Improving selectivity and organic solvent tolerance by introduction of an (R)-proline spacer

We report on our efforts to create efficient artificial metalloenzymes for the enantioselective hydrogenation of N-protected dehydroamino acids using streptavidin as host protein. Introduction of an (R)-proline spacer between the biotin anchor and the diphosphine moiety affords a versatile ligand Biot-(R)-Pro-1 which displays good (S)-selectivities in the presence of streptavidin (91% ee). The resulting artificial metalloenzyme [Rh(Biot-(R)-Pro-1)(COD)]⁺ ⊂ WT-Sav displays increased stability against organic solvents.     

Synthesis of the (1S,2S)- and (1R,2S)-stereoisomers of the respective E- and Z-isomers of ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoate using yeast whole cell bioreduction of the parent ketones

Saccharomyces cerevisiae, strain DBM 2115, was successfully employed in the reduction of the separated Z- and E-isomers of ethyl 4-[(2-oxocyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 1 and 2, in order to prepare the (1S,2S)- and (1R,2S)-enantiomers of the corresponding ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 3–6. The products were obtained with the required absolute configuration: (1S,2S)-3 (ee = 98%; yield 48%), (1R,2S)-4 (ee = >99%; yield 45%), (1S,2S)-5 (ee = 98.5%; yield 47%), and (1R,2S)-6 (ee = >99%; chemical yield 44%).