Practical and efficient synthesis of chiral 2,4-disubstituted oxazolines from β-phosphonoamides

Herein we report a practical and efficient method for the synthesis of optically active 2,4-disubstituted oxazolines (S)-1a–h in good to excellent yields. The target compounds were prepared in good yield through the Horner–Wadsworth–Emmons reaction of β-phosphonoamide 3 bearing l-phenylalaninol with commercially available aryl aldehydes followed by the cyclodehydration of the corresponding N-(cinnamoyl)-(S)-phenylalaninol derivatives (S)-2a–h. Additionally, the cyclodehydration of β-phosphonoamide (S)-3 followed by the Horner–Wadsworth–Emmons reaction of β-phosphono-oxazoline (S)-4 with aryl aldehydes also gave the 2,4-disubstituted oxazolines (S)-1a–h.     

Resolution of racemic 1-azido-3-aryloxy-2-propanols by lipase-catalyzed enantioselective acetylation

Kinetic resolution of racemic 1-azido-3-aryloxy-2-propanols 1a–g was performed using supported lipase of Candida antarctica-B (Novozym^® SP 435) in toluene at 4°C with isopropenyl acetate as the acyl donor to afford the optically active (S)-alcohols 2a–g and their corresponding (R)-acetates 3a–g with E values from 56 to 72.     

Asymmetric Friedel-Crafts alkylation of activated benzenes with methyl (E)-2-oxo-4-aryl-3-butenoates catalyzed by [Pybox/Sc(OTf)3]

The asymmetric Friedel-Crafts reaction between methyl (E)-2-oxo-4-aryl-3-butenoates (1a-c) and activated benzenes (2a-d) has been efficiently catalyzed by the Sc^III triflate complex of (4′S,5′S)-2,6-bis[4′-(triisopropylsilyl) oxymethyl-5′-phenyl-1′,3′-oxazolin-2′-yl]pyridine (pybox 3). The 4,4-diaryl-2-oxo-butyric acid methyl esters (4) are usually formed in good yields and the enantioselectivity is up to 99% ee. The sense of the stereoinduction can be rationalized with the same octahedral complex (10) between 1, pybox 3 and Sc triflate already proposed for other reactions involving pyruvates, and catalyzed by the same complex.     

Synthesis of arylated highly congested indans using a domino sequence

A novel and efficient regioselective synthesis of various arylated highly congested 7-aryl-5-methylsulfanylindan-4-carbonitriles (3a-f), methyl 7-aryl-5-methylsulfanylindan-4-carboxylates (10a-e) and 7-aryl-5-methylsulfanylindan-4-carboxylic acids (11a-e) through base-catalyzed reaction of 6-aryl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitriles (1a-f) and methyl 6-aryl-4-methylsulfanyl-2-oxo-2H-pyran-3-carboxylates (9a-e) by cyclopentanone (2) has been delineated. The synthetic potential of 2-pyranone was explored further to generate molecular diversity using 6-aryl-4-sec-amino-2-oxo-2H-pyran-3-carbonitriles (7a-h), 5,6-diaryl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitriles (5a,b) and methyl 5,6-diaryl-4-methylsulfanyl-2-oxo-2H-pyran-3-carboxylates (12a,b) as precursors for the ring transformation by cyclopentanone to assess the effects of substituents on the course of the reaction to obtain highly congested indans, 6,7-diaryl-5-methylsulfanylindan-4-carbonitriles (6a,b), 7-aryl-5-(piperidin-1-yl)indan-4-carbonitriles (8a-h) and methyl 6,7-diaryl-5-methylsulfanylindan-4-carboxylates (13a,b).     

Chemoenzymatic syntheses of N-trifluoroacetyl-l-daunosamine,N-trifluoroacetyl-l-acosamine,N-benzoyl-d-acosamine,and N-benzoyl-d-ristosamine from an achiral precursor,methyl sorbate

A conjugated addition of benzylamine to methyl (4R,5S)-4,5-(isopropylidenedioxy)-(2E)-hexenoate 8a followed by lactonization under acidic condition proceeds to the formal total syntheses of l-daunosamine 4 and l-acosamine 2. On the other hand, direct conjugated addition of benzylamine to methyl (4S,5S)-4,5-epoxy-(2E)-hexenoate 5 and the subsequent intramolecular nucleophilic attack by the ester carbonyl group on the epoxy ring of the substrates leads to the formal total syntheses of d-acosamine 2 and d-ristosamine 1.     

Stereoselective production of (S)-1-aralkyl- and 1-arylethanols by freshly harvested and lyophilized yeast cells

Substituted (S)-1-phenyl- 2a–h and (S)-1-benzyl-propan-2-ols 4a and b, and (S)-1-phenylethanol 6 were produced from prochiral ketones 1a–h, 3a,b and 5 by reductions with freshly harvested Zygosaccharomyces rouxii and Debaryomyces hansenii cells. The bioreductions were also performed by lyophilized cells. Comparison of the secondary alcohols from the bioreductions 2b–e,g,h and 4a and authentic (S)-alcohols (S)-2b–e,g,h and (S)-4a synthesized from enantiopure (S)-methyloxirane 7 proved the absolute configuration of the products.     

Synthesis of methyl 4-aryl-4-oxo-2-[(4-sulfamoylphenyl)amino]but-2-enoates and their reaction with ninhydrin

The reaction of methyl 4-aryl-2,4-dioxobut-2-enoates with 4-aminobenzenesulfonamide in acetic acid–ethanol (1: 1) afforded methyl (2Z)-4-aryl-4-oxo-2-(4-sulfamoylanilino)but-2-enoates which reacted with ninhydrin in glacial acetic acid to give 3-aroyl-4-(4-sulfamoylanilino]-5H-spiro[furan-2,2′-indene]-1′,3′,5-triones.     

Photochemistry of β, γ-unsaturated ketones-V: The direct irradiation of some γ-phenyl β, γ-enones

The photochemistry of some members of the two series of γ-phenyl substituted acyclic β, γ-unsaturated ketones 1 and 2 upon direct irradiation with γ 310nm has been investigated, viz 1c–1h and 2b+2c.The alkyl substituted (E)-5-phenyl-4-penten-2-ones 1c–1h yield the corresponding 1,3-acyl shift products and (Z)-isomers, and 1g and 1h in addition two decarbonylated products. 2b only yields the (Z)-isomer and some benzaldehyde, but 2c yields the 1,3-acyl shift product, the ODPM product, three hydrocarbons formed by disproportionation of the allyl radical, and some benzaldehyde. The β-phenyl β, γ-UK 3a proved to be photostable. The 1,3-acyl shift products of 1c–1h result mainly from the singlet excited state in a cage radical process. The exclusive formation of the (E)-configuration of the 1,3-acyl shift product is explained in terms of conformational preference of the intermediate allyl radical. It is proposed that the formation of the (Z)-isomer proceeds from ¹T(π -π^*) which is populated according to

. Evidence is presented which supports the proposed mechanism.The β,γ-UK 2b containing a benzoyl moiety leads to a higher degree of (E)-(Z) isomerization than the corresponding 1d which has an acetyl moiety.The triplet energies of (E)- and (Z)-1h are 56 and ca 70 kcal/mol respectively.     

Natural product synthesis from (8aR)- and (8aS)-bicyclofarnesols: synthesis of (+)-wiedendiol A, (+)-norsesterterpene diene ester and (−)-subersic acid

Both enantiomers (8aR)-7 and (8aS)-7 of bicyclofarnesol were synthesized from the enzymatic resolution products (1R,4aR,8aR)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aR)-5 (98% ee) and acetate of (1S,4aS,8aS)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aS)-6 (>99% ee), respectively. The formal synthesis of (+)-wiedendiol 1 was achieved via a coupling reaction of an ate complex derived from 1,2,4-trimethoxybenzene with allyl bromide (8aS)-8 derived from (8aS)-7. The total synthesis of (+)-norsesterterpene diene ester 2 was achieved, based on the synthesis of (13E,10S)-α,β-unsaturated aldehyde 12, derived from (8aS)-7, followed by the selective construction of the (3E,5E)-diene moiety including a C(2)-stereogenic centre in (+)-2. The total synthesis of (−)-subersic acid 3 was carried out based on a Stille coupling between allyl trifluoroacetate congener 25c, derived from (8aR)-7, corresponding to the diterpene part, and aryl stannane congener 26 in the presence of Pd catalyst and CuI as an additive.     

Resolution of 1-arylalkylamines with 3-O-hydrogen phthalate glucofuranose derivatives: role of steric bulk in a family of resolving agents

The development of three new acidic resolving agents which are hydrogen phthalates of 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose 1, 1,2:5,6-di-O-cyclohexylidene-α-d-glucofuranose 2 and 1,2-O-cyclohexylidene-5,6-O-diphenylmethylidene-α-d-glucofuranose 3 is shown for the resolution of 1-arylalkylamines 7a–k. The salts between 1, 2 and (RS)-1-arylalkylamines 7a–k selectively crystallize 1·(S) 7a–j and 2·(S) 7a–h salts, allowing us to recover the corresponding bases (S) 7a–j and (S) 7a–h, respectively, in good yield and enantiomeric excess (73–95% ee). Whereas, the salts between 3 and (RS)-1-arylalkylamines 7a–c,g–i,k selectively crystallize 3·(S)-7a–c,g–i salts to recover the corresponding bases (S)-7a–c,g–i in poor enantiomeric excess (4–35% ee). The difference between the resolving ability of 1 and 2 for 1-arylalkylamines 7a–h is very slight, but there is considerable difference compared to ortho-substituted 1-arylalkylamines 7i and 7j. The role of substituents on a family of resolving agents 1, 2 and 3 is also discussed to interpret their resolving ability.     

Novel thiosemicarbazone derivatives containing benzimidazole moiety: Green synthesis and anti-malarial activity

A series of (E)-2-[5-chloro-1-(1H-benzo[d]imidazol-2-yl)ethylidene] N-(substituted) hydrazine carbothioamide (7a–7t) and (E)-2-[1-(1H-benzo[d]imidazol-2-yl)ethylidene] N-(substituted) hydrazine carbothioamide (8a–8t) were prepared via the synthesis of 1-(substituted-1H-benzimidazol-2-yl) ethanol (3a–3b) which was synthesized by the condensation of substituted o-phenylenediamine (2a–2b) with dl-lactic acid (1) followed by oxidation with sodium hypochlorite in mild acidic condition to form the corresponding ketones 4a–4b. Final compounds were formed by condensation of 4a–4b with different thiosemicarbazides 6a–6t. A total of 40 compounds were synthesized and characterized by FT-IR, ¹H NMR, ¹³C NMR, Mass spectral technique and elemental analysis, in addition they were evaluated for anti-malarial properties. Among the compounds tested 7o, 7p, 7q, 7r, 7s, 8e and 8h exhibited good antimalarial activity in vitro.     

Preparation of various enantiomerically pure (benzotriazol-1-yl)- and (benzotriazol-2-yl)-alkan-2-ols

(S)-(−)-(Benzotriazol-1-yl)- and (S)-(−)-(benzotriazol-2-yl)-alkan-2-ols 7a–9a, 7b–9b and their (R)-(+)-acetates 10a–12a and 10b–12b were prepared in high enantiomeric excess via lipase from Pseudomonas fluorescens (Amano AK) catalyzed enantioselective acetylation of racemic alcohols 4a–6a and 4b–6b with vinyl acetate in tert-butyl methyl ether or toluene at 23 °C. The enantioselectivity of this transformation was dependent on the length of the alkyl chain with E-values ranging from 30 to 57. Several benzotriazole substituted ketones 1a–3a and 1b–3b were synthesized from 1H-benzotriazole and corresponding haloketones. These compounds were stereoselectively reduced with Baker’s yeast in water or in organic solvent containing 5% v/v of water at 30 °C to give the (S)-(−)-alcohol. Better stereoselectivity was observed in the kinetic resolution of racemic alcohols 4a–6a and 4b–6b (ee = 69–92% at 44–52% conversion) compared to reduction of corresponding prochiral ketones 1a–3a and 1b–3b with Baker’s yeast (ee = 40–67% at 39–89% conversion). Enhanced enantioselectivities were observed at lower temperatures.     

Optically active 1-(benzofuran-2-yl)ethanols and ethane-1,2-diols by enantiotopic selective bioreductions

Enantiotopic selective reduction of 1-(benzofuran-2-yl)ethanones 1a–d, 1-(benzofuran-2-yl)-2-hydroxyethanones 4a–c and 2-acetoxy-1-(benzofuran-2-yl)ethanones 3a–c was performed by baker's yeast for preparation of optically active (benzofuran-2-yl)carbinols [(S)-5a–d, (S)-6a–c and (R)-6a–c, enantiomeric excess from 55 to 93% ee].     

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.     

Acylation of Fischer’s Base with Methyl Aroylpyruvates

Methyl (Z)-4-aryl-2-hydroxy-4-oxobut-2-enoates (methyl aroylpyruvates) reacted with 1,3,3-trimethyl- 2-methylidene-2,3-dihydro-1H-indole (Fischer’s base) to give (2Z,5E)-1-aryl-3-hydroxy-5-(1,3,3-trimethyl-2,3-dihydro-1H-indol-2-ylidene)pent-2-ene-1,4-diones.     

Heterocycles 35. CaL-B mediated synthesis of enantiomerically pure (R)- and (S)-ethyl 3-(2-arylthiazol-4-yl)-3-hydroxypropanoates

Herein we present the lipase catalyzed synthesis of four new enantiomerically pure (R)- and (S)-ethyl 3-(2-arylthiazol-4-yl)-3-hydroxypropanoates and their butanoates by enzymatic enantioselective acylation of the racemic alcohols rac-1a–d and by ethanolysis of the corresponding racemic esters rac-2a–d mediated by lipase B from Candida antarctica (CaL-B) in organic solvents. In terms of stereoselectivity and activity, both procedures, the acylation and alcoholysis, are successful (50% conversion, E ≫ 200). The absolute configuration of the resolution products was determined by a detailed ¹H NMR study of the Mosher’s derivatives of (S)-1a.     

A convenient,rapid microwave assisted synthesis of some novel 3-[(4-chloro-3-methylphenoxy)methyl]-6-aryl-5,6-dihydro-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles using acidic alumina

A facile synthesis of 3-[(4-chloro-3-methylphenoxy)methyl]-6-aryl-5,6-dihydro-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles 3a–n has been achieved by microwave promoted condensation of 3-mercapto-4-amino-5-[(4-chloro-3-methylphenoxy)methyl]-4H-1,2,4-triazole 1 with various aromatic aldehydes 2a–n in presence of catalytic amount of p-TsOH (para-toluenesulphonic acid). The structures of 3a–n are supported by IR and ¹H and ¹³C NMR spectral data.     

Reactions of β-substituted amines-IV: Kinetics and stereochemistry of the thermal to (r) 3-chloro-1-ethylpiperidine,and the stereo-chemical course of their reactions with nucleophiles

The rate of the thermal rearrangement of (S) 2 chloromethyl-1-ethylpyrrolidine [(S)-1a] to (R)-3-chloro-1-ethylpiperidine [(R) 2a] has been examined at three temperatures in benzene by PMR and polarimetry. The rearrangement was shown to be completely stereospecific and to obey a simple first order rate law. The calculated E_a ΔH3 and ΔS3 were 22 ± 2 $\text{kcal}\text{mole}$ (25°), 21 ± 2.5 $\text{kcal}\text{mole}$ (25°) and - 10 ± 2 e.u. (0°K) respectively. The effect of solvents having differing dielectric constants was also studied. A transition state 9'a and an ion pair intermediate 3a are suggested for the rearrangement. The stereochemical course of the reactions of (S)-1a, (R)-2a and (S)-2a with hydroxide and methoxide ions have been shown to be 100% stereospecific with an uncertainty of about 1%. The absolute configurations of all optically active reactants and products [(S)- and (R)-4a, (S)-4b (R)- and (S)-5a, (R)-5b, (S,S')-6a, (S,R')-7a and (R,R')-8a] were established by chemical correlations with known compounds or by ORD and chemical inference. The ring opening of both the primary and secondary aziridinium ion positions of 1-azonia-1-ethylbicyclo [3.1.0]hexane [(S)-3a] by nucleophiles proceeds entirely by S_N2 processes. The conversion of (R)-1-ethyl-3-hydroxypiperidine [(R)-5a] to (S)-2a. HCl with thionyl chloride in chloroform proceeds by inversion with 4.8% racemization, whereas the thermal rearrangement of (S)-1a to (R)-2a occurs with complete retention of absolute configuration.     

Synthesis of substituted chiral chromans via organocatalytic kinetic resolution of racemic 3-nitro-2-aryl-2H-chromenes with ketones catalyzed by pyrrolidinyl-camphor-derived organocatalysts

Efficient kinetic resolution of racemic 2-aryl-3-nitro-2H-chromenes (1a–l) has been explored with the pyrrolidinyl-camphor derivative 2b as a bifunctional organocatalyst under neat conditions in the presence of AcOH at 0 °C. In general, the organocatalytic asymmetric Michael addition of ketones proceeded smoothly to give the functionalized Michael adducts (3a–n) with good-to-high diastereo- and enantioselectivities (up to 92:8 dr, 93% ee, and 47% yield). The less reactive chromenes (S)-1a–h, k, l and (R)-1i–j were recovered in high chemical yields and moderate optical purity (up to 42% chemical yield and 72% ee).     

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%).