d-Phg-l-Pro Dipeptide-derived prolinol ligands for highly enantioselective Reformatsky reactions

Optically pure N-aminoethyl prolinol derivatives 3a-c have been prepared from the dynamic kinetic resolution of N-(α-bromo-α-phenylacetyl) proline ester 1 in asymmetric nucleophilic substitution and subsequent reduction. The peptide-derived prolinols are tested as chiral ligands in the asymmetric addition of Reformatsky reagent to aromatic aldehydes. Chiral ligand 3c has been shown to be effective to produce enantioenriched β-hydroxy esters 5a-j with up to 98% ee.     

The preparation and alkylation of a butanedione-derived chiral glycine equivalent and its use for the synthesis of α-amino acids and α,α-disubstituted amino acids

A benzyloxycarbonyl protected glycine equivalent 2 has been prepared in enantiopure form and has been used in the synthesis of both α-substituted amino acids and α,α-disubstituted amino acids. The process involved deprotonation to form the corresponding enolates which underwent stereoselective alkylation with various electrophiles and upon hydrolysis gave the corresponding amino acid derivatives as enantiomerically pure products.     

Novel chiral Schiff base macrocycles containing azobenzene chromophore: gelation and guest inclusion

A novel chiral Schiff base macrocycle 1 was synthesized by [3+3] condensation of enantiomerically pure trans-1,2-diaminocyclohexane with azobenzene-4,4′-dicarbaldehyde. Subsequent reduction of 1 afforded macrocyclic hexamine 2 having three azobenzene units. The former could be converted into a benzene gel, while the latter could include several aromatic guest molecules.     

Study on enantiomerically pure 2-substituted N,N-dialkyl-1-naphthamides: resolution,absolute stereochemistry,and application to desymmetrization of cyclic meso anhydrides

The axially chiral 2-substituted N,N-diisopropyl-1-naphthamides 1 and 2 were resolved by HPLC over a chiral stationary phase to provide enantiomerically pure atropisomers. The absolute stereochemistry of (−)-syn-1 was determined by X-ray crystallographic analysis of the corresponding (1S)-camphanic acid ester derivative. Desymmetrization of cyclic meso anhydrides 5a and 5b using (−)-syn-1 gave a single diastereomer in good yield.     

Enantioselective synthesis of (S)-N,N-diethyl-2-formyl-2-(methoxymethoxy)butyramide,a key intermediate for 20(S)-camptothecin analogues,via asymmetric bromolactonization

A new enantioselective synthetic method for enantiomerically pure (S)-N,N-diethyl-2-formyl-2-(methoxymethoxy)butyramide 5, a versatile key intermediate has been developed employing asymmetric bromolactonization using (S)-proline as the chiral auxiliary.     

Synthesis of enantiomerically pure inherently chiral calix[4]arenes using the meta-substitution strategy

A meta-substituted aminocalix[4]arene 4 immobilized in the cone conformation was prepared via mercuration of the starting tetra-propoxy derivative, followed by nitrosation and reduction reactions. Acylation of the amino group by chiral amino acid residues ((S)-N-trifluoroacetyl-Ala or (S)-N-trifluoroacetyl-Phe) allowed for the preparation of diastereomeric amides that were separated by preparative TLC on silica gel. Subsequent cyclization under Bischler-Napieralski reaction conditions yielded calixarenes 7b and 7c bearing an oxazole moiety in the meta position instead of the expected upper rim-bridged compounds. The reaction sequence represents a straightforward approach towards enantiomerically pure inherently chiral calix[4]arene derivatives (without HPLC separation steps). The absolute configuration of the enantiomers were confirmed by single crystal structure determination (X-ray).     

(4R,5S)-1,5-Dimethyl-4-phenylimidazolidin-2-one as a chiral auxiliary for the diastereoselective alkylation of a new iminic glycine derivative: practical asymmetric synthesis of α-amino acids

The lithium enolate of enantiomerically pure N-[bis(methylthio)methylene] glycinate 11 derived from (4R,5S)-1,5-dimethyl-4-phenylimidazolidin-2-one reacts with alkyl halides giving the alkylated derivatives 12 with a high degree of control of the diastereoselectivity. These alkylated systems are easily hydrolyzed to the corresponding α-amino acids, the chiral auxiliary being recovered.     

Efficient method for the asymmetric reduction of α- and β-ketophosphonates

An efficient and versatile method for the asymmetric reduction of α- and β-ketophosphonates using chiral reactant derived from sodium borohydride and l-(+)- or d-(−)-tartaric acid is developed. The methodology was used for the preparation of a number of biologically interesting enantiomerically pure products: including 2,3-epoxypropylphosphonate 11, 2-hydroxy-3-aminopropylphosphonic acid 14 (phospho-GABOB), phospho-carnitine 19, and others in multigram scale.     

Oxazoline-mediated highly stereoselective synthesis of α,β-substituted-β-aminoalkanamides, potential precursors of unnatural β-amino acids

The reaction of α-lithiated-2-alkyl-2-oxazolines 1-Li with aliphatic, aromatic and heteroaromatic N-cumyl nitrones results in the stereoselective formation of N-cumyl-1,6-dioxa-2,9-diazaspiro[4,4]nonanes 3 which equilibrate with the hydroxylamino derivatives 4. Such equilibrating mixtures can be easily transformed into β-amino alkanamides 5 under reductive conditions, whereas acidic hydrolysis with trifluoroacetic acid (TFA) furnishes high yields of β-phenylamino alkanamides 6 via a cumene hydroperoxide-type rearrangement. Derivatives 5 and 6 provide a backbone of potentially useful unnatural β^2,2,3-amino acids.     

An efficient approach to 2-substituted N-tosylpiperdines: asymmetric synthesis of 2-(2-hydroxy substituted)piperidine alkaloids

We have developed an efficient and a general approach to chiral 2-substituted N-tosylpiperidines starting from chiral α-substituted-N-tosylaziridines. Using this approach, we have synthesized (+)-coniine. The synthesis of chiral N-tosyl-2-piperidinylethanol 15 and ent-15, was achieved from l- and d-aspartic acids, respectively in few steps. Piperidine 15 was converted into 2-(2-hydroxysubstituted)piperidines of type 2 in optically active form. By applying this strategy, asymmetric syntheses of halosaline (R,R)-2a, (+)- and (−)-sedamine 2b, (+)- and (−)-allosedamine 2c, (+)- and (−)-sedridine 2d, (+)- and (−)-allosedridine 2e, (+)-tetraponerine T-3 3a, T-4 3c, T-7 3b, and T-8 3d have been achieved in high yields. These stereoisomers can be interconverted via Mitsunobu inversion in excellent yields.     

Asymmetric synthesis of substituted 1-aminocyclopropane-1-carboxylic acids from a new chiral glycine equivalent with 3,6-dihydro-2H-1,4-oxazin-2-one structure

Condensation of the new chiral glycine equivalent 10 with aldehydes at room temperature in the presence of K₂CO₃ under solid–liquid phase-transfer-catalysed conditions afforded stereoselectively new chiral (Z)-α,β-didehydroamino acid (DDAA) derivatives with oxazinone structure 14. These systems have been used in diastereoselective cyclopropanation reactions for the synthesis of enantiomerically pure 1-aminocyclopropanecarboxylic acids (ACCs) such as (−)-allo-norcoronamic and (−)-allo-coronamic acids.     

Manganese(III)-mediated facile synthesis of 3,4-dihydro-2(1H)-quinolinones: selectivity of the 6-endo and 5-exo cyclization

The 4,4-bis(ethoxycarbonyl)-3,4-dihydro-2(1H)-quinolinones 2 were easily synthesized by the oxidative 6-endo-trig cyclization of 2-[2-(N-arylamino)-2-oxoethyl]malonates 1 with manganese(III) acetate in good to excellent yields. The same reaction of N-(2,4-dimethoxyphenyl)-substituted malonate 1t exclusively produced the 5-exo-cyclized 4,4-bis(ethoxycarbonyl)-1-azaspiro[4,5]deca-6,9-diene-2,8-dione 5t instead of the corresponding dihydroquinolinone. The regioselectivity during the cyclization could be explained by the difference in the activation energy of the transition state of the 6-endo/5-exo cyclization.     

Convenient preparation of chiral phase-transfer catalysts with conformationally fixed biphenyl core for catalytic asymmetric synthesis of α-alkyl- and α,α-dialkyl-α-amino acids: application to the short asymmetric synthesis of BIRT-377

Chiral phase-transfer catalysts (S)-1a, (S)-1b, and (S)-2 with conformationally fixed biphenyl cores were conveniently prepared from the known, easily available (S)-6,6′-dimethylbiphenyl-2,2′-diol 3 and (S)-4,5,6,4′,5′,6′-hexamethoxybiphenyl-2,2′-dicarboxylic acid 14, respectively, in five steps. The catalysts, (S)-1a and (S)-1b are readily applicable to asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester with excellent enantioselectivity. In particular, catalyst (S)-1b was found to exhibit the unique temperature effect on the enantioselectivity, and asymmetric alkylation of glycine derivatives at room temperature gave higher enantiomeric excess than that at 0 °C. In addition, the catalyst (S)-2 exhibited the high catalytic performance (0.01-1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester and N-(p-chlorophenylmethylene)alanine tert-butyl ester compared to the existing chiral phase-transfer catalysts, thereby allowing to realize a general and useful procedure for highly practical enantioselective synthesis of structurally diverse natural and unnatural α-alkyl-α-amino acids as well as α,α-dialkyl-α-amino acids. This approach is successfully applied to the short asymmetric synthesis of cell adhesion BIRT-377.     

Asymmetric synthesis of axially chiral benzamides and anilides utilizing planar chiral arene chromium complexes

Optically active axially chiral 2,6-disubstituted benzamides and anilides were stereoselectively prepared by utilizing planar chiral (arene)chromium complexes. Nucleophilic addition to enantiomerically pure planar chiral tricarbonyl(N,N-diethyl-2-methyl-6-formyl- (or 6-acyl)benzamide)chromium complex gave axially chiral 2-methyl-6-substituted N,N-diethyl benzamide chromium complexes with high selectivity. An alternative method for the preparation of axial chiral benzamides or anilides is an enantiotopic lithiation at the benzylic methyl of prochiral tricarbonylchromium complexes of N,N-diethyl-2,6-dimethylbenzamide and N-methyl-N-acyl-2,6-dimethylaniline with a chiral lithium amide followed by electrophilic substitution. The resulting axially chiral chromium-complexed benzamides and anilides were oxidized in air to give chromium-free axially chiral benzamides and anilides in enantiomerically enriched form without axial bond rotation at room temperature.     

Asymmetric Mukaiyama aldol reaction of silyl enol ethers with aldehydes using a polymer-supported chiral Lewis acid catalyst

Chiral N-sulfonylated α-amino acid monomer (5) derived from (S)-tryptophan was copolymerized with styrene and divinylbenzene under radical polymerization conditions to give a polymer-supported N-sulfonyl-(S)-tryptophan (6). Treatment of the polymer-supported chiral ligand with 3,5-bis(trifluoromethyl)phenyl boron dichloride afforded a polymeric Lewis acid catalyst (16) effective for asymmetric Mukaiyama aldol reaction of silyl enol ethers and aldehydes. Various aldehydes were allowed to react with silyl enol ethers in the presence of the polymeric chiral Lewis acid to give the corresponding aldol adducts in high yield with high levels of enantioselectivity.     

Asymmetric synthesis of 3-substituted unsaturated prolines from chiral sulfoximine substituted allyl titanium(IV) complexes

An asymmetric synthesis of the 3-substituted Δ^3,4-unsaturated prolines 7a-e and 3-substituted 4-methylene prolines 14a-c starting from the corresponding γ,δ-unsaturated α-amino acids 4a-e and 11a-c, respectively, is described. Amino acid derivatives 4a-e and 11a-d were obtained through aminoalkylation of the corresponding sulfoximine substituted allyl titanium(IV) complexes 2a-e and 10a-d, respectively, with the N-tert-butylsulfonyl imino ester 3. Activation of sulfoximines 4a-e and 11a-c through methylation of the sulfoximine group followed by a KF mediated isomerization of the vinyl aminosulfoxonium salts 5a-e and 12a-c, respectively, to the corresponding allyl aminosulfoxonium salt 6a-e and 13a-c, respectively, and a subsequent intramolecular nucleophilic substitution of the allylic aminosulfoxonium group afforded the enantio- and diastereomerically pure proline derivatives in medium to high yields.     

Organocatalytic Michael addition, a convenient tool in total synthesis. First asymmetric synthesis of (−)-botryodiplodin

The asymmetric Michael addition of propionaldehyde to (2E)-(3-nitro-but-2-enyloxymethyl)-benzene 8, catalyzed by the chiral diamine (S,S)-N-iPr-2,2′-bipyrrolidine, afforded, with 93% ee, a precursor 9 of (−)-botryodiplodin. The nitro functionality of 9 was converted to a ketone via a Nef reaction to give, after a few steps, the enantiomerically enriched (−)-botryodiplodin.     

Use of optically active cyclic diethyl sulfamidate 2-phosphonates as chiral synthons for the synthesis of β-substituted α-amino phosphonates

Optically active protected sulfamidate 2-phosphonates have been synthesized from either (R)- or (S)-N-benzyl-2-phosphonoserine for use as chiral synthons. These sulfamidates have been shown to undergo nucleophilic substitution with select nucleophiles, to afford following N-sulfate removal, the β-substituted α-amino-2-phosphonates. N-Sulfate removal was accomplished using boron trifluoride etherate in the presence of either n-propylthiol or N-hydroxysuccinimide allowing retention of the diethylphosphonate ester groups. Replacement of the unpleasant smelling n-propylthiol with N-hydroxysuccinimide provides higher yields of the desired products. Synthesis of β-S-substituted analogues required the use of cesium carbonate as a base. The sulfamidates described have excellent stability and have been demonstrated, using chiral HPLC, to be greater than 97% enantiomerically pure.     

Optically active imidazoles derived from enantiomerically pure trans-1,2-diaminocyclohexane

A new exploration of monoprotected derivatives of trans-1,2-diaminocyclohexane as a platform for the synthesis of enantiomerically pure imidazole derivatives is described. The primary amino group (-NH₂), present in the mono-imine derivative of salicylic aldehyde (hemi-salen derivative) 5 was used for sequential reactions with formaldehyde and the corresponding α-(hydroxyimino)ketone. (S)-(−)-1-Phenylethylamine was also used as starting material for the preparation of new imidazole N-oxides 7c and 10a-c, bearing a chiral N-(1-phenylethyl)carboxamido function at C(4). Imidazole N-oxides 10a,b possessing either a Me or i-Pr group at N(1), respectively, follow the known sulfur-transfer pathway to afford the corresponding imidazole-2-thiones 13a,b. However, in the case of imidazole N-oxide 10c with a bulky adamantan-1-yl substituent at N(1), the attempted ‘sulfur-transfer reaction’ led to the deoxygenated imidazole derivative 14. Finally, the same reaction with 7c, which bears an electron-withdrawing N-(1-phenylethyl)carboxamide residue at C(4) of the imidazole ring, yielded a mixture of deoxygenated imidazole 16 and imidazole-2-thione 15c.