Chiral linker. Part 3: Synthesis and evaluation of aryl substituted m-hydrobenzoins as solid supported open chain chiral auxiliaries for the diastereoselective reduction of α-keto esters

Five partly novel aryl substituted m-hydrobenzoins were synthesized and the corresponding desymmetrized hydrobenzoin ethers evaluated as open chain chiral auxiliaries in the L-Selectride^® mediated stereoselective reduction of phenylglyoxylates, resulting in de values of up to 91%. Two optimized auxiliary structures were immobilized on commercially available Wang-resin and applied as a reusable solid supported chiral auxiliary in the same type of reaction.     

Chiral linker. Part 4: Diastereoselective addition of RZnX to α-keto esters using m-hydrobenzoin derived chiral auxiliaries in solution and on solid support and their application in the stereoselective synthesis of frontalin

Two recently reported, m-hydrobenzoin derived open chain chiral auxiliaries, which were developed for application in either solution or immobilized on a solid support, were tested in the diastereoselective addition of RZnX to their corresponding phenylglyoxylates and pyruvates, respectively, resulting in diastereoisomeric excesses of up to >98% de. The optimized procedure was applied in the stereoselective synthesis of frontalin, the aggregation pheromone of pine bark beetles of the Dendroctonus family, by the use of both the solution phase and the polymer supported chiral auxiliary.     

New chiral NiII complexes of Schiff’s bases of glycine and alanine for efficient asymmetric synthesis of α-amino acids

New modified chiral auxiliaries (S)-N-(2-benzoylphenyl)-1-(2-chlorobenzyl)pyrrolidine-2-carboxamide (2-CBPB) and (S)-N-(2-benzoylphenyl)-1-(3,4-dimethylbenzyl)pyrrolidine-2-carboxamide (3,4-DMBPB) and their Ni^II complexes of Schiff’s base with glycine and alanine have been synthesized and tested in asymmetric C-alkylation and aldol condensation reactions of amino acid moieties. The tests proved that both new auxiliaries were efficient with the ee’s of the final amino acids as high as 98% even in case of α-methyl-α-amino acid synthesis.     

New chiral auxiliaries derived from (S)-α-phenylethylamine as chiral solvating agents for carboxylic acids

The two new diastereoisomeric chiral auxiliaries 1a and 1b were synthesized conveniently and effectively. ¹H NMR was employed to investigate their chiral recognition ability. Compared with (S)-PEA, these new chiral auxiliaries exhibited better enantioselectivity towards the carboxylic acids we had chosen.     

Cyclosulfamide as a chiral auxiliary: application to efficient asymmetric synthesis (alkylation/aldolization)

The chiral cyclosulfamide (S)-2-benzyl-4-isopropyl-1,2,5-thiadiazolidine 1,1-dioxide was designed as a chimera of Evans and Oppolzer chiral auxiliaries. The N-propionyl derivative appeared to be very powerful for the stereocontrolled synthesis of chiral building blocks through asymmetric aldolization and alkylation reactions.     

Synthesis of α-chiral-β,γ-unsaturated carboxylic acid derivatives using chiral auxiliaries

We report an efficient and reliable method for the synthesis of α-chiral-β,γ-unsaturated carboxylic acid derivatives using chiral auxiliaries and vinylacetic acid. Two well-established chiral auxiliaries ((S,S)-pseudoephedrine and (R)-benzyl-oxazolidinone) were chosen to test the merits of this method. Six different electrophiles were examined in the diastereoselective alkylation with both auxiliaries. The pseudoephedrine auxiliary provided isolated yields between 61 and 85% and diastereomeric ratios all greater than 96:4. Employing the same reactions as with the pseudoephedrine derivative, the corresponding oxazolidinone auxiliary provided isolated yields between 0 and 80% with diastereomeric ratios from 80:20 to 93:7.     

Synthesis of chiral sultams and their application as chiral auxiliaries in an asymmetric Diels–Alder reaction

A number of bicyclic chiral sultams were synthesized based on 1,3-dipolar cycloaddition reactions of nitrile oxides and nitrones with prop-1-ene-1,3-sultone. The corresponding N-enoyl sultams were prepared by acylation. Their relative effectiveness as new chiral auxiliaries in asymmetric synthesis was evaluated for the asymmetric Diels–Alder reactions with cyclopentadiene. Good chemical yield and excellent endo selectivity were observed. The relationship between the structure and their effectiveness in promoting asymmetric induction of the synthetic chiral sultams was investigated.     

Intermolecular Asymmetric Carboesterification of Alkenes by Using Chiral Amine Auxiliaries under O2: Synthesis of Enantioenriched α‐Methylene‐γ‐Lactones through Chloropalladation of Alkynes

Herein, the first example of chloropalladation‐initiated asymmetric intermolecular carboesterification of alkenes with alkynes by using chiral amine auxiliaries is reported. The use of (1S,2S)‐N¹,N¹‐dimethylcyclohexane‐1,2‐diamine auxiliaries is essential for providing α‐methylene‐γ‐lactones products in moderate to high yields and excellent enantioselectivities at room temperature. Moreover, the chiral amine auxiliaries can be readily removed by hydrolysis during the reaction process to keep the absolute configuration. This oxygen‐ and water‐promoted asymmetric reaction opens a new window to study asymmetric processes in halopalladation reactions.     

Syntheses and applications of C2-symmetric chiral diols

Synthetic procedures for a large variety of C₂-symmetric chiral diols are reviewed. Prominent among these procedures are enantioselective reductions, epoxide-cleavages, dihydroxylation of olefins, and synthetic transformations. Applications of these diols as chiral auxiliaries/ligands for several important reactions are also highlighted.     

Chiral linker. Part 2: Synthesis and evaluation of a novel,reusable solid-supported open chain chiral auxiliary derived from m-hydrobenzoin for the diastereoselective reduction of α-keto esters

Three novel m-hydrobenzoin derived chiral hydrobenzoin mono-alkyl ethers were synthesized and evaluated as open chain chiral auxiliaries in the L-selectride^R/ZnCl₂ mediated stereoselective reduction of their corresponding phenyl glyoxylates, resulting in des of up to 91%. The optimized auxiliary structure was immobilized on commercially available Wang-resin by using the ether substituent as a sublinking unit and applied as a reusable solid-supported chiral auxiliary in the same type of reaction with only little loss of stereofacial selectivity.     

Synthesis of novel chiral hydrobenzoin mono-tert-butyl ethers derived from m-hydrobenzoin and their application as chiral auxiliaries in the diastereoselective reduction of α-keto esters

Three m-hydrobenzoin derived chiral hydrobenzoin mono-tert-butyl ethers were synthesized by a new reaction pathway and tested as chiral auxiliaries in the L-selectride^® mediated stereoselective reduction of their corresponding phenyl glyoxylates. As a result, improved stereoselectivities of up to a ratio of 92:8 compared to 84:16 with the initially examined analogous benzyl ether were achieved.     

Synthesis of chiral isoquinuclidines and determination of their absolute configuration

A route to 1,2-dihydropyridines N¹-substituted with chiral auxiliaries has been developed starting from commercially available chiral amines. Cycloaddition between these dihydropyridines and methyl acrylate gave, in moderate d.e., isoquinuclidines of good enantiomeric purity whose absolute configuration has been established.     

Asymmetric Diels–Alder addition of cyclopentadiene to chiral naphthoquinones

Diels–Alder reactions of 1,4-naphthoquinones bearing a chiral auxiliary at C-2, with cyclopentadiene under Lewis acid conditions afforded the corresponding Diels–Alder adducts. High levels of diastereomeric excess were obtained using (R)-pantolactone, (S)-N-methyl-2-hydroxysuccinimide and trans-2-phenylcyclohexanol as auxiliaries. Moderate asymmetric induction was achieved using Oppolzer's camphorsultam and (R)-(+)-4-benzyl-2-oxazolidinone as auxiliaries. X-Ray crystallographic analysis of the pantolactone adduct enabled determination of the stereochemistry of all adducts obtained.     

New glyco-oxazolidin-2-ones as chiral auxiliaries in boron-mediated asymmetric aldol reactions

Two new chiral glyco-oxazolidin-2-one auxiliaries based on d-glucose are described. Some N-acyl derivatives were synthesized and used in dialkylboron-mediated asymmetric aldol reactions. All reactions delivered as the major diastereomer those predicted by the Zimmerman–Traxler model and were separated by column chromatography and mostly isolated in moderate to good yields.     

Asymmetric synthesis of methyl bacteriopheophorbide-d and analogues by stereoselective reduction of the 3-acetyl to the 3-(1-hydroxyethyl) group

Asymmetric borane-reduction of 3-acetyl-13¹-oxo-tetrapyrroles (=bacteriochlorin, chlorin and porphyrin) in the presence of a chiral source gave selectively chiral 3-(1-hydroxyethyl)-13¹-oxo-tetrapyrroles. Oxazaborolidines were effective as chiral auxiliaries. Reduction with (S)-oxazaborolidines led to the (3¹S)-alcohol as the major product, whose stereoselectivity was the opposite of that in the same asymmetric reduction of usual prochiral ketones ArCOR. The asymmetric reduction of the 3-acetyl group in methyl bacteriopyropheophorbide-a and the 7,8-oxidation afforded 3¹-epimeric methyl (3¹S)-bacteriopheophorbide-d (89% de).     

Synthesis of novel chiral NiII complexes of dehydroalanine Schiff bases and their reactivity in asymmetric nucleophilic addition reactions. Novel synthesis of (S)-2-carboxypiperazine

New chiral Ni^II complexes of Schiff bases of dehydroalanine with modified chiral auxiliaries (S)-2-N-[N′-(3,4-dichlorobenzyl)prolyl]aminobenzophenone (3,4-DCBPB), (S)-2-N-[N′-(3,4-dimethylbenzyl)prolyl]aminobenzophenone (3,4-DMBPB), (S)-2-N-[N′-(2-chlorobenzyl)prolyl]aminobenzophenone (2-CBPB), and (S)-2-N-[N′-(2-fluorobenzyl)prolyl]-aminobenzophenone (2-FBPB) have been synthesized. Asymmetric Michael addition reactions of primary and secondary amines and thiols to the dehydroalanine moieties of the complexes were studied. (S)-2-FBPB was found to be the best chiral auxiliary in terms of both selectivity of the reactions (de ～92–96%) and reactivity of the complexes. A novel synthetic route toward (S)-2-carboxypiperazine was developed based on the auxiliary.     

Highly diastereoselective preparation of anti-α,β-dialkyl β-amino acids containing natural α-amino acid side chains

An efficient synthesis of anti-2-alkyl β³-amino acids was developed starting from the fully protected β³-amino acids. The strategy allows the introduction of the side chain of natural α-amino acids such as Ala, Phe and Ser at the C-2 position, with high diastereoselectivity. The preparation of 2-methyliden-β³-amino acids is also reported. This methodology does not need the use of expensive chiral reagents and/or chiral auxiliaries, and leads to compounds with orthogonal protecting groups.     

Asymmetric cyclopropanation with diazoacetates induced by carbohydrate-derived chiral auxiliaries

α-Diazoacetates with carbohydrate-based chiral auxiliaries have been synthesized and screened in the asymmetric cyclopropanation of styrene in connection with the effect of the chiral Cu(I)-catalyzed reaction, showing the remarkable importance of the carbohydrate-based chiral auxiliaries on the enantioselectivities and an unexpected effect on the trans:cis ratios.     

Synthesis of two (S)-indoline-based chiral auxiliaries and their use in diastereoselective alkylation reactions

Two chiral auxiliaries, 2-[(S)-indolin-2-yl]propan-2-ol 1a and (S)-2-(2-methoxypropan-2-yl)indoline 1b, were synthesised from enantiomerically pure (S)-indoline-2-carboxylic acid 3. High diastereoselectivities in alkylations of enolates of the propanoylamides derived from the two auxiliaries are presented. Surprisingly, both auxiliaries induced the same selectivity at the newly created stereogenic centre. The benzyl bromide and n-butyl iodide alkylation reactions showed diastereomeric ratios that were moderate (81:19) to very good (96:4) and with very good yields (86–98%). When LiCl was used as an enolate coordinating agent, in the benzylation of the enolate from propanoylated auxiliary 1a, a very high crude diastereomeric ratio was obtained (99.7:0.3).     

Molecular-level chiral discrimination and induction

The review describes the mechanism of chiral discrimination of racemic amines upon crystallization and the induction of chirality in organic reactions by using them as chiral auxiliaries. In order to form conglomerates, which can be resolved into the two enantiomers upon alternative seeding, both formation and packing of 2₁-columns are essentially very important. On the other hand, in order to achieve high efficiency in resolution through diastereomeric salt formation, which is the most practical method, one of a pair of diastereomeric salts derived from a racemic amine and an enantiomerically pure resolving agent should at least have two 2₁-columns and planar boundary surfaces in its crystal structure. On the basis of this knowledge, we developed several artificial chiral auxiliaries such as erythro-2-amino-1,2-diphenylethanol,cis-2-amino-1-acenaphthenol, andcis-2-amino-3,3-dimethyl-1-indanol. These were found to be very efficient chiral auxiliaries in asymmetric inductions: alkylation of chiral imines, catalytic borane-reduction, and alkylation of chiral N-acylated oxazolidinone.