Preparation of chiral secondary boronic esters via copper-catalyzed enantioselective conjugate reduction of β-boronyl-β-alkyl α,β-unsaturated esters

A new methodology involving the copper(I)-catalyzed enantioselective conjugate reduction of β-boronyl-β-alkyl enoates was developed. Various chiral secondary boronate derivatives can be accessed in excellent yields and good to high levels of enantioselectivity through this efficient copper-catalyzed process using polymethylhydrosiloxane (PMHS) as hydride source.     

Enantioselective α-hydroxylation of β-keto esters catalyzed by chiral S-timolol derivatives

A screen of aryloxy aminopropanol organocatalysts derived from the β-blocker inhibitor S-timolol determined the most active catalyst of asymmetric α-hydroxylation of β-keto esters. (R)-1-(tert-butylamino)-3-(3,4,5-trimethoxyphenoxy) propan-2-ol (3k) was the most effective derivative, enantioselectively catalyzing α-hydroxylation of β-keto esters using tert-butyl hydroperoxide as the oxidant in hexane to afford the corresponding products in excellent yield and with good enantioselectivity (up to 96% yield, 88% ee).     

Enantioselective α-chlorination of β-oxo esters catalyzed by chiral diterpenoid alkaloid derivatives

Easily accessible diterpenoid alkaloid derivatives have been used as organocatalysts in the enantioselective α-chlorination of β-oxo esters. The treatment of β-oxo esters with N-chlorophthalimide (NCP) as a chlorine source under mild reaction conditions afforded the corresponding α-chlorinated β-oxo esters in excellent yields (up to 98%) and with moderate enantioselectivities (up to 68% ee) in 30 min.     

Copper-bipyridine-catalyzed enantioselective α-amination of β-keto esters

The Cu(OTf)₂-bipyridine-catalyzed, enantioselective, direct α-amination of β-keto esters and β-diketones with diethyl azodicarboxylate (DEAD) has been studied. Exceptionally high enantioselectivities of up to 99% ee were found for 1-indanone-based β-keto esters in particular even for substrates and reagents carrying conventional ester groups such as methyl and ethyl.     

Catalytic hydrolysis of α-amino esters in the presence of chiral palladacycles

The rate of hydrolysis of esters derived from optically active α-amino acids, catalyzed by chiral cyclopalladated benzylamines, depends on the configuration of chiral centers in the substrate and catalyst. The catalytic hydrolysis of sulfur-containing amino esters follows an intramolecular mechanism, and the difference in the reaction rates for the stereoisomers increases in going from ortho-palladated primary benzylamines (k _S/k _R = 1.1) to tertiary amines (k _S/k _R = 1.5); the strongest catalytic effect is observed for an ester and a complex with the same absolute configuration of the chiral centers. The efficiency of intermolecular catalysis is greater for a complex and ester with opposite absolute configurations of the chiral centers, and the rate constants of catalytic hydrolysis for two pairs of stereoisomers coincide within experimental error. The maximal difference in the reaction rates is observed for cyclopalladated secondary benzylamines; it reaches 2.3 for the phenylalanine ester.     

Preparation of chiral α-diazophosphonic acid derivatives

The chiral α-diazophosphonic acid derivatives 3, 6 and 8 were prepared from (−)-ephedrine and (S,S)-N,N′-dimethyl-1,2-diaminocyclohexane; preliminary experiments suggest that the chiral auxiliary exerts little influence in the dirhodium(II) acetate catalysed OH and NH insertion reactions.     

Highly enantioselective synthesis of α-fluoro-α-nitro esters via organocatalyzed asymmetric Michael addition

Primary amines catalyzed the asymmetric Michael addition of ethyl 2-fluoro-2-nitroacetate to enones to provide chiral α-fluoro-α-nitro ester ketones with two contiguous stereogenic centers, one of which is a fluorinated quaternary chiral center, with excellent enantioselectivities and in moderate to good yields.     

Synthesis of novel chiral lipophilic pyridyl-containing β-amino alcohol ligands and enantioselective hydrolysis of α-amino acid esters by chiral metallomicelles

Seven novel chiral lipophilic pyridyl-containing β-amino alcohol ligands have been synthesized by coupling of 6-alkoxymethyl-2-chloromethylpyridine 3 with the corresponding chiral β-amino alcohols or l-cysteine. Their metal ion complexes have been investigated as catalysts for the enantioselective hydrolysis of N-protected α-amino acid esters in aqueous comicellar solution. The results indicate that the hydrophobic interactions between substrate and metallocatalyst, the rigidity of the ligand, the hydroxyl group of the ligand acting as a nucleophile for the transacylation process, and the micellar microenvironment are important factors for the activity and enantioselectivity. Large rate accelerations (up to three orders of magnitude) and moderate enantioselectivities (up to 7.81 (k_R/k_S)) employing 4a–Cu²⁺ have been observed.     

Conversion of homochiral amines and α-amino esters to their chiral 1,2,3,5-substituted pyrrole derivatives via gold-catalysed amination/annulation reactions of 2-propynyl-1,3-dicarbonyl compounds

Homochiral primary amines, amino alcohols and α-amino esters have been reacted with 2-propynyl-1,3-dicarbonyl compounds under gold catalysis leading to 1,2,5-trisubstituted 3-acylpyrrole derivatives in moderate to high yields and high enantiomeric excess.     

Nickel-catalyzed formation of cyclopentenone derivatives via the unique cycloaddition of α,β-unsaturated phenyl esters with alkynes

Oxygen-containing organic compounds, such as ethers, carboxylates, and carbamates, have recently received increasing attention because of their newly discovered applications as electrophiles in cross-coupling reactions via transition metal-catalyzed C-O bond activation. However, no cycloaddition reaction involving their C-O bond activation has been demonstrated thus far. The present study developed a Ni(0)-catalyzed unique [3+2] cycloaddition reaction of α,β-unsaturated phenyl esters with alkynes in (i)PrOH to yield cyclopentenone derivatives.     

Highly enantioselective synthesis of anti aryl β-hydroxy α-amino esters via DKR transfer hydrogenation

An efficient preparation of highly enantiomerically enriched aryl β-hydroxy α-amino esters via dynamic kinetic resolution (DKR), asymmetric transfer hydrogenation of α-amino β-keto esters is described. The anti β-hydroxyl α-amino esters were obtained both in high yields and high diasteroselectivity. The observed high anti selectivity is inconsistent with the previous results in literature. The absolute stereochemistry of the aryl β-hydroxy α-amino esters was unambiguously confirmed via chemical derivatization as well as Vibrational Circular Dichroism (VCD) techniques.     

Stereoselective alkylation of chiral glycine enolate synthons. The enantioselective synthesis of α-amino acid derivatives.

The highly stereoselective alkylation (% de=99.6 to 97.6) of a new chiral glycine enolate synthon derived from D-2-phenylglycinol is described. Deprotection of the alkylation adducts in a one-pot three-step procedure provides the ethyl ester hydrochloride salts of the corresponding α-amino acids with no attending racemization.     

Diastereoselective photodeconjugation of chiral α,β-unsaturated esters

Chiral alcohols available in both enantiomeric forms have been tested for the diastereoselective photochemical deconjugation of 2,4-dimethylpentenoic acid esters. (R)-Pantolactone afforded selectively the (2R)-β,γ-unsaturated ester in good yield and high d.e. (89%), while analogous use of (S)-pantolactone gave the (2S)-stereoisomer with similar selectivity.     

Evaluation of the steric interactions responsible for the enantioselective photodeconjugation of α,β-unsaturated esters

A model is proposed to explain the configuration and the proportion of the preponderant enantiomer in the enantioselective photodeconjugation of α,β-unsaturated esters. The major interactions in the transition state are developed between the inductor and the β-carbon of the intermediate dienol.     

Camphor-based oxazaphospholanes as chiral templates for the enantioselective synthesis of α-chlorophosphonic acids

Constrained camphor-derived oxazaphospholanes have provided an efficient entry for the preparation of enantiomerically enriched α-chlorophosphonic acids.     

A very enantioselective photodeconjugation of α,β-unsaturated esters

An enantiomeric excess up to 70% can be obtained in the photodeconjugation of conjugated esters in the presence of (1R, 2S) 1-phenyl, 2-isopropylamino-propanol.     

Nitrobenzylation of α-carbonyl ester derivatives using TDAE approach

A series of 2-hydroxy-propionic acid ethyl ester derivatives was prepared in good yields by reaction of o- and p-nitrobenzyl chlorides (1, 8) with various α-carbonyl esters in presence of tetrakis(dimethylamino)ethylene (TDAE). This reaction was generalized to α-ketolactam and α-ketomalonate.     

SPANamine derivatives in the catalytic asymmetric α-fluorination of β-keto esters

The use of C₂-symmetric enantiopure nitrogen ligands in the asymmetric catalytic α-fluorination of β-ketoesters is described. SPANamine 1 in the presence of nickel salts gives up to 63% ee in the fluorination of tert-butyl 2-oxocyclopentanecarboxylate with N-fluorosuccinimide (NFSI). The same enantioselectivity is obtained when SPANamine 1 is used as an organocatalyst, although the reaction is much slower.     

Enantioselective photodeconjugation of α,β-unsaturated esters: Effect of the nature of the chiral agent

A synergism exists between the effect of the amino and hydroxy groups of chiral amino alcohols on the enantiomeric excess of the photodeconjugation of α,β-unsaturated esters.