Vasicine as tridentate ligand for enantioselective addition of diethylzinc to aldehydes

The first report of natural l-vasicine as tridentate chiral ligand for the enantioselective addition of diethylzinc to a variety of aliphatic and aromatic aldehydes is described. The ligand generates R-isomer of the secondary alcohols upto 98% ee. The quinazoline structure possibly imparts rigidity to the ligand and hence, consistently high enantioselectivity. The importance of the quinazoline ring was also supported by the reaction with other related ligands, partially lacking the structural features, thus resulting in poor enantioselectivity.     

3-Aminoquinazolinones as chiral ligands in catalytic enantioselective diethylzinc and phenylacetylene addition to aldehydes

A series of readily known enantiomerically pure 3-aminoquinazolinones 1a–d were synthesised from easily accessible chiral pool α-hydroxy acids and α-amino acids in only four steps without any requirement of chromatography. These quinazolinones were examined as chiral ligands for catalytic enantioselective diethylzinc and phenylacetylene additions to aldehydes. For enantioselective alkylations, the effects of temperature, solvent, diethylzinc and ligand criteria were analysed, and the desired chiral alcohols were obtained in up to 86% ee. 3-Aminoquinazolinones 1a–d were also shown to be very useful ligands in enantioselective alkynylations of aldehydes. Based upon the optimised conditions, the corresponding propargylic alcohols were obtained in up to 94% ee.     

Axially chiral tridentate isoquinoline derived ligands for diethylzinc addition to aldehydes

The synthesis and resolution of new tridentate isoquinoline-derived ligands has been developed. The key steps in the synthetic sequence include successive, chemo-selective Suzuki-Miyaura cross-couplings of 1,3-dichloroisoquinoline with suitable arylboronic acids. The new ligands prepared in this manner were resolved either via molecular complexation with N-benzylcinchonidinium chloride as with 1-[3-(2-hydroxyphenyl)isoquinolin-1-yl]naphthalen-2-ol or via chromatographic separation of its epimeric camphorsulfonates as for 1,3-bis-(2-hydroxynaphthalen-1-yl)isoquinoline. 4-tert-Butyl-2-chloro-6-[1-(2-hydroxymethylnaphthalen-1-yl)isoquinolin-3-yl]phenol was resolved by chiral semi-preparative HPLC. The application of these ligands in the diethylzinc addition to aldehydes was investigated. In certain cases, the desired secondary alcohols were obtained in high yield with excellent enantiomeric excess (ee?>?99%) at low catalyst loading (1?mol%).     

Stereocontrolled enantioselective addition of diethylzinc to aldehydes using new chiral aminoalcohols

The new chiral β-aminoalcohols of indolinylmethanols ( 1 ) and their reduced derivatives ( 2 ) were synthesized from (S)-indoline-2-carboxylic acid. Both (R) and (S) enantiomers of the optically active secondary alcohols have been successfully obtained in high enantiomeric excess from the stereoselective addition of diethylzinc to the aldehydes catalyzed by the chiral aminoalcohols ( 1 and 2 ). The sense of the asymmetric induction and the degrees of enantioselectivities turned out to be highly dependent on the structure of the catalysts: The presence of the catalyst 1 afforded the (S)-configuration of the corresponding alcohols; on the other hand, the presence of 2 afforded the (R)-configuration of the alcohols in high enantiomeric selectivity.     

Novel thiazolidine derivatives as chiral catalysts in the enantioselective addition of diethylzinc to aldehydes

New chiral thiazolidine derivatives were synthesized conveniently from natural l-cysteine and showed good enantioselectivity (up to 90% ee) in the addition of diethylzinc to aldehydes. The catalytic efficiency of the thiazolidine derivatives is influenced by the different structures of the thiazolidine rings and the bulkiness of R moiety in the ester groups.     

Camphor-based Schiff base ligand SBAIB: an enantioselective catalyst for addition of phenylacetylene to aldehydes

A series of Schiff base ligands were synthesized from (1R)-camphor. Under the optimal conditions, (+)-SBAIB-a, 10 was found to be an excellent catalyst for the enantioselective addition of phenylacetylene to various aldehydes without utilizing either achiral additives or Ti(O(i)Pr)(4). This approach yielded (R)-propargylic alcohols in extremely high yields (up to 99%) and excellent enantioselectivities (up to 92%). The corresponding (S)-propargylic alcohols were synthesized in good to high enantioselectivities (up to 91%) and excellent yields (up to 99%) using (-)-SBAIB-a, 41.     

Synthesis and application of oxadiazines as chiral ligands for the enantioselective addition of diethylzinc to aldehydes

A series of oxadiazines derived from l-phenylalanine bearing phenolic substituents have been synthesized in a multistep, one pot process. This process involves the reaction of a mixed anhydride with a β-hydrazino alcohol, methanesulfonylation of the alcohol moiety, and base induced cyclization. The resultant oxadiazines were employed in the asymmetric addition of diethylzinc to aldehydes.     

Catalytic enantioselective addition of diethylzinc to aldehydes using aziridine based chiral ligands

The readily available ferrocenyl substituted aziridinylmethanol 1 (FAM-1) was used as a chiral catalyst in the diethylzinc addition reaction to aromatic and aliphatic aldehydes to give secondary alcohols in high yields and up to 99% enantiomeric excess at room temperature. The catalyst can be recovered and used without losing its activity.     

The synthesis of chiral amino diol tridentate ligands and their enantioselective induction during the addition of diethylzinc to aldehydes

A series of C₂-symmetric chiral amino diol tridentate ligands 3a–g were prepared from achiral bulky organolithiums, achiral bulky primary amines, and optically active epichlorohydrin (ECH). The prepared C₂-symmetric chiral amino diol tridentate ligands were capable of inducing enantioselectivity in the model reaction of aromatic and aliphatic aldehydes with diethylzinc with an ee of up to 96%. The enantioselectivity can be modulated by adjusting the steric hindrance of the achiral reagents employed in the synthesis of the chiral ligand. The configuration of the addition product depended on the configuration of the amino diol ligands, which can be simply controlled as desired by using the ECH with the desired configuration during the preparation of the ligand.     

Synthesis of chiral plane-extended pyridyl alcohols for the enantioselective addition of diethylzinc to aldehydes

Enantiomerically pure pyridyl alcohols, 1 and 2, were prepared from readily available racemic pyridyl alcohol 3 and applied to the enantioselective addition of diethylzinc to aldehydes resulting in moderately high enantiomeric excess (90%) in the case of benzaldehyde. In this study the first example of an alteration in the configuration of the product upon change of a non-chiral and non-coordinating part of the catalyst was observed.     

Salicylhydrazone ligands for enantioselective addition of diethylzinc to aldehydes

A family of chiral salicylhydrazones was designed and synthesized for application in asymmetric catalysis. The ability of newly prepared binaphthyl-containing salicylhydrazones as the chiral ligand was examined in the addition of diethylzinc to aldehyde, with the desired product obtained in up to 80% ee.     

Modular amino acid amide chiral ligands for enantioselective addition of diethylzinc to aromatic aldehydes

Enantioselective addition of diethylzinc to a series of aromatic aldehydes was developed using a modular amino acid amide chiral ligand (2S)‐3‐phenyl‐N‐((R)‐1‐phenyl‐ethyl)‐2‐(tosylamino)propanamide without using titanium complex. The catalytic system employing 10 mol% of 1g was found to promote the addition of diethylzinc (ZnEt₂) to a wide range of aromatic aldehydes with electron‐donating and electron‐withdrawing substituents, giving up to 82% ee of the corresponding secondary alcohol under mild conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

An efficient method for catalytic enantioselective addition of diethylzinc to aryl aldehydes by a C2-symmetric chiral imino alcohol

The enantioselective alkylation of aryl aldehydes with diethylzinc in the presence of a C₂-symmetric chiral ligand derived from 2-hydroxy-3-pinanone was achieved at room temperature with high enantiomeric excesses.     

New highly efficient aziridine-functionalized tridentate sulfinyl catalysts for enantioselective diethylzinc addition to carbonyl compounds

New tridentate enantiomerically pure heteroorganic catalysts, containing hydroxyl, sulfinyl, and aziridine moieties, have proven to be highly efficient in the enantioselective diethylzinc addition to aryl and alkyl aldehydes to give the desired products in very high yields (up to 99%) and with ee’s up to 97%. The influence of the stereogenic centers located on the sulfinyl sulfur atom and in the aziridine moiety on the stereochemical course of the reaction are discussed.     

The application of chiral aminonaphthols in the enantioselective addition of diethylzinc to aryl aldehydes

[reaction: see text]. Optically active aminonaphthol 3 obtained by condensation of 2-naphthol, benzaldehyde, and (S)-methylbenzylamine followed by N-methylation was found to catalyze the enantioselective ethylation of aryl aldehydes to secondary alcohols with high enantioselectivities (up to 99.8%) at room temperature.     

Synthesis of Chiral N-Ferrocenylmethylaminoalcohols and their Application in Enantioselective Addition of Diethylzinc to Aldehydes

Three chiral N-ferrocenylmethylaminoalcohols were synthesized from readily available natural L-valine, leucine and phenylanine, and used as chiral ligands in the enantioselective addition of diethylzinc to aldehydes.     

New bis-steroidal axially chiral diols as ligands for the asymmetric addition of diethylzinc to aldehydes

New axially chiral steroidal binaphthols, prepared from estrone, were applied in the form of their Ti-alkoxides as catalysts for the addition of diethylzinc to aldehydes. The addition reactions proceeded with high yields and enantioselectivities up to 86%.     

Synthesis of novel chiral 1,3-aminophenols and application for the enantioselective addition of diethylzinc to aldehydes

Novel chiral 1,3-aminophenols were efficiently synthesized by applying a Friedel–Crafts reaction and optical resolution. The catalytic activity of the aminophenols was studied for the addition of diethylzinc to benzaldehyde. The results showed that (S)-5a with bulky tert-butyl groups on the stereogenic carbon atom and 4,6-positions of phenol favored higher enantioselectivity (94% ee). The same ligand was also used with other aldehydes, to give optically active alcohols in good chemical yields and ee values (up to 99%).     

New silica-immobilized chiral amino alcohol for the enantioselective addition of diethylzinc to benzaldehyde

[reaction: see text] A readily available chiral amino alcohol has been immobilized on silica by sol-gel synthesis and grafting. The solid prepared according to the latter method led to the best enantioselectivity (77% ee) described for the asymmetric addition of diethylzinc to benzaldehyde using inorganic solids.