Asymmetric synthesis of 3-substituted 8-hydroxy-3,4-dihydroisocoumarins from (S)-4-isopropyl-2-(2-methoxy-6-methylphenyl)oxazoline

Reaction of the laterally lithiated (S)-4-isopropyl-2-(2-methoxy-6-methylphenyl)oxazoline with p-tolualdehyde gave an inseparable mixture of the addition products in low diastereoselectivity. However, the (S,S)-product cyclized to the corresponding 3,4-dihydroisocoumarin faster than the (S,R)-product on silica gel, which allowed to be produced both enantiomers of 8-methoxy-3-(p-tolyl)-3,4-dihydroisocoumarin in moderate to good optical purity [S-enantiomer: 75% ee; R-enantiomer: 96% ee]. This procedure was applied to the short-step synthesis of optically active 3,4-dihydroisocoumarin natural products such as (R)-8-hydroxy-3-(1-tridecyl)-3,4-dihydroisocoumarin and (R)-phyllodulcin.     

(Salen)Ti(IV) complex catalyzed asymmetric ring-opening of epoxides using dithiophosphorus acid as the nucleophile

A series of chiral bis-Schiff bases were synthesized starting from (1R,2R)-(+)-diaminocyclohexane, (+)-cis-1,2,2-trimethyl-1,3-diaminocyclopentane, (R)-2,2^′-diamino-1,1^′-binaphthalene, and (1S,2S)-diphenyl-1,2-ethanediamine. The enantioselective ring-opening of meso epoxides with dithiophosphorus acids catalyzed by a (salen)Ti(IV) complex formed in situ upon the treatment of Ti(OPr-i)₄ and the aforementioned chiral Schiff base was realized. The resulting products were obtained with low to good enantioselectivities (up to 73% ee). The (salen)Ti(IV) complex containing the backbone of 1,2-diaminocyclohexane exhibited the best enantioselectivity. The substituents in dithiophosphorus acids and those on the salen aromatic ring have a significant influence on the reaction. Moderate enantioselectivity were obtained for the (salen)Ti(IV) complex catalyzed ring-opening of racemic monosubstituted epoxides. High regioselectivity was observed for the alkyl substituted epoxides, whereas poor regioselectivity was obtained for the aryl substituted ones.     

Enantioselective Michael addition of aldehydes to maleimides organocatalysed by chiral 1,2-diamines: an experimental and theoretical study

Simple and commercially available chiral 1,2-diamines were used as organocatalysts for the enantioselective conjugate addition of aldehydes, including α,α-disubstituted, to maleimides. The reaction was carried out in the presence of hexanedioic acid as an additive in aqueous solvents at room temperature. By employing (1S,2S)- and (1R,2R)-cyclohexane-1,2-diamine as organocatalysts, the corresponding Michael adducts bearing new stereocenters were obtained in high or quantitative yields with enantioselectivities of up to 92%, whereas the use of (1S,2S)-1,2-diphenylethane-1,2-diamine gave a much lower ee. Theoretical calculations were used to justify the observed sense of the stereoinduction.     

A new synthetic route to optically active cyclomercurated ferrocenylimines

An adaptation of Kagan’s method for preparing 2-substituted ferrocenecarboxaldehydes has allowed us to directly prepare enantiopure (S_p)-2-chloromercurio-ferrocenecarboxaldehyde, (S_p)-3. Subsequent condensation of this aldehyde with (1R,2R)-(+)-1,2-diphenyl-1,2-ethanediamine ((R,R)-4) yielded a novel, enantiopure bis-cyclomercurated ferrocenylimine, (S_p,S_p,R_c,R_c)-N,N-bis(2-(chloromercurio)ferrocenylidene)-1,2-diphenylethane-1,2-diimine ((S_p,S_p,R_c,R_c)-5). In addition to the chiroptical data collected for both (S_p)-3 and (S_p,S_p,R_c,R_c)-5, the solid-state structure and absolute configuration of (S_p,S_p,R_c,R_c)-5 were confirmed by X-ray crystallography.     

Stereospecific co-crystallization of reactant and palladium complex in the resolution of stilbendiamine using N*-chiral ortho-palladated matrix

Resolution of racemic 1,2-diphenyl-1,2-ethanediamine was performed through recrystallization of its diastereomeric adducts 2a,b with N*-chiral ortho-palladated complex (S_CR_N)-1. The structure and absolute configuration of the less soluble (S_CR_N,SS)-2a diastereomer was determined by X-ray diffraction. The complex has an unusual dimeric structure with a palladacycle:diamine ratio of 2:3; the association of 2a with an additional diamine molecule of the same (SS) absolute configuration is achieved due to hydrogen bonds between the coordinated and solvate diamine.     

Diastereoselectivity in the induction of planar chirality in prochiral cymantrenes

The carbonylation of the (S)-enantiomer of the palladated derivative of dimethyl-aminomethylferrocene in the presence of 1,2-bis(hydroxymethyl)cymantrene affords a product containing a chiral plane predominantly of the (R)-configuration. The extent of asymmetric induction amounts to 30–35 %. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1799–1802, October, 1993.     

Organic reactions in supramolecular gel media: reaction driven release of reagents in a macrocyclisation reaction

A bifunctional reactive organogelator containing p-nitrophenyl carbamate moieties has been reacted with (1R,2R)-(+)-1,2-diphenyl-1,2-ethanediamine to produce a l-valine derived macrocyclic bisurea. The macrocyclisation can be carried out very efficiently in the presence of the supramolecular gel formed by the biscarbamate, although the results indicate that the reaction takes place on the soluble species present in the system. The reversible nature of a supramolecular gel together with the high active surface of its fibrillar network permits the behaviour of the studied gel as an on-demand supplier of reagents to the solution phase.     

A Validated LC Method for the Determination of Chiral Purity of (1S)-6,11-Dioxo-1,2,3,4,6,11-Hexahydropyridazino[1,2-b]Phthalazine-1-Carboxylic Acid: A Key Intermediate of Cilazapril

A simple and accurate normal phase liquid chromatographic method was developed for the determination of chiral purity of (1S)-6,11-dioxo-1,2,3,4,6,11-hexahydropyridazino[1,2-b]phthalazine-1-carboxylic acid, S-enantiomer used as key intermediate in the manufacturing of cilazapril bulk drug. Chromatographic separation between (1S)-6,11-dioxo-1,2,3,4,6,11-hexahydropyridazino[1,2-b]phthalazine-1-carboxylic acid, and its opposite enantiomer (1R)-6,11-dioxo-1,2,3,4,6,11-hexahydropyridazino[1,2-b]phthalazine-1-carboxylic acid, R-enantiomer was achieved using a Chiralpak AD-H column using a mobile phase containing hexane, isopropyl alcohol and tri-fluoro acetic acid (80:20:0.1 v/v/v). The resolution between the two enantiomers was found to be more than 3.2. The limit of detection (LOD) and limit of quantitation (LOQ) of the R-enantiomer was 0.15 and 0.5 μg mL^?1, respectively, for 10 μL injection volume. The percentage recoveries of the R-enantiomer ranged from 96.5 to 105.3 in the bulk samples of (1S)-6,11-dioxo-1,2,3,4,6,11-hexahydropyridazino[1,2-b]phthalazine-1-carboxylic acid. The test solution and mobile phase was observed to be stable up to 24 h after the preparation. The developed method was validated as per International Conference on Harmonization guidelines in terms of LOD, LOQ, precision, linearity, accuracy, robustness and ruggedness.     

Synthesis and spectroscopic characterization of enantiopure protected trans-4-amino-1-oxyl-2,2,6,6-tetramethyl piperidine-3-carboxylic acid (trans β-TOAC)

Enantiomerically pure (3R,4S) and (3S,4R) protected 4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acids were synthesized by reduction of the enamines resulting from the condensation of 3-carboxymethyl-1-oxyl-2,2,6,6-tetramethyl-4-piperidone with (R) or (S)-α-methylbenzylamine. While NaBH₃CN/CH₃COOH reduction gave predominantly a mixture of the two possible cis-diastereomers, the use of NaBH₄/(CH₃)₂CHCOOH resulted in a mixture of only one trans- and one cis-diastereomer. Removal of the chiral auxiliary from the separated diastereoisomers by hydrogenolysis and regeneration of the nitroxide radical gave the desired β-amino esters. The ESR spectrum of the (3R,4S)-enantiomer is also reported.     

New non-C2-symmetric phosphine-phosphonites as ligands in asymmetric metal catalysis

Starting from 1,2-dibromobenzene, the synthesis of N,N,N′,N′-tetraethyl-[2-(diphenylphosphino)phenyl]phosphonous acid tetraamide is possible in two simple steps. This key compound reacts with a variety of chiral diols such as (R)- and (S)-binaphthol, 1,2:5,6-diisopropylidene-d-mannitol or (1R,2R)-1,2-diphenyl-1,2-ethane diol to form the corresponding non-C₂-symmetric phosphine-phosphonite compounds. These ligands react with Rh(COD)₂BF₄ to form bidentate Rh-complexes which serve as catalysts in the asymmetric hydrogenation of dimethyl itaconate with ee values of up to 88%.     

Diastereoselective cycloaddition of chiral 1-acryloyl-2-imidazolidinone and o-quinodimethane generated by reduction of 1,2-bis(bromomethyl)benzene with zinc

The reduction of 1,2-bis(bromomethyl)benzene with zinc powder followed by cycloaddition with the chiral dienophile (4R,5S)-1-acryloyl-3,4-dimethyl-5-phenyl-2-imidazolidinone in the presence of BF₃·Et₂O under ultrasound irradiation gave the corresponding Diels–Alder cycloadduct in high yield (90%) and high diastereoselectivity (R:S = 87:13).     

Investigation of the importance of nitrogen substituents in a N–P chiral ligand for enantioselective allylic alkylation

The synthesis of three chiral chelate nitrogen–phosphorus (S)-valine derived ligands with the potential for stereogenic nitrogen donation is described. In palladium catalysed allylic substitution reactions the ligands induced varying enantioselectivities ranging from 92% e.e. of the (R)-enantiomer to 83% e.e. of the (S)-enantiomer. Structural and spectroscopic investigations into the origin of this effect were conducted, but were inconclusive. However, the importance of the consideration of N-substituents in such systems is highlighted.     

A practical preparation of enantiomerically pure (R)- and (S)-2-bromohexadecanoic acids

The efficient preparation of enantiomerically pure (R)- and (S)-2-bromohexadecanoic acids with e.e.>95% through resolution with the use of a recoverable chiral auxiliary is described. The procedure involves three reactions: Steglich esterification, DIBAL reduction, and Sharpless oxidation. The assessment of the enantiomeric purity is based on NMR analysis by using (1R,2R)-(+)-diphenylethane-1,2-diamine as a chiral solvating agent.     

The absolute stereochemistry of grenadamide

3-(2S-Heptylcycloprop-1S-yl)propanoic acid 2-phenylethanamide was synthesised from cis-cyclopropan-1,2-dimethanol via enzymatic desymmetrisation of the dibutyrate; it gave identical NMR spectroscopic data to those reported for grenadamide but had an equal and opposite absolute rotation, indicating that the latter is the 2R,1R-enantiomer.     

Chiral nitrogen-containing calix[4]crown—an excellent receptor for chiral recognition of mandelic acid

A chiral nitrogen-containing calix[4]crown 2 bearing optically pure 1,2-diphenyl-1,2-oxyamino residue at lower rim showed excellent chiral recognition between enantiomers of mandelic acid. Using competitive ¹H NMR titration the ratio of association constants of (S)- and (R)-mandelic acid with the chiral calix[4]crown was determined to be 102, that is 98% de, which is the best result obtained from artificial receptors for the chiral recognition of mandelic acid up to now.     

Synthesis of enantiomeric pure lithium and potassium benzamidinate complexes

A new synthesis leading to the chiral amidines (S,S)- and (R,R)-N,N-bis-(1-phenylethyl)benzamidine ((S)- and (R)-HPEBA) in good yields is presented. Further reaction of (S)-HPEBA with n-BuLi gave the chiral lithium salt (S)-LiPEBA. Treatment of KH with (S)-HPEBA in boiling THF afforded the corresponding potassium salt (S)-KPEBA. In contrast by performing the reaction in boiling toluene a fast racemization was observed. In the solid state racemic KPEBA formed a dimer, in which all four nitrogen atoms are in a plane. To each potassium atom a toluene molecule is η⁶-coordinated.     

Preparation of new chiral pyridine–phosphine ligands and their Pd-catalyzed asymmetric allylic alkylations

Enantiomerically pure 2-(diphenylphosphino)methyl-N-[1-(2-pyridinyl)ethyl]pyrrolidines 1 and 2 have been prepared by the stereospecific substitution of enantiomerically pure 1-(2-pyridinyl)ethyl methanesulfonate 6 with enantiomerically pure 2-(diphenylphosphino)methylpyrrolidine. Asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate 11 with dimethyl malonate sodium salt using the (S,S)-ligand 1 affords the (R)-product 12 with up to 86% e.e. in good yield.     

Synthesis of (R)- and (S)-4-hydroxyisophorone by ruthenium-catalyzed asymmetric transfer hydrogenation of ketoisophorone

The first synthesis of (R)- and (S)-4-hydroxyisophorone by catalytic transfer hydrogenation of ketoisophorone is reported. Ruthenium catalysts containing commercially available chiral amino alcohols afforded 4-hydroxyisophorone in up to 97% selectivity and 97% ee. (R)- or (S)-4-Hydroxyisophorones with >99% ee were isolated by crystallization. The catalyst precursors [RuCl₂((S,R)-ADPE)(η⁶-p-cymene)] ((S,R)-ADPE=(1S,2R)-amino-1,2-diphenylethanol-N) and (R_Ru)-[RuCl((S,R)-ADPE⁻¹)(η⁶-p-cymene)] (ADPE⁻¹=amino-1,2-diphenylethanolato-N,O) were isolated for the first time and the X-ray crystal structure of the latter determined.     

First synthesis of enantiomerically pure (1S,2S)- and (1R,2R)-1,2-diaminocyclobutanecarboxylic acid–ornithine derivative–, from racemic 2-aminocyclobutanone

An easy and efficient one-pot reaction from readily available 2-(N-Cbz) aminocyclobutanone selectively gave, by means of an asymmetric Strecker synthesis in the presence of a chiral benzylic amine, the thermodynamic 1,2-diamino nitriles. Basic hydrolysis, cleavage of the benzylic group and acidic hydrolysis of the resulting trans-1,2-diaminocyclobutanecarbonitrile gave, in a four-step sequence from the ketone, (1S,2S)- or (1R,2R)-1,2-diaminocyclobutanecarboxylic acid, ornithine derivatives. The absolute configuration has been established by X-ray analysis of the corresponding trans-diamino amide.     

Stereoselective synthesis of (R)-(+)-1-methoxyspirobrassinin, (2R,3R)-(−)-1-methoxyspirobrassinol methyl ether and their enantiomers or diastereoisomers

Stereoselective synthesis of cruciferous indole phytoalexin (R)-(+)-1-methoxyspirobrassinin and its unnatural (S)-(−)-enantiomer was achieved by spirocyclization of 1-methoxybrassinin in the presence of (+)- and (−)-menthol and subsequent oxidation of the obtained menthyl ethers. Methanolysis of menthyl ethers in the presence of TFA afforded (2R,3R)-(−)-1-methoxyspirobrassinol methyl ether as well its unnatural (2S,3S)-, (2R,3S)-, and (2S,3R)-isomers.