Design of N-spiro C2-symmetric chiral quaternary ammonium bromides as novel chiral phase-transfer catalysts: synthesis and application to practical asymmetric synthesis of alpha-amino acids

A series of C(2)-symmetric chiral quaternary ammonium bromides 10 and 11 have been designed as a new, purely synthetic chiral phase-transfer catalyst, and readily prepared from commercially available optically pure 1,1'-bi-2-naphthol as a basic chiral unit. The details of the synthetic procedures of each requisite chiral binaphthyl subunit have been disclosed, and the structures of the assembled N-spiro chiral quaternary ammonium bromides 11a and 11f were unequivocally determined by single-crystal X-ray diffraction analysis. The reactivity and selectivity of these chiral ammonium bromides as chiral phase-transfer catalysts have been evaluated in the asymmetric alkylation of the benzophenone Schiff base of glycine ester 7 under mild liquid-liquid phase-transfer conditions, and the optimization of the reaction variables (solvent, base, and temperature) has also been conducted. Further, the scope and limitations of this asymmetric alkylation have been thoroughly investigated with a variety of alkyl halides, in which the advantage of the unique N-spiro structure of 11 and dramatic effect of the steric as well as the electronic properties of the aromatic substituents on the 3,3'-position of one binaphthyl moiety have been particularly emphasized. Finally, the potential synthetic utility of the present method for the practical asymmetric synthesis of structurally diverse natural and unnatural alpha-amino acids has been demonstrated by its successful application to the facile asymmetric syntheses of (S)-N-acetylindoline-2-carboxylate, a key intermediate in the synthesis of the ACE inhibitor, and l-Dopa (l-3,4-dihydroxyphenylalanine) ester and its analogue.     

Design, synthesis, and application of tartaric acid derived N-spiro quaternary ammonium salts as chiral phase-transfer catalysts

A novel class of tartaric acid-derived N-spiro quaternary ammonium salts was synthesised starting from known TADDOLs. These compounds were found to catalyse the asymmetric α-alkylation of glycine Schiff bases with high enantioselectivities and in good yields.     

Highly enantioselective conjugate addition of nitroalkanes to alkylidenemalonates using efficient phase-transfer catalysis of N-spiro chiral ammonium bromides

Highly enantioselective conjugate addition of nitroalkanes to alkylidenemalonates has been accomplished for the first time by the utilization of efficient phase-transfer catalysis of N-spiro C2-symmetric chiral quaternary ammonium bromide 1. For instance, simple mixing of nitropropane (2, R1 = Et), diisopropyl benzylidienemalonate (3, R2 = Ph), Cs2CO3 (1 equiv), and (S,S)-1 (1 mol %) in toluene at 0 degrees C for 2.5 h gave rise to the desired conjugate addition product 4 (R1 = Et, R2 = Ph) quantitatively (anti/syn = 86:14) with 97% ee (anti isomer). The applicability of this procedure has been demonstrated with other representative alkylidenemalonates and nitroalkanes. Since 4 can be readily transformed into the corresponding gamma-amino acid hydrochloride 5 without loss of diastereo- and enantioselectivity, the present method provides a new and practical access to various optically active gamma-amino acid derivatives.     

Enantioselective synthesis of DIANANE, a novel C2-symmetric chiral diamine for asymmetric catalysis

DIANANE (endo,endo-2,5-diaminonorbornane) is a novel chiral C(2)-symmetric diamine, based on the rigid bicyclo[2.2.1]heptane scaffold. Schiff-base ligands derived from DIANANE have already found use in asymmetric catalysis, e.g., in the highly enantioselective Nozaki-Hiyama-Kishi reaction. We herein describe a practical synthesis of enantiomerically pure DIANANE, starting from norbornadiene in four steps: (i) Pd-MOP catalyzed Hayashi-hydrosilylation/Tamao-Fleming oxidation, (ii) oxidation to norbornane-2,5-dione, (iii) endo-selective reductive amination with benzylamine, and (iv) hydrogenolytic debenzylation. None of the steps involves chromatographic purification. For the Tamao-Fleming oxidation, the use of hydrogen peroxide in the form of its urea clathrate instead of aqueous solution proved beneficial. By the above sequence, enantiomerically pure (ee >or=99%) DIANANE was obtained from norbornadiene in 40-50% overall yield. The relative and absolute configuration of DIANANE was confirmed by X-ray crystallography of the DIANANE bis-tosylamide, and of its bis-camphorsulfonamide. Furthermore, the synthesis and X-ray crystal structure of the Schiff-base ligand derived from DIANANE and 3,5-di-tert-butyl salicylic aldehyde are reported.     

N-spiro chiral quaternary ammonium bromide catalyzed diastereo- and enantioselective conjugate addition of nitroalkanes to cyclic alpha,beta-unsaturated ketones under phase-transfer conditions

[reaction, structure: see text] Conjugate addition of various prochiral nitroalkanes to cyclic alpha,beta-unsaturated ketones was found to be efficiently catalyzed by N-spiro C2-symmetric chiral quaternary ammonium bromide 1b possessing a 3,5-bis(3,4,5-trifluorophenyl)phenyl substituent under solid-liquid phase-transfer conditions to afford the corresponding gamma-nitro ketones in excellent chemical yields with unprecedented levels of diastereo- and enantiocontrol.     

Enantioselective synthesis of cyclopropylphosphonates containing quaternary stereocenters using a D2-symmetric chiral catalyst Rh2(S-biTISP)2

[reaction: see text] Rh(2)(S-biTISP)(2)-catalyzed reactions of dimethyl aryldiazomethylphosphonates generate donor/acceptor-substituted rhodium carbenoid intermediates capable of cyclopropanation of styrenes in high yields (85-96%), diastereoselectivity (> or =98% de), and enantioselectivity (76-92% ee).     

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.     

A sterically-encumbered,C2-symmetric chiral acetal for enhanced asymmetric induction in the Pauson-Khand reaction

High levels of diastereoselection were achieved in the PKR of 1,6- and 1,7-cyclopropylidenynes bearing a bulky propargylic C(2)-symmetric acetal.     

Novel C2-symmetric macrocycles bearing diamide-diester groups: synthesis and enantiomeric recognition for primary alkyl ammonium salts

We synthesized a series of novel macrocycles with diamide-diester groups (S,S)-1, (S,S)-2, (S,S)-3, and (R,R)-1, derived from dimethyloxalate and amino alcohols by high dilution technique, and evaluated enantiomeric recognition properties of these macrocycles toward primary alkyl ammonium salts by 1H NMR titration. Taking into account the host employed, important differences were observed in the Ka values of (R)-Am and (S)-Am for (S,S)-1 and (R,R)-1 hosts, KS/KR = 5.55 and KR/KS = 3.65, Delta Delta Go = 0.43 and -0.32 kJ mol-1, respectively. There seems a general tendency for the host to include the guests with the same absolute configuration.     

An improved procedure for the synthesis of 2-morpholinoethanamine

2-Morpholinoethanamine is prepared rapidly mainly in aqueous conditions from inexpensive and commercially accessible starting materials. The process development of an effective synthetic route by utilizing morpholine as the starting material via Michael addition, hydrazinolysis, and Curtius rearrangements was undertaken. The optimal conditions were selected in the experiments. The total yield was 81.8% and it was a convenient process.     

C2-symmetric chiral disulfoxide ligands in rhodium-catalyzed 1,4-addition: from ligand synthesis to the enantioselection pathway

A family of chiral C(2)-symmetric disulfoxide ligands possessing biaryl atropisomeric backbones has been synthesized by using the Andersen methodology. Complete characterization includes X-ray crystallographic studies of all ligands and some of their rhodium complexes. Their synthesis, optical purity, electronic properties, and catalytic behavior in the prototypical rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclohexen-1-one are presented through an in depth study of this ligand class. Density functional theory calculations on the step of the catalytic cycle that determines the enantioselectivity are presented and reinforce the first hypothetical explanations for the high levels of asymmetric induction observed.     

Enantioselective recognition of a chiral quaternary ammonium ion by C3 symmetric cyclic hexapeptides

C3 Symmetric cyclic hexapeptides containing alternating L-proline and 3-aminobenzoic acid derivatives as subunits possess different affinities towards the two enantiomers of the N,N,N-trimethyl-1-phenylethyl ammonium cation.     

Molecular design of chiral quaternary ammonium polymers for asymmetric catalysis applications

Repeated reaction between a chiral quaternary ammonium dimer and disodium disulfonate gave a chiral ionic polymer, which showed excellent catalytic activity in the asymmetric benzylation of N-diphenylmethylene glycine tert-butyl ester.     

Double C-H activation strategy for the asymmetric synthesis of C2-symmetric anilines

Rh(2)(S-DOSP)(4)-catalyzed C-H activation to N,N-dimethylanilines is described. A double C-H activation was possible by using an excess of methyl aryldiazoacetate, and this provided a very direct approach to C(2)-symmetric anilines. [reaction--see text]     

新型C2对称的手性双(亚)唑啉化合物的合成

手性双噁唑啉化合物因其在不对称合成领域的广泛应用,而成为有机合成的一个研究热点。具有C2对称的手性双噁唑啉化合物因其在不对称合成反应中表现出较好的对映选择活性而倍受关注,其合成方法有由羧基转化为噁唑啉基;     

A new class of chiral organogermanes derived from C2-symmetric dithiols: synthesis,characterization and stereoselective free radical reactions

A new class of dithiostannanes and dithiogermanes have been prepared from 1,1'-binaphthyl-2,2'-dithiol and 3,3'-bis(trimethylsilyl)-1,1'-binaphtho-2,2'-dithiol. While reduction of 4-butyl-4-chloro-3,5-dithia-4-stanna-cyclohepta[2,1-a;3,4-a']dinaphthalene to the corresponding tin hydride was unsuccessful, 4-tert-butyl-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene and 4-tert-butyl-2,6-bis(trimethylsilyl)-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene were obtained by reduction of the parent germanium chlorides with NaBH(4) and LiBH(4), respectively. Kinetic constants for hydrogen transfer to a primary alkyl radical were measured for both germanium hydrides. Reduction of alpha-halo carbonyl compounds by these germanium hydrides occurs with moderate ee values (up to 42%), while hydrogermylation of methyl methacrylate occurs with low selectivity (<3/1) for the former hydride but high selectivity (>10/1) for the latter.     

First asymmetric synthesis of a C2-symmetric 2-endo,6-endo-disubstituted bispidine

The enantioselective synthesis of a C₂-symmetric 2-endo,6-endo-disubstituted bispidine (3,7-diazabicyclo[3.3.1]nonane) has been accomplished for the first time. The key step was a Michael addition of the protected β-amino ester methyl (R)-3-{N-benzyl-N-[(S)-1-phenylethyl]amino}-3-phenylpropionate to its α-methylene derivative delivering an anti,anti-configured α,α′-methylene-bridged bis(β-amino ester) as the major diastereomer. Deprotection, reduction and cyclisation furnished (1R,2R,5R,6R)-2,6-diphenyl-3,7-bis((S)-1-phenylethyl)-3,7-diazabicyclo[3.3.1]nonane in six steps and 15% overall yield.