Biomimetic Asymmetric 1,3-Dioplar Cycloaddition: Amino Acid Precursors in Biosynthesis Serve as Latent Azomethine Ylides

Fortuitous Enantiomeric Self-Rectification of an Unreported Partial Racemisation in the Synthesis of a Chiral Phosphoric Acid: A Warning to Practitioners

Racemisation without Consequence: MOM deprotection during a routine synthesis of phosphoric acid 1 promoted partial racemisation of the product BINOL 5. Surprisingly, however, after phosphorylation of the partially racemised BINOL, enantiopure acid 1 was isolated. Further inspection revealed that during phosphorylation, unhydrolysed racemic phosphorochloridate 6 precipitated, thus restoring homochirality in the product phosphoric acid 1. Moreover, MOM deprotection partial racemisation was avoided by conducting reactions at lower temperatures for no longer than the required deprotection time, and/or by application of other deprotection conditions from the literature.

Asymmetric Organocatalytic 1,3-Dipolar Cycloaddition of Azomethine Ylides with β-Substituted Cyclic Enones

The enantioselective and diastereoselective control of 1,3-dipolar cycloaddition reactions to β-substituted cyclic enones has been developed. The 1,3-dipolar cycloaddition of phthalazinium dicyanomethanides with cyclic dienones affords chiral tetrahydropyrrolo[2,1-a]phthalazine derivatives 3 through vinylogous iminium ion activation by combining a cinchona-based primary amine C3 and a chiral camphorsulfonic acid additive. Conversely, with a weaker 3,5-bis(trifluoromethyl)benzoic acid additive, the 1,3-dipolar cycloaddition of phthalazinium dicyanomethanides with β-substituted cyclic enones leads to chiral hexahydroisoindolo[1,2-a]phthalazin-10(8H)-one derivatives 4 with excellent stereocontrol via endo-dienamine activation.

Multifunctional Organocatalysts - Singly-Linked and Macrocyclic Bisphosphoric Acids for Asymmetric Phase-Transfer and Brønsted-Acid Catalysis

The linking of phosphoric acids via covalent or mechanical bonds has proven to be a successful strategy for the design of novel organocatalysts. Here, we present the first systematic investigation of singly-linked and macrocyclic bisphosphoric acids, including their synthesis and their application in phase-transfer and Brønsted acid catalysis. We found that the novel bisphosphoric acids show dramatically increased enantioselectivities in comparison to their monophosphoric acid analogues. However, the nature, length and number of linkers has a profound influence on the enantioselectivities. In the asymmetric dearomative fluorination via phase-transfer catalysis, bisphosphoric acids with a single, rigid bisalkyne-linker give the best results with moderate to good enantiomeric excesses. In contrast, bisphosphoric acids with flexible linkers give excellent enantioselectivities in the transfer-hydrogenation of quinolines via cooperative Brønsted acid catalysis. In the latter case, sufficiently long linkers are needed for high stereoselectivities, as found experimentally and supported by DFT calculations.

Selective C3-Allylation and Formal [3 + 2]-Annulation of Spiro-Aziridine Oxindoles: Synthesis of 5'-Substituted Spiro[pyrrolidine-3,3'-oxindoles] and Coerulescine

Brønsted acid- and/or Lewis acid-catalyzed selective C3-allylation and formal [3 + 2]-annulation of spiro-aziridine oxindoles with allylsilanes have been demonstrated to deliver direct access to 3-allyl-3-aminomethyl oxindoles and 5-silyl methyl spiro[pyrrolidine-3,3'-oxindoles], respectively. The acid-catalyzed methods do not provide any stereoselectivity when chiral spiroaziridines are used. However, the reaction of nonracemic sprioaziridines with allyl-Grignard reagent under catalyst-free conditions afforded 3-allyl-3-aminomethyl oxindoles with good stereoselectivity (ee up to 80%). The allylation protocol is utilized for the short synthesis of coerulescine and various 5'-substituted spiro[pyrrolidine-3,3'-oxindoles].

Catalytic asymmetric umpolung reactions of imines via 2-azaallyl anion intermediates

The imine umpolung is a relatively new and interesting strategy, especially in catalytic asymmetric synthesis. A significant development in organo- and transition metal-catalyzed umpolung of imines took place only in the recently concluded decade. A majority of the reports on the asymmetric umpolung of imines involve the initial generation of 2-azaallyl anion intermediates with the chiral catalysts, which serve as a significant driving force for the umpolung addition/substitution reactions. A variety of organocatalysts such as bifunctional cinchona alkaloids including squaramides and thioureas, chiral BINOL derived phosphoric acids, phase transfer catalysts (PTCs), phosphines, and transition metal-complexes of iridium, copper and palladium have been employed to achieve the excellent level of asymmetric induction in such types of umpolung reactions. The asymmetric imine umpolung strategy has been applied successfully to synthesize synthetic amino-acid derivatives and other useful chiral amines, including drugs and potentially bioactive molecules. This review summarizes all the significant recent development in catalytic umpolung reactions of imines involving a 2-azaallyl anion intermediate.

Recent advances in organocatalytic asymmetric multicomponent cascade reactions for enantioselective synthesis of spirooxindoles

Catalytic asymmetric MCCRs for enantioselective synthesis of spirooxindoles by using chiral phosphoric acids, amines, bifunctional thiourea/squaramides and metal-based reagents as catalysts.

Ag(i)/(S,S)-ip-FOXAP catalyzed diastereo- and enantioselective 1,3-dipolar cycloaddition of azomethine ylides with benzosultam-3-ylidenes

The AgOAc/ip-FOXAP complex catalyzed the highly diastereo- and enantioselective 1,3-dipolar cycloaddition of azomethine ylides with benzosultam-3-ylidenes to give a single isomer of the exo-cycloadduct in good yields.

Synthesis of chiral anti-1,2-diamine derivatives through copper(I)-catalyzed asymmetric α-addition of ketimines to aldimines

Chiral 1,2-diamines serve as not only common structure units in bioactive molecules but also useful ligands for a range of catalytic asymmetric reactions. Here, we report a method to access anti-1,2-diamine derivatives. By means of the electron-withdrawing nature of 2- or 4-nitro-phenyl group, a copper(I)-catalyzed asymmetric α-addition of ketimines derived from trifluoroacetophenone and 2- or 4-NO₂-benzylamines to aldimines is achieved, which affords a series of chiral anti-1,2-diamine derivatives in moderate to high yields with moderate to high diastereoselectivity and high to excellent enantioselectivity. Aromatic aldimines, heteroaromatic aldimines, and aliphatic aldimines serve as suitable substrates. The nitro group is demonstrated as a synthetical handle by several transformations, including a particularly interesting Fe(acac)₃-catalyzed radical hydroamination with a trisubstituted olefin. Moreover, the aryl amine moiety obtained by the reduction of the nitro group serves as a synthetically versatile group, which leads to the generation of several functional groups by the powerful Sandmeyer reaction, such as -OH, -Br, -CF₃, and -BPin.

Chiral 1,2-diamines are useful chemicals used as ligands in asymmetric catalysis. Here, the authors report a copper(I)-catalyzed asymmetric α-addition of ketimines derived from trifluoroacetophenone and 2- or 4-NO2-benzylamines to aldimines, affording a series of chiral anti-1,2-diamine derivatives.

Kitamura Electrophilic Fluorination Using HF as a Source of Fluorine

This review article focused on the innovative procedure for electrophilic fluorination using HF and in situ generation of the required electrophilic species derived from hypervalent iodine compounds. The areas of synthetic application of this approach include fluorination of 1,3-dicarbonyl compounds, aryl-alkyl ketones, styrene derivatives, α,β-unsaturated ketones and alcohols, homoallyl amine and homoallyl alcohol derivatives, 3-butenoic acids and alkynes.

Organocatalytic synthesis of chiral CF3-containing oxazolidines and 1,2-amino alcohols: asymmetric oxa-1,3-dipolar cycloaddition of trifluoroethylamine-derived azomethine ylides

A series of CF₃-containing oxazolidines were constructed via organocatalytic asymmetric oxa-1,3-dipolar cycloaddition. These oxazolidines could undergo facile conversion to CF₃-containing 1,2-amino alcohols with vicinal stereogenic centers.

Pd-Catalyzed Allylation of Imines to Access α-CF3-Substituted α-Amino Acid Derivatives

We herein report a high yielding protocol for the direct α-allylation of easily accessible trifluoropyruvate-derived imines using Pd-catalysis. The reaction gives access to a variety of different α-allylated-α-CF3-amino acids in a straightforward manner, starting from commercially available trifluoropyruvate. We also provide a proof-of-concept for an enantioselective protocol (up to er = 75:25) by using chiral phosphane ligands.

Synthesis of α-CF3-proline derivatives by means of a formal (3 + 2)-cyclisation between trifluoropyruvate imines and Michael acceptors

We herein report the first formal (3 + 2)-cyclisation between 3,3,3-trifluoropyruvate-derived imines and indandione-based Michael acceptors. This reaction gives access to a novel class of spirocyclic α-CF3-α-proline derivatives with complete control of the diastereoselectivity under phase transfer-catalysed reaction conditions.

Synthesis of Furan-Annelated BINOL Derivatives: Acid-Catalyzed Cyclization Induces Partial Racemization

In this account, we describe the synthesis of a series of BINOL-based bis- and trisphosphoric acids 11d/e/f, which commonly feature an unusual phosphoric acid monoester motif. This motif is generated by an acid-catalyzed 5- endo- dig cyclization of the 3-alkynyl-substituted BINOL precursors to give the corresponding Furan-annelated derivatives, followed by phosphorylation of the remaining phenolic alcohols. In the cyclization reaction, we observed an unexpected partial racemization in the bis- and tris-BINOL scaffolds, leading to mixtures of diastereomers that were separated and characterized spectroscopically and by X-ray crystal structure analyses. The cyclization and racemization processes were investigated both experimentally and by DFT-calculations, showing that although the cyclization proceeds faster, the barrier for the acid-catalyzed binaphthyl-racemization is only slightly higher.

Intramolecular Umpolung Synthesis of Exocyclic β-Amino Alcohols through Decarboxylative Amination

An intramolecular aminative Umpolung cyclization strategy has been developed by using α,α-diphenylglycine (2) as the amination and Umpolung reagent. Aldehydes (1) bearing an additional carbonyl group underwent condensation with α,α-diphenylglycine to form an imine, decarboxylation to generate a delocalized 2-azaallylanion, and subsequent intramolecular Umpolung cyclization to produce a variety of exocyclic β-amino alcohols (6) in 60-93% yields with up to >20:1 trans/cis selectivity under mild conditions.

Aminative Umpolung cyclization for synthesis of chiral exocyclic vicinal diamines

Chiral exocylic vicinal diamines are biologically and chemically important compounds, but they are not easy to make. In this paper, an interesting aminative Umpolung cyclization process has been developed. Aromatic aldehydes 6 bearing an electrophilic chiral sulfinimine group underwent imine formation with 2,2-diphenylglycine (2), decarboxylation, and subsequent Umpolung cyclization, producing various trans-diamines 10 in 84-96% yields with high trans/cis ratios under very mild conditions. This work not only provides an efficient, clean, and mild method for the synthesis of chiral exocyclic vicinal diamines in one step but also represents a new application of aminative Umpolung strategy on intramolecular reactions.

Enantioselective aza-Friedel-Crafts reaction of furan with α-ketimino esters induced by a conjugated double hydrogen bond network of chiral bis(phosphoric acid) catalysts

Chiral C 2- and C 1-symmetric BINOL-derived bis(phosphoric acid) catalysts, which have OP([double bond, length as m-dash]O)(OH)2/OP([double bond, length as m-dash]O)(OH)(OR) moieties at the 2,2'-positions, were developed and used for the enantioselective aza-Friedel-Crafts reaction of 2-methoxyfuran with α-ketimino esters for the first time. The intramolecular conjugated double hydrogen bond network is a key to increasing the Brønsted acidity and preventing deactivation of the catalysts. Highly functionalized α-amino acid derivatives with a chiral quaternary carbon center could be transformed into versatile optically active N- and O-heterocycles and an α-aryl-substituted serine.

Ir/PTC cooperatively catalyzed asymmetric umpolung allylation of α-imino ester enabled synthesis of α-quaternary amino acid derivatives bearing two vicinal stereocenters

A novel Ir/PTC (phase-transfer catalyst) cooperatively catalyzed asymmetric umpolung allylation of simple α-imino esters is developed and it provides facile access to α-quaternary amino acid derivatives bearing two vicinal stereocenters. Both the metal catalyst and the phase-transfer catalyst are crucial for this methodology. The reaction features good yields, high diastereo- and enantio-selectivities, ease of scale-up to gram scale and further transformation of the products (e.g. to quaternary proline analogues with multi stereocenters).

Highly diastereoselective C → N acyl rearrangement in polysubstituted pyrrolidine 2,2-dicarboxylates. Stereocontrolled synthesis of densely functionalized prolines

A fully diastereoselective C–N acyl transfer reaction operates in enantiopure densely substituted pyrrolidine 2,2-dicarboxylates.

Catalytic synthesis of 3-aminooxindoles via addition to isatin imine: an update

3-Substituted-3-aminooxindoles have attracted the attention of organic chemist to develop different synthetic methodologies for their synthesis using catalysts. This review covers the recent developments.

Catalytic asymmetric construction of spiropyrrolidines via 1,3-dipolar cycloaddition of azomethine ylides

In this review, recent progress on the catalytic enantioselective synthesis of chiral spiropyrrolidine derivatives via 1,3-dipolar cycloaddition of azomethine ylides has been discussed.

Organocatalytic diastereoselective synthesis of spirooxindoles via [3+2] cycloadditions of azomethine ylides with α,β-unsaturated esters

Isatin-derived azomethine ylides react diastereoselectively with unsaturated esters to afford densely substituted spirooxindols.

Decarboxylative Umpolung of conjugated enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde

Decarboxylative Umpolung converts enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde group with excellent regio- and diastereoselectivities.

Catalytic Asymmetric Synthesis of Chiral γ-Amino Ketones via Umpolung Reactions of Imines

The first direct catalytic asymmetric synthesis of γ-amino ketones was realized by the development of a highly diastereoselective and enantioselective C-C bond-forming umpolung reaction of imines and enones under the catalysis of a new cinchona alkaloid-derived phase-transfer catalyst. In a loading ranging from 0.02 to 2.5 mol %, the catalyst activates a broad range of trifluoromethyl imines and aldimines as nucleophiles to engage in chemo-, regio-, diastereo- and enantioselective C-C bond-forming reactions with acyclic and cyclic enones, thereby converting these readily available prochiral starting materials into highly enantiomerically enriched chiral γ-amino ketones in synthetically useful yields. Enabled by this unprecedented umpolung reaction of imines, conceptually new and concise routes were developed for the asymmetric synthesis of nitrogen-heterocycles such as pyrrolidines and indolizidines.