Organocatalytic Aza-Friedel-Crafts/Lactonization Domino Reaction of Naphthols and Phenols with 2-Acetamidoacrylate to Naphtho- and Benzofuranones Bearing a Quaternary Center at the C3 Position

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

Chiral Brønsted Acid Catalyzed Cascade Alcohol Deprotection and Enantioselective Cyclization

The protection-deprotection sequence is vital to organic synthesis. Here, we describe a novel catalytic cascade where a chiral Brønsted acid selectively removes ether protecting groups and catalyzes intramolecular cyclization in one pot. We tested three model substrates from our previous work and investigated the rate of deprotection through gas chromatography (GC) studies. This work builds on our stereoselective synthesis of lactones by streamlining our synthesis. It also opens the door for additional investigations into other catalytic cascade reactions using chiral Brønsted acid catalysts.

Efficient construction of a β-naphthol library under continuous flow conditions

A β-naphthol library has been efficiently constructed utilizing a mild continuous flow procedure, relying on a tandem Friedel-Crafts reaction and starting from readily available arylacetyl chloride and alkynes. Multiple functionalized β-naphthols can be acquired within 160 s in generally high yields (up to 83%). Using an electron-rich phenylacetyl chloride derivative (4-OH- or 4-MeO-) provides spirofused triene dione as the primary product. A scale-up preparation affords a throughput of 4.70 g h-1, indicating potential large-scale application. Herein, we present a rapid, reliable, and scalable method to obtain various β-naphthols in the compound library.

Brønsted Acid-Catalyzed Tandem Double Friedel-Crafts Alkylation to Construct a Dihydrophenalene Skeleton Bearing an All-Carbon Quaternary Center

An efficient Brønsted acid-catalyzed tandem reaction has been developed for the construction of a dihydrophenalene skeleton bearing an all-carbon quaternary center. Starting with 2-naphthol-tethered ketones and indoles, the tandem reaction catalyzed by TsOH monohydrate proceeded smoothly with good to excellent efficiency through a double Friedel-Crafts alkylation process. Moreover, the synthetic utility of this method was demonstrated by easy gram-scale preparation and product transformations to fused hexacyclic compounds.

Aromatic C-H bond functionalization through organocatalyzed asymmetric intermolecular aza-Friedel-Crafts reaction: a recent update

The aza-Friedel-Crafts reaction allows an efficient coupling of electron-rich aromatic systems with imines for the facile incorporation of aminoalkyl groups into the aromatic ring. This reaction has a great scope of forming aza-stereocenters which can be tuned by different asymmetric catalysts. This review assembles recent advances in asymmetric aza-Friedel-Crafts reactions mediated by organocatalysts. The mechanistic interpretation with the origin of stereoselectivity is also explained.

Asymmetric Alkylation of Cyclic Ketones with Dehydroalanine via H-Bond-Directing Enamine Catalysis: Straightforward Access to Enantiopure Unnatural α-Amino Acids

The growing importance of structurally diverse and functionalized enantiomerically pure unnatural amino acids in the design of drugs, including peptides, has stimulated the development of new synthetic methods. This study reports the challenging direct asymmetric alkylation of cyclic ketones with dehydroalanine derivatives via a conjugate addition reaction for the synthesis of enantiopure ketone-based α-unnatural amino acids. The key to success was the design of a bifunctional primary amine-thiourea catalyst that combines H-bond-directing activation and enamine catalysis. The simultaneous dual activation of the two relatively unreactive partners, confirmed by mass spectrometry studies, results in high reactivity while securing high levels of stereocontrol. A broad substrate scope is accompanied by versatile downstream chemical modifications. The mild reaction conditions and consistently excellent enantioselectivities (>95 % ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical enantiopure α-amino-acid building blocks.

Photocatalytic Reductive Olefin Hydrodifluoroalkylation Enabled by Tertiary Amine Reductants Compatible with Complex Systems

Noncanonical amino acids (NCAAs) are imperative to many facets of chemistry and biology. Herein, we report a method for the reductive hydrodifluoroalkylation of olefins that utilizes triethylamine base as the terminal reductant. The alkene acceptors include a range of electronically diverse alkenes, chief among them, dehydroalanine in variously protected forms, which provides access to synthetically relevant NCAA scaffolds under mild and general reaction conditions. We have demonstrated that a chiral auxiliary may be incorporated to provide diastereocontrol for pro-stereogenic substrates. Mechanistically motivated experiments provide some insight into the reaction mechanism, which supports a terminal step involving proton transfer for electron-poor olefins, while H atom transfer assisted by a thiol cocatalyst may complete the catalytic cycle for electron-rich olefins. The protocol is found to be compatible with additions to complex molecules, including the natural product thiostrepton.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Enantioselective and regioselective aza-Friedel–Crafts reaction of electron-rich phenols with isatin-derived ketimines

An efficient asymmetric aza-Friedel–Crafts reaction of phenols with isatin-derived ketimines is described.

Concise and Convergent Enantioselective Total Syntheses of (+)- and (-)-Fumimycin

The concise and convergent total syntheses of (+)- and (-)-Fumimycin have been achieved by taking advantage of strategies for the asymmetric aza-Friedel-Crafts reaction of a highly substituted hydroquinone and N-fumaryl ketimine generated from the corresponding dehydroalanine. The enantiomerically pure natural product and its enantiomer were prepared in seven steps and 22% overall yield by employing both enantiomers of a BINOL-derived chiral phosphoric acid (CPA) catalyst.

Total synthesis of (±)-fumimycin and analogues for biological evaluation as peptide deformylase inhibitors

A concise 7-step total synthesis of (±)-fumimycin in 11.6 % overall yield is reported. An acid-catalyzed intramolecular aza-Friedel-Crafts cyclization was developed to construct the benzofuranone skeleton of the natural product bearing an α,α-disubstituted amino acid moiety in a single step. Regioselective chlorination followed by a Suzuki-Miyaura cross-coupling rapidly enabled the preparation of a library of analogues which were evaluated against peptide deformylase for antibacterial activity.