Gold(I)-catalyzed one-pot reaction between 2-alkynylanilines and alkynols leading to the formation of C-3-substituted indoles: a case of formal carboamination of alkynes

First fragment-based screening identifies new chemotypes inhibiting ERAP1-metalloprotease

Inhibition of endoplasmic reticulum aminopeptidase 1 (ERAP1) by small-molecules is being eagerly investigated for the treatment of various autoimmune diseases and in the field of immuno-oncology after its active involvement in antigen presentation and processing. Currently, ERAP1 inhibitors are at different stages of clinical development, which highlights its significance as a promising drug target. In the present work, we describe the first-ever successful identification of several ERAP1 inhibitors derived from a fragment-based screening approach. We applied an enzymatic activity assay to a large library of ∼3000 fragment entries in order to retrieve 32 hits. After a multi-faceted selection process, we prioritized 3 chemotypes for SAR optimization and strategic modifications provided 2 series (2-thienylacetic acid and rhodanine scaffolds) with improved analogues at the low micromolar range of ERAP1 inhibition. We report also evidence of selectivity against homologous aminopeptidase IRAP, combined with complementary in silico docking studies to predict the binding mode and site of inhibition. Our compounds can be the starting point for future fragment growing and rational drug development, incorporating new chemical modalities.

B(C6F5)3-Catalyzed Intramolecular Hydroalkoxylation Deuteration Reactions of Unactivated Alkynyl Alcohols

Using D2O as a deuterium source, a method for the deuteration of intra- and extra-cyclic methylene has been developed for cyclic ethers with moderate yield and excellent deuterium incorporation. This transformation features superb functional group tolerance in a wide range of alkynols. Notably, the critical factor to achieve high deuterium incorporation is determined by the hydrogen isotope exchange reaction of an unstable oxonium ion. This novel methodology provides an efficient and concise synthetic route to a number of valuable deuterated cyclic ethers that are often difficult to prepare with other methods.

1,2-Difunctionalizations of alkynes entailing concomitant C–C and C–N bond-forming carboamination reactions

Vicinal carboamination of alkynes is a highly reliable and efficient practical strategy for the quick preparation of valuable and diverse amine derivatives starting from simple synthons. The last decade has witnessed numerous practical methods employing transition-metal-based/metal-free carboamination approaches using alkynes for the synthesis of these N-bearing entities. Driven by the renaissance of transition metal catalysis, intermolecular and intramolecular carboamination of alkynes comprising concomitant C–N and C–C bond formation has been studied extensively. In contrast to metal catalysis, though analogous metal-free approaches have been relatively less explored in the literature, they serve as alternatives to these expensive approaches. Despite this significant progress, reviews documenting such examples are sporadic; as a result, most reports of this type remained scattered throughout the literature, thereby hampering further developments in this escalating field. In this review, different conceptual approaches will be discussed and examples from the literature will be presented. Further, the reader will get insight into the mechanisms of different transformations.

The 1,2-difunctionalization of alkynes happening through concomitant C–C and C–N bond formation strategies have provide an unified access to diversely functionalized N-bearing heterocycles.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Assembly-Induced Strong Circularly Polarized Luminescence of Spirocyclic Chiral Silver(I) Clusters

Spirocyclic Ag₉ clusters, as a new form of intrinsically chiral metal clusters, were constructed through vertex-sharing of two in-situ-generated heteroaryl diide-centered metal rings. Such core-peripheral type clusters exhibit versatile photoluminescent and chiroptical behavior under different aggregation conditions. In contrast to a ligand-based fluorescence emission in a diluted solution of the clusters, a solvent polarity-caused assembly gives rise to new cluster-based phosphorous luminescence owing to radiative mode switching and aggregation-induced emission. Assembly of cluster enantiomers leads to micrometer-long helical nanofibers, whose handedness is determined by absolute configuration of individual spirocyclic clusters. Benefiting from exciton couplings of helical arrangements of chelating ligands at molecular and microscopic levels, the assembled film of cluster enantiomers exhibits circularly polarized luminescence with a high anisotropy factor (0.16).

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Acid catalysed rearrangement of isobenzofurans to angularly fused phthalides

An acid catalysed, cascade process for the construction of angularly fused polycyclic phthalides from isobenzofurans under transition metal free conditions has been reported. This process is very general for diverse gem-disubstituted isobenzofuran substrates. Control experiments supported the mechanism as the nucleophilic attack of the carboxylate onto the acid activated furan ring for the simultaneous ring closing-ring opening cascade followed by dehydration. This method serves as a greener alternative for the synthesis of angularly fused polycyclic phthalides.

Ring strain strategy for the control of regioselectivity. Gold-catalyzed anti-Markovnikov cycloisomerization initiated tandem reactions of alkynes

Gold-catalyzed nucleophilic addition to terminal alkynes has received considerable interest in the past decade, as this chemistry offers a highly efficient and regioselective way for CC, CH and CX bond formation. However, such a nucleophilic addition mainly involves a Markovnikov addition. In this short review, the recent progress of the gold-catalyzed 5-endo-dig cycloisomerization-initiated tandem reactions by utilizing the steric strain in ring formation to achieve an anti-Markovnikov regioselectivity was reviewed, including the scope of reactions, mechanism and synthetic applications.

Lewis Acid Promoted Oxonium Ion Driven Carboamination of Alkynes for the Synthesis of 4-Alkoxy Quinolines

Lewis acid mediated multisegment coupling cascade is designed for the synthesis of densely substituted 4-alkoxy quinolines via an oxonium ion triggered alkyne carboamination sequence involving C-C and C-N bond formations. Cyclic ether fused-quinolines could also be accessed using this fast, operationally simple, high yielding, chemoselective and functional group tolerant method. Versatility and utility of this methodology is demonstrated by postfunctionalization of products obtained and its use in synthesis of potent drug molecules.

Metal free carboamination of internal alkynes – an easy access to polysubstituted quinolines

A synergistic Brønsted acid catalyst was used to intermolecularly generate vinyl cations for a C–C bond formation – Schmidt rearrangement – sequence.

Oxidative coupling between C(sp2)–H and C(sp3)–H bonds of indoles and cyclic ethers/cycloalkanes

Cross-dehydrogenative-coupling (CDC) between C–H/C–H bonds of indoles and cyclic ethers/cycloalkanes is made viable through a simple transition-metal-free pathway.

A one-pot catalysis: the strategic classification with some recent examples

In this "Emerging Area", the strategic classification of one-pot catalysis, i.e. cooperative, relay and sequential catalysis, is described. In order to illustrate this classification, we take the readers through a series of recent examples which utilize either metal-metal, metal-organo and organo-organo catalysts. The compilation clearly demonstrates the explosive growth and power of this field, which has become, in the last few years, an important technique particularly in the case of enantioselective catalysis.

Intramolecular hydroamination of alkynic sulfonamides catalyzed by a gold-triethynylphosphine complex: Construction of azepine frameworks by 7-exo-dig cyclization

The gold-catalyzed, seven-membered ring forming, intramolecular hydroamination of alkynic sulfonamides has been investigated. The protocol, with a semihollow-shaped triethynylphosphine as a ligand for gold, allowed the synthesis of a variety of azepine derivatives, which are difficult to access by other methods. Both alkynic sulfoamides with a flexible linear chain and the benzene-fused substrates underwent 7-exo-dig cyclization to afford the nitrogen-containing heterocyclic seven-membered rings, such as tetrahydroazepine and dihydrobenzazepine, in good yields.

One-pot gold-catalyzed synthesis of 3-silylethynyl indoles from unprotected o-alkynylanilines

The Au(III)-catalyzed cyclization of 2-alkynylanilines was combined in a one-pot procedure with the Au(I)-catalyzed C3-selective direct alkynylation of indoles using the benziodoxolone reagent TIPS-EBX to give a mild, easy and straightforward entry to 2-substituted-3-alkynylindoles. The reaction can be applied to unprotected anilines, was tolerant to functional groups and easy to carry out (RT, and requires neither an inert atmosphere nor special solvents).