From Heteroaromatic Acids and Imines to Azaspirocycles: Stereoselective Synthesis and 3D Shape Analysis

Reactivity of ethyl nitrosoacrylate toward pyrrole, indole and pyrrolo[3,2-c]carbazole: an experimental and theoretical study

Nitrosoalkenes react with 8-methyl-1,6-dihydropyrrolo[3,2-c]carbazole to give both 2- and 3-alkylated products via hetero-Diels-Alder reaction followed by the cycloadduct ring-opening. Quantum chemical calculations, at DFT level of theory, were carried out to investigate the regioselectivity of the cycloaddition of ethyl nitrosoacrylate with 1,6-dihydropyrrolo[3,2-c]carbazoles as well as with pyrrole and indole, allowing a more comprehensive analysis of the reactivity pattern of nitrosoalkenes with five-membered heterocycles. Furthermore, theoretical calculations confirmed that ethyl nitrosoacrylate reacts with these heterocycles via a LUMOheterodiene-HOMOdienophile controlled cycloaddition. The reactivity of one of the oxime-functionalized 1,6-dihydropyrrolo[3,2-c]carbazole was explored and a new hexahydropyrido[4',3':4,5]pyrrolo[3,2-c]carbazole system was obtained in high yield via a one-pot, two-step procedure.

A unified "top-down" approach for the synthesis of diverse lead-like molecular scaffolds

A "top-down" synthetic approach enabled the step-efficient synthesis of 21 diverse novel molecular scaffolds. The scaffolds were derived from four complex intermediates that had been prepared using cycloaddition chemistry. Scaffold-hopping of these intermediates was achieved through attachment of an additional ring, ring cleavage, ring expansion and/or ring fusion. It was shown that the resulting scaffolds could be decorated to yield diverse lead-like screening compounds.

Strategies and methodologies for the construction of spiro-γ-lactams: an update

This review gives an insight into the most recent synthetic methodologies towards spiro-γ-lactams, a class of compounds that are present in a wide range of synthetic bioactive and naturally occurring molecules.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

A modular and divergent approach to spirocyclic pyrrolidines

An efficient three-step sequence to afford a valuable class of spirocyclic pyrrolidines is reported. A reductive cleavage/Horner-Wadsworth-Emmons cascade facilitates the spirocyclisation of a range of isoxazolines bearing a distal β-ketophosphonate. The spirocyclisation precursors are elaborated in a facile and modular fashion, via a [3 + 2]-cycloaddition followed by the condensation of a phosphonate ester, introducing multiple points of divergence. The synthetic utility of this protocol has been demonstrated in the synthesis of a broad family of 1-azaspiro[4,4]nonanes and in a concise formal synthesis of the natural product (±)-cephalotaxine.

Exploring the Chemistry of Spiroindolenines by Mechanistically-Driven Reaction Development: Asymmetric Pictet-Spengler-type Reactions and Beyond

The Pictet-Spengler reaction is a fundamental named reaction in organic chemistry, and it is the most straightforward method for the synthesis of tetrahydro-β-carbolines, a core structure embedded in numerous alkaloids. Spiroindolenines are often proposed as possible intermediates in Pictet-Spengler reactions. However, whether the spiroindolenine species is an intermediate in the mechanism of the asymmetric Pictet-Spengler reaction remains unclear. Questions about the role of the spiroindolenine species regarding the mechanism include the following: Can the spiroindolenine species be formed effectively under Pictet-Spengler conditions? If so, what is its fate? Is the delivery of the enantioenriched tetrahydro-β-carboline product related to the spiroindolenine intermediate? Previous studies regarding these questions have not reached a consensus. Therefore, elucidating these questions will advance the field of synthetic organic chemistry.The first highly enantioselective synthesis of spiroindolenines that have the same molecular scaffold as the proposed key intermediate of the Pictet-Spengler reaction was accomplished by an Ir-catalyzed intramolecular asymmetric allylic substitution reaction of an indol-3-yl allylic carbonate. In this reaction, a piperidine, pyrrolidine, or cyclopentane ring can be introduced in conjunction with the indolenine structure.Spiroindolenines were found to undergo ring-expansive migration reactions when treated with a catalytic amount of an acid, leading to tetrahydro-β-carbolines or related tetrahydrocarbazoles. Comprehensive DFT calculations and Born-Oppenheimer molecular dynamics simulations have provided insight into the mechanism of the migration process. It has been found that the stereochemistry is strongly correlated with the electronic properties of the migratory group along with the acidity of the catalyst. Close interactions between the positively charged migratory group and the electron-rich indole ring favor the stereospecificity of the migration. Furthermore, a continuous mechanistic spectrum of the Pictet-Spengler reactions can be obtained on the basis of two readily accessible energetic parameters that are derived from computed energies for competing transition states relative to a key intermediate species. This theoretical model provides a unified mechanistic understanding of the asymmetric Pictet-Spengler reaction, which has been further supported by rationally designed prototype reactions. Chemically and stereochemically controllable migration can be achieved when multiple potential migratory groups are available.The reactivity of spiroindolenines has also been explored beyond Pictet-Spengler reactions. A one-pot Ir-catalyzed asymmetric allylic dearomatization/stereoconvergent migration allows the facile synthesis of enantioenriched tetrahydro-β-carbolines from racemic starting materials. An unprecedented six- to seven-membered ring-expansive migration can be achieved when a vinyliminium moiety is involved as a highly reactive migratory group. This reaction facilitates the stereoselective synthesis of thermodynamically challenging indole-annulated seven-membered rings. It has also been found that the migration process can be interrupted. The electrophilic migratory group released from the retro-Mannich reaction of a spiroindolenine can be captured by an inter- or intramolecular nucleophile, thus providing new entries into structurally diverse polycyclic indole derivatives.Therefore, the mechanism of the Pictet-Spengler reaction can be probed by manipulating the reactivity of the spiroindolenine species. In turn, the mechanistic insights gained herein will aid in chemical transformations toward various target molecules. This study serves as a vivid example of the positive interplay between experimental and theoretical investigations in synthetic organic chemistry.

Visible-light-induced intramolecular charge transfer in the radical spirocyclisation of indole-tethered ynones

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols. This initiates a novel radical chain sequence, based on dearomatising spirocyclisation with concomitant C–S bond formation. Sulfur-containing spirocycles are formed in high yields using this simple and mild synthetic protocol, in which neither transition metal catalysts nor photocatalysts are required. The proposed mechanism is supported by various mechanistic studies, and the unusual radical initiation mode represents only the second report of the use of an intramolecular electron donor–acceptor complex in synthesis.

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols.

A marine viral halogenase that iodinates diverse substrates

Oceanic cyanobacteria are the most abundant oxygen-generating phototrophs on our planet, and therefore, important to life. These organisms are infected by viruses called cyanophages, recently shown to encode metabolic genes that modulate host photosynthesis, phosphorus cycling and nucleotide metabolism. Herein, we report the characterisation of a wild type flavin-dependent viral halogenase (VirX1) from a cyanophage. Notably, halogenases have been previously associated with secondary metabolism, tailoring natural products. Exploration of this viral halogenase reveals it capable of regioselective halogenation of a diverse range of substrates, with a preference for forming aryl iodide species; this has potential implications for the metabolism of the infected host. Until recently, a flavin-dependent halogenase (FDH) capable of iodination in vitro had not been reported. VirX1 is interesting from a biocatalytic perspective showing strikingly broad substrate flexibility, and a clear preference for iodination, as illustrated by kinetic analysis. These factors together render it an attractive tool for synthesis.

Dearomatizing Spiroannulation Reagents: Direct Access to Spirocycles from Indoles and Dihalides

Unfunctionalized indoles can be directly converted into 3,3'-spirocyclic indolenines and indolines upon reaction with electrophilic dihalides in the presence of t-BuOK/BEt₃. This double C-C bond forming reaction, which simultaneously generates a quaternary spirocyclic center, typically proceeds in high yield and has good functional group tolerance. In contrast to existing dearomatizing spirocyclization approaches, there is no need to prepare a prefunctionalized aromatic precursor, enabling faster access to valuable spirocyclic products from simple, commercially available aromatics in one step.

Synthesis of 4,5-Diazaspiro[2.3]hexanes and 1,2-Diazaspiro[3.3]heptanes as Hexahydropyridazine Analogues

4,5-Diazaspiro[2.3]hexanes are made by dihalocarbene addition across the exocyclic double bond of readily accessible 3-alkylidene-1,2-diazetidines. Using difluorocarbene, generated from TMSCF₃/NaI, these spirocycles were produced in yields up to 97% by stereospecific addition across the alkene. Lower yields (up to 64%) were observed using more reactive dichlorocarbene, due to competitive insertion of the carbene into the N-N bond. Larger 1,2-diazaspiro[3.3]heptanes are produced by [2 + 2] cycloaddition of 3-alkylidene-1,2-diazetidines with tetracyanoethylene (TCNE) in up to 99% yield.

Catalyst-free [3 + 2] cyclization of dihydroisoquinoline imines and isatin-derived Morita–Baylis–Hillman carbonates via 1,5-electrocyclization: synthesis of tetrahydroisoquinoline-fused spirooxindoles

The incorporation of tetrahydroisoquinoline and spirooxindole frameworks has been achieved.

Synthesis of spiroindolenines by intramolecular ipso-iodocyclization of indol ynones

A high-yielding fast spirocyclization of easily available indol ynones has been developed by applying N-iodosuccinimide.

Photoredox-catalyzed procedure for carbamoyl radical generation: 3,4-dihydroquinolin-2-one and quinolin-2-one synthesis

A reductive approach for carbamoyl radical generation from N-hydroxyphthalimido oxamides under photoredox catalysis is outlined. This strategy was applied to the synthesis of 3,4-dihydroquinolin-2-ones via the intermolecular addition/cyclization of carbamoyl radicals with electron deficient olefins in a mild, redox-neutral manner. Under a general set of reaction conditions, diversely substituted 3,4-dihydroquinolin-2-ones, including spirocyclic systems can be prepared. By using chlorine-substituted olefins, aromatic quinolin-2-ones can also be accessed.

β-Functionalization of Indolin-2-one-Derived Aliphatic Acids for the Divergent Synthesis of Spirooxindole γ-Butyrolactones

β-Functionalization of indolin-2-one-derived aliphatic acids has been applied in formal [3 + 2] annualtions for catalyst-free and divergent synthesis of two series of structurally interesting 3,3'-spirooxindole γ-butyrolactones that may be attractive for potential drug discovery. These findings also pave the way for further diversity-oriented synthesis of spirooxindoles starting from indolin-2-one-derived aliphatic acids or their derivatives.

Ring-Expansion Approach to Medium-Sized Lactams and Analysis of Their Medicinal Lead-Like Properties

Medium-sized rings are widely considered to be under-represented in biological screening libraries for lead identification in medicinal chemistry. To help address this, a library of medium-sized lactams has been generated by using a simple, scalable and versatile ring-expansion protocol. Analysis of the library by using open-access computational tool LLAMA suggested that these lactams and their derivatives have highly promising physicochemical and 3D spatial properties and thus have much potential in drug discovery.

Selective synthesis of three product classes from imine and carboxylic acid precursors via direct imine acylation

Three divergent procedures allow the selective generation of δ-lactams, β-lactams and tetrahydropyrimidinones from common imine and carboxylic acid precursors.

Evaluating New Chemistry to Drive Molecular Discovery: Fit for Purpose?

As our understanding of the impact of specific molecular properties on applications in discovery-based disciplines improves, the extent to which published synthetic methods meet (or do not meet) desirable criteria is ever clearer. Herein, we show how the application of simple (and in many cases freely available) computational tools can be used to develop a semiquantitative understanding of the potential of new methods to support molecular discovery. This analysis can, among other things, inform the design of improved substrate scoping studies; direct the prioritization of specific exemplar structures for synthesis; and substantiate claims of potential future applications for new methods.

From Heteroaromatic Acids and Imines to Azaspirocycles: Stereoselective Synthesis and 3D Shape Analysis

Heteroaromatic carboxylic acids have been directly coupled with imines using propylphosphonic anhydride (T3P) and NEt(iPr)2 to form azaspirocycles via intermediate N-acyliminium ions. Spirocyclic indolenines (3H-indoles), azaindolenines, 2H-pyrroles and 3H-pyrroles were all accessed using this metal-free approach. The reactions typically proceed with high diastereoselectivity and 3D shape analysis confirms that the products formed occupy areas of chemical space that are under-represented in existing drugs and high throughput screening libraries.