Streamlining the Synthesis of Pyridones through Oxidative Amination of Cyclopentenones

Herein we report the development of an oxidative amination process for the streamlined synthesis of pyridones from cyclopentenones. Cyclopentenone building blocks can undergo in situ silyl enol ether formation, followed by the introduction of a nitrogen atom into the carbon skeleton with successive aromatisation to yield pyridones. The reaction sequence is operationally simple, rapid, and carried out in one pot. The reaction proceeds under mild conditions, exhibits broad functional group tolerance, complete regioselectivity, and is well scalable. The developed method provides facile access to the synthesis of 15N-labelled targets, industrially relevant pyridone products and their derivatives in a fast and efficient way.

Chemoselective Construction of Polycyclic Heterocycles Containing a [6-6-5] or [7-6-5] Tricyclic Core Skeleton from a 2-Isocyanophenyl Propargylic Ester

Functionalized isocyanide chemistry represents an important research area in organic synthesis. A structurally unique 2-isocyanophenl propargylic ester has been designed to incorporate the reactivity of isocyanide and propargylic ester. Thus, the reaction of 2-isocyanophenyl propargylic ester and 2-aminoaromatic aldimine facilitates the synthesis of a wide range of polycyclic benzo[b]indolo[3,2-h][1,6]naphthyridine derivatives. Furthermore, reacting with 2-hydroxyaromatic aldimine enables the divergent synthesis of both the aforementioned scaffolds and another structurally distinctive diazabenzo[f]naphtho[2,3,4-ij]azulenes featuring a [7-6-5] core skeleton. Experimental results and DFT calculations suggest that these transformations likely proceed via the in situ generation of a strained cyclopropen-imine species followed by [3+2] cycloaddition. Next, switchable nucleophilic attack/ring-expansion/aromatization and nucleophilic addition/ring-expansion/elimination account for the observed selectivity.

Skeletal Editing of Isatins for Heterocycle Molecular Diversity

Isatins have been widely used in the preparation of a variety of heterocyclic compounds, where the skeletal editing of isatins has shown significant advantages for the construction of diverse heterocycles. This review highlights the progress made in the last decade (2013-2023) in the skeletal editing of the isatin scaffold. A series of ring expansion reactions for the construction of quinoline skeleton, quinolone skeleton, polycyclic quinazoline skeleton, medium-sized ring skeleton, as well as a series of ring opening reactions for the generation of 2-(azoly)aniline skeleton by the cleavage of C-C bond and C-N bond are highlighted. It is hoped that this review will provide some understanding of the chemical transformations of isatins and contribute to the further realization of its molecular diversity.

Bioinspired Total Synthesis of Cephalotaxus Diterpenoids and Their Structural Analogues

Herein, we present a unified chemical synthesis of three subgroups of cephalotaxus diterpenoids. Key to the success lies in adopting a synthetic strategy that is inspired by biosynthesis but is opposite in nature. By employing selective one-carbon introduction and ring expansion operations, we have successfully converted cephalotane-type C18 dinorditerpenoids (using cephanolide B as a starting material) into troponoid-type C19 norditerpenoids and intact cephalotane-type C20 diterpenoids. This synthetic approach has enabled us to synthesize cephinoid H, 13-oxo-cephinoid H, 7-oxo-cephinoid H, fortalpinoid C, 7-epi-fortalpinoid C, cephanolide E, and 13-epi-cephanolide E. Furthermore, through the development of an intermolecular asymmetric Michael reaction between β-oxo esters and β-substituted enones, we have achieved the enantioselective synthesis of advanced intermediates within our synthetic sequence, thus formally realizing the asymmetric total synthesis of the cephalotaxus diterpenoids family.

Catalytic Ring Expanding Difluorination: An Enantioselective Platform to Access β,β-Difluorinated Carbocycles

Cyclic β,β-difluoro-carbonyl compounds have a venerable history as drug discovery leads, but limitations in the synthesis arsenal continue to impede chemical space exploration. This challenge is particularly acute in the arena of fluorinated medium rings where installing the difluoromethylene unit subtly alters the ring conformation by expanding the internal angle (∠C-CF2-C>∠C-CH2-C): this provides a handle to modulate physicochemistry (e.g. pKa). To reconcile this disparity, a highly modular ring expansion has been devised that leverages simple α,β-unsaturated esters and amides, and processes them to one-carbon homologated rings with concomitant geminal difluorination (6 to 10 membered rings, up to 95 % yield). This process is a rare example of the formal difluorination of an internal alkene and is enabled by sequential I(III)-enabled O-activation. Validation of enantioselective catalysis in the generation of unprecedented medium ring scaffolds is reported (up to 93 : 7 e.r.) together with X-ray structural analyses and product derivatization.

Skeletal Editing: Ring Insertion for Direct Access to Heterocycles

Skeleton editing has rapidly advanced as a synthetic methodology in recent years, significantly streamlining the synthesis process and gaining widespread acceptance in drug synthesis and development. This field encompasses diverse ring reactions, many of which exhibit immense potential in skeleton editing, facilitating the generation of novel ring skeletons. Notably, reactions that involve the cleavage of two distinct rings followed by the reformation of new rings through ring insertion play a pivotal role in the construction of novel ring skeletons. This article aims to compile and systematize this category of reactions, emphasizing the two primary reaction types and offering a thorough exploration of their associated complexities and challenges. Our endeavor is to furnish readers with comprehensive reaction strategies, igniting research interest and injecting fresh impetus into the advancement of this domain.

Ring Expansion toward Disila-carbocycles via Highly Selective C-Si/C-Si Bond Cross-Exchange

Herein, we successfully inhibited the preferential homodimerization and C-Si/Si-H bond cross-exchange of benzosilacyclobutenes and monohydro-silacyclobutanes and achieved the first highly selective C-Si/C-Si bond cross-exchange reaction by deliberately tuning the Ni-catalytic system, which constitutes a powerful and atom-economical ring expansion method for preparing medium-sized cyclic compounds bearing two silicon atoms at the ring junction, which are otherwise inaccessible. The DFT calculation explicitly elucidated the pivotal role of Si-H bond at silacyclobutanes and the high ring strain of two substrates in realizing the two C-Si bonds cleavage and reformation in the catalytic cycle.

Benzo-fused Nitrogen Heterocycles by Asymmetric Ring Expansion and Stereochemically Retentive Re-contraction of Cyclic Ureas

Benzo-fused nitrogen heterocycles are common features of bioactive molecules, and the enantioselective synthesis of their substituted analogues is an important goal. In this paper we demonstrate a practical and mechanistically intriguing approach to the enantioselective synthesis of 1-arylbenzazepines and their analogues. The reaction sequence starts with an asymmetric migratory ring expansion of indoline, tetrahydroquinoline, or tetrahydrobenzazepine ureas on treatment with a chiral lithium amide base. Treatment of the ring-expanded ureas with acid triggers a two-atom ring contraction-an 'azatropic shift' in which one urea nitrogen displaces the other-with almost complete retention of stereochemistry. Aminolysis of the urea products provides enantioenriched 1-aryl-tetrahydrobenzazepine derivatives and their congeners, including an analogue of an intermediate in the synthesis of the drug solifenacin. Deuteration, in situ IR, and DFT studies provide evidence for the mechanisms of the reaction steps.

Rhodium-Catalyzed One-Carbon Ring Expansion of Aziridines with Vinyl-N-triftosylhydrazones for the Synthesis of 2-Vinyl Azetidines

Azetidines, being four-membered N-heterocycles, possess significant potential in contemporary medicinal chemistry owing to their favorable pharmacokinetic properties. Regrettably, the incorporation of functionalized azetidines into pharmaceutical lead structures has been impeded by the absence of efficient synthetic methods for their synthesis. In this study, a Rh-catalyzed one-carbon ring expansion of aziridines with vinyl-N-triftosylhydrazones is presented, which facilitates the synthesis of high value-added 2-alkenyl azetidine products. This research represents the first example of ring expansion of aziridines enabled by vinyl carbenes. Additionally, a one-pot two-step protocol, initiated from cinnamaldehyde, was successfully achieved, offering a step-economical and facile approach for the synthesis of these compounds. The pivotal aspect of this successful transformation lies in the in situ formation of an alkenyl aziridinium ylide intermediate. Experimental investigations, coupled with computational studies, suggest that a diradical pathway is involved in the reaction mechanism.

Methylene Insertion into Nitrogen-Heteroatom Single Bonds of 1,2-Azoles via a Zinc Carbenoid: An Alternative Tool for Skeletal Editing

The nitrogen-heteroatom single bonds of 1,2-azoles and isoxazolines underwent methylene insertion in the presence of CH2 I2 (6 equiv.) and diethylzinc (3 equiv.) to produce a wide variety of the ring-expanded six-membered heterocycles. Density functional theory calculations suggest that the methylene insertion proceeds via cleavage of nitrogen-heteroatom single bonds followed by ring closure.

Divergent Cascade Ring-Expansion Reactions of Acryloyl Imides

Macrocyclic and medium-sized ring ketones, lactones and lactams can all be made from common acryloyl imide starting materials through divergent, one-pot cascade ring-expansion reactions. Following either conjugate addition with an amine or nitromethane, or osmium(VIII)-catalysed dihydoxylation, rearrangement through a four-atom ring expansion takes place spontaneously to form the ring expanded products. A second ring expansion can also be performed following a second iteration of imide formation and alkene functionalisation/ring expansion. In the dihydroxylation series, three- or four-atom ring expansion can be performed selectively, depending on whether the reaction is under kinetic or thermodynamic control.

Synthesis of 1,2-Disubstituted Adamantane Derivatives by Construction of the Adamantane Framework

This review summarizes achievements in the synthesis of 1,2-disubstituted adamantane derivatives by the construction of the tricyclic framework either by total synthesis or by ring expansion/contraction reactions of corresponding adamantane homologues. It is intended to complement reviews focusing on the preparation of 1,2-disubstituted derivatives by C-H functionalization methods.

A Domino Radical Amidation/Semipinacol Approach to All-Carbon Quaternary Centers Bearing an Aminomethyl Group

A photoredox-mediated radical amidation ring-expansion sequence that enables the generation of all-carbon quaternary centers bearing a protected aminomethyl substituent is described. The methodology can be applied to both styrene and unactivated alkene substrates generating structurally diverse sp3 -rich amine derivatives in a concise manner.

Synthesis of imidazo[4,5-e][1,3]thiazino[2,3-c][1,2,4]triazines via a base-induced rearrangement of functionalized imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazines

A series of imidazo[4,5-e][1,3]thiazino[2,3-c][1,2,4]triazines was synthesized via a cascade sequence of hydrolysis and skeletal rearrangement of imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazin-7(8H)-ylidene)acetic acid esters in methanol upon treatment with excess KOH. Imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazin-6(7H)-ylidene)acetic acid esters are also suitable substrates for the reaction. In this case hydrolysis and thiazole ring expansion were accompanied with the change of the thiazolotriazine junction type from thiazolo[3,2-b][1,2,4]triazine to thiazino[2,3-c][1,2,4]triazine.

Photochemically Mediated Ring Expansion of Indoles and Pyrroles with Chlorodiazirines: Synthetic Methodology and Thermal Hazard Assessment

We demonstrate that arylchlorodiazirines serve as photo-activated halocarbene precursors for the selective one-carbon ring expansion of N-substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N-substituted pyrazoles to pyrimidinium salts. The N-substituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co-product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring-expanded products with different degrees of increased C(sp3) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing - rather than thermolyzing - these reagents.

Simultaneous Ring Contraction and Expansion Reaction: Electrosynthesis of Heterocycle-Fused Fulleroids and Photovoltaic Application

Simultaneous electrochemical ring contraction and expansion reactions remain unexplored to date. Herein, the reductive electrosynthesis of heterocycle-fused fulleroids from fullerotetrahydropyridazines and electrophiles in the presence of a trace amount of oxygen has been achieved with concurrent ring contraction and ring expansion. When trifluoroacetic acid and alkyl bromides are employed as electrophiles, heterocycle-fused fulleroids with a 1,1,2,6-configuration are regioselectively formed. In contrast, heterocycle-fused fulleroids with a 1,1,4,6-configuration are regioselectively produced as two separable stereoisomers if phthaloyl chloride is used as the electrophile. The reaction proceeds through multiple steps of electroreduction, heterocycle ring-opening, oxygen oxidation, heterocycle contraction, fullerene cage expansion, and nucleophilic addition. The structures of these fulleroids have been determined by spectroscopic data and single-crystal X-ray diffraction analyses. The observed high regioselectivities have been rationalized by theoretical calculations.  Representative fulleroids have been applied in organic solar cells as the third component and exhibit good performance.

Construction of a Diverse Range of Boron Heterocycles via Ring Expansion of a Carboranyl-Substituted 9-Borafluorene

Direct insertion of unsaturated substrates into a five-membered borole ring is a useful method to obtain valuable heterocycles containing one or more three-coordinate boron atoms. A 9-o-carboranyl-9-borafluorene, in which the o-carboranyl substituent is connected via one of the cluster carbon atoms to the boron atom of the 9-borafluorene unit, was found to react with a vast array of unsaturated molecules, such as alkynes, aldehydes and various organic azides, to form larger boraheterocyclic products. The ring expansion reactions of the central borole ring proceed rapidly at room temperature, cementing the role of the o-carboranyl substituent in enhancing the insertion reactivity of 9-borafluorenes.

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe₃. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš-Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

Synthesis of Azocanes from Piperidines via an Azetidinium Intermediate

α-Trifluoromethyl azocanes are accessible from 2-(trifluoropropan-2-ol) piperidines by metal-free ring-expansion involving a bicyclic azetidinium intermediate. The opening of the azetidinium intermediate was achieved by various nucleophiles (amines, alcoholates, carboxylates, phosphonates, halides and pseudo-halides) with an excellent regio- diastereo- and enantioselectivity and in good yields. The relative configuration of the piperidines and azocanes were assigned and the deprotected azocanes offer opportunities for further derivatization.

Another structural correction for 1-oxo-1H-phenalene-2,3-dicarbonitriles: Synthesis of a potent BCL-2 inhibiting 7-phenoxy derivative

Aromatic scaffolds are an important part of biologically active compounds and molecular probes used to study biochemical pathways and the involved targeted proteins of interest. 1-Oxo-1H-phenalene-2,3-dicarbonitrile-based compounds have been described as inhibitors of the BCL-2 family of proteins, and this core structure represents numerous possibilities for modifications that could lead to improved inhibitory potencies. Many studies demonstrated intriguing characteristics of these compounds in terms of reactivity and, interestingly, some contradictory literature reports appeared about reaction outcomes to synthesize them. Here, we initially provide a condensed overview of transformations performed on the phenalene scaffold, followed by the resynthesis of a 6-phenoxy-substituted derivative. We show that the initial determination of this particular structure was wrong and provide two-dimensional nuclear magnetic resonance (NMR) evidence to assign the structure properly. When preparing new derivatives using the same synthetic route, we observed 6- and 7-substituted regioisomers. After confirming their structures by NMR experiments, the ability of these compounds to inhibit BCL-2 was evaluated. The most potent 1-oxo-1H-phenalene-2,3-dicarbonitrile derivatives inhibited BCL-2 in the nanomolar range and showed double-digit micromolar cytotoxicity against four different cancer cell lines.

Synthesis and Multiple Subsequent Reactivity of Anionic cyclo-E3 Ligand Complexes (E=P, As)

A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E3 (E=P, As) ligand as an end deck was developed giving [Cp'''Co(η3 -E3 )]- (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, E=P ([5]), As ([6])) in good yields suitable for further reactivity studies. In the reaction with the chlorophosphanes R2 PCl (R=Ph, Cy, t Bu), neutral complexes with a disubstituted cyclo-E3 P (E=P, As) ligand in [Cp'''Co(η3 -E3 PR2 )] (E=P (7 a-c), As (9 a-c)) were obtained. These compounds can be partially or completely converted into complexes with a cyclo-E3 (E=P, As) ligand with an exocyclic {PR2 } unit in [Cp'''Co(η2 :η1 -E3 PR2 )] (E=P (8 a-c), As (10 a-c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=(t BuN)2 CPh) into the cyclo-E3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp'''Co(η3 -E3 SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations.

Trifluorinated Tetralins via I(I)/I(III)-Catalysed Ring Expansion: Programming Conformation by [CH2 CH2 ] → [CF2 CHF] Isosterism

Saturated, fluorinated carbocycles are emerging as important modules for contemporary drug discovery. To expand the current portfolio, the synthesis of novel trifluorinated tetralins has been achieved. Fluorinated methyleneindanes serve as convenient precursors and undergo efficient difluorinative ring expansion with in situ generated p-TolIF2 (>20 examples, up to >95 %). A range of diverse substituents are tolerated under standard catalysis conditions and this is interrogated by Hammett analysis. X-ray analysis indicates a preference for the CH-F bond to occupy a pseudo-axial orientation, consistent with stabilising σC-H →σC-F * interactions. The replacement of the symmetric [CH2 -CH2 ] motif by [CF2 -CHF] removes the conformational degeneracy intrinsic to the parent tetralin scaffold leading to a predictable half-chair. The conformational behavior of this novel structural balance has been investigated by computational analysis and is consistent with stereoelectronic theory.

Reductive Rearrangement of a 1-Phospha-2-azanorbornene

The reduction of the 1‐phospha‐2‐azanorbornene derivate endo‐1 with lithium aluminium hydride leads to an unprecedented 1‐phosphabicyclo[3.2.1]octa‐2,5‐diene, while a phospholide anion is formed with lithium. The latter can be protonated resulting in formation of an unusual 2H‐phosphole dimer. Furthermore, 3H‐phospholes, previously assumed to have no synthetic relevance as intermediates, were proposed to act as dienophile in the dimerisation of 3,4‐dimethyl‐1‐phenylphosphole at elevated temperatures based on theoretical calculations.

Methanol-Driven Oxidative Rearrangement of Biogenic Furans - Enzyme Cascades vs. Photobiocatalysis

The oxidative ring expansion of bio-derived furfuryl alcohols to densely functionalized six-membered O-heterocycles represents an attractive strategy in the growing network of valorization routes to synthetic building blocks out of the lignocellulosic biorefinery feed. In this study, two scenarios for the biocatalytic Achmatowicz-type rearrangement using methanol as terminal sacrificial reagent have been evaluated, comparing multienzymatic cascade designs with a photo-bio-coupled activation pathway.