Pd-catalyzed regio- and stereoselective cyclization-heck reaction of monoesters of 1,2-allenyl phosphonic acids with alkenes

Rh(III)-Catalyzed Regioselective Alkyne Insertion in the Context of C-H Activation and Lactonization Reactions: Synthesis of Fused 1,2-Oxaphospholenes

Herein, we report an expedient synthesis of fused phosphorus-containing heterocycles via Rh-catalyzed cascade C-H activation, alkyne insertion, and lactonization reactions. The substrate scope and the group tolerance are good, as various substituted benzoic acids, N-methoxybenzamides, and 2-phenylindoles could go through the reactions smoothly to afford the corresponding products in moderate to high yields. Additionally, excellent regioselectivities are shown in the alkyne insertion with the assistance of an electron-withdrawing phosphonate group.

Transition-Metal-Free Electrochemical Selenylative Cyclization of Alkynyl Phosphonates

Unprecedented regioselective electrochemical tandem selenation/cyclization of alkynyl phosphonates with diselenide is described here. These obtained selenoether products can be chemo-selectively converted into halogen-functionalized cyclic enol phosphonates under our electrochemical conditions. These protocols provide straightforward access to valuable cyclic enol phosphonate or phosphaisocoumarins under the electrochemical and transition-metal-free conditions. The robustness of these transformations was illustrated by their compatibility with various complex natural products and bioactive molecules. The selenoether and halogen functional groups allow the further diversification of the phosphorus heterocycles thus obtained.

Palladium-Catalyzed Aminocyclization-Coupling Cascades: Preparation of Dehydrotryptophan Derivatives and Computational Study

Dehydrotryptophan derivatives have been prepared by palladium-catalyzed aminocyclization-Heck-type coupling cascades starting from o-alkynylaniline derivatives and methyl α-aminoacrylate. Aryl, alkyl (primary, secondary, and tertiary), and alkenyl substituents have been introduced at the indole C-2 position. Further variations at the indole benzene ring, as well as the C-2-unsubstituted case, have all been demonstrated. In the case of C-2 aryl substitution, the preparation of the o-alkynylaniline substrate by Sonogashira coupling and the subsequent cyclization-coupling cascade have been performed in a one-pot protocol with a single catalyst. DFT calculations have revealed significant differences in the reaction profiles of these reactions relative to those involving methyl acrylate or methacrylate, and between the reactions of the free anilines and their corresponding carbamates. Those calculations suggest that the nature of the alkene and of the acid HX released in the HX/alkene exchange step that precedes C-C bond formation could be responsible for the experimentally observed differences in reaction efficiencies.

Synthesis and Anti-Pancreatic Cancer Activity Studies of Novel 3-Amino-2-hydroxybenzofused 2-Phospha-γ-lactones

A series of 3-amino-2-hydroxybenzofused 2-phosphalactones (4a–l) has been synthesized from the Kabachnik–Fields reaction via a facile route from a one-pot three-component reaction of diphenylphosphite with various 2-hydroxybenzaldehyes and heterocyclic amines in a new way of expansion. The in vitro anti-cell proliferation studies by MTT assay have revealed them as potential Panc-1, Miapaca-2, and BxPC-3 pancreatic cell growth inhibitors, and the same is supported by molecular docking, QSAR, and ADMET studies. The MTT assay of their SAHA derivatives against the same cell lines evidenced them as potential HDAC inhibitors and identified 4a, 4b, and 4k substituted with 1,3-thiazol, 1,3,4-thiadiazol, and 5-sulfanyl-1,3,4-thiadiazol moieties on phenyl and diethylamino phenyl rings as potential ones. Additionally, the flow cytometric analyses of 4a, 4b, and 4k against BxPC-3 cells revealed compound 4k as a lead compound that arrests the S phase cell cycle growth at low micromolar concentrations. The ADMET properties have ascertained their inherent pharmacokinetic potentiality, and the wholesome results prompted us to report it as the first study on anti-pancreatic cancer activity of cyclic α-aminophosphonates. Ultimately, this study serves as a good contribution to update the existing knowledge on the anticancer organophosphorus heterocyclic compounds and elevates the scope for generation of new anticancer drugs. Further, the studies like QSAR, drug properties, toxicity risks, and bioactivity scores predicted for them have ascertained the synthesized compounds as newer and potential drug candidates. Hence, this study had augmented the array of α-aminophosphonates by adding a new collection of 3-amino-2-hydroxybenzofused 2-phosphalactones, a class of cyclic α-aminophosphonates, to it, which proved them as potential anti-pancreatic cancer agents.

Modern Synthetic Methods for the Stereoselective Construction of 1,3-Dienes

The 1,3-butadiene motif is widely found in many natural products and drug candidates with relevant biological activities. Moreover, dienes are important targets for synthetic chemists, due to their ability to give access to a wide range of functional group transformations, including a broad range of C-C bond-forming processes. Therefore, the stereoselective preparation of dienes have attracted much attention over the past decades, and the search for new synthetic protocols continues unabated. The aim of this review is to give an overview of the diverse methodologies that have emerged in the last decade, with a focus on the synthetic processes that meet the requirements of efficiency and sustainability of modern organic chemistry.

Reactivity of allenylphosphonates/allenylphosphine oxides – some new addition/cycloaddition and cyclization pathways

In this paper, we highlight some addition/cycloaddition reactions of allenylphosphonates and allenylphosphine oxides, mostly based on the work done in our laboratory. Thus the electrophilic addition of iodine monochloride (ICl) with allenylphosphine oxides affords cyclic phosphonium salts rather than γ-chloro-β-iodovinylphosphine oxides (NMR, HRMS, X-ray) that exhibit rather unusual downfield shifts in the 31P NMR spectra. These compounds undergo hydrolysis to afford γ-hydroxy-β-iodovinylphosphine oxides; the hydroxymethyl group in these compounds can be oxidized by Dess-Martin periodinane to afford the corresponding aldehyde-substituted vinylphosphine oxides. A [2+2] cycloaddition product of an allenylphosphonate has also been structurally characterized. Other reactions that are highlighted include those leading to (Z)/(E)-β-aminovinylphosphonates, β-ketophosphonates (and their utility in Horner-Wadsworth-Emmons reaction), indolyl/furanyl/isocoumaranyl/naphthyl phosphine oxides, thiophosphorylated phosphonates and azo-substituted coumarin phosphonates.

Highly functionalised (γ-azido/γ-fluoro-β-iodo/)vinyl derivatives from phosphorus based allenes or allenoates: I⋯O halogen bonding interactions

γ-Azido/γ-fluoro-β-iodo-vinyl phosphine oxides/phosphonates/esters/sulphone and a γ-diodoallene were synthesised from phosphorus-based allenes or allenoates or a sulphur based allene; in many cases, I⋯O halogen bonding is observed in the solid state.

Exploring allene chemistry using phosphorus-based allenes as scaffolds

In this paper, we review some of our results on cycloaddition and cyclization reactions of allenylphosphonates/and allenyl phosphine oxides. Thus nitro-substituted propargylic alcohols react with P(III)–Cl substrates to lead to unprecedented phosphono-benzazepines or -hydroxyindolinones. A similar reaction using a higher stoichiometry of P(III)–Cl precursor has led to the first observation of spontaneous resolution by crystallization in allene chemistry. In the reaction of these phosphorus based allenes with diphenyl isobenzofuran (DPBF), depending on the substituents, both [α, β] and [β, γ]-cycloaddition products are formed. Extension of this work to sulfur based allenes and leading to a new [4+2] cycloadduct with DPBF is reported. A novel cyclization reaction of a functionalized allenylphosphine oxide with diethylamine leading to 3-diethylamino-4-diphenylphosphinoyl-1-naphthol is discovered. Base-catalyzed reactions of allenylphosphonates with 2-substituted benzaldehdes lead to novel thiochromans, chromenes, and quinolines. Allenylphosphonates undergo phosphonylation in the presence of tetrabutyl ammonium fluoride (TBAF). Vinyl azides derived from allenylphosphonates undergo thermolysis to form bis-phosphonopyrazines. [Pd]-catalyzed reactions of allenes with 2-iodophenols, 2-iodobenzyl alcohol and 3-iodo-indole-2-carboxylic acids lead to a variety of benzofurans, benzopyrans and benzopyranones, respectively.

Silver-catalyzed phosphonylation of unprotected propargylic alcohols for the synthesis of allenylphosphoryl compounds

A facile and efficient method for the synthesis of a variety of allenylphosphine oxides by silver-catalyzed phosphonylation of unprotected propargylic alcohols with P(O)H compounds has been established.

Base catalysed intermolecular cyclisation of N-protected o-amino benzaldehyde/acetophenone with phosphorus/sulphur based allenes: facile synthesis of substituted quinolines

Base catalyzed reaction of N-Bz protected o-aminobenzaldehyde with allenylphosphonates affords O-phosphorylated quinolines via a route involving a thermal rearrangement.

Convenient synthesis of allenylphosphoryl compounds via Cu-catalysed couplings of P(O)H compounds with propargyl acetates

A novel Cu-catalysed substitution reaction of propargyl acetates with P(O)H compounds is developed to afford allenylphosphoryl compounds in high yields. A plausible mechanism involving the nucleophilic interception of Cu-allenylidene intermediates is proposed.

Palladium-Catalyzed Domino Addition and Cyclization of Arylboronic Acids with 3-Hydroxyprop-1-yn-1-yl Phosphonates Leading to 1,2-Oxaphospholenes

A novel and efficient palladium-catalyzed domino addition-cyclization of a wide range of arylboronic acids with various 3-hydroxyprop-1-yn-1-yl phosphonates has been developed, affording a convenient and powerful tool for the preparation of valuable 1,2-oxaphospholenes with mild reaction conditions, broad substrate applicability, and good to excellent yields. Mechanistic studies revealed that the reaction might involve Michael addition and nucleophilic substitution.

Copper-mediated reaction of oxazirconacyclopentenes with dichlorophenylphosphine: a new pathway for the formation of 1,2-oxaphosphole derivatives

CuCl-mediated reaction of oxazirconacyclopentenes with dichlorophenylphosphine afforded dihydrooxaphospholes in high yields, in which the reaction was performed conveniently in one-pot from an alkyne, carbonyl compound and dichlorophosphine.

Spontaneous resolution upon crystallization of allenyl-bis-phosphine oxides

The first example of ‘spontaneous resolution by crystallization’ in allene chemistry, by means of crystal structures and solid state CD spectra of the R and S enantiomers, is presented.

α-Allenyl Ethers as Starting Materials for Palladium Catalyzed Suzuki-Miyaura Couplings of Allenylphosphine Oxides with Arylboronic Acids

We disclose here the first palladium-catalyzed Suzuki-Miyaura couplings of aryl ethers functionalized allenylphosphine oxides with arylboronic acids. This new methodology with α-allenyl ethers as starting materials provides a novel approach to generate phosphinoyl 1,3-butadienes and derivatives with medium to excellent yields. The reaction tolerates a variety of functional groups to afford ranges of structurally diverse substituted phosphionyl 1,3-butadienes. For unsymmetrical allene substrates, high stereospecific additions to give E-isomers are usually observed. On the basis of the known palladium and allene chemistry, a mechanism is proposed.

Regio- and stereoselective synthesis of 2-amino-dienes via decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO

A metal-free decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO via base-induced aza-Michael addition/elimination has been developed. A variety of substituted N-tosyl 1,3-dien-2-yl amines were obtained in good yields and excellent regio- and stereoselectivity. Moreover, this transformation could be applied in preparation of 2-amino-trienes.

Palladium-catalyzed cyclization reactions of allenes in the presence of unsaturated carbon-carbon bonds

Modern synthetic chemists have looked for rapid and efficient ways to construct complex molecules while minimizing synthetic manipulation and maximizing atom-economy. Over the last few decades, researchers have made considerable progress toward these goals by taking full advantage of transition metal catalysis and the diverse reactivities of allenes, functional groups which include two cumulative carbon-carbon double bonds. This Account describes our efforts toward the development of Pd-catalyzed cyclization reactions of allenes in the presence of compounds that contain unsaturated carbon-carbon bonds such as alkenyl halides, simple alkenes, allenes, electron-deficient alkynes, or propargylic carbonates. First, we discuss the coupling-cyclization reactions of allenes bearing a nucleophilic functionality in the presence of alkenyl halides, simple alkenes, functionalized and nonfunctionalized allenes, or electron-deficient alkynes. These processes generally involve a Pd(II)-catalyzed sequence: cyclic nucleopalladation, insertion or nucleopalladation, and β-elimination, reductive elimination, cyclic allylation or protonation. We then focus on Pd(0)-catalyzed cyclization reactions of allenes in the presence of propargylic carbonates. In these transformations, oxidative addition of propargylic carbonates with Pd(0) affords allenylpalladium(II) species, which then react with allenes via insertion or nucleopalladation. These transformations provide easy access to a variety of synthetically versatile monocyclic, dumbbell-type bicyclic, and fused multicyclic compounds. We have also prepared a series of highly enantioenriched products using an axial-to-central chirality transfer strategy. A range of allenes are now readily available, including optically active ones with central and/or axial chirality. Expansion of these reactions to include other types of functionalized allenes, such as allenyl thiols, allenyl hydroxyl amines, and other structures with differing steric and electronic character, could allow access to cyclic skeletons that previously were difficult to prepare. We anticipate that other studies will continue to explore this promising area of synthetic organic chemistry.

Palladium-catalysed inter- and intramolecular formation of C–O bonds from allenes

This review highlights the Pd-catalysed formation of a C–O bond from allenes via the inter- or intramolecular reaction.

Synthesis of alkynyl/alkenyl-substituted pyridine derivatives via heterocyclization and Pd-mediated Sonogashira/Heck coupling process in one-pot: a new MCR strategy

The present MCR strategy opens new avenues in the development of i) a diversity-oriented new cyanopyridine based compound library and ii) new chemical entities other than the present reported molecules.

Synthesis and stereochemistry of 6-membered ring phosphonates

Background: Organophosphorus compounds have important industrial and biomedical applications as pharmaceutical and agrochemical agents, as well as transition state analogs for the production of monoclonal antibodies.

Methods: Two diastereomers of a 6-membered ring, cyclic phenyl phosphonate were synthesized in 8 steps from 1,3-butanediol.

Results: The stereochemistry of the diastereomers was elucidated on the basis of 3 1 P NMR signals, together with 1 H NMR nuclear Overhauser effects (NOE) difference experiments.

Conclusions: Such cyclic phosphonates may have utility serving as transition state analogs for the production of monoclonal antibodies.

To stay as allene or go further? Synthesis of novel phosphono-heterocycles and polycyclics via propargyl alcohols

Novel (tetrahydro)dibenzazepines, N-hydroxyindolinones and polycyclic compounds are generated by a simple reaction of a chlorophosphite with functionalized propargyl alcohols.

The Heck reaction applied to 1,3- and 1,2-unsaturated derivatives, a way towards molecular complexity

This review is an overview of the last ten years' use of the Mizoroki-Heck coupling applied to 1,2- and 1,3-dienes. Since both these systems form pi-allyl palladium intermediates in Pd(0) coupling, they show particular chemical behavior. Many examples of 1,2-dienes Heck reactions are presented. 1,2-Dienes are important substrates because of their high reactivity that makes them useful building blocks for the synthesis of biologically relevant structures.