Allenylphosphine oxides as simple scaffolds for phosphinoylindoles and phosphinoylisocoumarins

Bisphosphonylallenes as Suitable Scaffolds for Unprecedented 4,5-Diphosphonyldihydropyridazines and 3,4-Diphosphonylpyrroles Displaying Antimelanoma Activity

An efficient and simple approach has been developed for the synthesis of unprecedented 4,5-diphosphonyldihydropyridazines and 3,4-diphosphonylpyrroles, through the condensation of bisphosphonylallenes with hydrazines and primary amines, respectively. The reactions proceed under operationally simple, mild, and catalyst-free conditions, for a wide substrate scope. The synthesized compounds were screened for their antiproliferative activity against melanoma cancer cells, and they showed promising growth inhibition.

Oxidative Route to Indoles via Intramolecular Amino-Hydroxylation of o-Allenyl Anilines

A new intramolecular oxidative amino-hydroxylation of o-allenyl anilines is reported. Treatment of carbamate-protected anilines with lead(IV) carboxylates in dichloromethane at room temperature results in facile tandem C-N (allene cyclization) and C-O bond formation (carboxylate trapping) to form indole products. Detailed reaction scope, mechanistic and kinetic studies suggest a reaction pathway involving an initial Wessely dearomatization step followed by cyclization and rearomatization.

Base-promoted, CBr4-mediated tandem bromination/intramolecular Friedel-Crafts alkylation of N-aryl enamines: a facile access to 1H- and 3H-indoles

Described here is a general and highly efficient method for the synthesis of 1H- and 3H-indoles. In the presence of CBr₄ and a suitable base, the cyclization of N-aryl enamines proceeds with high efficiency. Unlike previous intramolecular cross dehydrogenative coupling (CDC) of the same substrates, this process does not require the use of either a transition metal or a stoichiometric amount of oxidant. This method also features operational simplicity, easy scalability and good substrate tolerability. Control experiments indicate the reactions may proceed in a tandem sequence of bromination and intramolecular Friedel-Crafts alkylation in a simple one-pot procedure.

Recent advances in the direct transformation of propargylic alcohols to allenes

This review summaries a view of the advances in the direct transformation of propargylic alcohols to functionalized allenes.

Regio- and chemo-selective cyclization of allenic-Ugi products for the synthesis of 3-pyrroline skeletons

A highly efficient and stable novel class of allenic-Ugi products through a Crabbé homologation reaction is successfully demonstrated. Then, a regio- and chemo-selective cyclization of allenic-Ugi derivatives in a 5-exo-dig fashion to access 3-pyrroline skeletons is developed. Also, computational studies were performed and explained to provide insights into the reaction mechanism. This approach displays high bond-forming efficiency and atom economy with high yields.

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.

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.

Synthesis of 1,4-dihydrophosphinoline 1-oxides by acid-promoted cyclization of 1-(diphenylphosphoryl)allenes

Phosphonoallenes in Brønsted acids (TfOH, FSO₃H, H₂SO₄) gave the corresponding 1,2-oxaphosphol-3-enium ions, that were studied by means of NMR and DFT calculations.

Synthesis of γ-hydroxypropyl P-chirogenic (±)-phosphorus oxide derivatives by regioselective ring-opening of oxaphospholane 2-oxide precursors

The synthesis of P-chirogenic (±)-phosphine oxides and phosphinates via selective nucleophilic ring opening of the corresponding oxaphospholanes is described. Two representative substrates: the phosphonate 2-ethoxy-1,2-oxaphospholane 2-oxide and the phosphinate 2-phenyl-1,2-oxaphospholane 2-oxide were reacted with various Grignard reagents to produce a single alkyl/aryl product. These products may possess further functionalities in addition to the phosphorus center such as the γ-hydroxypropyl group which results from the ring opening and π-donor moieties such as aryl, allyl, propargyl and allene which originates from the Grignard reagent.

Acid-promoted transformations of 1-(diphenylphosphoryl)allenes: synthesis of novel 1,4-dihydrophosphinoline 1-oxides

1-(Diphenylphosphoryl)alka-1,2-dienes in acids (TfOH, FSO₃H, H₂SO₄) give (3-hydroxyalk-1-en-1-yl)diphenylphosphine oxides, that are further converted into 1-phenyl-1,4-dihydrophosphinoline 1-oxides. The latter compounds are directly formed from these 1,2-dienes under the action of AlCl₃.

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.