金属有机化合物催化的环化反应新进展

综述了近年来金属有机化合物催化的环化反应在合成碳20环及杂环化合物中的应用。     

Phosphorus-based allenes as scaffolds in cycloaddition and cyclization reactions

ABSTRACT

In this article, we describe some of our results on the reactions of allenylphosphonates/allenyl phosphine oxides and allenyl diazaphosphole oxides, primarily cycloaddition and cyclization reactions. Depending on the substituents, both the α,β and β,γ -cycloaddition products have been observed. A novel cyclization reaction of a functionalized allenylphosphine oxide with diethylamine leading to 3-diethylamino-4-diphenylphosphinoyl-1-naphthol is reported. Representative compounds have been characterized by X-ray crystallography.     

Chiral oxazolidinones as electrophiles: Intramolecular cyclization reactions with carbanions and preparation of functionalized lactams

The intramolecular cyclizations of oxazolidinones with carbanions adjacent to sulfones, sulfoxides, and phosphonates proceed in high yields to obtain functionalized γ and δ lactams. The chiral oxazolidinone precursors can be readily synthesized from commercial amino acids. The lactams from this study are useful synthetic intermediates, as demonstrated by the synthesis of a precursor for levetiracetam, an antiepileptic drug.     

Atom-transfer radical addition (ATRA) and cyclization (ATRC) reactions catalyzed by a mixture of [RuCl(2)Cp*(PPh3)] and Magnesium

A new catalytic procedure for atom-transfer radical addition (ATRA) and cyclization (ATRC) reactions is described. The combination of the ruthenium(III) complex [RuCl(2)Cp*(PPh3)] (Cp*: pentamethylcyclopentadienyl) with magnesium allows these reactions to be performed under mild conditions with high efficiency. In most cases, the catalyst concentrations required are significantly lower than those used in previously reported procedures. It is suggested that magnesium acts as a reducing agent that generates and regenerates the catalytically active ruthenium(II) species. The precatalyst [RuCl(2)Cp*(PPh3)] has been analyzed by X-ray crystallography.     

Our Contributions in Synthesis of Diverse Heterocyclic Scaffolds by Using Mixed Oxides as Heterogeneous Catalysts

This personal account mainly introduces and reviews our recent contributions in developing different catalyst materials involving mixed oxides and their scope as renewable catalysts in multicomponent reactions to synthesize various novel heterocyclic scaffolds under green conditions. The application of various mixed oxides and their composites in the organic synthesis is emphasized through this review, in order to reveal the versatility, scope and importance of mixed oxides and their interactions during the reaction. We have also briefed the limitations of mixed oxides as catalysts, to put forward the broader prospective in the field.     

Synthesis and cyclization reactions with pyrazolopyrimidinyl keto esters part I. Reactivity of pyrazolopyrimidinyl β-keto ester and pyrazolopyrimidinyl α,β-unsaturated ketones

5-Acetyl-4,5-dihydro-1-phenylpyrazolo[3,4-d]pyrimidin-4-one was prepared and subjected to various chemical transformations to give novel 5-heterocyclic pyrazolopyrimidinone derivatives of expected important biological activity. Then, the latter compounds were used to obtain β-keto ester and α,β-unsaturated carbonylpyrazolopyrimidinones which were used as alternate precursors to produce new pyrazolopyrimidinones substituted with five-membered heterocycles such as pyrazole and isoxazole. The structure of these compounds was identified on the basis of their chemical behaviour as well as elemental and spectral analysis.     

Dinickel catalyzed carbonylation reactions using metal carbonyl reagents as CO sources

Dinickel complexes promote stoichiometric Pauson–Khand reactions of enynes and CO (g). However, catalytic turnover is precluded by the strong binding of CO to the Ni₂ active site. This article describes the use of M(CO)_x reagents as stoichiometric CO precursors in Ni₂-catalyzed carbonylation reactions. As part of these studies, well-defined heterotrinuclear Ni₂Fe, Ni₂Co, and Ni₂Mn carbonyl clusters are synthesized using the [NDI]Ni₂ platform as a template (NDI = naphthyridine–diimine).     

Enantioselective Palladium‐Catalyzed Dearomative Cyclization for the Efficient Synthesis of Terpenes and Steroids

A novel enantioselective palladium‐catalyzed dearomative cyclization has been developed for the efficient construction of a series of chiral phenanthrenone derivatives bearing an all‐carbon quaternary center. The effectiveness of this method in the synthesis of terpenes and steroids was demonstrated by a highly efficient synthesis of a kaurene intermediate, the facile construction of the skeleton of the anabolic steroid boldenone, and the enantioselective total synthesis of the antimicrobial diterpene natural product (?)‐totaradiol.     

Total synthesis of ent-(−)-azonazine using a biomimetic direct oxidative cyclization and structural reassignment of natural product

A biomimetic approach has been investigated and developed for the total synthesis of azonazine, an unusual marine natural cyclopeptide containing a rigid transannular 10-membered ring. A hypervalent iodine-mediated direct oxidative cyclization was successfully developed and applied to construct the highly strained core, which was the key step in the first total synthesis of ent-(−)-azonazine. Based on the physical evidences of synthesized diastereomer and enantiomer of azonazine, both the relative and absolute configurations of the natural product were revised. Two fluorinated azonazine derivatives were also synthesized in short convenient steps utilizing the same intermediate in this work. The established total synthesis opens a potential opportunity to study the structure–activity relationship of natural azonazine.     

Chiral imidates as a new class of nitrogen-based chiral ligands: synthesis and catalytic activity in asymmetric aziridinations and diethylzinc additions

Chiral imidates were efficiently synthesized in one step and with high yields (seven examples). These chiral imidates were used as ligands in the Cu(I)-catalyzed asymmetric aziridination of methyl cinnamate and in the asymmetric diethylzinc additions to benzaldehyde as a proof of principle. The imidate catalyst system showed high catalytic activities and induced encouraging selectivities. An X-ray structure analysis of an imidate-Cu(I) complex is included, showing a distorted tetrahedral arrangement with two bidentate ligand molecules surrounding the metal.     

A coordinatively unsaturated ruthenium methoxide as a highly effective catalyst for the halogen atom-transfer radical cyclization of N-allyl dichloroacetamides and related reactions

Atom-transfer radical cyclization (ATRC) catalyzed by coordinatively unsaturated ruthenium alkoxides 4, [(η⁵-C₅Me₅)Ru(OR)]₂, is investigated, and ruthenium methoxide 4a (R=Me) is found to exhibit excellent catalytic activity for the cyclization of N-allyl-α,α-dichloroacetamides at ambient temperature. Addition of some amounts of two-electron donor ligands such as pyridine and triphenylphosphine improves the catalyst efficiency to afford the corresponding γ-lactams in high yields. The high catalytic activity of this catalyst system enables to control the diastereoselectivity of this 5-exo cyclization kinetically. The present 4a/pyridine system is also effective for the 4-exo cyclization of N-vinylacetamides to afford the corresponding β-lactams in quantitative yields. The 4a/pyridine system is also active towards the ATRP of methyl methacrylate (MMA) at room temperature to afford the poly(MMA) with narrow molecular weight distributions (M_w/M_n=1.2) at the initial stage.     

Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates

Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen.     

Facile and straightforward synthesis of the racemic version of 2-amino-4H-chromene-3-carbonitriles using K2HPO4 as an eco-friendly base

A simple and facile two-step synthetic route has been developed for the preparation of 2-amino-4H-chromenes via chalcone intermediate under an ambient condition, along with the crystallographic analysis of the racemic mixture so obtained. The crystal orientation can be very useful for generic drug development in future. The described non-hazardous optimized methodology also provides an alternative simple route to prepare functionalized 2-amino-4H-chromenes of biological interest.     

A general and efficient synthesis of substituted fluorenes and heterocycle-fused indenes containing thiophene or indole rings utilizing a Suzuki-Miyaura coupling and acid-catalyzed Friedel-Crafts reactions as key steps

A general and efficient synthesis of fluorenes or heterocycle-fused indenes including 3-thia-cyclopenta[a]indenes, 9-thia-indeno[1,2-a]indenes, 5,6-dihydroindeno[2,1-b]indoles has been developed. This methodology is realized by a multistep protocol involving first preparation of ortho-formylbiaryls through Suzuki-Miyaura coupling of o-bromobenzaldehydes with arylboronic acids or the coupling of aryl halides with 2-formylbenzene boronic acid, this is followed by Grignard addition and Friedel-Crafts cyclization reactions catalyzed by Brønsted or Lewis acid to form the desired fluorene or indene rings. The method offers several advantages such as high yields, high selectivities, mild reaction conditions, easily accessible starting materials, and so on.     

Palladium-catalyzed cyclization reaction of N-(2-Haloaryl)alkynylimines: Synthesis of 3-acylindoles using water as the sole solvent and oxygen source

A simple and efficient strategy for the preparation of 3-acylindoles via palladium-catalyzed cyclization reaction of N-(2-haloaryl)alkynylimines in water has been developed. The reaction tolerates a wide range of functional groups, and the corresponding 3-acylindoles were obtained in high yields using water as the sole solvent and oxygen sources. Additionally, this method could provide a short synthesis route for Pravadoline, a phase II analgesic drug.     

Chemo‐selective copper‐catalyzed C‐O coupling reactions of phenols with aryl/vinyl halides using enaminone as efficient ligand

The copper‐catalyzed Ullmann C‐O coupling reactions between phenols and aryl/vinyl halides have been efficiently performed by employing (E)‐3‐(dimethylamino)‐1‐(2‐hydroxyphenyl)prop‐2‐en‐1‐one, an easily available enaminone, as ligand. This new ligand is advantageous for its easy availability, broad applicability and good efficiency. Copyright © 2012 John Wiley & Sons, Ltd.