Indium-catalyzed inter- and intramolecular dithianyl-alkyne metathesis reactions

Herein, we reported an efficient indium catalyzed dithianyl-alkyne metathesis (DAM) reaction. This strategy allows for the formation of a new C-C double bond and valuable C-S bonds during the metathesis event, and was successfully applied to the synthesis of diverse vinyl dithianyl substituted organic molecules.

Gold(I)-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis for the Synthesis of Butenolides

Alkyne-carbonyl metathesis is a type of carbon-carbon forming reaction involving the construction a carbon-carbon double bond and a carbonyl group in one transformation. Herein, a Au(I)-catalyzed ring-closing alkyne-carbonyl metathesis protocol has been developed to make densely substituted γ-butenolides from propargyl α-ketoesters. It features 100 % atom economy, excellent substrate flexibility and benign functional group tolerance. Mechanistic studies demonstrate that the coordinative interaction between the gold catalyst and the alkyne might initiate the transfer of an oxygen atom and the formation of the carbon-carbon double bond. By using this gold-catalyzed ring-closing alkyne-carbonyl metathesis as a key step reaction, four naturally occurring butenolide-type compounds including decumbic acid (45 % yield for 3 steps), deoxyisosporothric acid (32 % yield for 5 steps), lichesterinic acid (34 % yield for 5 steps) and isomuronic acid (6 % yield for 8 steps) have been synthesized starting from commercially available starting materials.

Enantioselective synthesis of α,α-diarylketones by sequential visible light photoactivation and phosphoric acid catalysis

Chiral ketones and their derivatives are useful synthetic intermediates for the synthesis of biologically active natural products and medicinally relevant molecules. Nevertheless, general and broadly applicable methods for enantioenriched acyclic α,α-disubstituted ketones, especially α,α-diarylketones, remain largely underdeveloped, owing to the easy racemization. Here, we report a visible light photoactivation and phosphoric acid-catalyzed alkyne-carbonyl metathesis/transfer hydrogenation one-pot reaction using arylalkyne, benzoquinone, and Hantzsch ester for the expeditious synthesis of α,α-diarylketones with excellent yields and enantioselectivities. In the reaction, three chemical bonds, including C═O, C─C, and C─H, are formed, providing a de novo synthesis reaction for chiral α,α-diarylketones. Moreover, this protocol provides a convenient and practical method to synthesize or modify complex bioactive molecules, including efficient routes to florylpicoxamid and BRL-15572 analogs. Computational mechanistic studies revealed that C-H/π interactions, π-π interaction, and the substituents of Hantzsch ester all play crucial roles in the stereocontrol of the reaction.

Construction of 4-Acyl-2-quinolones through Sc(OTf)3-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis

An atom-economical protocol to construct densely substituted 4-acyl-2-quinolones from N-(2-alkynylphenyl)-α-ketoamides has been developed through Sc(OTf)₃-catalyzed ring-closing alkyne-carbonyl metathesis. Mechanistic experimental studies support that coordinative interaction between Sc(OTf))₃ and the substrate, the formation of an oxetene intermediate, and an electrocyclic ring-opening of the oxetene might be involved.

Synthesis of α-(aminoethyl)-α,β-enones via alkyne aza-Prins cyclization and their synthetic application to pyrrolidines

We developed a synthetic method for α-(aminoethyl)-α,β-enones from aryl-substituted homopropargyl sulfonamides and aldehydes, representing the first synthesis of conjugated enones via alkyne aza-Prins cyclization. These products could be converted into pyrrolidines by a formal 5-endo-trig cyclization.

Iron-Catalyzed Carbonyl–Alkyne and Carbonyl–Olefin Metathesis Reactions

Construction of carbon–carbon bonds is one of the most important tools for the synthesis of complex organic molecules. Among multiple possibilities are the carbonyl–alkyne and carbonyl–olefin metathesis reactions, which are used to form new carbon–carbon bonds between carbonyl derivatives and unsaturated organic compounds. As many different approaches have already been established and offer reliable access to C=C bond formation via carbonyl–alkyne and carbonyl–olefin metathesis, focus is now shifting towards cost efficiency, sustainability and environmentally friendly metal catalysts. Iron, which is earth-abundant and considered as an eco-friendly and inexpensive option in comparison to traditional metal catalysts, fulfils these requirements. Hence, the focus of this review is on recent advances in the iron-catalyzed carbonyl–alkyne, carbonyl–olefin and related C–O/C–O metathesis reactions. The still large research potential for ecologically and economically attractive and sustainable iron-based catalysts is demonstrated.

Oxophosphonium-Alkyne Cycloaddition Reactions: Reversible Formation of 1,2-Oxaphosphetes and Six-membered Phosphorus Heterocycles

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R₂E═O compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus-oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.

Catalytic C-C Cleavage/Alkyne-Carbonyl Metathesis Sequence of Cyclobutanones

A ring-opening/alkyne-carbonyl metathesis sequence of alkyne-tethered cyclobutanones catalyzed by AgSbF₆ is realized for the first time to furnish multisubstituted naphthyl ketones under mild conditions. A range of substrates decorated with various substituents at different positions were all well accommodated. Preliminary mechanistic studies show that silver salt acted as a Lewis acid to facilitate both C-C cleavage of the cyclobutanone moiety and the subsequent metathesis between C═O and C≡C bonds.

Zinc-Catalyzed Alkyne-Carbonyl Metathesis of Ynamides with Isatins: Stereoselective Access to Fully Substituted Alkenes

A zinc-catalyzed intermolecular alkyne-carbonyl metathesis reaction of ynamides with isatins followed by an amide to ester conversion has been developed, which produces the indolone derivatives with a fully substituted alkene species in good to high yields. The salient features of this reaction include the following: mild reaction conditions, an inexpensive zinc catalyst, a broad substrate scope, the excellent regiocontrol and stereoselectivity, and amenable to the gram scale.

Inter- and Intramolecular Alkyne-Carbonyl Metathesis/1,6-Addition/Oxidative Aromatization Tandem Reactions between 1,6-Diynes and Aldehydes

We report an efficient synthetic route to 9-arylfluorenes and biaryl compounds from 1,6-diynes and aldehydes via inter- and intramolecular alkyne-carbonyl metathesis/1,6-addition/oxidative aromatization reactions. These tandem reactions are initiated by a BF₃·Et₂O-promoted tandem inter- and intramolecular alkyne-carbonyl metathesis of 1,6-diynes with carbonyl compounds followed by an In(OTf)₃-catalyzed 1,6-addition/oxidative aromatization with iodobenzene diacetate.

Beyond olefins: new metathesis directions for synthesis

The olefin-olefin metathesis reaction has emerged as one of the most important carbon-carbon bond-forming reactions, as illustrated by its wide use in the synthesis of complex molecules, natural products and pharmaceuticals. The corresponding metathesis reaction between carbonyls and olefins or alkynes similarly allows for the formation of carbon-carbon bonds. Although these variants are far less developed and utilized in organic synthesis, they possess attractive qualities that have prompted chemists to incorporate and explore these modes of reactivity in complex molecule synthesis. This review highlights selected examples of carbonyl-olefin and carbonyl-alkyne metathesis reactions in organic synthesis, in particular in the total synthesis of natural products and complex molecules, and provides an overview of current advantages and limitations.

Domino Synthesis of 2,3-Dialkylidenetetrahydrofurans via Tandem Prins Cyclization-Skeletal Reorganization

A domino synthesis of 2,3-dialkylidenetetrahydrofurans based on Prins-type cyclization of 3,5-diynols and aldehydes is described. In the present reaction, skeletal reorganization of the Prins-cyclized intermediates proceeds via a ring-opening reaction followed by intramolecular (hemi)acetalization of the resulting 4-en-1-yn-3-ones. Furthermore, a representative product undergoes a Diels-Alder reaction with dimethyl acetylenedicarboxylate (DMAD) to afford a highly substituted 2,3-dihydrobenzofuran.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Barluenga's reagent with HBF4as an efficient catalyst for alkyne-carbonyl metathesis of unactivated alkynes

Barluenga's reagent (IPy₂BF₄, Py = pyridine) treated with HBF₄efficiently catalyzes the inter- and intramolecular alkyne-carbonyl metathesis of unactivated alkynes. This work represents the first catalytic application of the Barluenga's reagent.