Ru- and Co-Catalyzed Intermolecular Carbonyl-Alkyne Metathesis Reactions of 1H-Indene-1,2,3-triones with Internal Alkynes

The catalyst-dependent intermolecular carbonyl-alkyne metathesis (CAM) reaction of 1H-indene-1,2,3-triones with internal alkynes was realized using Ru and Co catalysts. 2-(2-Oxo-1,2-diphenylethylidene)-1H-indene-1,3(2H)-dione derivatives were obtained using a Ru catalyst, whereas S-alkyl 2-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)-2-phenylethanethioates were prepared using a Co catalyst. These transformations led to the synthesis of α,β-unsaturated carbonyl compounds with a broad substrate scope, excellent regiocontrol, and gram-scale amenability. This catalytic strategy using a Co or Ru catalyst has rarely been described for other established CAM catalysts.

Functionalized Chromans from ortho-Quinone Methides and Arylallenes

ortho-Quinone methides (o-QMs) underwent formal [4 + 2]-cycloaddition reactions with arylallenes regioselectively at the styrenyl olefin to furnish the corresponding 3-methylene-2-arylchromans in moderate to good yields (up to 88%). When R ≠ H, the reactions also proceeded with moderate stereoselectivity (up to 5:1) which was governed by the nature of the R group. The 3-methylene-2-arylchromans could serve as common intermediates for further functionalization including epoxidation, oxidative cleavage/Baeyer-Villiger oxidation, Riley oxidation, acid-catalyzed rearrangement, and Pd-catalyzed cross-coupling reactions to furnish the corresponding derivatives in moderate to good yields.

Regioselective Synthesis of Functionalized Pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones via Tandem Post-Ugi Cyclization and Gold(I)-Catalyzed Annulation

A convenient synthesis of less explored pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones is described in two steps from Ugi adducts. The method involves acid-mediated cyclization of Ugi adducts to form dihydropyrazinones followed by gold(I)-catalyzed regioselective annulation. The generality of the transformation was established by reacting a variety of substituted dihydropyrazinones under the optimized reaction conditions to form densely functionalized pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones in good-to-excellent yields. It was also observed some of the acetone-derived Ugi adducts furnish 7-acyl-pyrroloimidazolones as a byproduct during TFA-mediated cyclization via alkyne-carbonyl metathesis and condensation.

Ag(I)-Promoted homo-dimerization of 2-(alk-2-yn-1-onyl)-1-alkynylbenzenes via a [4 + 2] cycloaddition of benzopyrylium ions: access to structurally unique naphthalenes

Herein we report a Ag(I)-promoted homo-dimerization of 2-(alk-2-yn-1-onyl)-1-alkynylbenzenes for the synthesis of structurally novel and functionalized naphthalene derivatives. This transformation exhibits a broad scope for the alkyl as well as aryl groups present on alkynes. Observations made from control experiments suggest the possible mechanism as (i) the homo-dimerization of the in situ generated benzopyrylium ion intermediates through a head-tail [4 + 2] cycloaddition, followed by (ii) the competitive ring-opening vs. decarbonylative aromatization of the adduct to give formylated and deformylated naphthalenes, respectively.

Iron-catalyzed alkyne-carbonyl metathesis for the synthesis of 6,7-dihydro-5H-dibenzo[c,e]azonines

The synthesis of nine-membered rings via alkyne-carbonyl metathesis is reported. The alkyne and acetal units contained in a biaryl substrate undergo the intramolecular alkyne-carbonyl metathesis reaction under FeCl₃-catalysis to furnish the unexplored 6,7-dihydro-5H-dibenzo[c,e]azonines. The method provides an alternative to existing routes to nine-membered rings with cheap, non-toxic iron catalysts and milder conditions.

Synthesis of 2H-chromenes: recent advances and perspectives

As one type of chromene, 2H-chromenes represent important oxygen heterocycles, which not only widely exist in natural products, pharmaceutical agents and biologically relevant molecules, but have also been used broadly in materials science and organic synthesis. Two major synthetic strategies have been developed towards such compounds. This review mainly focuses on the recent advances in this field, including benzopyran ring formation involving cyclization reactions and the late-stage functionalization of the parent 2H-chromenes.

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.

BF3-Catalyzed Skeletal Rearrangement of 7-En-2-ynones to endo-Type Cyclic Dienes

Transition-metal-catalyzed cycloisomerization reactions with skeletal rearrangement of 1,n-enynes to afford cyclic dienes (particularly exo-cyclization products) have been well studied. However, there are few reports on the nonmetal-catalyzed skeletal rearrangement of enynes and skeletal rearrangement of electron-deficient enynes such as n-en-2-ynones. Here, we describe BF₃·MeCN-catalyzed synthesis of 3-alkylidenecyclohexenes from 7-en-2-ynones, representing the first nonmetal-catalyzed skeletal rearrangement of 1,6-enynes to endo-type cyclic dienes.

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.

Halogen Bonding Helicates Encompassing Iodonium Cations

The first halonium-ion-based helices were designed and synthesized using oligo-aryl/pyridylene-ethynylene backbones that fold around reactive iodonium ions. Halogen bonding interactions stabilize the iodonium ions within the helices. Remarkably, the distance between two iodonium ions within a helix is shorter than the sum of their van der Waals radii. The helical conformations were characterized by X-ray crystallography in the solid state, by NMR spectroscopy in solution and corroborated by DFT calculations. The helical complexes possess potential synthetic utility, as demonstrated by their ability to induce iodocyclization of 4-penten-1-ol.

Alkyne aza-Prins cyclization of N-(hexa-3,5-diynyl)tosylamides with aldehydes using triflic acid and a binuclear aluminum complex

The alkyne aza-Prins cyclization of 3,5-diynyl amides and various aldehydes was developed as a first example of the aza-Prins cyclization with the introduction of TfO groups. This method could be applied to homopropargyl amides.

2-Picoline catalyst-triggered [2 + 2 + 2] cycloaddition-type reaction of acetylenedicarboxylates, aldehydes and alkenes

2-Picoline efficiently catalyzes the formation of α,β-enones from acetylenedicarboxylates and aldehydes in the presence of alkenes, thereby leading to pyrans in moderate to good yields with complete regioselectivities. This synthetic method of pyrans is represented as a first example of catalytic and direct [2 + 2 + 2] cycloaddition of three different components under the metal-free conditions.

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.