Rhodium-Catalyzed Carbonylative [3 + 2 + 1] Cycloaddition of Alkyne-Tethered Alkylidenecyclopropanes to Phenols in the Presence of Carbon Monoxide

Synthesis of 1,2,4,5-tetra-substituted benzenes via copper-catalyzed dimerization of γ,δ-unsaturated ketones

An efficient cyclization for the synthesis of 1,2,4,5-tetra-substituted benzenes via copper catalyzed dimerization of γ,δ-unsaturated ketones has been described. This one-pot procedure employs the γ,δ-unsaturated ketones as the sole substrate with multiple C-C bond formation. This protocol features broad substrate scope and provides a facile and robust method to construct polysubstituted benzene derivatives under mild conditions.

Electrochemical oxidative C-C bond cleavage of methylenecyclopropanes with alcohols

Herein, an electrochemical approach toward the ring opening functionalization of methylenecyclopropanes (MCPs) via C-C bond cleavage in the presence of alcohols is reported. The methodology avoids the usage of external oxidants and shows good functional group tolerance. The mechanistic studies suggest that the reaction proceeds via direct single electron oxidation of the C-C bond of MCPs followed by ring opening to form the desired product.

Cobalt(I)-Catalyzed (3 + 2 + 2) Cycloaddition between Alkylidenecyclopropanes, Alkynes, and Alkenes

Cobalt(I) catalysts equipped with bisphosphine ligands can be used to promote formal (3 + 2 + 2) intramolecular cycloadditions of enynylidenecyclopropanes of type 1. The method provides synthetically appealing 5,7,5-fused tricyclic systems in good yields and with complete diastereo- and chemoselectivity. Interestingly, its scope differs from that of previously reported annulations based on precious metal catalysts, specifically rhodium and palladium. Noticeably, density functional theory calculations confirm that the mechanism of the reaction is also different from those proposed for these other catalysts.

Chemoenzymatic Synthesis of 13-Oxoverruculogen

Verruculogens are rare fumitremorgin alkaloids that contain a highly unusual eight-membered endoperoxide. In this paper, we report a concise chemoenzymatic synthesis of 13-oxoverruculogen using enzymatic C-H peroxidation and rhodium-catalyzed C-C bond activation reactions to install the eight-membered endoperoxide and the pentacyclic core of the natural product, respectively. Our strategy involves the use of 13-epi-fumitremorgin B as a substrate analog for endoperoxidation by verruculogen synthase, FtmOx1. The resulting product, 13-epi-verruculogen, is the first unnatural endoperoxide generated by FtmOx1 and is used in the first synthesis of 13-oxoverruculogen. This strategy enables a 10-step synthesis of this natural product from commercially available starting materials and illustrates a hybrid approach utilizing biocatalytic and transition-metal-catalyzed reactions to access challenging alkaloid architectures. Moreover, this work demonstrates the use of native enzyme promiscuity as a viable strategy for the chemoenzymatic synthesis of natural products.

[2+2+1+1] Cycloaddition for de novo Synthesis of Densely Functionalized Phenols

A unique benzannulation strategy for regioselective de novo synthesis of densely functionalized phenols is described. Through metal-mediated formal [2+2+1+1] cycloaddition of two different alkynes and two molecules of CO, a series of densely functionalized phenols were obtained. The benzannulation strategy allows efficient regioselective installation up to five different substituents on a phenol ring. The resulting phenols have a substitution pattern different from those obtained from Dötz and Danheiser benzannulations.

Functionalized 10-Membered Aza- and Oxaenediynes through the Nicholas Reaction

The scope and limitations of the Nicholas-type cyclization for the synthesis of 10-membered benzothiophene-fused heterocyclic enediynes with different functionalities were investigated. Although the Nicholas cyclization through oxygen could be carried out in the presence of an ester group, the final oxaenediyne was unstable under storage. Among the N-type Nicholas reactions, cyclization via an arenesulfonamide functional group followed by mild Co-deprotection was found to be the most promising, yielding 10-membered azaendiynes in high overall yields. By contrast, the Nicholas cyclization through the acylated nitrogen atom did not give the desired 10-membered cycle. It resulted in the formation of a pyrroline ring, whereas cyclization via an alkylated amino group resulted in a poor yield of the target 10-membered enediyne. The acylated 4-aminobenzenesulfonamide nucleophilic group was found to be the most convenient for the synthesis of functionalized 10-membered enediynes bearing a clickable function, such as a terminal triple bond. All the synthesized cyclic enediynes exhibited moderate activity against lung carcinoma NCI-H460 cells and had a minimal effect on lung epithelial-like WI-26 VA4 cells and are therefore promising compounds in the search for novel antitumor agents that can be converted into conjugates with tumor-targeting ligands.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Visible light mediated synthesis of 4-aryl-1,2-dihydronaphthalene derivatives via single-electron oxidation or MHAT from methylenecyclopropanes

A new synthetic strategy of a single-electron oxidation and MHAT of methylenecyclopropanes (MCPs) for the rapid construction of 4-aryl-1,2-dihydronaphthalene derivatives by merging photoredox catalysis and cobalt catalysis has been developed.

Visible-light-promoted divergent functionalizations of methylenecyclopropanes

Visible-light-induced monofunctionalization and difunctionalization of MCPs via ring-opening and cyclization processes have been developed for the syntheses of difluoromethyl compounds, alkyl compounds, halides, and sulfonyl compounds.

Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes: DFT analysis of origins of regio- and diastereo-selectivities

A unconventional reaction mechanism can be responsible for the regio- and diastereo-selectivities of rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes.

Recent Advances in Phthalan and Coumaran Chemistry

Oxygen-containing heterocycles are common in biologically active compounds. In particular, phthalan and coumaran cores are found in pharmaceuticals, organic electronics, and other useful medical and technological applications. Recent research has expanded the methods available for their synthesis. This Minireview presents recent advances in the chemistry of phthalans and coumarans, with the goal of overcoming synthetic challenges and facilitating the applications of phthalans and coumarans.

α-Tetrasubstituted Aldehydes through Electronic and Strain-Controlled Branch-Selective Stereoselective Hydroformylation

Hydroformylation utilizes dihydrogen, carbon monoxide, and a catalyst to transform alkenes into aldehydes. This work applies chiral bisdiazaphospholane (BDP)- and bisphospholanoethane-ligated rhodium complexes to the hydroformylation of a variety of alkenes to produce chiral tetrasubstituted aldehydes. 1,1'-Disubstituted acrylates bearing electron-withdrawing substituents undergo hydroformylation under mild conditions (1 mol % of catalyst/BDP ligand, 150 psig gas, 60 °C) with high conversions and yields of tetrasubstituted aldehydes (e.g., 13:1 regioselectivity, 85% ee, and <1% hydrogenation for 1-fluoromethyl acrylate). The scope also encompasses both acyclic 1,1'-disubstituted and trisubstituted, electron-poor alkenes as well as di- and trisubstituted alkenes composed of small rings with exocyclic and endocyclic unsaturation. For example, 1-methylene-β-lactam furnished the tetrasubstituted aldehyde with 98% selectivity and up to 83% ee. Notably, chiral trisubstituted bicyclic methyleneaziridines are transformed with >99% regioselectivity and >19:1 diastereoselectivity to tetrasubstituted aldehydes at rates >50 catalyst turnovers/hour. NMR studies of the noncatalytic reaction of HRh(BDP)(CO)₂ with methyl 1-fluoroacrylate enable interception of tertiary alkylrhodium intermediates, demonstrating migratory insertion to acyl species is slower than formation of secondary and primary alkylrhodium intermediates. Overall, these investigations reveal how the interplay of sterics, electronics, and ring strain are harnessed to provide access to valuable α-tetrasubstituted aldehyde synthetic building blocks by promoting branched-selective hydroformylation.

Rhodium-catalyzed [(3+2)+1] carbocyclizations of alkynylidenecyclopropanes with carbon monoxide: construction of polysubstituted bicyclohexa-2,5-dienones

A rhodium-catalyzed carbocyclization reaction of alkynylidenecyclopropanes with carbon monoxide to prepare bicyclohexa-2,5-dienones is described. This protocol offers convenient access to doubly conjugated cyclic enones and is tolerant of functionalized alkynes and cyclopropanes. Furthermore, a photochemical rearrangement of a bicyclohexa-2,5-dienone facilitates the construction of a highly functionalized bicyclopentenone containing two contiguous stereogenic centres, which represents a versatile intermediate for target directed synthesis.

Fused-Ring Formation by an Intramolecular "Cut-and-Sew" Reaction between Cyclobutanones and Alkynes

The development of a catalytic intramolecular "cut-and-sew" transformation between cyclobutanones and alkynes to construct cyclohexenone-fused rings is described herein. The challenge arises from the need for selective coupling at the more sterically hindered proximal position, and can be addressed by using an electron-rich, but less bulky, phosphine ligand. The control experiment and 13 C-labelling study suggest that the reaction may start with cleavage of the less hindered distal C-C bond of cyclobutanones, followed by decarbonylation and CO reinsertion to enable Rh insertion at the more hindered proximal position.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Transition metal mediated carbonylative benzannulations

This review summarizes novel building blocks recently developed for transition metal-catalyzed carbonylative benzannulations.

Recent advances in the chemical transformations of functionalized alkylidenecyclopropanes (FACPs)

Classification of functionalized alkylidenecyclopropanes (FACPs) and recent chemical transformations for the synthesis of novel and useful polycyclic and heterocyclic compounds.

An unexpected three-component reaction of 2-alkylenecyclobutanone and N′-(2-alkynylbenzylidene)hydrazide with water

A silver(i)-catalyzed three-component reaction of 2-alkylenecyclobutanone, N′-(2-alkynylbenzylidene)hydrazide with water gives rise to 3-(pyrazolo[5,1-a]isoquinolin-1-yl)propanoic acids in good yields through 6-endo cyclization, [3 + 2] cycloaddition, and rearrangement.