Enantioselective synthesis of (-)-paeonilide

Lewis Acid Catalyzed Cyclopropane Ring-Opening-Cyclization Cascade Using Thioureas as a N,N-bisnucleophile: Synthesis of Bicyclic Furo-, Pyrano-, and Pyrrololactams via a Formal [4+1]-Addition

Fused bicyclic cyclopropanes were converted by Lewis acid-catalysis with thioureas to furo-, pyrano, and pyrrololactams with yields of up to 99 % and high diastereoselectivity. The formation of the title compounds, representing a formal [4+1]-cycloaddition to a donor-acceptor substituted cyclopropane, follows a cascade reaction involving SN1-type ring-opening addition and cyclization. Thiourea, being a cost-effective and odorless reagent, acts as an N,N-bis-nucleophile to generate bicyclic compounds containing an N-substituted γ-lactam moiety.

Secondary Metabolites from Marine-Derived Fungus Penicillium rubens BTBU20213035

Two new polyketide derivatives, penirubenones A and B (1 and 2), and two naturally rare amino-bis-tetrahydrofuran derivatives, penirubenamides A and B (3 and 4), together with nine known compounds (5-13) were isolated from the marine-derived fungus Penicillium rubens BTBU20213035. The structures were identified by HRESIMS and 1D and 2D NMR analyses, and their absolute configurations were determined by a comparison of experimental and calculated electronic circular dichroism (ECD) spectroscopy and 13C NMR data. We found that 6 exhibited antibacterial activity against Staphylococcus aureus, with an MIC value of 3.125 μg/mL, and 1 and 2 showed synergistic antifungal activity against Candida albicans at 12.5 and 50 μg/mL with 0.0625 μg/mL rapamycin.

Eosin, blue LEDs and DIPEA are employed in a simple synthesis of (poly)cyclic O,O- and N,O-acetals

A simple procedure for the synthesis of (poly)cyclic O,O- and N,O-acetals from various enol ethers, N-acyl enamines or Boc-protected enamines has been developed. The key step is a photocatalytic Stork-Ueno-type cylization using the very simple metal-free conditions of catalytic eosin, diisopropylamine in the green solvent ethanol with blue LED irradition.

Open shell versus closed shell bonding interaction in cyclopropane derivatives: EDA-NOCV analyses

Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent-Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the CC bond of the cyclopropane (splitting two bonds at a time out of three CC bonds) ring is preferred to form two dative covalent CC bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3 -rings. These corresponding QTAIM parameters of C3 -rings are quite different for CC single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal-olefin systems.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.

A one-pot ring-closure and ring-opening sequence for the cascade synthesis of dihydrofurofurans and functionalized furans

We have developed a simple novel ring-closure and ring-opening pathway using an organo-base system for the synthesis of highly substituted dihydrofurofuran and furan frameworks via a triethylamine-catalyzed one-pot three-component reaction. The protocol involved a Knoevenagel and Michael adduct via Paal-Knorr cyclization with aromatic/aliphatic glyoxal and 2-cyanoacetophenone under mild and heating conditions with excellent yields through a simple filtration method. The merits of this methodology, including the use of easily available feedstocks and an inexpensive catalyst, Gram-scale synthesis, wide functional group tolerance, an open-air reaction setup, and no need for workup and column-chromatography procedures, make the developed methodology a practical way to access dihydrofurofurans and functionalized furans.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Diversity oriented multi-component reaction (DOS-MCR) approach to access natural product analogues: regio- and chemo-selective synthesis of polyheterocyclic scaffolds via one-pot cascade reactions

Skeletally diverse and complex aza-cyclopenta(cd)diindene, pyrrolo(3,4-d)pyridine-13-carboxamide, and furo-pyrrolo(1,2-a)imidazole-4-carboxamide fused polyheterocyclic hybrid scaffolds and a furo(2,3-b)furan core have been accessed via one-pot three-component reaction by exploiting the build/couple/pair strategy of diversity oriented synthesis (DOS). This protocol is metal free, has a good substrate scope and affords products with good to excellent yields and regio- and chemo-selectivity. The heterocyclic skeletons obtained in this study mimic natural products such as eupolauramine, gracilamine and presilphiperfolanol.

Regio- and Stereoselective Synthesis of Functionalized Dihydropyridines, Pyridines, and 2H-Pyrans: Heck Coupling of Monocyclopropanated Heterocycles

A palladium-catalyzed coupling between aryl halides and monocyclopropanated pyrroles or furans has been developed, leading to valuable six-membered N- and O-heterocycles. As the key step, a selective cleavage of the non-activated endocyclic C-C bond of the 2-heterobicyclo-[3.1.0]hexane framework is achieved. The developed method offers access to highly functionalized piperidines, pyridines, and pyrans that are challenging to access by traditional methods.

Rh(II)-Catalyzed Cyclopropanation of Furans and Its Application to the Total Synthesis of Natural Product Derivatives

Rh(II)-catalyzed enantioselective cyclopropanations of furans, providing access to synthetically useful building blocks, are reported. After screening of 10 Rh(II) catalysts, Rh₂(S-TCPTTL)₄ was identified as a highly efficient and selective catalyst (up to 98% ee, TON 88000, and TOF 24/s) for the cyclopropanation of furans. These cyclopropanes were successfully applied to the enantioselective synthesis of novel paraconic acid derivatives.

Catalytic Asymmetric Synthesis of Butenolides and Butyrolactones

γ-Butenolides, γ-butyrolactones, and derivatives, especially in enantiomerically pure form, constitute the structural core of numerous natural products which display an impressive range of biological activities which are important for the development of novel physiological and therapeutic agents. Furthermore, optically active γ-butenolides and γ-butyrolactones serve also as a prominent class of chiral building blocks for the synthesis of diverse biological active compounds and complex molecules. Taking into account the varying biological activity profiles and wide-ranging structural diversity of the optically active γ-butenolide or γ-butyrolactone structure, the development of asymmetric synthetic strategies for assembling such challenging scaffolds has attracted major attention from synthetic chemists in the past decade. This review offers an overview of the different enantioselective synthesis of γ-butenolides and γ-butyrolactones which employ catalytic amounts of metal complexes or organocatalysts, with emphasis focused on the mechanistic issues that account for the observed stereocontrol of the representative reactions, as well as practical applications and synthetic potentials.

Enantio- and diastereoselective synthesis of tetrahydrofuro[2,3-b]furan-2(3H)-one derivatives and related oxygen heterocycles via an asymmetric organocatalytic cascade process

Tetrahydrofuro[2,3-b]furan-2(3H)-one derivatives were formed via a novel asymmetric organocatalytic cascade process with excellent stereoselectivity in one single operation.