Redox-neutral α-functionalization of pyrrolidines: facile access to α-aryl-substituted pyrrolidines

α-Aryl-substituted pyrrolidine moiety is found in many natural alkaloids. Starting from pyrrolidine, we were able to synthesize α-aryl-substituted pyrrolidines in one step using quinone monoacetal as the oxidizing agent and DABCO as the base. We also discovered the reaction condition needed to efficiently remove the N-aryl moiety from the α-arylated product. When the above reaction was carried out without the addition of an aryl nucleophile, the reaction of pyrrolidine and quinone monoacetal in 2,2,2-trifluoroethanol afforded octahydro-dipyrroloquinoline in high yield, which has the same skeleton as that of natural product incargranine B.

Synthesis of Nitrogen-Containing Heterocyclic Scaffolds through Sequential Reactions of Aminoalkynes with Carbonyls

Sequential reactions of aminoalkynes represent a powerful tool to easily assembly biologically important polyfunctionalized nitrogen heterocyclic scaffolds. Metal catalysis often plays a key role in terms of selectivity, efficiency, atom economy, and green chemistry of these sequential approaches. This review examines the existing literature on the applications of reactions of aminoalkynes with carbonyls, which are emerging for their synthetic potential. Aspects concerning the features of the starting reagents, the catalytic systems, alternative reaction conditions, pathways and possible intermediates are provided.

Diastereoselective intramolecular cyclization/Povarov reaction cascade for the one-pot synthesis of polycyclic quinolines

The method for the synthesis of complex alkaloid-like aza-heterocycles has been developed through the Povarov reaction of in situ generated 2-oxoimidazolium cations. The reaction lead to the formation of 2 C-N, 2 C-C bonds and three stereocentres, and features excellent regio- and diastereoselectivity. Based on the controlled experiments and quantum chemistry data, the mechanism of the reaction cyclization was proposed and diastereoselectivity origins were rationalized. Additionally, a straightforward synthesis of hardly accessible 4,4'-bi(imidazol-2-one) derivatives has been achieved using the same methodology.

Discovery of seneciobipyrrolidine derivatives for the amelioration of glucose homeostasis disorders through 4E-BP1/Akt/AMPK signaling activation

Modulating the glucose transport in skeletal muscle is a promising strategy for ameliorating glucose homeostasis disorders. However, the complicated mechanisms of glucose transport make it difficult to find compounds therapeutically relevant molecular mechanisms of action, while phenotypic screening is thought to be an alternative approach to mimic the cell state of interest. Here, we report (±)-seneciobipyrrolidine (1a) is first found to enhance glucose uptake in L6 myotubes through phenotype-based screening. Further SAR investigation led to the identfication of compound A27 (EC₅₀ = 2.7 μM). Proteomiic analysis discloses the unique function mechanism of A27 through upregulating the level of the eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), subsequently enhancing the Akt and AMPK phosphorylation, thereby promoting the glucose uptake. Chronic oral administration of A27 significantly lowers blood glucose and improves glucose tolerance in db/db mice. This work is new research on seneciobipyrrolidine derivatives, providing a promising avenue for ameliorating glucose homeostasis.

Metal-Free Synthetic Shortcut to Octahydro-Dipyrroloquinoline Skeletons from 2,5-Cyclohexadienone Derivatives and l-Proline

The tandem decarboxylative condensation-dimerization reaction of l-proline with 2,5-cyclohexadienones including p-quinone monoacetals, p-quinol ethers, and p-quinols is reported to provide a concise and rapid synthesis of octahydro-dipyrroloquinoline compounds. The reaction features the use of cost-effective and readily available starting materials, high efficiency, metal-free and green reaction conditions. The reaction is applied to the synthesis of incargranine B aglycone. The discovery of this reaction may suggest a biosynthetic pathway from 2,5-cyclohexadienones and proline for natural ingredients containing pyrroloquinoline moieties.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Synthesis of the Hexahydropyrrolo-[3,2-c]-quinoline Core Structure and Strategies for Further Elaboration to Martinelline, Martinellic Acid, Incargranine B, and Seneciobipyrrolidine

Natural products are rich sources of interesting scaffolds possessing a plethora of biological activity. With the isolation of the martinella alkaloids in 1995, namely martinelline and martinellic acid, the pyrrolo[3,2-c]quinoline scaffold was discovered. Since then, this scaffold has been found in two additional natural products, viz. incargranine B and seneciobipyrrolidine. These natural products have attracted attention from synthetic chemists both due to the interesting scaffold they contain, but also due to the biological activity they possess. This review highlights the synthetic efforts made for the preparation of these alkaloids and formation of analogues with interesting biological activity.

One-pot three-component reaction of p-quinone monoacetals, l-proline and naphthols to afford N-aryl-2-arylpyrrolidines

A one-pot three-component reaction of p-quinone monoacetals (or p-quinol ethers), l-proline and naphthols is developed for the synthesis of N-aryl-2-arylpyrrolidines under mild conditions with high chemo- and regioselectivity.

Gold-catalysed regioselective cascade cycloisomerisation reactions of aza-enediynes for the synthesis of substituted benzoisoquinoline derivatives

A facile, regioselective, gold-catalysed cascade cycloisomerisation of easily assembled aza-enediynes to afford dihydrobenzo[f]isoquinoline derivatives is reported.

Modular Synthesis of Dipyrroloquinolines: A Combined Synthetic and Mechanistic Study

A straightforward synthesis of [1,2-a][3',2'-c]dipyrroloquinolines has been developed generating up to eight new σ-bonds and five new stereogenic centers in a simple and modular one-pot operation. Generally good to excellent yields and moderate to good stereoselectivities in favor of the all-cis stereoisomer were observed. A detailed investigation combining synthetic studies, analytical measurements, and theoretical calculations has been conducted to elucidate the reaction mechanism using ESI- and liquid-beam IR-laser desorption mass spectrometry as well as DFT calculations. Key steps of this sequential transformation include a Lewis acid-catalyzed vinylogous Mukaiyama-Mannich reaction of bis(silyl) dienediolate 1 and a Brønsted acid-promoted Mannich-Pictet-Spengler reaction cascade reaction to complete the synthesis of the dipyrroloquinoline core of the target compounds.

A Au(i)-catalyzed hydrogen bond-directed tandem strategy to synthesize indeno-chromen-4-one and indeno-quinolin-4-one derivatives

A gold-catalyzed hydrogen bond-directed tandem cyclization strategy to synthesize indeno-chromen-4-one and indeno-quinolin-4-one derivatives has been developed.