Total synthesis of (-)-martinellic acid via radical addition-cyclization-elimination reaction

Palladium-Catalyzed Cascade Cyclization for the Synthesis of Fused Benzo-Aza-Oxa-[5-6-5] Tetracycles

A novel and expedient cascade strategy has been demonstrated for the synthesis of fused benzo-aza-oxa-[5-6-5] tetracycles in high yields and diastereoselectivities (up to 20 : 1 dr). The strategy was fulfilled through palladium-catalyzed oxidative convergent assembly of functionally divergent anilines and 3-butenoic acid with five chemical bonds constructed. Coupled with control experiments and deuterium labelled studies, DFT calculations were performed for the proposed mechanism. The utility of the illustrated strategy is emphasized by gram-scale syntheses, late-stage functionalization, and the transformation to a key core of natural products such as martinellic acid and seneciobipyrrolidine.

Rapid Generation of Tetrahydropyridines and Tetrahydroquinolines by Dearomative Cyanation/Grignard Addition

A rapid, practical and scalable method for the reductant and tansition-metal-free synthesis of a variety of novel 2,4-disubstituted tetrahydropyridines and tetrahydroquinolines is disclosed. The method is based upon dearomative functionalization of pyridines or quinolines to generate amino nitrile intermediates as masked iminium ions, which then reacted rapidly with various Grignard reagents in complete stereocontrol.

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.

Metal-Free Radical [2 + 2 + 1] Cyclization of ortho-Cyanoarylacrylamides with Alkyl Nitriles: Synthesis of Pyrrolo[3,2- c]quinolines

A novel radical [2 + 2 + 1] cyclization of ortho-cyanoarylacrylamides with dual α-C-H bonds in alkyl nitriles has been developed. The reaction provides new facile and straightforward access to cyano-substituted pyrrolo[3,2- c]quinolines, which are important nitrogen-containing polyheterocycles. A possible mechanism for the transformation is proposed.

A Facile and Convenient Approach for the Synthesis of Novel Sesamol–Oxazine and Quinoline–Oxazine Hybrids

A series of functionalized sesamol–oxazine and quinoline–oxazine hybrids have been synthesized via one-pot reaction of sesamol/6-hydroxyquinoline, aromatic amines, and methanal. The structures of all the novel compounds were confirmed by spectral data. The structures of the synthesized hybrids were also confirmed by X-ray crystallographic studies. Mild reaction conditions, operational simplicity, short reaction times, simple workup, and high yields of products are salient features of this methodology.

Diastereoselective Intramolecular [3 + 2]-Annulation of Donor-Acceptor Cyclopropane with Imine-Assembling Hexahydropyrrolo[3,2-c]quinolinone Scaffolds

An intramolecular [3 + 2]-annulation of amide-linked donor-acceptor cyclopropane with in situ-generated imine is described. As a result, diverse hexahydropyrrolo[3,2-c]quinolinones, as the tricyclic core of martinellines, were efficiently assembled in good to excellent yield (up to 93%) with a good diastereomeric ratio (up to 98:2).

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

Copper(i) catalyzed C(sp2)–N bond formation: synthesis of pyrrolo[3,2-c]quinolinone derivatives

An intramolecular copper-catalyzed direct C(sp²)–H activation/C(sp²)–N bond formation reaction has been developed for the synthesis of pyrrolo[3,2-c]quinolinone derivatives under an oxygen atmosphere. This approach presents an alternative route to the tricyclic core of martinellic acid or martinelline.

Enantioselective Total Synthesis of (-)-Martinellic Acid

An enantioselective total synthesis of martinellic acid is described. The pyrroloquinoline alkaloid core is efficiently prepared from a quinoline, employing a method which relies on a newly developed Cu-catalyzed enantioselective alkynylation using the chiral imidazole-based biaryl P,N ligand StackPhos to establish the absolute stereochemistry. The remaining carbon atoms are then installed by means of a diastereoselective Pd-catalyzed decarboxylative allylation and the synthesis is completed after straightforward functional-group manipulation. This new synthetic method enables the most concise enantioselective synthesis of this important class of molecules to date.

Novel 1,3,4-oxadiazole motifs bearing a quinoline nucleus: synthesis, characterization and biological evaluation of their antimicrobial, antitubercular, antimalarial and cytotoxic activities

Quinoline–oxadiazole hybrids: a new class of antimicrobial, antitubercular and antimalarial compounds.

Modular, flexible, and stereoselective synthesis of pyrroloquinolines: rapid assembly of complex heterocyclic scaffolds

A novel 1,2-dinucleophile engages two imines in a sequential vinylogous Mannich-Mannich-Pictet-Spengler process to generate complex hexahydropyrrolo[3,2-c]quinolines in a one-pot operation. This methodology provides a rapid, highly modular, and flexible access toward a wide range of products and forms four new σ-bonds and chiral centers each. The diastereoselectivity may be inverted by fine-tuning of reaction conditions and the electronic nature of the imines.

Highly stereoselective synthesis of functionalized pyrrolo[3,2-c]quinolines via N-heterocyclic carbene catalyzed cascade sequence

An N-heterocyclic carbene-catalyzed stereoselective Michael-Mannich-lactamization cascade reaction of tosyl-protected o-amino aromatic aldimines and 2-bromoenals for the construction of functionalized pyrrolo[3,2-c]quinolines with three consecutive stereocenters was achieved in good yields with excellent diastereo- and enantioselectivities.

An expedient approach to pyrrolo[3,2-c]quinolines via regioselective formation of the pyrrole nucleus over indoles

The regioselective synthesis of pyrrolo[3,2-c]quinolines from protected 2-alkynylanilines by Heck and intramolecular Michael addition has been described.

Asymmetric synthesis of (-)-martinellic acid

A high-yielding total asymmetric synthesis of (-)-martinellic acid is reported. The conjugate addition of lithium (R)-N-allyl-N-(α-methyl-4-methoxybenzyl)amide to tert-butyl (E)-3-[2'-(N,N-diallylamino)-5'-bromophenyl]propenoate and alkylation of the resultant β-amino ester have been used as the key steps to install the C(9b) and C(3a) stereogenic centers, respectively, and a highly diastereoselective Wittig reaction/intramolecular Michael addition was then used to create the C(4) stereogenic center within this tricyclic molecular architecture.

A one-pot multicomponent strategy for stereospecific construction of tricyclic pyrrolo[1,2-a]quinolines

A novel multicomponent strategy for the efficient synthesis of tricyclic pyrrolo[1,2-a]quinolines has been described. The bond-forming efficiency, accessibility and generality of this synthesis make it highly attractive to assemble tri-heterocyclic scaffolds.

Chiral phosphoric acid-catalyzed enantioselective three-component Povarov reaction using cyclic enethioureas as dienophiles: stereocontrolled access to enantioenriched hexahydropyrroloquinolines

Three become one: Phosphoric acid-catalyzed enantioselective three-component Povarov reactions of aldehydes, anilines, and endocyclic enethioureas have been developed (see scheme). This process afforded hexahydropyrroloquinolines in high yields with excellent diastereo- and enantioselectivities. The presence of the thiourea functionality is crucial for the enantioselectivity of the reaction.

Palladium-catalyzed cross coupling reaction of N-alkoxyimidoyl bromides and its application to one-pot synthesis of N-arylamines

The synthetic utility of N-alkoxyimidoyl halides is demonstrated using the palladium-catalyzed cross-coupling reaction. The Sonogashira and Suzuki-Miyaura coupling reactions of N-alkoxyimidoyl bromides produced versatile ketoxime ethers in good to excellent yields. A one-pot reaction of the imidoyl bromides with arylboronic acid and allylmagnesium bromide to produce N-arylamines via Suzuki-Miyaura coupling followed by domino reaction involving sequential addition-eliminative rearrangement-addition reactions was developed.

Efficient entry into 2-substituted tetrahydroquinoline systems through alkylative ring expansion: stereoselective formal synthesis of (+/-)-martinellic acid

A new efficient synthesis of 2-substituted tetrahydroquinolines has been achieved by the domino reaction of N-indanyl(methoxy)amines, which consists of three types of reactions: elimination of an alcohol, the rearrangement of an aryl group, and the addition of an organolithium or magnesium reagent. The synthetic utility of this approach is demonstrated by the stereoselective formal synthesis of (+/-)- martinellic acid.