Total synthesis of (+/-)-martinellic acid

Development of a Highly Selective Synthesis of 4-Substituted Tetrahydroquinolines: Substrate Scope and Mechanistic Study

Herein, we describe a general and selective deprotonation functionalization reaction of tetrahydroquinolines at the 4-position using organolithiums and phosphoramide ligands. In addition to the development of a direct deprotonation alkylation reaction with primary and secondary alkyl halides, a Negishi cross-coupling protocol was realized to afford products with a range of aromatic halides. These methods were applied to the late-stage installation of tetrahydroquinolines into a variety of substrates including pharmaceuticals as well as natural product analogues. The use of thorough mechanistic investigations revealed the aggregation state of the newly formed tetrahydroquinoline anion to be a separated ion pair, which proved critical to optimizing the reaction conditions.

Decarboxylative 1,3-dipolar cycloaddition of amino acids for the synthesis of heterocyclic compounds

The [3 + 2] cycloadditions of stabilized azomethine ylides (AMYs) derived from amino esters are well-established. However, the reactions of semi-stabilized AMYs generated from decarboxylative condensation of α-amino acids with arylaldehydes are much less explored. The [3 + 2] adducts of α-amino acids could be used for a second [3 + 2] cycloaddition as well as for other post-condensation modifications. This article highlights our recent work on the development of α-amino acid-based [3 + 2] cycloaddition reactions of N-H-type AMYs in multicomponent, one-pot, and stepwise reactions for the synthesis of diverse heterocycles related to some bioactive compounds and natural products.

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.

Synthesis of 1,5-Substituted Pyrrolidin-2-ones from Donor-Acceptor Cyclopropanes and Anilines/Benzylamines

We developed a straightforward synthetic route to pharmacologically important 1,5-substituted pyrrolidin-2-ones from donor-acceptor cyclopropanes bearing an ester group as one of the acceptor substituents. This method includes a Lewis acid-catalyzed opening of the donor-acceptor cyclopropane with primary amines (anilines, benzylamines, etc.) to γ-amino esters, followed by in situ lactamization and dealkoxycarbonylation. The reaction has a broad scope of applicability; a variety of substituted anilines, benzylamines, and other primary amines as well as a wide range of donor-acceptor cyclopropanes bearing (hetero)aromatic or alkenyl donor groups and various acceptor substituents can be involved in this transformation. In this process, donor-acceptor cyclopropanes react as 1,4-C,C-dielectrophiles, and amines react as 1,1-dinucleophiles. The resulting di- and trisubstituted pyrrolidin-2-ones can be also used in subsequent chemistry to obtain various nitrogen-containing polycyclic compounds of interest to medicinal chemistry and pharmacology, such as benz[g]indolizidine derivatives.

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.

Valproic Acid Thermally Destabilizes and Inhibits SpyCas9 Activity

The clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system plays an important role in prokaryotic adaptive immunity. Due to its capacity for sequence-specific gene editing, CRISPR-Cas9 has become one of the most important tools widely used for genome editing in molecular biotechnology. However, its clinical application is currently limited by unwanted mutations at off-target sites. Various strategies have been developed for precise control of Cas9 in order to reduce these off-target effects, including chemical-based approaches. From a chemical screening, I observed that valproic acid (VPA) binds to and destabilizes Streptococcus pyogenes Cas9 (SpyCas9) protein in vitro, as well as in cells, while within its therapeutical concentration range under conditions of hyperthermia as demonstrated. Conditions were generated either by an external heat bag or in combination with the photothermal therapeutic agent indocyanine green activated by a near-infrared laser. Use of other histone deacetylase inhibitors failed, suggesting a histone deacetylase inhibition-independent function of VPA. Thus, this finding provides an uncomplicated thermotherapeutical approach for timely regulation of the activity of the CRISPR-Cas9 system at desired locations.

Synthesis of highly substituted tetrahydroquinolines using ethyl cyanoacetate via aza-Michael-Michael addition

A three-component cascade reaction involving 2-alkenyl aniline, aldehydes, and ethyl cyanoacetate in the presence of DBU to synthesize highly substituted 1,2,3,4-tetrahydroquinolines is reported. The reaction proceeded through the Knoevenagel condensation of ethyl cyanoacetate with aldehydes followed by the aza-Michael-Michael addition with 2-alkenyl anilines to prepare the tetrahydroquinoline scaffolds.

P-Chiral, N-phosphoryl sulfonamide Brønsted acids with an intramolecular hydrogen bond interaction that modulates organocatalysis

Brønsted acids exemplified by OttoPhosa I (5c) were designed and evaluated in the asymmetric transfer hydrogenation of quinolines. Their catalytic properties are modulated by an intramolecular hydrogen bond that rigidifies their catalytic cavity, accelerates the reaction rate and improves enantioselectivity.

Double 1,3-Dipolar Cycloadditions of Two Nonstabilized Azomethine Ylides for Polycyclic Pyrrolidines

Nonstabilized azomethine ylides derived from decarboxylation of oxazolidin-5-ones were used for double 1,3-dipolar cycloadditions in diastereoselective synthesis of pyrrolidine-containing tetracyclic compounds. This is the first example of one-pot and five-component reactions involving two nonstabilized azomethine ylides. Only CO₂ and water were generated as byproducts.

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.

Heterogeneous graphene oxide as recyclable catalyst for azomethine ylide mediated 1,3 dipolar cycloaddition reaction in aqueous medium

Graphene oxide (GO) catalyzed regio and diastereoselective synthesis of spiro-indenoquinoxaline pyrrolizidines and spiro-oxindoles pyrrolizidines is described with good substrate scope and yield using azomethine ylide under aq. EtOH condition at RT.

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).

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.

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.

Recent syntheses of 1,2,3,4-tetrahydroquinolines, 2,3-dihydro-4(1H)-quinolinones and 4(1H)-quinolinones using domino reactions

A review of the recent literature is given focusing on synthetic approaches to 1,2,3,4-tetrahydroquinolines, 2,3-dihydro-4(1H)-quinolinones and 4(1H)-quinolinones using domino reactions. These syntheses involve: (1) reduction or oxidation followed by cyclization; (2) S_NAr-terminated sequences; (3) acid-catalyzed ring closures or rearrangements; (4) high temperature cyclizations and (5) metal-promoted processes as well as several less thoroughly studied reactions. Each domino method is presented with a brief discussion of mechanism, scope, yields, simplicity and potential utility.

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.

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.