Asymmetric synthesis of (-)-martinellic acid

A highly enantioselective intramolecular 1,3-dipolar cycloaddition yields novel pseudo-natural product inhibitors of the Hedgehog signalling pathway

De novo combination of natural product (NP) fragments by means of efficient, complexity- and stereogenic character-generating transformations to yield pseudo-natural products (PNPs) may explore novel biologically relevant chemical space. Pyrrolidine- and tetrahydroquinoline fragments rarely occur in combination in nature, such that PNPs that embody both fragments might represent novel NP-inspired chemical matter endowed with bioactivity. We describe the synthesis of pyrrolo[3,2-c]quinolines by means of a highly enantioselective intramolecular exo-1,3-dipolar cycloaddition catalysed by the AgOAc/(S)-DMBiphep complex. The cycloadditions proceeded in excellent yields (up to 98%) and with very high enantioselectivity (up to 99% ee). Investigation of the resulting PNP collection in cell-based assays monitoring different biological programmes led to the discovery of a structurally novel and potent inhibitor of the Hedgehog signalling pathway that targets the Smoothened protein.

Cu(I)-Catalyzed Alkynylation of Quinolones

Herein we report the first alkynylation of quinolones with terminal alkynes under mild reaction conditions. The reaction is catalyzed by Cu(I) salts in the presence of a Lewis acid, which is essential for the reactivity of the system. The enantioselective version of this transformation has also been explored, and the methodology has been applied in the synthesis of the enantioenriched tetrahydroquinoline alkaloid cuspareine.

Enantioselective Synthesis of 2-Functionalized Tetrahydroquinolines through Biomimetic Reduction

Biomimetic asymmetric reduction of 2-functionalized quinolines has been successfully developed with the chiral and regenerable NAD(P)H model CYNAM in the presence of transfer catalyst simple achiral phosphoric acids, providing the chiral 2-functionalized tetrahydroquinolines with up to 99% ee. Using this methodology as a key step, a chiral and potent opioid analgesic containing a 1,2,3,4-tetrahydroquinoline motif was synthesized with high overall yield.

Taxonomic revision of Martinella Baill. (Bignonieae, Bignoniaceae)

Martinella Baill. is a genus of Neotropical lianas in tribe Bignonieae (Bignoniaceae). The genus is monophyletic and well supported by morphological and molecular characters. Members of Martinella are characterized by a continuous interpetiolar ridge surrounding the stem, bilobed or 4-5-parted calyces, and minute triangular prophylls of the axillary buds. Generic circumscription remained unchanged since the description of the genus, although unclear species limits remained. Based on extensive fieldwork, herbarium work, and a molecular phylogenetic hypothesis for the genus, we here recognize five species of Martinella. Of these, three were recognized in earlier treatments for the genus, while two represent new species described here, Martinella lanuginosa Kataoka & L.G.Lohmann, sp. nov. and Martinella tomentosa Kataoka & L.G.Lohmann, sp. nov. Martinella iquitoensis A.Samp. is treated as a synonym of M. insculpta Sprague & Sandwith. In addition, one second-step lectotype is designated for Bignonia martini DC., and neotypes are designated for Doxantha longisiliqua Miers and Martinella gollmeri K.Schum. This work provides a full taxonomic treatment for Martinella, including a complete list of synonyms, morphological descriptions, illustrations, photographs, distribution maps, conservation status, and comments for all five species recognized.

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.

Facile syntheses of tetrahydroquinolines and 1,2-dihydroquinolines via vinylogous cascade hydride transfer/cyclization

The medicinally significant 3-monosubstituted tetrahydroquinolines and 1,2-dihydroquinolines were controllably constructed via redox-neutral vinylogous cascade condensation/[1,5]-hydride transfer/cyclization in EtOH.

Gas-phase basicity of cyclic guanidine derivatives – a DFT study

Density functional theory calculations (B3LYP) were employed in the study of gas-phase basicity (GB) and pK_a of three different types of cyclic guanidines differing in the number of nitrogen atoms incorporated in rings.

Serendipitous base catalysed condensation-heteroannulation of iminoesters: a regioselective route to the synthesis of 4,6-disubstituted 5-azaindoles

A serendipitous discovery of a novel one-pot synthesis of 4,6-disubstituted 5-azaindoles is reported herein. In the presence of Hunig's base, various N-substituted pyrrole-2-carboxaldehydes have been efficiently transformed into their corresponding 4,6-disubstituted 5-azaindoles through an imine mediated cascade condensation-heteroannulation. The synthetic value of the methodology is established by preparing a novel chemical analogue of a cannabinoid receptor type 2 (CB2) agonist.

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.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Intramolecular Umpolung Allylation of Imines

An intramolecular umpolung allylation of imines is reported. This reaction occurs via the intermediacy of 2-azaallyl anions. It could proceed either under transition-metal-catalyzed conditions or under transition-metal-free conditions. Importantly, this approach afforded trans-3-vinyl-4-aminochromanes with high diastereoselectivity, while conventional, nonumpolung methods often display high cis-selectivity.

PIFA-Mediated oxidative cyclization of 1-aroyl-N-arylcyclopropane-1-carboxamides and their application in the synthesis of pyrrolo[3,2-c]quinolinones

A PIFA-promoted ring-closing reaction for the synthesis of spirocyclopropane quinolinediones, and their application toward pyrrolo[3,2-c]quinolinones.

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).

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.

A silver-promoted solid-phase guanidylation process enables the first total synthesis of stictamide A

First total synthesis of stictamide A, a structurally unique peptide with a statine motif and a N-prenyl modified arginine in the side chain, is disclosed with a novel silver-promoted solid-phase strategy for the first time.

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.