Organocatalytic Asymmetric Construction of 2,6-Diazabicyclo-[2.2.2]octanes by Harnessing the Potential of an 3-Oxindolium Ion Intermediate

Due to its structural complexity and intrinsic sensitivity of bridged aminal junction, 2,6-diazabicyclo[2.2.2]octane (2,6-DABCO) has remained a highly desirable target in synthetic chemistry. However, the asymmetric access to this unit is still insufficient and hampered by the need for meticulously created functionalities for intricate double aza-cyclizations. Herein, we have developed a novel enantio- and diastereoselective protocol to access polycyclic chiral 2,6-DABCOs under metal-free conditions. This domino process involves the amine-catalyzed [4+2] annulation between glutaraldehyde and 2-arylindol-3-ones, followed by an acid-mediated Pictet-Spengler reaction/intramolecular aza-cyclization cascade sequence with tryptamine by trapping of in situ generated 3-oxindolium ion intermediate for the first time. Overall, 2,6-DABCOs fused with medicinally relevant scaffolds were isolated with good yield and excellent stereoselectivity by constructing five new bonds and four stereocenters in a one-pot operation.

Copper Phenylacetylide and TiO2 Modification for an Efficient Visible-Light-Driven Oxidative Coupling of Amines

The selective oxidation of amines to imines under mild conditions has attracted much attention. Our study reveals that copper phenylacetylide (PhC2Cu) could serve as an efficient photocatalyst for imine synthesis under visible-light irradiation (>400 nm). Utilizing benzylamine as a model reactant, PhC2Cu achieves an imine yield of 50.4%, which is 5 times higher than that of P25 under the same conditions and comparable to the yield obtained by the 3 wt % Au/P25 photocatalyst (55.4%). Further loading 3.9 nm TiO2 onto PhC2Cu through tetrabutyl titanate hydrolysis increases the imine yield to 84.7%, with a Ti:Cu atomic ratio of 3.65%. Control experiments, photoluminescence (PL) spectra, optical pump terahertz probe (OPTP) spectra, and electron spin resonance (ESR) tests confirm that the optimized TiO2 modification promotes the separation of excited carriers and electron transfer in PhC2Cu and facilitates the activation of surface oxygen, thereby enhancing the formation of superoxide radicals, a key active oxygen species in the reaction system. This work presents a promising strategy for efficient imine synthesis via amine coupling and expands the application field of PhC2Cu-based photocatalysts.

Harnessing Dpp-Imine as a Powerful Achiral Cocatalyst to Dramatically Increase the Efficiency and Stereoselectivity in a Magnesium-Mediated Oxa-Michael Reaction

Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa-cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.

Enantioselective synthesis of α-tetrasubstituted (1-indolizinyl) (diaryl)-methanamines via chiral phosphoric acid catalysis

An enantioselective Friedel-Crafts reaction of cyclic α-diaryl N-acyl imines with indolizines catalyzed by a chiral spirocyclic phosphoric acid has been developed. The asymmetric transformation proceeds smoothly to afford α-tetrasubstituted (1-indolizinyl) (diaryl)methanamines in good yields with up to 98% ee under mild conditions.

Thiourea fused γ-amino alcohol organocatalysts for asymmetric Mannich reaction of β-keto active methylene compounds with imines

Catalytic functionality of new optically active thiourea fused γ-amino alcohols was examined in the asymmetric Mannich reaction of β-keto active methylene compounds with imines to afford chiral Mannich products, β-amino keto compounds, with continuous chiral centers, that are versatile synthetic intermediates for deriving various biologically active compounds. In particular, the thiourea fused γ-amino alcohols showed satisfactory catalytic activity in this reaction and afforded chiral Mannich products in excellent chemical yield (up to 88%) and stereoselectivities (up to syn : anti/93 : 7 dr, up to 99% ee).

Enantioselective synthesis of α-tetrasubstituted (3-indolizinyl) (diaryl)methanamines via chiral phosphoric acid catalysis

An enantioselective Friedel-Crafts reaction of cyclic α-diaryl N-acyl imines with indolizines catalyzed by a chiral spirocyclic phosphoric acid has been developed. The asymmetric transformation proceeds smoothly to afford α-tetrasubstituted (3-indolizinyl) (diaryl)methanamines in good yields with up to 98% ee under mild conditions.

Enantioselective hydrophosphonylation of N-Boc imines using chiral guanidine-thiourea catalysts

The enantioselective hydrophosphonylation of N-Boc imines was investigated using a new family of pseudo-symmetric guanidine-thiourea catalysts, providing α-amino phosphonates in moderate to high yields with good enantioselectivity. The catalyst was heterogenized by polymerization with styrene and the resulting catalyst was applied to reactions under continuous-flow conditions.

A Three-Component Ugi-Type Reaction of N-Carbamoyl Imines Enables a Broad Scope Primary α-Amino 1,3,4-Oxadiazole Synthesis

A general synthesis of N-protected primary α-amino 1,3,4-oxadiazoles, from N-carbamoyl imines, N-isocyaniminotriphenylphosphorane (NIITP), and carboxylic acids, is described. Featuring an isocyanide addition reaction with N-carbamoyl imines, this efficient three-component Ugi-type reaction was found to be broad in scope with respect to imine, and carboxylic acid coupling partners. Furthermore, the versatility of this method was demonstrated by α-amino 1,2,4-triazole synthesis, the late-stage functionalization of seven drug molecules, and five divergent derivatizations of a primary α-amino 1,3,4-oxadiazole.

Tyrosinase-mediated synthesis of larvicidal active 1,5-diphenyl pent-4-en-1-one derivatives against Culex quinquefasciatus and investigation of their ichthyotoxicity

1,5-diphenylpent-4-en-1-one derivatives were synthesised using the grindstone method with Cu(II)-tyrosinase used as a catalyst. This method showed a high yield under mild reaction conditions. The synthesised compounds were identified by FTIR, 1H NMR, 13C NMR, mass spectrometry, and elemental analysis. In this study, a total of 17 compounds (1a-1q) were synthesised, and their larvicidal and antifeedant activities were evaluated. Compound 1i (1-(5-oxo-1,5-diphenylpent-1-en-3-yl)-3-(3-phenylallylidene)thiourea) was notably more active (LD50: 28.5 µM) against Culex quinquefasciatus than permethrin(54.6 µM) and temephos(37.9 µM), whereas compound 1i at 100 µM caused 0% mortality in Oreochromis mossambicus within 24 h in an antifeedant screening, with ichthyotoxicity determined as the death ratio (%) at 24 h. Compounds 1a, 1e, 1f, 1j, and 1k were found to be highly toxic, whereas 1i was not toxic in antifeedant screening. Compound 1i was found to possess a high larvicidal activity against C. quinquefasciatus and was non-toxic to non-target aquatic species. Molecular docking studies also supported the finding that 1i is a potent larvicide with higher binding energy than the control (- 10.0 vs. - 7.6 kcal/mol) in the 3OGN protein. Lead molecules are important for their larvicidal properties and application as insecticides.

N,N-Diacylaminals as Emerging Tools in Synthesis: From Peptidomimetics to Asymmetric Catalysis

N,N-Diacylaminals are flexible molecular scaffolds that have commonly been utilized as amide surrogates in peptidomimetics. The singularities of this motif as an N-acyl imine equivalent and as hydrogen-bond donor have recently opened new synthetic opportunities, especially in the field of asymmetric catalysis. This concept article highlights this diverse synthetic potential and provides the elements necessary for further developments.

Enantioselective Divergent Syntheses of (+)-Bulleyanaline and Related Isoquinoline Alkaloids from the Genus Corydalis

The isoquinoline alkaloids isolated from the genus Corydalis possess potent and diverse biological activities. Herein, a concise, divergent, and enantioselective route to access these natural products is disclosed. Key transformations of our approach include a challenging Zn-ProPhenol-catalyzed asymmetric Mannich reaction to build a quaternary stereogenic center and a rapid cationic Au-catalyzed cycloisomerization to the common structural skeleton of these natural products. Subsequent late-stage oxidations and modifications allow efficient access to the targeted alkaloids. Overall, seven natural products have been successfully synthesized in 6 to 10 steps from readily available starting materials, including (+)-corynoline, (+)-anhydrocorynoline, (+)-12-hydroxycorynoline, (+)-12-hydroxycorynoloxine, (+)-corynoloxine, (+)-6-acetonylcorynoline, and (+)-bulleyanaline.

2-Azadienes as Enamine Umpolung Synthons for the Preparation of Chiral Amines

The development of new strategies for the preparation of chiral amines is an important objective in organic synthesis. In this Synpacts, we summarize our approach for catalytically accessing nucleophilic aminoalkyl metal species from 2-azadienes, and its application in generating a number of important but elusive chiral amine scaffolds. Reductive couplings with ketones and imines afford 1,2-amino tertiary alcohols and 1,2-diamines, respectively, whereas fluoroarylations of gem-difluoro-2-azadienes deliver α-trifluoromethylated benzylic amines.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Highly enantioselective addition of sulfur-containing heterocycles to isatin-derived ketimines

In this study, we report a highly stereoselective addition of sulfur-containing heterocyclic compounds to isatin-derived ketimines efficiently catalyzed by cinchonidine-derived bifunctional tertiary aminothiourea (1 mol%).

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

The enantioselective addition of 1-fluoro-1-nitro(phenylsulfonyl)methane to isatin-derived ketimines

An asymmetric organocatalytic addition of fluorinated phenylsulfonylnitromethane to isatin-derived ketimines was developed. The reaction was efficiently catalyzed by a chiral tertiary amine, cinchonine. This methodology provides a new type of optically active compound with two adjacent quaternary carbon stereocenters in good yield (up to 96%), with moderate diastereoselectivity (up to 5.7 : 1 dr) and excellent enantioselectivity (up to 98/96% ee).

Chiral phosphoric acid catalyzed enantioselective N-alkylation of indoles with in situ generated cyclic N-acyl ketimines

A chiral SPINOL derived phosphoric acid-catalyzed asymmetric N-alkylation reaction of indoles with cyclic α-diaryl-substituted N-acyl imines, which are generated in situ from 3-aryl 3-hydroxyisoindo-linones, has been demonstrated.

Enantioselective Brønsted Acid Catalysis as a Tool for the Synthesis of Natural Products and Pharmaceuticals

Synthesis of biologically active molecules (whether at laboratory or industrial scale) remains a highly appealing area of modern organic chemistry. Nowadays, the need to access original bioactive scaffolds goes together with the desire to improve synthetic efficiency, while reducing the environmental footprint of chemical activities. Long neglected in the field of total synthesis, enantioselective organocatalysis has recently emerged as an environmentally friendly and indispensable tool for the construction of relevant bioactive molecules. Notably, enantioselective Brønsted acid catalysis has offered new opportunities in terms of both retrosynthetic disconnections and controlling stereoselectivity. The present report attempts to provide an overview of enantioselective total or formal syntheses designed around Brønsted acid-catalyzed transformations. To demonstrate the versatility of the reactions promoted and the diversity of the accessible motifs, this Minireview draws a systematic parallel between methods and retrosynthetic analysis. The manuscript is organized according to the main reaction types and the nature of newly-formed bonds.

Chiral Primary Amine Catalysis for Asymmetric Mannich Reactions of Aldehydes with Ketimines: Stereoselectivity and Reactivity

The application potential of primary amine catalysts in the context of Mannich reactions of aldehydes with ketimines is exemplified by isatin-derived ketimines and cyclic trifluoromethyl ketimines. Primary amine catalysts exhibit either unusual stereoselectivity or reactivity, which is not observable with secondary amine catalysts. Moreover, reversal of diastereofacial selectivity between primary and secondary amine catalysts is disclosed. These new reactions provide useful methods for the syntheses of chiral 3-substituted 3-amino-2-oxindoles and dihydroquinazolinones bearing a trifluoromethylated quaternary stereocenter. The synthetic utility of the reactions is demonstrated through the formal synthesis of AG-041R and DPC 083 and the total synthesis of (-)-psychotriasine.

Chiral Brønsted Acid Catalyzed Enantioselective aza-Friedel-Crafts Reaction of Cyclic α-Diaryl N-Acyl Imines with Indoles

Asymmetric addition of indoles to cyclic α-diaryl-substituted N-acyl imines, which are generated in situ from 3-aryl 3-hydroxyisoindolinones, is described. The transformation proceeds smoothly with a broad range of indoles and isoindolinone alcohols using a SPINOL-derived chiral Brønsted acid catalyst to afford α-tetrasubstituted (3-indolyl)(diaryl)methanamines in excellent yields and enantioselectivities (up to 98% yield, up to >99:1 e.r.). The origin of stereochemical induction is supported by DFT calculations and experimental data.

In situ generation of less accessible Boc-imines from aldehydes: construction of a quaternary carbon by the Mannich reaction or unprecedented aldol reaction

Mannich reactions of β-dicarbonyls with in situ generated less accessible Boc-imines from aldehydes were achieved.

Construction of 2,3,4,5-tetrahydro-1,2,4-triazines via [4 + 2] cycloaddition of α-halogeno hydrazones to imines

In the presence of sodium carbonate, the [4 + 2] cycloaddition of α-halogeno hydrazones to imines proceeded readily, and furnished 2,3,4,5-tetrahydro-1,2,4-triazines in moderate to high chemical yields.

In situ generation of N-Boc-protected alkenyl imines: controlling the E/Z geometry of alkenyl moieties in the Mukaiyama–Mannich reaction

Readily available Boc-protected Z-alkenyl aminals were used as Z-alkenyl and E-alkenyl imine precursors under acidic conditions. In the Mukaiyama–Mannich reaction of Z-alkenyl Boc-aminals, the E/Z geometry of the products was controlled by the catalyst used.

An asymmetric Brønsted acid-catalyzed Friedel–Crafts reaction of indoles with cyclic N-sulfimines

A highly enantioselective organocatalytic Friedel–Crafts reaction of indoles with cyclic N-sulfimines using a chiral phosphoric acid as an organocatalyst has been developed.

Why different ligands can control stereochemistry selectivity of Ni-catalyzed Suzuki–Miyaura cross-coupling of benzylic carbamates with arylboronic esters: a mechanistic study

Transmetallation is the rate-determining step for the whole catalytic cycle, and oxidative addition controls the stereoselectivity of products.