No abstract

A highly enantioselective synthesis of 1,12‐disubstituted [4]carbohelicenes is reported. The key step for the developed synthetic route is a Au‐catalyzed intramolecular alkyne hydroarylation, which is achieved with good to excellent regio‐ and enantioselectivity by employing TADDOL‐derived (TADDOL=α,α,α,α‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) α‐cationic phosphonites as ancillary ligands. Moreover, an appropriate design of the substrate makes the assembly of [4]helicenes of different substitution patterns possible, thus demonstrating the synthetic utility of the method. The absolute stereochemistry of the newly prepared structures was determined by X‐ray crystallography and characterization of their photophysical properties is also reported.

A series of cationic phosphonites, all sharing a TADDOL skeleton but decorated with different positively charged substituents at phosphorus, were synthesized and tested as chiral ancillary ligands on the Au-catalyzed intramolecular hydroarylation of appropriate diynes toward carbo[6]­helicenes with different substitution patterns. Our studies showed that the Au complexes derived from phosphonites bearing 1,3-dimesityl-1,2,3-triazolium and 1,4-dimesityl-1,2,4-triazolium substituents are the best precatalysts for the desired cyclization in terms of regio- and enantioselectivity of the products obtained. In contrast, all of our attempts to prepare Au complexes from cationic phosphonites derived from CAACs failed, and only ligand decomposition products could be isolated.

The synthesis of a series of 5-(alkynyl) dibenzothiophenium triflates, prepared from dibenzo[b,d]thiophene 5-oxide and the corresponding trimethylsilyl-substituted alkynes is reported. Their structures were determined by X-ray crystallography, and their reactivities as electrophilic alkynylation reagents evaluated. Their broad substrate scope and functional-group tolerance illustrate their potential to become an alternative to the broadly used EBX reagents. Isotope labeling studies reveal that alkynyldibenzothiophenium salts may undergo attack by nucleophiles at either the α- or β-carbon atom depending on the nature of their substitution pattern. Subsequent elimination of the dibenzothiophene unit and 1,2-migration of one of the groups (in case of β-attack) affords the desired alkynes.

S -Aryl dibenzothiophenium salts, obtained through a highly regioselective C–H sulfenylation of o -benzyl-protected phenols, are used as precursors of 6 H -benzo[ c ]chromenes. The reaction starts with a photocatalytically triggered single-electron transfer to the sulfonium salt, which promotes the formation of an aryl radical via selective mesolitic cleavage of the S–Ar exo bond. Mechanistic studies reveal that this initial radical species cyclizes following a kinetically favored 5-exo-trig pathway. Subsequent ring expansion, favored by rearomatization, delivers the desired tricyclic systems.

The one-pot synthesis of a series of sulfonium salts containing transferable diazomethyl groups is described, and the structure of these compounds is elucidated by X-ray crystallography. Under photochemical conditions, reaction of these salts with N,N-dialkyl hydrazones affords 1-(dialkylamino)-1,2,3-triazoles via diazomethyl radical addition to the azomethine carbon followed by intramolecular ring closure. The straightforward transformation of the structures thus obtained into mesoionic carbene-metal complexes is also reported and the donor properties of these new ligands characterized.

A series of original spiropyrrolizidine derivatives has been prepared by a one-pot three-component [3 + 2] cycloaddition reaction of (E)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones, l-proline, and the cyclic ketones 1H-indole-2,3-dione (isatin), indenoquinoxaline-11-one and acenaphthenequinone. We disclose an unprecedented isomerization of some spiroadducts leading to a new family of spirooxindolepyrrolizidines. Furthermore, these cycloadducts underwent retro-1,3-dipolar cycloaddition yielding unexpected regioisomers. Upon treatment of the dipolarophiles with in situ generated azomethine ylides from l-proline or acenaphthenequinone, formation of spiroadducts and unusual polycyclic fused piperazines through a stepwise [3 + 3] cycloaddition pathway is observed. The stereochemistry of these N-heterocycles has been confirmed by several X-ray diffraction studies. Some of these compounds exhibit extensive hydrogen bonding in the crystalline state. To enlighten the observed regio- and stereoselectivity of the [3 + 2] cycloaddition, calculations using the DFT approach at the B3LYP/6-31G(d,p) level were carried out. It was found that this reaction is under kinetic control.

Three new compounds, colletotrichones A-C (1-3), and one known compound, chermesinone B (4a), were isolated from an endophytic fungus, Colletotrichum sp. BS4, harbored in the leaves of Buxus sinica, a well-known boxwood plant used in traditional Chinese medicine (TCM). Their structures were determined by extensive spectroscopic analyses including 1D and 2D NMR, HRMS, ECD spectra, UV, and IR, as well as single-crystal X-ray diffraction, and shown to be azaphilones sharing a 3,6a-dimethyl-9-(2-methylbutanoyl)-9H-furo[2,3-h]isochromene-6,8-dione scaffold. Owing to the remarkable antibacterial potency of known azaphilones coupled to the usage of the host plant in TCM, we evaluated the antibacterial efficacy of the isolated compounds against two commonly dispersed environmental strains of Escherichia coli and Bacillus subtilis, as well as against two human pathogenic clinical strains of Staphylococcus aureus and Pseudomonas aeruginosa. Compound 1 exhibited marked antibacterial potencies against the environmental strains that were comparable to the standard antibiotics. Compound 3 was also active against E. coli. Finally, compound 2a exhibited the same efficacy as streptomycin against the clinically relevant bacterium S. aureus. The in vitro cytotoxicity of these compounds on a human acute monocytic leukemia cell line (THP-1) was also assessed. Our results provide a scientific rationale for further investigations into endophyte-mediated host chemical defense against specialist and generalist pathogens.