A pyridinium anionic ring-opening reaction applied to the stereodivergent syntheses of Piperaceae natural products

Reductive Zincke Reaction: Opening of Pyridinium Rings to δ-Amino Ketones via Transfer Hydrogenation

The Zincke reaction and Birch reduction have been one of the few reactions that allow for ring opening of pyridines ever since the discovery of pyridine more than a century ago. This paper presents a new addition to the list of pyridine ring-opening reactions, reductive Zincke reaction, which affords saturated δ-amino ketones. Under the catalysis of a simple rhodium complex, pyridinium salts with diverse substituents are reduced with formic acid, ring-opened with water, transaminated with a secondary amine and further reduced to afford a wide range of δ-amino ketones, including those in which the alkane chain of the ketones is selectively deuterated or fluorinated. The applicability of the reaction is exemplified by the synthesis of drug analogues and late-stage modification of drug molecules.

Unexpected amine-triggered skeletal modification of fascaplysin and its derivatives: rapid access to δ,γ-biscarbolines

An unsuccessful attempt to directly construct alkaloid iheyamine A from a fascaplysin framework resulted in the discovery of a novel synthetic approach to alkylated δ,γ-biscarboline derivatives. With the easy-to-operate reaction conditions of this protocol, 18 alkylated biscarbolines have been prepared in moderate to good yields. An unexpected domino transformation was proposed to involve a pyridinium ring-opening/aza-electrocyclization/oxidative deformylation sequence.

Photoinduced stereoselective reactions using pyridinium salts as radical precursors

Pyridinium salts are amine surrogates that are abundant in nature and the redox active nature of the pyridinium salts allows them to serve as precursors for generating radical species under mild conditions that can be initiated by light, heat or metal catalysis. The stereoselective formation of products has always been a topic of interest for synthetic chemists worldwide. In this context, pyridinium salts can readily undergo single electron reduction to form a neutral radical, and the N-X bond's subsequent fragmentation furnishes the X radical without any harsh reaction conditions. As a consequence, the past decade has witnessed an increased effort in utilizing pyridinium salts to photocatalytically generate radicals for the regioselective, diastereoselective as well as enantioselective formation of products that have been summarised in this review.

Anti-inflammatory activity and cytotoxicity against ovarian cancer cell lines by amide alkaloids and piperic esters isolated from Piper longum fruits: In vitro assessments and molecular docking simulation

Three new amide alkaloids, piperlongumamides D-F (14, 19, and 32); a new piperic ester, piperlongumester A (45); and two new natural compounds, methyl (2E,4Z)-5-(1,3-benzodioxol-5-yl)penta-2,4-dienoate (46) and trans-piperolein B ester (47), along with 41 known compounds were isolated from the fruits of Piper longum L. Their structures were identified by analyzing spectroscopic data, including mass spectrometry, 1D, and 2D NMR data. The anti-inflammatory and cytotoxic activities of all isolated compounds (1-47) were evaluated. Compounds 3, 6, and 19 inhibited nitric oxide production with IC₅₀ values of 16.1 ± 0.94, 14.5 ± 0.57, and 27.3 ± 1.11 μM, respectively, whereas compound 1 exhibited strong cytotoxic activity toward three ovarian cancer cell lines A2780, TOV-112D, and SK-OV3, with IC₅₀ values of 6.7 ± 0.77, 5.8 ± 0.29, and 48.3 ± 0.40 μM, respectively. Molecular docking simulations were performed to identify the interaction and binding mechanisms of these active metabolites with proteins related to inflammation and cancer.

On the Structure of Thailandene A: Synthetic Examination of the Cryptic Natural Product Aided by a Theoretical Approach

Phenotype-guided transposon mutagenesis has emerged as a valuable tool to access cryptic metabolites encoded in bacterial genomes. Recently, the method was demonstrated by inducing silent biosynthetic gene clusters in Burkholderia thailandensis. Amongst the isolated metabolic products, thailandene A exhibited promising antibiotic activity. By assignment, the linear polyenic aldehyde contained a labile motif, where an ostensible chiral secondary alcohol was interlaced in an allylic and a homoallylic constellation. Our attention was drawn to the pseudo-symmetric relationship between the heterofunctionalities, indicating the transformation of a dodecapentaenedial scaffold. Centering on an iterative cross-coupling protocol, the assigned all-E-(12R)-hydroxydodecapentaenal moiety was assembled by combining Zincke chemistry with the MIDA-attenuated Suzuki reaction developed in the Burke laboratory. Thus, according to the devised strategy, the mixed 1,2-bisborylated vinyl linchpin was consecutively functionalized with 5-bromodienal derivatives in a doubly orthogonal fashion. However, when the synthetic material was matched against the bacterial isolate, inconsistencies were observed. A re-examination of the cryptic natural product was conducted by juxtaposing analytical data from experiment and density functional theory calculations, in which hydroperoxide was evaluated as a candidate metabolite present in the bacterial isolate.