A divergent strategy for the synthesis of secologanin derived natural products

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Total Syntheses of (+)-Villocarine A, (-)-Apogeissoschizine, and (+)-Geissoschizine

Total syntheses of geissoschizine-type monoterpenoid indole alkaloids (MTIAs) are reported. An intramolecular Pictet-Spengler cyclization was developed for the selective construction of the 3R stereocenter. The first total synthesis of (+)-villocarine A was then achieved. Furthermore, the first total synthesis of the highly strained (-)-apogeissoschizine was also accomplished in an aza-Michael cyclization/E1cB elimination/stereoselective olefin isomerization sequence. Finally, (+)-geissoschizine, a common biosynthetic intermediate of MTIAs, was obtained from apogeissoschizine through ring-opening along with a release of ring strain.

Therapeutic Potentials of Secoiridoids from the Fruits of Ligustrum lucidum Aiton against Inflammation-Related Skin Diseases

Ligustrum lucidum Aiton is a flowering plant of the Oleaceae family, and its fruits have been traditionally used for skin nourishment and the treatment of skin diseases. However, the anti-inflammatory constituents for skin disease are not well-characterized. Phytochemical investigation of L. lucidum fruits resulted in the isolation of a new secoiridoid, secoligulene (1), together with (E)-3-(1-oxobut-2-en-2-yl)pentanedioic acid (2) and trans-(E)-3-(1-oxobut-2-en-2-yl)glutaric acid (3). Secoligulene (1) displayed the potent inhibitory effect on NO production with an IC₅₀ value of 12.0 μg/mL. Secoligulene (1) also downregulated mRNA transcriptional levels of pro-inflammatory cytokines such as IL-1 α, IL-1β, IL-6 and COX-2 in LPS-stimulated RAW264.7 cells. Further investigation showed that secoligulene (1) inhibited the phosphorylation of IκB and JNK activated by LPS. In addition, secoligulene (1) downregulated the expression of chemokines such as CXCL8 and CCL20 in the TNF-α/IL-17/IFN-γ induced HaCaT psoriasis model. Taken together, these findings support the beneficial effects of L. lucidum and its constituents on inflammation-related skin diseases and can be further developed as therapeutic treatments for related diseases.

A synthetic route to 1,4-disubstituted tetrahydro-β-carbolines and tetrahydropyranoindoles via ring-opening/Pictet–Spengler reaction of aziridines and epoxides with indoles/aldehydes

A route to 1,4-disubstituted tetrahydro-β-carbolines and tetrahydropyrano[3,4-b]indoles in high yields and stereoselectivity via S_N2-type ring opening/Pictet–Spengler reaction of aziridines and epoxides with indoles is described.

Unveiling the presence of biosynthetic pathways for bioactive compounds in the Thalassiosira rotula transcriptome

Diatoms are phytoplankton eukaryotic microalgae that are widely distributed in the world’s oceans and are responsible for 20–25% of total carbon fixation on the planet. Using transcriptome sequencing here we show for the first time that the ubiquitous diatom Thalassiosira rotula expresses biosynthetic pathways that potentially lead to the synthesis of interesting secondary metabolites with pharmaceutical applications such as polyketides, prostaglandins and secologanin. We also show that these pathways are differentially expressed in conditions of silica depletion in comparison with standard growth conditions.

Ring-Fused Cyclic Aminals from Tetrahydro-β-carboline-Based Dipeptide Compounds

An acid- and oxidant-promoted intramolecular cyclization of a tetrahydro-β-carboline-based dipeptide has been developed to prepare new indole-fused aminoacetals. This approach involves N-acyliminium formation from readily available precursors and cyclization under mild reaction conditions. The diastereoselectivity in the formation of the products is influenced by the specific substituents of the starting reagents, which has been rationalized analyzing the energy profile of the related reactions and the relative stability of the proposed structures based on DFT computational methods.

Total Synthesis of (-)-Geissoschizol through Ir-Catalyzed Allylic Amidation as the Key Step

The iridium-catalyzed, Zn(OTf)₂ promoted asymmetric allylic amidation of a secondary alcohol derived from tryptamine has been utilized to forge a tetrahydro-β-carboline. Based on this key step, a novel, facile, and enantioselective total synthesis of (-)-geissoschizol was achieved in 10 steps.

A general strategy for the synthesis of indoloquinolizine alkaloids via a cyanide-catalyzed imino-Stetter reaction

A general synthetic strategy applicable to indoloquinolizine alkaloids has been developed using a cyanide-catalyzed imino-Stetter reaction as a key step.

Asymmetric Redox-Annulation of Cyclic Amines

Cyclic amines such as 1,2,3,4-tetrahydroisoquinoline undergo regiodivergent annulation reactions with 4-nitrobutyraldehydes. These redox-neutral transformations enable the asymmetric synthesis of highly substituted polycyclic ring systems in just two steps from commercial materials. The utility of this process is illustrated in a rapid synthesis of (−)-protoemetinol. Computational studies provide mechanistic insights and implicate the elimination of acetic acid from an ammonium nitronate intermediate as the rate-determining step.

Enantioselective synthesis of indoloquinolizidines via asymmetric catalytic hydrogenation/lactamization of imino diesters

We have developed a highly efficient cascade sequence for asymmetric synthesis of indoloquinolizidines with absolute control of cis-H2/H12b relative geometry in good to excellent yields and excellent enantioselectivities. This cascade was triggered by the Ru(II)-TsDPEN-catalyzed asymmetric transfer hydrogenation of imino diesters, with subsequent spontaneous lactamization with discrimination between the two diastereotopic 2-alkoxy-2-oxoethyl groups. The synthetic utility of this strategy was demonstrated by the asymmetric preparation of dihydrocorynantheol, geissoschizol, and isogeissoschizol.

Access to electron-rich arene-fused hexahydroquinolizinones through a gold-catalysis-initiated cascade process

Golden Cascade: With a tethered, electron-rich arene as the internal nucleophile, a gold-catalyzed amide cyclization to an alkyne initiates a cascade process that ends with a Ferrier rearrangement. Electron-rich arene-bearing hexahydroquinolizin-2-ones are formed in good yields and can be converted into indole alkaloids in only a few steps.

Organocatalytic approach for the syntheses of corynantheidol, dihydrocorynantheol, protoemetinol, protoemetine, and mitragynine

O-Trimethylsilyl (TMS)-protected diphenylprolinol-catalyzed Michael addition of a functionalized alkylidene malonate and n-butanal affords an aldehyde. This adduct can serve as the common intermediate for the assembly of secologanin tryptamine and dopamine alkaloids; this is demonstrated by the total syntheses of corynantheidol, dihydrocorynantheol, protoemetinol, and protoemetine, and the formal synthesis of mitragynine. The key steps include reductive amination of this aldehyde with tryptamine, condensation of this aldehyde with 4-methoxytryptamine, condensation of dimethoxyphenethylamine with a lactone derived from this aldehyde, and subsequent Bischler-Napieralski cyclization and reduction.