Diastereoselective Access to Triazolo[1,2-a]indolines via a Bio-Inspired Oxidative Cyclization of NH-Indoles

Establishing three-dimensional chemicals by using the C₂-C₃ π bond of indoles has always been a research hotspot in organic synthesis; however, employing the oxidative C₂-C₃ π bond of indoles to generate imine which would lead to the N₁-C₂ π bond cyclization under metal-free conditions is still rare. Here, we report a bio-inspired synthesis of triazolo[1,2-a]indolines by the oxidative cyclization between NH-indoles and azomethine imines with 3,3-dimethyldioxirane as the sole oxidant under metal-free and mild conditions. This finding represents an elegant instance of tri-functionalization of NH-indoles, which provides rapid access to a broad range of triazolo[1,2-a]indolines with tetrahydroisoquinolines in one single step. Up to 86% yield and above 20:1 dr value are observed. The radical mechanism and proton migration process have been speculated.

Copper-catalyzed aerobic oxidative radical alkoxycyclization of tryptamines to access 3-alkoxypyrroloindolines

We report a copper-catalyzed alkoxycyclization of tryptamine derivatives with O₂ as the sole oxidant, leading to a variety of C3a-alkoxypyrroloindolines in good yields with high diastereoselectivities. This reaction involves an interesting double catalytic cycle in which copper-catalyzed carboamination cyclization is favored to form the C-3 radical pyrrolidinoindoline intermediate, then a copper-catalytic radical alkoxylation reaction proceeds smoothly.

Bromotryptamine and Imidazole Alkaloids with Anti-inflammatory Activity from the Bryozoan Flustra foliacea

Chemical investigation of the marine bryozoan Flustra foliacea collected in Iceland resulted in isolation of 13 new bromotryptamine alkaloids, flustramines Q-W (1-7) and flustraminols C-H (8-13), and two new imidazole alkaloids, flustrimidazoles A and B (14 and 15), together with 12 previously described compounds (16-27). Their structures were established by detailed spectroscopic analysis using 1D and 2D NMR and HRESIMS. Structure 2 was verified by calculations of the 13C and 1H NMR chemical shifts using density functional theory. The relative and absolute configurations of the new compounds were elucidated on the basis of coupling constant analysis, NOESY, [α]D, and ECD spectroscopic data, in addition to chemical derivatization. The compounds were tested for in vitro anti-inflammatory activity using a dendritic cell model. Eight compounds (1, 3, 5, 13, 16, 18, 26, and 27) decreased dendritic cell secretion of the pro-inflammatory cytokine IL-12p40, and two compounds (4 and 14) increased secretion of the anti-inflammatory cytokine IL-10. Deformylflustrabromine B (27) showed the most potent anti-inflammatory effect (IC50 2.9 μM). These results demonstrate that F. foliacea from Iceland expresses a broad range of brominated alkaloids, many without structural precedents. The potent anti-inflammatory activity in vitro of metabolite 27 warrants further investigations into its potential as a lead for inflammation-related diseases.

Procedure-Economical, Enantioselective Total Syntheses of Polycyclic Natural Products and Analogues Containing a 3a-Hydroxyhexahydropyrrolo[2,3-b]indole-2-carboxylic Acid Residue

The 3a-hydroxyhexahydropyrrolo[2,3-b]indole-2-carboxylic acid (HPIC) residue and its aza-analogue are found in many bioactive natural products. In this account, short divergent total syntheses of several such natural products, diastereomers and analogues are described. It is demonstrated that by appropriate combination of different efficient tactics such as biomimetic/bio-inspired synthesis, chemo/regioselective reactions, umpolung of regioselectivity and/or reactivity, and tandem reactions, the enantioselective syntheses of polycyclic molecules such as (+)-asperlicin E and (–)-robustanoids A and B can be achieved in a protecting-group-free and redox-economical manner, in only three to four steps starting from l-tryptophan.

1 Introduction

2 Strategic Considerations

2.1 Occurrence of HO-HPIC and HO-aza-HPIC Residues in Natural Products

2.2 Biosyntheses of HO-HPIC and HO-aza-HPIC Residues

2.3 Chemical Syntheses of HO-HPIC and HO-aza-HPIC Residues

3 Procedure-Economical Syntheses of HO-HPIC-Containing Natural Products

3.1 Protecting-Group-Free Syntheses of Asperlicin E, Its Diastereomer, and an Analogue

3.2 Divergent Syntheses of (–)-Robustanoids A and B, a Diastereomer, and Analogues

4 Conclusion and Future Perspectives

Applanaic acids A-C, three new highly oxygenated lanostane triterpenoids from the fruiting bodies of Ganoderma applanatum

Three new highly oxygenated lanostane triterpenoids, applanaic acids A-C (1-3), were isolated from the fruiting bodies of the basidiomycete Ganoderma applanatum. Among them, applanaic acid B (2) possessed the Δ¹⁷⁽²⁰⁾-double bond connection between the side chain and the tetracyclic skeleton, which was not common in the natural lanostane triterpenoids. Their structures were determined by 1D, 2D NMR and HRESIMS spectroscopic analysis. Compound 3 showed a weak acetylcholinesterase (AchE) inhibitory activity with 33.5% inhibition rate at 50 μM.[Figure: see text].

Biomimetic Enantioselective Total Synthesis of (-)-Robustanoids A and B and Analogues

We report a step-economical, enantioselective total synthesis of (-)-robustanoid B and (-)-robustanoid A and four novel natural product-like compounds. Our strategy relied on our biosynthetic hypothesis and on a novel complexity generation methodology, namely, the one-pot hydroxylative double cyclization reaction. The latter consists of a modified 3,3-dimethyldioxirane-triggered epoxidation-epoxide-ring-opening cyclization reaction cascade and Trost's regioselectivity umpolung methodology ("anti-Michael addition").