Daphnillonins A and B: Alkaloids Representing Two Unknown Carbon Skeletons from Daphniphyllum longeracemosum

Synthesis of the ABC Core of Daphniphyllum Alkaloids with a [5-6-7] Azatricyclic Scaffold via Ring Expansion of Azabicyclic and Azatricyclic Building Blocks

The [5-6-7] azatricyclic ABC core, found in several Daphniphyllum alkaloids, has been synthesized through a novel route involving ring expansion of a perhydroindolone to afford the AC ring system and a radical B ring closure as key steps. The level of functionalization of the reported octahydro-1,7-ethanocyclohepta[b]pyrroles suggests that they can serve as valuable building blocks in this alkaloid field. Also reported is the first synthesis of homomorphans by the ring enlargement of 2-azabicyclo[3.3.1]nonanes.

Gold-Catalyzed Oxidative Rearrangement Strategy to Yield 2-Hydroxycyclohepta-1,3-diene-1-carbonyl Compounds

A gold-catalyzed oxidative rearrangement of propargyl alcohols, derived from commercially available cyclohex-2-en-1-ones and alkynes, was successfully developed for the efficient synthesis of seven-membered rings. Thorough investigations were conducted to optimize the reaction conditions and evaluate its compatibility with various functional groups. Additionally, this methodology was applied to the formal total synthesis of guanacastepene A, demonstrating its practical utility in complex natural product synthesis. This versatile and efficient approach opens up new possibilities for the construction of diverse seven-membered ring systems, providing valuable building blocks for further exploration in drug discovery and the synthesis of intricate molecules.

Total Synthesis of (±)- and (-)-Daphnillonin B

The first total synthesis of (±)- and (-)-daphnillonin B, a daphnicyclidin-type alkaloid with a new [7-6-5-7-5-5] A/B/C/D/E/F hexacyclic core, has been achieved. The [6-5-7] B/C/D ring system was efficiently and diastereoselectively constructed via a mild type I intramolecular [5+2] cycloaddition, followed by a Grubbs II catalyst-catalyzed radical cyclization. The [5-5] fused E/F ring system was synthesized via a diastereoselective intramolecular Pauson-Khand reaction. Notably, the synthetically challenging [7-6-5-7-5-5] hexacyclic core was reassembled by a unique Wagner-Meerwein-type rearrangement from the [6-6-5-7-5-5] hexacyclic framework found in calyciphylline A-type Daphniphyllum alkaloids.

Dolabrane-Type Diterpenoids with Immunosuppressive Activity from Koilodepas hainanense

Thirteen new dolabrane-type diterpenoids, koilodenoids A-M (1-13), including a chlorinated congener (2), along with six known analogues, were isolated from Koilodepas hainanense. The structures were determined by analysis of spectroscopic data, ECD spectra, and X-ray crystallographic studies. The absolute configuration of C-15 in the 15,16-diol unit of compounds 4 and 5 was established by using the dimolybdenum tetraacetate [Mo₂(AcO)₄]-induced ECD method. Compounds 4, 7, 16, 17, and 19 showed moderate to significant immunosuppressive activities against the proliferation of T and B lymphocytes in vitro, with compound 16 being the most potent (IC₅₀ 0.86 and 0.29 μM, respectively).

Survey of natural products reported by Asian research groups in 2020

The new natural products reported in 2020 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2020 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Narjatamolide, an Unusual Homoguaiane Sesquiterpene Lactone from Nardostachys jatamansi

Narjatamolide (1), an unusual homoguaiane sesquiterpene lactone, was isolated from the roots and rhizomes of Nardostachys jatamansi DC. It represents the new carbon skeleton of a homoguaiane sesquiterpenoid possessing an additional acetate unit spiro-fused with C-4 and C-15 to form a cyclopropane ring. The structure of 1 was elucidated by extensive spectroscopic analyses, and the absolute configuration was confirmed by the electronic circular dichroism (ECD) calculations and X-ray single-crystal diffraction analysis. Compound 1 showed antiproliferative effects against BEL-7402 cell lines with an IC₅₀ value of 5.67 ± 1.43 μM, and the mechanism study showed that 1 induces cell cycle of BEL-7402 cell lines arrest at G2/M phase.

Discovery of four modified classes of triterpenoids delineated a metabolic cascade: compound characterization and biomimetic synthesis

Chemical studies on Dichapetalum gelonioides have afforded 18 highly modified complex triterpenoids belonging to four compound classes as defined by the newly adapted functional motifs associated with the A ring of the molecules. Their structures were determined by solid data acquired by diverse methods. The biosynthetic pathway for the four compound classes was rationalized via cascade modifications involving diverse chemical events. The subsequent biomimetic syntheses afforded all the desired products, including compounds 16 and 19 that were not obtained in our purification, which validated the proposed biosynthetic pathway. Besides, some compounds exhibited strong cytotoxic activities, especially 2 and 4 showed nanomolar potency against the NAMALWA tumor cell line, and a gross structure–activity relationship (SAR) of these compounds against the tested tumor cell lines was delineated.

Characterization of four classes of highly modified triterpenoids from Dichapetalum gelonioides sheds light on an unprecedented biosynthetic cascade, which was validated by the subsequent biomimetic syntheses. Moreover, some isolates exhibited nanomolar cytotoxic activities.

The chemistry of Daphniphyllum alkaloids

The triterpenoids Daphniphyllum alkaloids share the unique fused hexacyclic ring framework are isolated from the genus Daphniphyllum. These natural products possess comprehensive biological activities and exhibit excellent potential medicinal appliment. This review covers the reported isolation studies and biological activities of Daphniphyllum alkaloids spanning the period from 1966 to the beginning of 2020, In the meantime, the total synthesis of Daphniphyllum alkaloids will be emphatically summarized for supplement over this review series.

Bungsteroid A: One Unusual C34 Pentacyclic Steroid Analogue from Zanthoxylum bungeanum Maxim

Bungsteroid A (1), possessing an unreported carbon skeleton, was isolated from the pericarps of Zanthoxylum bungeanum Maxim. It represents the first carbon skeleton of a C₃₄ steroid analogue featuring a unique 6/6/6/6/5-fused pentacyclic skeleton, which has been determined by spectroscopic methods, quantum-chemical ¹³C NMR, ECD calculations, and calculations of optical rotations. Bungsteroid A showed the antiproliferative effects against HepG2, MCF-7, and HeLa cell lines with the IC₅₀ values of 56.3 ± 1.1, 64.2 ± 0.9, and 74.2 ± 1.3 μM, respectively.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as flavipeside A from Aspergillus flavipes.