A unified strategy for enantioselective total synthesis of cladiellin and briarellin diterpenes: total synthesis of briarellins E and F and the putative structure of alcyonin and revision of its structure assignment

Total Synthesis of (+)-Aberrarone

The structurally intriguing diterpene (+)-aberrarone has been assembled in only 12 steps from the commercially available (S,S)-carveol without protecting group manipulations. This concise synthesis features a Cu-catalyzed asymmetric hydroboration to generate the chiral methyl group, a Ni-catalyzed reductive coupling to link two fragments, and a Mn-mediated radical cascade cyclization to construct the triquinane system.

Trends of antimalarial marine natural products: progresses, challenges and opportunities

This review provides an overview of the antimalarial marine natural products, focusing on their chemistry, malaria-related targets and mechanisms, and highlighting their potential for drug development.

Structural characterisation of natural products by means of quantum chemical calculations of NMR parameters: new insights

In this review, we focus in all aspects of NMR simulation of natural products, from the fundamentals to the new computational toolboxes available, combining advanced quantum chemical calculations with upstream data processing and machine learning.

The Discreet Structural Diversity of Briarellins: DU8+ Guided Multiple Structure Revisions Yielded Two Unknown Structural Types

Briarellins, a subset of C2-C11 cyclized cembranoids, were proposed to contain a C3-C14 ether or lactone bridge, similar to asbestinins. However, the total synthesis of the proposed structure of briarellin J revealed a misassignment. We revisited briarellins, computationally, with the help of a recently developed hybrid DFT/parametric method, DU8+, and revised the structures of briarellin C14-C3 ε-lactones to new structural types containing either a C14-C11 or C14-C12 lactone bridge. The original structures of briarellin and asbestinin ethers were confirmed.

Construction of a 6/5/9-membered tricyclic structure of cladiellins via radical-polar crossover reaction

A three-component coupling reaction of structurally simple 6-8 was successfully applied for expeditious synthesis of the 6/5/9-membered tricyclic structure 3 of cladieunicellin D (1) and klysimplexin U (2). Upon treatment with the Et₃B/O₂ reagent system, α-alkoxyacyl telluride 6, six-membered enone 7, and (Z)-4-hexenal (8) were linked in one pot to provide the densely functionalized 5 via sequential decarbonylative radical generation, radical addition, boron enolate formation, and intermolecular aldol reaction. Subsequent Lewis acid-promoted reductive etherification and SiO₂-induced C10-epimerization gave rise to the cis-fused five-membered ether of 4. Finally, cyclization of the nine-membered ring was achieved by the ring-closing metathesis reaction, giving rise to 3. Compound 3 possesses the six stereocenters of 1 and 2, and would thus serve as an advanced intermediate for their total syntheses.

Recent Advances of Total Syntheses of Diterpenoids Starting from Carvone

This review focuses on total syntheses of diterpenoids starting from carvone (1) since 2006 in the alphabetical order of compound names.

Synthesis and Evaluation of a 2,11-Cembranoid-Inspired Library

The 2,11-cembranoid family of natural products has been used as inspiration for the synthesis of a structurally simplified, functionally diverse library of octahydroisobenzofuran-based compounds designed to augment a typical medicinal chemistry library screen. Ring-closing metathesis, lactonisation and SmI2 -mediated methods were exemplified and applied to the installation of a third ring to mimic the nine-membered ring of the 2,11-cembranoids. The library was assessed for aqueous solubility and permeability, with a chemical-space analysis performed for comparison to the family of cembranoid natural products and a sample set of a screening library. Preliminary investigations in cancer cells showed that the simpler scaffolds could recapitulate the reported anti-migratory activity of the natural products.

Stereospecific total synthesis of the indole alkaloid ervincidine. Establishment of the C-6 hydroxyl stereochemistry

The total synthesis of the indole alkaloid ervincidine (3) is reported. This research provides a general entry into C-6 hydroxy-substituted indole alkaloids with either an α or a β configuration. This study corrects the errors in Glasby’s book (GlasbyJ. S.Encyclopedia of the Alkaloids; Plenum Press: New York, 1975) and Lounasmaa et al.’s review (LounasmaaM.; HanhinenP.; WestersundM. In The Alkaloids; CordellG. A., Ed.; Academic Press: San Diego, CA, 1999; Vol. 52, pp 103–195) as well as clarifies the work of Yunusov et al. (MalikovV. M.; SharipovM. R.; YunusovS. Yu.Khim. Prir. Soedin.1972, 8, 760−761. RakhimovD. A.; SharipovM. R.; AripovKh. N.; MalikovV. M.; ShakirovT. T.; YunusovS. Yu.Khim. Prir. Soedin.1970, 6, 724–725). It establishes the correct absolute configuration of the C-6 hydroxyl function in ervincidine. This serves as a structure proof and corrects the misassigned structure reported in the literature.

Strategies towards the synthesis of calyciphylline A-type Daphniphyllum alkaloids

The Daphniphyllum alkaloids are a diverse family of natural products rich in number and structural diversity that have been known for many decades. However, the structurally unique subclass of calyciphylline A-type alkaloids has only recently been discovered and is relatively unexplored. Several noteworthy core syntheses and the development of a wide range of novel synthetic strategies have been achieved. This includes strategies based on intramolecular Michael addition, Pd-catalysis, cycloaddition, and Mannich-type reactions. This review will provide an overview of these synthetic studies.

Identification of an Unexpected 2-Oxonia[3,3]sigmatropic Rearrangement/Aldol Pathway in the Formation of Oxacyclic Rings. Total Synthesis of (+)-Aspergillin PZ

This paper reports the first unambiguous evidence that the cascade synthesis of tetrahydrofuran-containing oxacyclic molecules depicted in Scheme 12 can take place by a 2-oxonia[3,3]sigmatropic/aldol mechanism rather than by a Prins cyclization/pinacol rearrangement sequence. The 8-oxabicyclo[3.2.1]octyl aldehyde products of this reaction, 20 and 29, were employed to complete the first total synthesis of the structurally remarkable isoindolone alkaloid (+)-aspergillin PZ (1). The lack of activity seen in two tumor cell lines for synthetic (+)-aspergillin PZ calls into question the suggestion that aspergillin PZ, like many aspochalasin diterpenes, might exhibit useful antitumor properties.

Survey of marine natural product structure revisions: a synergy of spectroscopy and chemical synthesis

The structural assignment of new natural product molecules supports research in a multitude of disciplines that may lead to new therapeutic agents and or new understanding of disease biology. However, reports of numerous structural revisions, even of recently elucidated natural products, inspired the present survey of techniques used in structural misassignments and subsequent revisions in the context of constitutional or configurational errors. Given the comparatively recent development of marine natural products chemistry, coincident with modern spectroscopy, it is of interest to consider the relative roles of spectroscopy and chemical synthesis in the structure elucidation and revision of those marine natural products that were initially misassigned. Thus, a tabulated review of all marine natural product structural revisions from 2005 to 2010 is organized according to structural motif revised. Misassignments of constitution are more frequent than perhaps anticipated by reliance on HMBC and other advanced NMR experiments, especially when considering the full complement of all natural products. However, these techniques also feature prominently in structural revisions, specifically of marine natural products. Nevertheless, as is the case for revision of relative and absolute configuration, total synthesis is a proven partner for marine, as well as terrestrial, natural products structure elucidation. It also becomes apparent that considerable 'detective work' remains in structure elucidation, in spite of the spectacular advances in spectroscopic techniques.

The 2,11-cyclized cembranoids: cladiellins, asbestinins, and briarellins (period 1998-2010)

The 2,11-cyclized cembranoids are isolated from marine invertebrates of Octocorallia species. They are a very interesting class of natural products sharing a common oxatricyclo[6.6.1.0(2,7)]pentadecane core and carrying a varied substituent pattern. This review presents their structural diversity along with the reported biological activities. The 2,11-cyclized cembranoids were comprehensively reviewed previously in 1998, and this contribution will serve as an update of that work. Since 1998 a number of structural assignments of the isolated products have been revised, some as a result of total synthesis efforts. The chemical reactivity of several of the natural compounds has been studied, and the relevance of these findings to the biosynthesis or the generation of isolation artifacts is discussed. The wide range of biological activities displayed by the 2,11-cyclized cembranoids justifies the interest shown within the synthetic chemistry community and suggests that this class of natural products remains a fruitful area for future synthetic and biological research.

Total synthesis of the proposed structure of briarellin J

The total synthesis of the originally proposed structure of briarellin J is reported in 15 steps from a known compound and in 23 steps from readily available materials. Key reactions include an exo-selective intramolecular Diels-Alder and a substrate-controlled hydroboration. Discrepancies in the spectroscopic data of the synthetic and natural material led to a revision of the assigned structure.

Enantioselective synthesis of (-)-sclerophytin A by a stereoconvergent epoxide hydrolysis

The cytotoxic natural product (-)-sclerophytin A was constructed in 13 steps from geranial. Highlights from the synthesis are a stereoselective Oshima-Utimoto reaction, a Shibata-Baba indium-promoted radical cyclization, and a novel stereoconvergent epoxide hydrolysis.

Synthesis and anticancer activity of sclerophytin-inspired hydroisobenzofurans

Three structurally related sets of hydroisobenzofuran analogs of sclerophytin A were prepared in three or four steps from (S)-(+)-carvone via an aldol-cycloaldol sequence. The most potent members of each set of analogs exhibited IC(50)'s of 1-3 microM in growth inhibitory assays against KB3 cells. The NCI 60-cell line 5-dose assay for analog 6h revealed a GI(50)=0.148 microM and LC(50)=9.36 microM for the RPMI-8226 leukemia cell line, and a GI(50)=0.552 microM and LC(50)=26.8 microM for the HOP-92 non-small cell lung cancer cell line.