Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (-)-Nitidasin

Synthetic study toward the diterpenoid aberrarone

An approach to aberrarone, an antimalarial diterpenoid natural product with tetracyclic skeleton is reported. Key to the stereoselective preparation of the 6-5-5 tricyclic skeleton includes the mediation of Nagata reagent for constructing the C1 all-carbon quaternary centers and gold-catalyzed cyclopentenone synthesis through C–H insertion.

Studies towards the Synthesis of (+)‐Dictyoxetane

The dolabellane-type diterpene dictyoxetane represents a significant challenge to synthetic organic chemistry. Methodology directed towards the total synthesis of naturally occurring (+)-dictyoxetane is reported. Catalytic asymmetric synthesis of the trans-hydrindane ring system is achieved through chemoselective deoxygenation of the Hajos-Parrish ketone. An alternative to the Garst-Spencer furan annulation is developed for the synthesis of a 2,5-dimethyl, tetrasubstituted furan, employing a tandem 5-exo-dig alcohol to alkyne cyclisation/aromatisation reaction as a key step. The (4+3) cycloaddition reaction of an oxyallyl cation with a tetrasubstituted furan is established on a cyclohexanone-derived model system, and a range of related (4+3) cycloadditions investigated on a homochiral, trans-hydrindane-fused furan, where regio- and diastereoselectivity is required for the natural product synthesis. In an alternative (4+2) Diels-Alder approach, a C₂ -symmetric vinyl sulfoxide-based chiral ketene equivalent is used to prepare oxanorbornenes with the same oxygen bridge stereochemistry found in the 2,7-dioxatricyclo[4.2.1.0^3,8 ]nonane ring system of the natural product.

Sequential iodine-mediated diallylsilane rearrangement/asymmetric dihydroxylation: Synthesis and reactions of enantioenriched oxasilacycles

Products from an iodine-mediated diallylsilane rearrangement were taken into an asymmetric dihydroxylation (AD) reaction resulting in the formation of diastereomeric 6-membered oxasilacycles. Removal of the epimeric stereocenter among this mixture of diastereomers by elimination of iodine produced a single enantioenriched cyclic allyl silyl ether. The resulting allyl silane was then successfully engaged in several further transformations, providing an alternative means to prepare useful intermediates for enantioselective synthesis.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Synthesis of Nβ-Substituted α,β-Diamino Acids via Stereoselective N-Michael Additions to a Chiral Bicyclic Dehydroalanine

The highly diastereoselective 1,4-conjugate additions of several nitrogen nucleophiles to chiral bicyclic dehydroalanines have been assessed effectively at room temperature in good to excellent yields without needing any catalyst or additional base. This methodology is general, simple, oxygen and moisture tolerant, high-yielding, totally chemo- and stereoselective. This procedure offers an efficient and practical approach for the synthesis of N_β-substituted α,β-diamino acids, such as 1-isohistidine, τ-histidinoalanine, β-benzylaminoalanine, β-(piperidin-1-yl)alanine, β-(azepan-1-yl)alanine, and fluorescent and ciprofloxacin-containing amino acid derivatives.

Vinylogous acyl triflates as an entry point to α,β-disubstituted cyclic enones via Suzuki-Miyaura cross-coupling

An alternative protocol for the B-alkyl Suzuki-Miyaura reaction to produce cyclic α,β-disubstituted enones is reported. The use of β-triflyl enones as coupling partners in lieu of their halogenated analogs provides enhanced substrate stability to light and chromatography without adversely affecting reactivity. This protocol allows efficient access to the synthetically challenging α,β-disubstituted enone motif under mild conditions.

Total synthesis of sesterterpenoids

Total syntheses of biologically and structurally fascinating sesterterpenoids published between Jan. 2012 and Jan. 2018 are summarized and discussed here.

Asymmetric total synthesis of (+)-astellatol and (−)-astellatene

Here we describe the full account of the total synthesis of (+)-astellatol, as well as the first total synthesis of (−)-astellatene.

Highly oxygenated isoprenoid lipids derived from fungi and fungal endophytes: Origin and biological activities

This mini review is devoted to highly oxygenated isoprenoid lipids (HOIL) that are produced by fungi and fungal endophytes from various ecological niches, both terrestrial and aquatic. Steroids were distributed as from edible cultivated fungi, as well as fungi collected in forests. Fungal endophytes were generally isolated from plants and cultured to obtain sufficient biomass. Marine fungi were obtained from marine brown and red algae and marine invertebrates such as sponges, corals, worms, crustacea or from marine sediments. HOIL isolated from the terrestrial ecosystem have the pharmacological potential on anti-hypercholesterolemic, anti-neoplastic, anti-eczematic and anti-inflammatory activity estimated with a confidence of 84-90%. HOIL that produced by marine fungal species are predicted as having anti-inflammatory and anti-hypercholesterolemic activity with a confidence of 82-91%. In addition, they may have potential acetylcholinesterase and cell adhesion molecule inhibitors estimated with a confidence of 86-88%.

Stereoselective Synthesis of the ABC Ring System of Aspterpenacids

Aspterpenacids A and B are sesterterpenoids that possess a unique and highly congested 5/3/7/6/5 fused ring system. These compounds also contain a sterically encumbered isopropyl trans-hydrindane motif and a cyclopropane motif bearing two quaternary centers, which make them remarkably challenging synthetic targets. Herein, we report the successful construction of the key highly substituted ABC ring system in a stereoselective manner.

Total Synthesis of Astellatol

A nearly-30-year-old unanswered synthetic puzzle, astellatol, has been solved in an enantiospecific manner. The highly congested pentacyclic skeleton of this rare sesterterpenoid, which possesses a unique bicyclo[4.1.1]octane motif, ten stereocenters, a cyclobutane that contains two quaternary centers, an exo-methylene group, and a sterically encumbered isopropyl trans-hydrindane motif, makes astellatol arguably one of the most challenging targets for sesterterpenoid synthesis. An intramolecular Pauson-Khand reaction was exploited to construct the right-hand side scaffold of this sesterterpenoid. An unprecedented reductive radical 1,6-addition, mediated by SmI₂ , forged the cyclobutane motif. Last, a strategic oxidation/reduction step provided not only the decisive solution for the remarkably challenging late-stage transformations, but also a highly valuable unravelling of the notorious issue of trans-hydrindane synthesis. Importantly, the synthesis of astellatol showcases a rapid, scalable strategy to access diverse complex isopropyl trans-hydrindane sesterterpenoids.

Bioinspired synthesis of pentacyclic onocerane triterpenoids

The first chemical synthesis of pentacyclic onocerane triterpenoids (+)-cupacinoxepin and (+)-onoceranoxide is described.

The first chemical synthesis of pentacyclic onocerane triterpenoids has been achieved. A putative biomimetic tricyclization cascade is employed to forge a fused decalin-/oxepane ring system. The synthetic route proceeds to (+)-cupacinoxepin in seven steps and to (+)-onoceranoxide in eight steps in the longest linear sequence, when starting from geranyl chloride and (+)-sclareolide. The bioinspired epoxypolyene cyclization is supported by computational and enzymatic studies.

Methodology for the Construction of the Bicyclo[4.3.0]nonane Core

The bicyclo[4.3.0]nonane scaffold, commonly known as a hydrindane, is a common structural motif found in many terpenoid structures and one that remains a challenge for synthetic chemists to elaborate with appropriate regio- and stereo-selectivity. Over the course of the study of terpene natural products, the elaboration of the hydrindane structure has seen progress on the utilization of both old and newer methods to achieve the desired outcomes. This review seeks to serve as a general overview of these methods, and detail specific examples.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Constructing densely functionalized Hajos–Parrish-type ketones with six contiguous stereogenic centers and two quaternary carbons in a formal [2 + 2 + 2] cycloaddition cascade

Efficient construction of Hajos–Parrish-type ketones with six contiguous stereogenic centers in a formal [2 + 2 + 2] cycloaddition cascade. The spontaneous resolution of racemic product without the need for chiral agent has been achieved.