Synthesis of Tricyclo[4,3,1,01,5]decane Core of Plumisclerin A Using Pauson–Khand Annulation and SmI2-Mediated Radical Cyclization

Cytotoxic analogues of marine diterpenoid plumisclerin A by shifting the lipophilic branch on the characteristic tricyclic core

Plumisclerin A is one of the most complex cytotoxic xenicane diterpenes from marine sources, featuring a unique congested and rigid tricyclo[4.3.1.0^1,5]decane core and a lipophilic acyl chain. This work explored a number of new analogues of plumisclerin A through modifying the characteristic tricyclo[4.3.1.0^1,5]decane core with lipophilic chains starting from a common lactone intermediate. Bioactivity examination of all the synthetic analogues shows that new analogues 2a, 18 and 21 exhibited comparable inhibitory potencies to that of the natural product against the proliferation of cancer cells. Structural comparison of these bioactive natural and unnatural compounds reveals that the location of lipophilic substituent(s) on the tricyclo[4.3.1.0^1,5]decane core is spatially flexible, and this work thus offers a new channel to diverse bioactive analogues of plumisclerin A.

Total syntheses of strained polycyclic terpenes

Terpenoids constitute a broad class of natural compounds with tremendous variability in structure and bioactivity, which resulted in a strong interest of the chemical community to this class of natural products over the last 150 years. The presence of strained small rings renders the terpenoid targets interesting for chemical synthesis, due to limited number of available methods and stability issues. In this feature article, a number of recent examples of total syntheses of terpenoids with complex carbon frameworks featuring small rings are discussed. Specific emphasis is given to the new developments in strategical and tactical approaches to construction of such systems.

Application of Pauson-Khand reaction in the total synthesis of terpenes

The Pauson-Khand reaction (PKR) is a formal [2 + 2 + 1] cycloaddition involving an alkyne, an alkene and carbon monoxide mediated by a hexacarbonyldicobaltalkyne complex to yield cyclopentenones in a single step. This versatile reaction has become a method of choice for the synthesis of cyclopentenone and its derivatives since its discovery in the early seventies. The aim of this review is to point out the applications of PKR in the total synthesis of terpenes.

Total synthesis of sesterterpenoids

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

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

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.

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.

Synthesis of Rings DEF of Solanoeclepin A

An improved synthesis of rings DEF of solanoeclepin A has been achieved from ent-Hajos Parrish ketone. A key tricyclo[5.3.2.0^1,6]decene intermediate having an additional vinyl group as a precursor of a hydroxyl functionality was synthesized, in which the key steps included (i) a [2,3]-Wittig rearrangement to provide trans-hydroindene with C11(R)-configuration, (ii) the introduction of a vinyl group as a masked OH at C6, (iii) an oxymercurative aldol to synthesize the tricyclo[5.3.2.0^1,6]decene moiety, (iv) an oxidative C-C bond cleavage to yield an aldehyde and an unsaturated methyl ketone, and (v) a radical cyclization for the cyclobutane ring formation to provide the tricyclo[5.2.1.0^1,6]decene compound.

Synthesis of Xenia diterpenoids and related metabolites isolated from marine organisms

This review describes strategies for the chemical synthesis of xenicane diterpenoids and structurally related metabolites. Selected members from the four different subclasses of the Xenia diterpenoid family, the xenicins, xeniolides, xeniaphyllanes and xeniaethers, are presented. The synthetic strategies are discussed with an emphasis on the individual key reactions for the construction of the uncommon nine-membered carbocycle which is the characteristic structural feature of these natural products. Additionally, the putative biosynthetic pathway of xenicanes is illustrated.