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A 2018 synthesis reported a four-step route to gram quantities of a fused 5–8–5 carbocyclic scaffold—the three-ring core found in more than 30 di- and sesterterpene natural products. Its key move is a light-driven cycloisomerization that reorganizes a strained intermediate to form the central eight-membered ring. It builds a chemical framework, not a finished drug.
What “carbotricycle” means in this route
Here, the term refers to a fused carbon-ring framework whose rings contain five, eight, and five atoms, respectively. The specific scaffold is dicyclopenta[a,d]cyclooctene, a shared core found in a range of natural products.
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Salvati, Law, Liriano, and Frederich reported the method in “Modular access to functionalized 5–8–5 fused ring systems via a photoinduced cycloisomerization reaction,” published in Chemical Science in 2018. They describe access to gram quantities of the scaffold in four steps. The paper is open access: read the article in Chemical Science.
How the four-step strategy forms the 5–8–5 core
The route starts from a more readily accessible isomer and uses light to initiate a sequence that changes the molecule’s ring connectivity. The authors’ proposed pathway is:
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- Set up the precursor: The initial material is designed to undergo an intramolecular photocycloaddition.
- Build a cyclobutane: Light promotes a [2+2] photocycloaddition, joining two alkene units within the molecule and forming a stereodefined four-membered ring.
- Expand the strained intermediate: The cyclobutane’s accumulated strain enables a Cope rearrangement, which forms the eight-membered ring of the target framework.
- Form the conjugated enone: Isomerization gives the conjugated enone and completes access to the 5–8–5 scaffold.
The paper presents this sequence as the reaction design; it should not be read as proof that every mechanistic detail was directly established. The strategy also installs a C11 quaternary stereocenter as part of building the scaffold.
Why chemists care about this ring system
The 5–8–5 framework is a recurring structural core in more than 30 di- and sesterterpene natural products, according to the authors. Examples illustrate how different groups attached around the shared core can accompany very different biological effects:
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- Fusicoccin A is described as an orthosteric stabilizer of 14–3–3 protein–protein interactions.
- Ophiobolin A is described as a cytotoxin that modulates calmodulin activity.
- Cyclooctatin is described as an inhibitor of lysophospholipases.
These activities belong to the named natural products; they do not show that the isolated synthetic core is a medicine or that any of these compounds has clinical efficacy. The route’s significance is synthetic: it gives chemists a way to reach and functionalize a structurally important framework.
What the four-step result does—and does not—claim
The reported achievement is a concise route to gram quantities of a scaffold. It is not a four-step preparation of a finished therapeutic, nor does the step count alone establish that the method is best for every target. The paper frames the approach as modular and as an alternative to earlier methods; the appropriate comparison depends on the target and reaction conditions, including scale, substituent variation, photochemical requirements, stereochemical control, and any need for metal catalysis or high temperatures.
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