# Biosynthetic rearrangement reactions
Biosynthetic rearrangement reactions are skeletal reorganizations, cationic cascades, and pericyclic sigmatropic shifts (pericyclic reaction where a bond migrates along a molecule) that are carried out inside living systems by enzymes, which generate highly reactive intermediates such as carbocations and then steer them to one dominant product. Class I [terpene synthases](https://www.edgepedia.org/terpene-synthase) do this with a generally α-helical fold that simply ionizes an allylic diphosphate and confines the resulting carbocation; the chorismate mutase [Claisen rearrangement](https://www.edgepedia.org/claisen-rearrangement) achieves its rate acceleration with no covalent catalysis, no acid-base chemistry, and no cofactors at all.[1](https://doi.org/10.1002/anie.202318913) • [2](https://pubs.rsc.org/en/content/articlehtml/2026/np/d5np00066a) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/) [Laboratory](https://www.edgepedia.org/laboratory) cation chemistry, exemplified by the acid-catalyzed isomerization of pinene, is carried out with acidic catalysts and gives wide mixtures of monoterpenoids; the enzyme versions embed the same physical-organic steps inside a binding pocket that preorganizes the substrate and blocks competing trajectories.[1](https://doi.org/10.1002/anie.202318913)
| Key fact | Detail |
|---|---|
| Carbocation generation in terpene synthases | Class I enzymes ionize an allylic diphosphate; class II enzymes protonate an alkene by Brønsted acid catalysis.[1](https://doi.org/10.1002/anie.202318913) |
| Skeleton-building economy | Complex terpene frameworks with multiple rings and stereocenters arise from achiral, ring-free precursors in one or two enzyme-promoted reactions.[4](https://www.beilstein-journals.org/bjoc/articles/12/41) |
| Pericyclase catalysis | Chorismate mutase, the first documented pericyclase, needs no protons, electrons, cofactors, or covalent intermediates; binding alone organizes the substrate into the transition state.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6461539/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/) |
| Near-attack conformation control | Chorismate mutases limit the types of near-attack conformations, in which reacting atoms are within van der Waals contact and within ±15° of the forming-bond angle, so one product dominates.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/) |
| Engineered benchmark | A squalene-hopene cyclase octuple mutant converts pinene to (+)-borneol at 90.2% selectivity (from 1.2%), 99.6% de, on gram scale, without a diphosphate substrate.[1](https://doi.org/10.1002/anie.202318913) |
| Post-2023 revision | Plant cytochrome P450 enzymes can induce terpene backbone rearrangements, including methyl and alkyl group shifts along rings, not just oxidations.[6](https://www.mdpi.com/1420-3049/30/17/3540) |
## Terpene cyclase cationic cascades
**Two entry points** exist for the terpene cation chemistry. Class I terpene synthases, which generally feature an α-helical fold (α-domain), catalyze ionization of the allylic diphosphate substrate to generate the initial carbocation by loss of the diphosphate leaving group; class II enzymes instead initiate by protonation of an alkene via [Brønsted acid catalysis](https://www.edgepedia.org/br-nsted-acid-catalysis).[1](https://doi.org/10.1002/anie.202318913) • [2](https://pubs.rsc.org/en/content/articlehtml/2026/np/d5np00066a)
Once formed, the cation cascades through cyclization, alkyl shift, hydride shift, and/or proton transfer reactions, each step generating a new, more complex carbocation. The cascade ends when the cation is trapped by a nucleophile (water or diphosphate) or is deprotonated to form an alkene.[4](https://www.beilstein-journals.org/bjoc/articles/12/41)
Negative catalysis is the accepted description of that geometric control: terpene synthases are believed to obstruct rearrangement along the more favorable reaction pathways, preventing bifurcation of the reaction trajectory so that a single cascade route wins.[1](https://doi.org/10.1002/anie.202318913) Which fate a given cation meets, rearrangement, elimination, or water quench, is therefore decided by the active-site scaffold, including the placement of water: molecular dynamics on engineered [squalene-hopene cyclases](https://www.edgepedia.org/squalene-hopene-cyclase) shows that reorganization of key aromatic residues restructures the enzyme-anchored water network, guiding the cationic pathway and selectively terminating the secondary isobornyl cation while avoiding premature water quenching of intermediate carbocations.[1](https://doi.org/10.1002/anie.202318913)
The economy of this chemistry explains much terpene structural diversity: complex hydrocarbon frameworks carrying multiple stereocenters and rings derive from simple achiral, ring-free precursors transformed in only one or two enzyme-promoted reactions.[4](https://www.beilstein-journals.org/bjoc/articles/12/41)
## The chorismate mutase Claisen step
**Chorismate mutase** is recognized as the first documented pericyclase: it catalyzes the Claisen rearrangement of chorismate to prephenate in primary metabolism, a [3,3]-sigmatropic shift.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6461539/) The catalytic strategy is unusual in enzymology. No protons or electrons are donated or accepted from the enzyme or cofactors, and there are no covalent intermediates; instead, the binding event organizes the substrate into the transition state, and product formation follows.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/)
Quantitatively, the selectivity comes from near-attack conformation (NAC) selection. A NAC is a structure in which the reacting atoms are within van der Waals contact distance and approach at an angle within ±15° of the bond that is formed; chorismate mutases limit the types of NACs that form, so catalysis proceeds through a particular transition state to a single product.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/) High-level QM/MM calculations indicate that the catalysis arises from a combination of binding the reactive conformation and electrostatic transition-state stabilization, and the computed activation parameters agree well with experiment.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/)
The pericyclase family extends beyond chorismate mutase. Isochorismate-pyruvate lyase (PchB) catalyzes a concerted but asynchronous [1,5]-sigmatropic hydrogen shift from C2 to C9 with \(k_{\mathrm{cat}}/K_{\mathrm{m}}\) of 4.11×104 M−1 s−1, and can also act nonphysiologically as a chorismate mutase, catalyzing the chorismate-to-prephenate Claisen with \(k_{\mathrm{cat}}/K_{\mathrm{m}}\) = 1.96×102 M−1 s−1.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/) Precorrin-8x methyl mutase and dimethylallyltryptophan synthase have also been proposed to involve pericyclic steps, and multiple putative pericyclase-catalyzed 1,3-dipolar cycloadditions have been proposed in recent years, so the category continues to expand.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6461539/)
## Wagner–Meerwein and CYP-induced shifts in natural product skeletons
**1,2-Shifts** of alkyl, methyl, and hydride groups are core steps inside terpene cation cascades, as described above: cyclization, alkyl shift, hydride shift, and proton transfer successively generate new, more complex carbocations that define the final skeleton.[4](https://www.beilstein-journals.org/bjoc/articles/12/41)
A recent addition is that cytochrome P450 enzymes in plants do not simply oxidize terpene backbones; in some cases they induce backbone rearrangements, including C–C scission, ring contraction, transannular cyclization, and methyl or alkyl group shifts along rings, across mono-, sesqui-, di-, and triterpenes.[6](https://www.mdpi.com/1420-3049/30/17/3540) Group shifts of this kind were observed only most recently; for the abietane diterpene backbone, a hypothetical [Wagner–Meerwein rearrangement](https://www.edgepedia.org/wagner-meerwein-rearrangement) has been invoked to explain the C18-to-C3 methyl shift.[6](https://www.mdpi.com/1420-3049/30/17/3540)
## How it compares with laboratory rearrangements
**Selectivity is the clearest difference.** The acid-catalyzed isomerization of pinene, representative of laboratory cation chemistry, is catalyzed with low selectivity by common Brønsted and Lewis acids, acid clays, and zeolites, producing a wide mixture of different monoterpenoids that is hard to separate. Natural monoterpene cyclases exert high control over the same cation chemistry but rely on phosphorylated (diphosphate-activated) substrates.[1](https://doi.org/10.1002/anie.202318913)
[Protein engineering](https://www.edgepedia.org/protein-engineering) can close the substrate gap. After 10 rounds of mutagenesis, the octuple squalene-hopene cyclase variant V10 (A306W/W312G/G315I/F365W/Y420F/V448I/Y609M/Y612F) increased selectivity for (+)-borneol from 1.2% to 90.2%, a 186-fold increase in formation of the product, with 99.6% de on gram scale from pinene, without a diphosphate-activated substrate.[1](https://doi.org/10.1002/anie.202318913)
## References
1. Controlling Monoterpene Isomerization by Guiding Challenging Carbocation Rearrangement Reactions in Engineered Squalene-Hopene Cyclases. https://doi.org/10.1002/anie.202318913
2. Engineering class I terpene synthases for skeletal diversity: strategies and applications. https://pubs.rsc.org/en/content/articlehtml/2026/np/d5np00066a
3. Pericyclic reactions catalyzed by chorismate-utilizing enzymes. https://pmc.ncbi.nlm.nih.gov/articles/PMC3164438/
4. Dynamic behavior of rearranging carbocations – implications for terpene biosynthesis. https://www.beilstein-journals.org/bjoc/articles/12/41
5. The Expanding World of Biosynthetic Pericyclases: Cooperation of Experiment and Theory for Discovery. https://pmc.ncbi.nlm.nih.gov/articles/PMC6461539/
6. Cytochrome P450-Induced Backbone Rearrangements in Terpene Biosynthesis of Plants. https://www.mdpi.com/1420-3049/30/17/3540
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Organic reactions, structure and reference › Organic reactions and synthetic methods › Rearrangement reactions › Biological and biosynthetic rearrangements*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license