Isorosmanol

Isorosmanol at a Glance

Isorosmanol is a phenolic abietane diterpene — C₂₀H₂₆O₅, MW 346.4, CAS 93780-80-4, PubChem CID 13820511, ChEBI:175403 — found in the leaves of rosemary and sage. Like carnosol, rosmanol and epirosmanol, it is not a primary plant product: it is what carnosic acid becomes when it is oxidised, and the amount present in any given leaf, extract or bottle is set by light, heat, solvent and shelf age at least as much as by the plant ^{1,11Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}.

It is also, of the four, the one that keeps turning out to be different.

Three things frame the page.

First, the isomer relationship is routinely stated wrongly, including in sources that ought to know better. Rosmanol and epirosmanol are genuine C-7 epimers — same connectivity, opposite configuration. Isorosmanol is not an epimer of either. It is a constitutional isomer: its lactone runs from C-20 to C-7, not to C-6, and its free hydroxyl sits at C-6. That makes it, structurally, carnosol with an extra hydroxyl rather than a stereochemical variant of rosmanol. This is settled below with the primary structural literature and with machine-readable connectivity from PubChem, because the point is load-bearing for everything that follows.

Second, the structural distinction predicts the pharmacology. In three independent studies — muscle-cell hypertrophy ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}, catalytic antioxidant behaviour in the presence of thiol ^{4Reference 4Higashino H et al. · 2024Catalytic antioxidant activity of two diterpenoid polyphenols of rosemary, carnosol, and isorosmanol, against lipid oxidation in the presence of cysteine thiolView study →}, and a purified-compound antibacterial screen ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →} — carnosol and isorosmanol behave alike, and rosmanol does not. That is not a coincidence; it is the shared six-membered lactone doing the work, and it is the strongest argument here that the “iso-” prefix marks a real chemical boundary rather than a naming quirk.

Third, its two best-known claims need care. The melanin result is real, was measured on purified compound, and is genuinely interesting — but the comparison that made it famous is unquantified, and cosmetic depigmenting claims are commercially loaded. The cholinesterase literature, meanwhile, contains a 500,000-fold contradiction between two published papers, and that has to be dealt with before anyone repeats either number.

PubMed indexes 18 records that mention isorosmanol. Three carry it in the title, and a fourth title paper (2025) is not PubMed-indexed at all. That is a small literature by any standard — but it is a materially better one than its sibling epirosmanol’s, which has 18 records and zero titles.

The Isomer Question, Settled

The three “rosmanol” diterpenes are used interchangeably in reviews, supplier catalogues and database synonym lists far more often than the chemistry allows. The relationships are not symmetrical, and the odd one out is not the one most people assume.

Compound	Lactone	Free OH	Relationship to isorosmanol
Carnosol	C-20 → C-7 (six-membered, δ)	none on B-ring	Same ring system. Isorosmanol is carnosol plus a 6α-hydroxyl
Isorosmanol	C-20 → C-7 (six-membered, δ)	6α	—
Rosmanol	C-20 → C-6 (five-membered, γ)	7α	Constitutional isomer — different connectivity, not a stereoisomer
Epirosmanol	C-20 → C-6 (five-membered, γ)	7β	Constitutional isomer; itself the C-7 epimer of rosmanol

Nakatani and Inatani assigned isorosmanol in 1984 as 6α,11,12-trihydroxy-7,10-(epoxymethano)abieta-8,11,13-trien-20-one, in the same paper that named epirosmanol (7β,11,12-trihydroxy-6,10-(epoxymethano)…) and revised rosmanol’s own published structure to the 7α isomer using NOE experiments and X-ray analysis ^{1Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →}. The bridge locant is the whole story: 7,10-(epoxymethano) for isorosmanol against 6,10-(epoxymethano) for the rosmanol pair.

That is confirmed independently, and mechanically, by the ring nomenclature PubChem generates from the deposited structures. Carnosol (CID 442009) and isorosmanol (CID 13820511) are both 16-oxatetracyclo[6.6.2.0¹,¹⁰.0²,⁷]hexadeca-2,4,6-trien-15-ones. Rosmanol (CID 13966122) and epirosmanol (CID 9884612) are both 16-oxatetracyclo[7.5.2.0¹,¹⁰.0²,⁷]hexadeca-2,4,6-trien-15-ones ^{21Reference 21PubChem Compound Summaries: Isorosmanol et al.National Library of MedicineView study →}. Rosmanol and epirosmanol have identical connectivity strings and differ only in stereodescriptors — the definition of epimers. Isorosmanol’s connectivity string is different from both, and matches carnosol’s ring skeleton ^{21Reference 21PubChem Compound Summaries: Isorosmanol et al.National Library of MedicineView study →}.

Practically: rosmanol → epirosmanol is a stereochemical change; rosmanol → isorosmanol is a bond change. Anything that depends on ring geometry — enzyme fit, quinone stability, lactone hydrolysis — can differ between them for reasons that have nothing to do with the C-7 hydroxyl everyone argues about.

This is not a pedantic distinction, and the muddle is documented. A 2016 review of rosemary diterpenes for Alzheimer’s disease describes isorosmanol as the structure “where the lactone ring is formed via the C-6 instead of the C-7 hydroxyl position” — the reverse of Nakatani and Inatani’s assignment, and self-contradictory within its own paragraph, which had already given rosmanol the C-20–C-6 lactone ^{27Reference 27Habtemariam S · 2016The therapeutic potential of rosemary (Rosmarinus officinalis) diterpenes for Alzheimer’s diseaseView study →}. If a dedicated review inverts it, a supplier catalogue or a supplement label certainly can.

A Note on the Greek Letters — and a Grouping to Fix

Our carnosic acid and rosmanol pages both draw the oxidation cascade as carnosol (γ-lactone) → rosmanol / epirosmanol / isorosmanol / 7-methyl-epirosmanol (δ-lactones). Isorosmanol is on the wrong side of that arrow — by connectivity it belongs with carnosol, not with the rosmanol pair — and that is the part worth fixing.

The Greek letters are murkier, and the blame is not ours. Counting ring atoms from the deposited structures ^{21Reference 21PubChem Compound Summaries: Isorosmanol et al.National Library of MedicineView study →}, carnosol’s lactone ring contains six atoms (C-7, C-6, C-5, C-10, C-20, O) while rosmanol’s and epirosmanol’s contain five (C-6, C-5, C-10, C-20, O) — which would make carnosol a δ-lactone and the rosmanol pair γ-lactones. But two well-cited rosemary reviews call both carnosol and rosmanol “γ-lactones” ^{26,27Reference 26Lešnik S · 2021Mechanistic insights into biological activities of polyphenolic compounds from rosemary obtained by inverse molecular dockingView study →Reference 27Habtemariam S · 2016The therapeutic potential of rosemary (Rosmarinus officinalis) diterpenes for Alzheimer’s diseaseView study →}, using the term loosely for the whole series. Because the labels are unreliable in the source literature, ring sizes are stated and bridge locants rather than Greek letters, and readers should do the same.

Where It Sits in the Cascade

Carnosic acid carries a catechol. On oxidation that catechol becomes an electrophilic ortho-quinone, which is intercepted intramolecularly by the C-20 carboxyl to close a lactone ^{2,24Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →Reference 24Birtić S et al. · 2015Carnosic acidView study →}. Depending on which carbon the oxygen lands on and what happens next, the products diverge:

carnosic acid
   → carnosol                  (20,7-lactone, six-membered)
   → isorosmanol               (20,7-lactone + 6alpha-OH)
   → rosmanol / epirosmanol    (20,6-lactones, five-membered, + 7alpha/7beta-OH)
   → galdosol, rosmadial and further ring-oxidised products

In the living rosemary leaf the flux is not evenly split. Munné-Bosch and Alegre’s subcellular work found that carnosic acid scavenging free radicals inside the chloroplast gives rise to diterpene alcohols “mainly isorosmanol” — making isorosmanol the principal in-planta oxidation product, not an incidental one ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}. That is a real difference from epirosmanol, which is largely a marker of processing.

The Melanin Claim, Checked

This is the finding the compound is known for, and it needs to be stated at exactly its strength — no more, and no less.

Sallam and colleagues (2016) isolated seven phenolic diterpenes from the methanolic extract of Salvia officinalis aerial parts — sageone, 12-methylcarnosol, carnosol, 7β-methoxyrosmanol, 7α-methoxyrosmanol, isorosmanol and epirosmanol — and tested all seven for two things: inhibition of melanin biosynthesis in B16 melanoma cells, and inhibition of acetylcholinesterase by Ellman’s method ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}.

What the paper reports:

Compound	Melanin synthesis (B16 cells)	AChE (Ellman’s)
Isorosmanol	”as potent as that of arbutin”	65% inhibition at 500 µM
7α-Methoxyrosmanol	not among the actives	50% inhibition at 500 µM
Epirosmanol	not among the actives	not among the actives
Sageone, 12-methylcarnosol, carnosol, 7β-methoxyrosmanol	not among the actives	not among the actives

Three things are worth pulling out.

It was a purified-compound result, and its sibling failed the same test. Isorosmanol and epirosmanol were isolated in the same fractionation, tested in the same assays, on the same day, by the same hands — and only isorosmanol cleared the bar on either endpoint ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}. Panels that test isomers side by side are rare and are the only clean way to attribute activity within a series. This one says the activity belongs to isorosmanol specifically.

What was measured was cellular melanin, not tyrosinase. The paper’s stated method is the effect on melanin biosynthesis in B16 melanoma cell lines ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}. There is no direct mushroom- or human-tyrosinase enzyme assay in the reported design, so the mechanism — whether isorosmanol inhibits the enzyme, suppresses its expression, or interferes with melanogenic signalling — is not established by this study. Anyone describing isorosmanol as a “tyrosinase inhibitor” is going beyond the source.

The arbutin comparison is unquantified, and arbutin is a low bar. No concentration, no IC₅₀ and no percentage figure for the melanin endpoint appears in the published abstract, and the full text is behind a paywall; secondary reviews that repeat the finding repeat it in exactly the same unquantified form ^{22Reference 22Enkhtaivan E · 2021Role of amine neurotransmitters and their receptors in skin pigmentation: therapeutic implicationView study →}. Arbutin is a genuine but weak depigmenting agent that is typically used in B16 assays at high micromolar to low millimolar concentrations. “As potent as arbutin” is therefore a comparative statement about a modest reference, not evidence of a potent one — and it says nothing about topical delivery, skin penetration or stability.

A missing control matters here. Melanin content per cell falls when cells are stressed or dying, so a melanin-inhibition assay without a matched viability curve cannot distinguish depigmentation from cytotoxicity. No cell-viability control is reported in the abstract. Until the full text or a replication supplies one, the honest reading is: isorosmanol reduced melanin in a melanoma cell line to roughly the degree arbutin does, in a single study, with the mechanism and the concentration unpublished.

That is a legitimate reason to be interested in the molecule. It is not a reason to put it in a skin-lightening product, and it is nowhere near the evidence bar for a cosmetic claim.

The Cholinesterase Contradiction

Two papers have measured isorosmanol against acetylcholinesterase. Their answers are five and a half orders of magnitude apart, and the site of the discrepancy is not a detail.

Study	Preparation	Result for isorosmanol	Comparator
Sallam 2016 2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →	Isolated from S. officinalis; Ellman’s method	65% inhibition at 500 µM	7α-methoxyrosmanol 50% at 500 µM
Köksal & Şenol 2025 6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →	Sigma-Aldrich standard; colorimetric acetylthiocholine assay	IC₅₀ 0.98 ± 0.02 nM	Tacrine IC₅₀ 46.62 nM

A compound cannot plausibly need 500 µM to reach 65% inhibition in one laboratory and inhibit half the enzyme at 1 nM in another. The gap is roughly 500,000-fold.

There are reasons to weight the 2025 numbers with considerable caution:

• Sub-nanomolar IC₅₀ values are implausible for a small phenolic against AChE in a standard colorimetric assay. Reported IC₅₀ cannot fall meaningfully below the enzyme concentration in the cuvette, and this paper reports the whole panel — carnosic acid, carnosol, rosmanol, isorosmanol — in the 0.7–5 nM range, all of them 10–60× more potent than tacrine, a clinically used drug ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}.
• Two compounds in the panel return identical IC₅₀ values, to three significant figures, against four unrelated enzymes. 12-Methoxy-carnosic acid is reported at 2.64 nM against AChE, BChE, CA I and CA II; 7-methoxy-rosmanol at 4.81 nM against all four ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}. Four unrelated active sites do not produce the same number by chemistry.
• The paper’s own discussion does not match its own table. It describes isorosmanol’s performance “against AChE, BChE, and CAII” as “less impressive” while its table gives 0.98 nM for AChE — thirty-fold better than the reference drug ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}.
• Nothing about the source is wrong: the compounds were authentic Sigma-Aldrich standards (isorosmanol, SMB00183) ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}. The problem is in the numbers, not the material.

The defensible statement is the weak one. Isorosmanol inhibits acetylcholinesterase measurably at 500 µM — a concentration far above anything a diterpene could reach in a brain, and one at which many phenols inhibit many enzymes non-specifically. It is a chemical observation, not a nootropic finding, and it should not be reported as one. The nanomolar dataset needs independent replication before it can be used for anything at all.

What It's Used For

Five studies have put purified isorosmanol into a biological assay, and a sixth used it as a synthetic intermediate. That is thin, but — unlike its siblings — the results are not all the same result.

Evidence by IndicationStrength of support · Beta

Support is an experimental score — weighted by study type (human > animal > in vitro) and volume. A beta, not a validated metric. With no mammalian in-vivo and no human study anywhere in this literature, every score is capped low by design. Each application links to its write-up.

01Antioxidant & Lipid ProtectionShaky38%Four isolate studies including a distinctive catalytic mechanism ^{1,4,9,20Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →Reference 4Higashino H et al. · 2024Catalytic antioxidant activity of two diterpenoid polyphenols of rosemary, carnosol, and isorosmanol, against lipid oxidation in the presence of cysteine thiolView study →Reference 9Miura K et al. · 2002Antioxidant activity of chemical components from sage (Salvia officinalis L.) and thyme (Thymus vulgaris L.) measured by the oil stability index methodView study →Reference 20Hamasaki H · 2025Enhancement of antioxidant activity of isorosmanol in aqueous micellar solution by a cysteine thiolView study →}

02Skeletal MuscleShaky25%One isolate SAR study, human cells plus a zebrafish in-vivo arm ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}

03Skin PigmentationShaky15%One isolate study, unquantified comparator, no viability control ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}

04NeuroprotectionShaky14%One isolate study in PC12 cells ^{5Reference 5Park SY · 2009Neuroprotective and neurotrophic effects of isorosmanolView study →}

05Cholinesterase InhibitionShaky8%Two studies contradicting each other by ~500,000-fold ^{2,6Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}

06Antibacterial (Plant Pathogen)Shaky12%One purified-compound agronomic assay, no human pathogen tested ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →}

Mechanisms

Pharmacokinetics

There is no pharmacokinetic study of isorosmanol. No absorption figure, no oral bioavailability, no plasma concentration, no half-life, no tissue distribution, in any species. isorosmanol AND pharmacokinetics returns a single PubMed record, and it is a computational ADMET prediction inside the cholinesterase paper discussed above — not a measurement ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}.

Two adjacent observations are worth recording, and neither is a substitute:

• In the muscle study, isorosmanol produced a functional effect in zebrafish larvae in water, which implies some uptake in that model but yields no numbers ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}.
• The compound is smaller and more polar than its carnosic acid parent, which is the direction that generally favours passive membrane transit. Work on the closely related epirosmanol found roughly 50% crossing an artificial blood–brain-barrier membrane model, better than carnosol or carnosic acid — a peak-area observation in a PAMPA assay, not a permeability coefficient, and not an isorosmanol result.

Until someone doses an animal and measures plasma, every concentration here is a concentration in a dish.

Clinical Trials

None. There are no clinical trials of isorosmanol — a ClinicalTrials.gov search returns 0 studies under that name (checked August 2026). Human exposure is entirely incidental, through culinary rosemary and sage and through rosemary antioxidant extracts.

Last checked: August 2026.

Isolate vs. Plant

The boundary is drawable here, and isorosmanol comes out better than most compounds of its size.

Genuine isolate evidence: the 1984 isolation and antioxidant report ^{1Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →}, the oil-stability-index panel ^{9Reference 9Miura K et al. · 2002Antioxidant activity of chemical components from sage (Salvia officinalis L.) and thyme (Thymus vulgaris L.) measured by the oil stability index methodView study →}, the melanin and AChE panel ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}, the catalytic-antioxidant work ^{4,20Reference 4Higashino H et al. · 2024Catalytic antioxidant activity of two diterpenoid polyphenols of rosemary, carnosol, and isorosmanol, against lipid oxidation in the presence of cysteine thiolView study →Reference 20Hamasaki H · 2025Enhancement of antioxidant activity of isorosmanol in aqueous micellar solution by a cysteine thiolView study →}, the PC12 neuroprotection study ^{5Reference 5Park SY · 2009Neuroprotective and neurotrophic effects of isorosmanolView study →}, the muscle SAR study ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}, the Pectobacterium screen ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →} and the cholinesterase/carbonic-anhydrase panel ^{6Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}. In all of them isorosmanol was purified or purchased as a standard before testing. That is unusually good coverage for a compound with 18 papers.

What does not transfer. Nothing here supports a claim about rosemary or sage as herbs. Isorosmanol is a minor member of a diterpene fraction whose recognised actives — the compounds carrying the food-preservative function and most of the pharmacology — are carnosic acid and carnosol. The EU food additive extracts of rosemary (E 392) is specified, and its maximum permitted levels expressed, as the sum of carnosol and carnosic acid ^{25Reference 25EFSA Panel on Food Additives et al. · 2018Refined exposure assessment of extracts of rosemary (E 392) from its use as food additiveView study →}; isorosmanol, though formed from both, is not in the specification and is not measured in commerce.

Derivatives that are not isorosmanol. Four distinct entities in this literature carry the name inside theirs and must not be conflated with the parent: 11,12-di-O-methylisorosmanol, the plant’s own O-methylated export form, which the authors state does not display antioxidant activity ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}; dimethylisorosmanol, the synthetic analogue that lost both the antioxidant and the hypertrophic effect ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}; isorosmaquinone, a semisynthetic oxidation product prepared from isorosmanol ^{16Reference 16Marrero JG et al. · 2009Semisynthesis and biological evaluation of abietane-type diterpenesView study →}; and epiisorosmanol, a separate natural diterpene reported in rosemary extracts and with only three PubMed records of its own ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →}. None of their results transfers to isorosmanol.

Prevalence in Nature

Isorosmanol is a Lamiaceae compound, and within the family a Salvia compound. Its distribution follows carnosic acid’s, because it is made from it.

Source	Part	Basis
Rosemary (Salvia rosmarinus, syn. Rosmarinus officinalis)	Leaves, aerial parts	Type source — first isolation 1984 1Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →; re-isolated as a known compound from aerial parts 19Reference 19Zhang Y et al. · 2014New terpenoid glycosides obtained from Rosmarinus officinalis L. aerial partsView study →; quantified across the season and across 96 Italian accessions 8,12,13,14Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →Reference 12Munné-Bosch S et al. · 1999Enhanced formation of α-tocopherol and highly oxidized abietane diterpenes in water-stressed rosemary plantsView study →Reference 13Munné-Bosch S · 2000Changes in carotenoids, tocopherols and diterpenes during drought and recovery, and the biological significance of chlorophyll loss in Rosmarinus officinalis plantsView study →Reference 14Munné-Bosch S et al. · 1999Response of abietane diterpenes to stress in Rosmarinus officinalis L.: new insights into the function of diterpenes in plantsView study →
Sage (Salvia officinalis)	Leaves, aerial parts	Isolated repeatedly from methanolic extracts 2,10Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →Reference 10Miura K et al. · 2001Apianane terpenoids from Salvia officinalisView study →; among the compounds in the sage/thyme antioxidant panel 9Reference 9Miura K et al. · 2002Antioxidant activity of chemical components from sage (Salvia officinalis L.) and thyme (Thymus vulgaris L.) measured by the oil stability index methodView study →
Salvia pachyphylla	Aerial parts	Isolated as a known diterpene in a cytotoxicity study 17Reference 17Guerrero IC et al. · 2006Abietane diterpenoids from Salvia pachyphylla and S. clevelandii with cytotoxic activity against human cancer cell linesView study →
Salvia clevelandii	Aerial parts	Same study; the S. clevelandii diterpene set is reported separately 17Reference 17Guerrero IC et al. · 2006Abietane diterpenoids from Salvia pachyphylla and S. clevelandii with cytotoxic activity against human cancer cell linesView study →
Vervain (Verbena officinalis)	Extract	HPLC-PDA-HRMS annotation only, reported as new for the species 18Reference 18Sun Y et al. · 2017Identification of the chemical compositions of Verbena officinalis L. extract by high performance liquid chromatography-photodiode array-high resolution mass spectrometryView study → — see caveat below

Quantitative data are relative, not absolute. The best figures come from Munné-Bosch’s rosemary field work. In field-grown rosemary leaf, carnosic acid and carnosol are present at roughly 5 and 1 mg/g dry weight respectively ^{14Reference 14Munné-Bosch S et al. · 1999Response of abietane diterpenes to stress in Rosmarinus officinalis L.: new insights into the function of diterpenes in plantsView study →}, and isorosmanol was found at “slightly lower concentrations than carnosol” ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →} — so on the order of, but below, 1 mg/g dry weight. No paper puts a direct number on it.

What does have hard numbers is the stress response, and it is large:

Condition	Isorosmanol change	Ref
Mediterranean summer drought, field-grown rosemary	+25% (dimethyl isorosmanol +40%), while carnosic acid fell 22%	12Reference 12Munné-Bosch S et al. · 1999Enhanced formation of α-tocopherol and highly oxidized abietane diterpenes in water-stressed rosemary plantsView study →
Severe prolonged drought	8-fold increase, from oxidation of carnosic acid	13Reference 13Munné-Bosch S · 2000Changes in carotenoids, tocopherols and diterpenes during drought and recovery, and the biological significance of chlorophyll loss in Rosmarinus officinalis plantsView study →
Summer stress (low precipitation, high radiation, high temperature)	Increased, while carnosic acid and carnosol fell ~50%	14Reference 14Munné-Bosch S et al. · 1999Response of abietane diterpenes to stress in Rosmarinus officinalis L.: new insights into the function of diterpenes in plantsView study →

That is the practical takeaway for anyone sourcing rosemary: isorosmanol content is an index of how stressed the plant was and how oxidised the material is. A drought-grown, sun-exposed, long-stored rosemary will be richer in isorosmanol and poorer in carnosic acid than a shaded, irrigated, freshly dried one — and Iobbi’s Italian survey found exactly that pattern geographically, with southern accessions shifted toward isorosmanol and northern ones toward carnosic acid ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →}.

One occurrence claim deserves a caveat. The Verbena officinalis report is an LC-MS annotation, not an isolation: a Chinese HPLC-PDA-HRMS profiling study reported carnosic acid, carnosol, rosmanol, isorosmanol, rosmarinic acid and acacetin-7-O-rutinoside as previously unreported in vervain extract ^{18Reference 18Sun Y et al. · 2017Identification of the chemical compositions of Verbena officinalis L. extract by high performance liquid chromatography-photodiode array-high resolution mass spectrometryView study →}. Rosmarinic acid in Verbena is well established; a full rosemary abietane set in a Verbenaceae species on MS evidence alone is a stronger claim, and until someone isolates the compound from vervain it should be held loosely.

Its Role in Plants

This is the one area where isorosmanol, rather than carnosic acid, is the compound the literature is actually about — and the story is unusually complete for a molecule this obscure.

Carnosic acid is rosemary’s stored lipophilic antioxidant, concentrated in leaf chloroplasts, and it protects the photosynthetic apparatus by being sacrificially oxidised under light and oxidative stress ^{24Reference 24Birtić S et al. · 2015Carnosic acidView study →}. Munné-Bosch and Alegre traced where the products go. Their subcellular fractionation of rosemary leaves found that carnosic acid scavenges free radicals within the chloroplasts, giving rise to diterpene alcohols, mainly isorosmanol ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}. Isorosmanol is then O-methylated inside the chloroplast, and the resulting 11,12-di-O-methylisorosmanol is transferred out to the plasma membrane, where it accumulates at 16.6-fold the chloroplast level ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}.

The authors read this as a controlled disposal route — in their phrase, a way out for free radicals from the chloroplast. And they note the cost: O-methylated diterpenes do not display antioxidant activity, though they may influence plasma-membrane stability ^{11Reference 11Munné-Bosch S · 2001Subcellular compartmentation of the diterpene carnosic acid and its derivatives in the leaves of rosemaryView study →}. The plant is not recycling its antioxidant; it is capping it and exporting it.

The field data fit. Across a Mediterranean year, carnosic acid and carnosol fell by about half during summer while rosmanol, isorosmanol and dimethyl isorosmanol rose in response to low precipitation, high radiation and high temperature ^{14Reference 14Munné-Bosch S et al. · 1999Response of abietane diterpenes to stress in Rosmarinus officinalis L.: new insights into the function of diterpenes in plantsView study →}. Under experimental drought, isorosmanol rose 25% and dimethyl isorosmanol 40% as carnosic acid fell 22% ^{12Reference 12Munné-Bosch S et al. · 1999Enhanced formation of α-tocopherol and highly oxidized abietane diterpenes in water-stressed rosemary plantsView study →}; under severe prolonged drought, isorosmanol rose eight-fold, and recovered along with the photosynthetic pigments after the autumn rains ^{13Reference 13Munné-Bosch S · 2000Changes in carotenoids, tocopherols and diterpenes during drought and recovery, and the biological significance of chlorophyll loss in Rosmarinus officinalis plantsView study →}.

So isorosmanol is not a waste product and not merely an extraction artefact. In the living leaf it is the main downstream marker of carnosic acid actually having done its job.

Discovery & Synthesis

Isorosmanol was isolated and named in 1984 by Nobuji Nakatani and Reiko Inatani at Osaka City University, from the leaves of rosemary, in a paper reporting two new antioxidative diterpenes — isorosmanol and epirosmanol — and revising the published structure of rosmanol itself ^{1Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →}. Its structure was assigned as 6α,11,12-trihydroxy-7,10-(epoxymethano)abieta-8,11,13-trien-20-one, and both new compounds were shown to be roughly four times as active as BHA and BHT in a lard system ^{1Reference 1Nakatani N · 1984Two antioxidative diterpenes from rosemary (Rosmarinus officinalis L.) and a revised structure for rosmanolView study →}.

Unlike epirosmanol, isorosmanol has a published laboratory synthesis. Li, Zhang, Yang, Zhang and Xie (2018) built it biomimetically from (+)-carnosic acid, using epoxidation, epoxide ring opening and lactonisation in one pot to reach 11,12-dimethoxy isorosmanol; they then carried isorosmanol forward, in ten steps overall, to przewalskin A — a C₂₃ terpenoid with anti-HIV-1 activity from Salvia przewalskii — via a bismuthonium-ylide-induced ring expansion of an ortho-quinone ^{15Reference 15Li Z et al. · 2018Biomimetic synthesis of isorosmanol and przewalskin AView study →}. The strategy was explicitly inspired by the biogenetic hypothesis, so the synthesis doubles as chemical support for the proposed biosynthetic route.

Isorosmanol has also served as a starting material: Marrero and colleagues (2009) prepared the new semisynthetic isorosmaquinone from it, in a paper that also reassigned the proton resonances of rosmaquinone ^{16Reference 16Marrero JG et al. · 2009Semisynthesis and biological evaluation of abietane-type diterpenesView study →}. Note that the widely cited 2002 semisynthesis from carnosol produced rosmanol, rosmaquinone, 7-methoxyrosmanol, 7-ethoxyrosmanol, galdosol and epirosmanol — but not isorosmanol ^{23Reference 23Marrero JG et al. · 2002Semisynthesis of rosmanol and its derivativesView study →}, which is consistent with the constitutional difference: carnosol’s own lactone is already in isorosmanol’s position, so reaching isorosmanol requires hydroxylating C-6 rather than shuffling stereochemistry at C-7.

Commercially, isorosmanol is sold only as an analytical reference standard, in milligram quantities, by several suppliers ^{6,7Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →Reference 7Liu S · 2024An integrated approach of bioassays and non-target screening for the assessment of endocrine-disrupting activities in tap water and identification of novel endocrine-disrupting chemicalsView study →}. It is not produced at scale and is not a component of any commercial product.

Patents: not yet researched.

Toxicity & Safety

The verdict is not characterised, and the unknown grading is deliberate. There has never been a toxicology study of isorosmanol: no acute or repeat-dose study, no LD₅₀, no genotoxicity panel, no reproductive or developmental work, no interaction study, and no observation in a mammal of any kind. isorosmanol AND toxicity returns two PubMed records — the PC12 neuroprotection paper, and one screening study described below.

That screening study is the reason low would be wrong.

Isorosmanol antagonises four steroid hormone receptors at low-micromolar concentrations. Liu and Liu (2024), publishing in Toxics, ran non-target screening of tap water alongside a panel of nuclear-receptor reporter bioassays, and isorosmanol emerged as one of four novel potential endocrine-disrupting chemicals identified in drinking-water samples. Tested as a purchased ≥98% standard, it showed ^{7Reference 7Liu S · 2024An integrated approach of bioassays and non-target screening for the assessment of endocrine-disrupting activities in tap water and identification of novel endocrine-disrupting chemicalsView study →}:

Receptor	Effect	RIC₂₀	RIC₅₀	Maximum inhibition
Mineralocorticoid (MR)	Antagonist	7.71 µM	12.1 µM	60.8%
Glucocorticoid (GR)	Antagonist	5.07 µM	10.0 µM	59.9%
Progesterone (PR)	Antagonist	8.15 µM	—	41.0%
Oestrogen (ERα)	Antagonist	9.99 µM	—	23.3%

The highest non-cytotoxic concentration used was 10 µM ^{7Reference 7Liu S · 2024An integrated approach of bioassays and non-target screening for the assessment of endocrine-disrupting activities in tap water and identification of novel endocrine-disrupting chemicalsView study →}. Two caveats belong with this: the identification of isorosmanol in tap water is a tentative annotation (confidence level 2a, library match), and it is genuinely odd — the paper offers no explanation for a rosemary diterpene in Hangzhou drinking water. But the bioassay data are not tentative: they were generated on an authentic purchased standard, and they are the only receptor-level pharmacology anyone has measured for this molecule.

Read plainly, this says isorosmanol is a broad-spectrum steroid-receptor antagonist in vitro at concentrations similar to those used in the muscle and antioxidant work. That is not a demonstration of harm — reporter-assay antagonism at 5–12 µM with 60% maximum effect is a screening signal, not a toxicological finding, and there is no in-vivo follow-up. But it does mean the compound is not a bland antioxidant, and it is the reason a concentrated isolate should not be treated as benign by default. It is also consistent with what is known of its relative carnosol, which has a documented dual androgen/oestrogen-receptor antagonism.

Reproductive and developmental effects have never been investigated for this molecule. Decisions about rosemary or sage in pregnancy belong on those herb pages, where intake is actually decided, and turn on thujone and on whole-preparation data — not on this compound.

Safety at a Glance

❔ Toxicity unknown

• No adverse effect has ever been reported in a person or an animal. It has also never been looked for — those are different statements, and the unknown grading reflects the second.
• Dietary exposure is long-standing and incidental. Anyone who cooks with rosemary or sage has been consuming small amounts indefinitely, with no signal attached.
• Antagonises glucocorticoid, mineralocorticoid, progesterone and oestrogen receptors in vitro at 5–12 µM ^{7Reference 7Liu S · 2024An integrated approach of bioassays and non-target screening for the assessment of endocrine-disrupting activities in tap water and identification of novel endocrine-disrupting chemicalsView study →} — overlapping the concentrations at which its useful effects are reported. This is the single reason not to treat a concentrated isolate as an innocuous antioxidant.
• The published cholinesterase data are contradictory by roughly 500,000-fold ^{2,6Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →Reference 6Köksal Z · 2025In vitroAnticholinesterase and carbonic anhydrase inhibitory activities of natural carnosic acid derivatives: a comprehensive in vitro and in silico studyView study →}, so any product claim built on the nanomolar figures rests on numbers that do not hold up.
• No toxicology, no pharmacokinetics, no mammalian in-vivo data, no human data. Every statement above is from a dish or a zebrafish.
• No reproductive or developmental data exist. Nothing is known about use in pregnancy or breastfeeding.
• Drug interactions have never been assessed — no CYP, no P-glycoprotein, no transporter work of any kind for this molecule.

Dosage

There is no dose. Isorosmanol has never been administered to a person as an isolate, no mammalian dosing study exists to extrapolate from, and no pharmacokinetic study has established whether an oral dose would reach the bloodstream at all. Every published figure is an in-vitro concentration — 10 µM in the muscle work ^{3Reference 3Berger A et al. · 2025Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle functionView study →}, 500 µM in the cholinesterase assay ^{2Reference 2Sallam A et al. · 2016Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalisView study →}, 250–1,000 µg/mL in the antibacterial screen ^{8Reference 8Iobbi V et al. · 2024Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorumView study →} — which is the concentration at a cell or in a cuvette, not an intake.

Real-world exposure comes from rosemary and sage leaf and from rosemary antioxidant extracts, where isorosmanol is one unquantified minor diterpene in a fraction defined and regulated by carnosic acid and carnosol ^{25Reference 25EFSA Panel on Food Additives et al. · 2018Refined exposure assessment of extracts of rosemary (E 392) from its use as food additiveView study →}. The plant, not the molecule, is what anybody is actually taking. Nothing here is a recommendation.

References

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2. Sallam A, Mira A, Ashour A, Shimizu K (2016). Acetylcholine esterase inhibitors and melanin synthesis inhibitors from Salvia officinalis. Phytomedicine 23(10):1005–1011. https://pubmed.ncbi.nlm.nih.gov/27444345/
3. Berger A, Sarniguet J, Hugon G, Cubedo N, Morel S, Chabi B, Vernus B, Bonnieu A, Brun A, Matecki S, Rossel M, Carnac G (2025). Advances in identifying functional groups in carnosol analogues to address their efficacy in skeletal muscle function. Biomedicine & Pharmacotherapy 190:118397. https://pubmed.ncbi.nlm.nih.gov/40753935/
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