Tropinone

Tropinone is a naturally occurring bicyclic tropane alkaloid with the molecular formula C₈H₁₃NO and the IUPAC name 8-methyl-8-azabicyclo[3.2.1]octan-3-one, featuring a characteristic ketone group at the 3-position of its bridged nitrogen-containing ring system.^[1]^[2] It appears as a colorless crystalline solid with a melting point of 40–44 °C and a boiling point of 113 °C at reduced pressure, exhibiting sparing solubility in water but good solubility in organic solvents such as ethanol, ether, and chloroform.^[2] As a central intermediate in the biosynthesis of pharmacologically significant tropane alkaloids—including atropine, scopolamine, and cocaine—tropinone is produced in plants of the Solanaceae family, notably Atropa belladonna (deadly nightshade), where it forms the core 8-azabicyclo[3.2.1]octane scaffold.^[1]^[3] In its biosynthesis, tropinone arises from the condensation of N-methyl-Δ¹-pyrrolinium cation and malonyl-CoA, catalyzed by the atypical type III polyketide synthase AbPYKS through two rounds of decarboxylative condensation to yield 4-(1-methyl-2-pyrrolidinyl)-3-oxobutanoic acid, followed by cyclization mediated by the cytochrome P450 enzyme AbCYP82M3.^[4] This pathway underscores tropinone's role as the first committed intermediate possessing the full bicyclic structure essential for downstream modifications into bioactive alkaloids used in treatments for motion sickness, gastrointestinal disorders, and as local anesthetics.^[3]^[2] Tropinone's synthetic history is marked by Richard Willstätter's total synthesis in 1901, but it gained prominence through Robert Robinson's biomimetic approach in 1917, which involved a one-pot condensation of succinaldehyde, methylamine, and acetone via sequential Mannich-type reactions, achieving yields up to 70–85% under optimized conditions and demonstrating thermodynamic favorability with an activation energy reduction of approximately 69 kcal/mol when using acetonedicarboxylic acid.^[1]^[5] This synthesis not only facilitated atropine production during World War I but also highlighted tropinone's versatility as a precursor in pharmaceutical and agrochemical applications, including the development of plant growth regulators, insecticides, and herbicides.^[5]^[2] While generally safe in controlled medicinal contexts, tropinone derivatives can cause side effects such as dizziness, dry mouth, and tachycardia if mishandled.^[2]

Overview

Chemical structure and identity

Tropinone is an organic compound classified as a tropane alkaloid, with the molecular formula C₈H₁₃NO and a molar mass of 139.195 g/mol.^[6] Its systematic IUPAC name is 8-methyl-8-azabicyclo[3.2.1]octan-3-one, reflecting the bicyclic nature of its core structure.^[7] Common synonyms include tropan-3-one and tropanone, which emphasize its ketone functionality and relation to the tropane family.^[7] The molecular structure of tropinone consists of a rigid bicyclic 8-azabicyclo[3.2.1]octane framework, where a piperidine ring is bridged by a pyrrolidine ring at positions 1 and 5, with the nitrogen atom located at the 8-bridgehead position bearing a methyl substituent.^[8] A ketone group is present at the 3-position on the six-membered ring, contributing to its reactivity as a precursor in alkaloid synthesis.^[7] This tropane skeleton is characteristic of several pharmacologically active alkaloids, including atropine and cocaine, which share the same bridged bicyclic motif but differ in substituents and stereochemistry.^[8]

Historical context

Tropinone's historical significance is rooted in the broader study of tropane alkaloids, which began with the isolation of cocaine from coca leaves in 1860 by German chemist Albert Niemann.^[9] This discovery sparked interest in the structural elucidation of natural alkaloids, as cocaine and related compounds like atropine exhibited potent pharmacological effects, including local anesthesia and mydriasis. By the late 19th century, chemists recognized tropinone as a key precursor in the tropane family, essential for understanding the biosynthesis and chemical makeup of these substances, though its own isolation and synthesis remained challenging.^[10] The first synthesis of tropinone was achieved in 1901 by Richard Willstätter, a pioneering organic chemist who used it as an intermediate in the total synthesis of cocaine, thereby confirming the alkaloid's structure through a combination of degradation studies and constructive synthesis.^[11] Willstätter's approach involved a laborious multi-step process starting from cycloheptanone, culminating in 15 steps with an overall yield of about 0.75%, and notably produced pseudotropine as a critical intermediate during the reduction of tropinone to form the tropane core.^[10] This work not only marked the inaugural preparation of tropinone but also advanced the field of alkaloid chemistry, earning Willstätter the Nobel Prize in Chemistry in 1915 for his contributions to organic synthesis. A landmark advancement came in 1917 with Robert Robinson's elegant total synthesis of tropinone, developed amid World War I to address a critical shortage of atropine, a tropane-derived antidote for chemical warfare agents.^[12] Motivated by the need for scalable production, Robinson employed a biomimetic one-pot reaction involving succindialdehyde, methylamine, and acetonedicarboxylic acid, achieving an initial yield of 17% that was later optimized.^[10] This synthesis exemplified an early application of biosynthetic principles to organic chemistry, enabling efficient access to tropane alkaloids and influencing subsequent pharmaceutical developments. Robinson's achievements in alkaloid synthesis, including this work, contributed to his receipt of the 1947 Nobel Prize in Chemistry for investigations into plant products of biological importance.^[13]

Physical and chemical properties

Physical characteristics

Tropinone is a colorless crystalline solid at room temperature.^[2] It has a melting point of 43 °C.^[1] Tropinone has a reported boiling point of 113 °C at 25 mm Hg, though it may decompose at higher temperatures.^[14] The compound exhibits slight solubility in water (approximately 10 mg/mL at pH 7.2) and is soluble in chloroform, while being more soluble in ethanol.^[15]^[16] At physiological pH 7.3, the predominant form is its protonated conjugate acid, known as tropiniumone.^[17] Tropinone carries a GHS classification of Danger, with key hazard statements including H302 (harmful if swallowed) and H314 (causes severe skin burns and eye damage).^[18] Safe handling necessitates the use of protective equipment, such as gloves, goggles, and appropriate ventilation, to mitigate risks of ingestion, skin contact, and inhalation.^[19]

Stability and reactivity

Tropinone features a cyclic ketone functional group at the C3 position within its bicyclic structure and a tertiary amine incorporated into the bridged 8-azabicyclo[3.2.1]octane system.^[8] The pKa of its conjugate acid is approximately 8.9, reflecting the basicity of the tertiary amine.^[8] The compound exhibits stability under neutral conditions and during storage as a solid for periods exceeding four years.^[20] High temperatures lead to decomposition.^[1] In terms of reactivity, tropinone undergoes nucleophilic addition at the carbonyl group, as demonstrated by reactions with Grignard reagents to form tertiary alcohols.^[21] The tertiary amine readily protonates to form salts or reacts with alkyl halides to yield quaternary ammonium compounds, such as those derived from alkylation with benzyl bromide.^[22] Spectroscopic characterization supports these structural and reactive features: the infrared spectrum displays a characteristic carbonyl stretch at approximately 1710 cm⁻¹, typical for cyclic ketones.^[23] The ¹H NMR spectrum reveals distinctive signals for the bridged protons, appearing around 2.67–3.45 ppm in CDCl₃.^[24] In mass spectrometry, the base peak occurs at m/z 82.^[6]

Biosynthesis

Natural sources

Tropinone occurs naturally as a key biosynthetic intermediate in the production of tropane alkaloids within several plant families, most notably the Solanaceae and Erythroxylaceae. In the Solanaceae family, it is present in species such as Atropa belladonna (deadly nightshade), Datura stramonium (jimsonweed), and Hyoscyamus niger (henbane), where it serves as a precursor to pharmacologically active compounds like hyoscyamine and scopolamine. Similarly, in the Erythroxylaceae family, tropinone is found in Erythroxylum coca (coca plant), contributing to the formation of cocaine and related alkaloids. These plants accumulate tropane alkaloids derived from tropinone primarily in their roots, leaves, and aerial parts, with total concentrations reaching up to 0.1-0.2% of dry weight in root tissues of Atropa belladonna. Tropinone itself, as a transient intermediate, accumulates at low levels.^[25]^[26]^[27] The ecological distribution of tropinone is closely tied to the prevalence of tropane alkaloids in tropical and subtropical regions, particularly in the Americas, Europe, and Asia. Tropane alkaloids, including tropinone as an intermediate, are reported across multiple angiosperm families in numerous plant species, with over 200 distinct compounds identified, reflecting a polyphyletic evolutionary origin. This widespread occurrence underscores tropinone's role in the chemical ecology of these plants, which are often adapted to environments with high herbivore pressure.^[28]^[29] Isolation of tropinone from natural sources typically involves alkaloid fractionation techniques, such as solvent extraction with methanol or ethanol followed by acid-base partitioning and chromatographic purification from plant material like roots or leaves. However, due to its low natural abundance as a transient intermediate and resulting poor yields—often below 0.1% recovery—chemical synthesis remains the preferred method for obtaining sufficient quantities for research or pharmaceutical applications.^[22] From an evolutionary perspective, tropinone contributes to plant defense mechanisms by enabling the synthesis of toxic tropane alkaloids that deter herbivores, acting as antifeedants and neurotoxins that disrupt insect feeding and locomotion. This protective function is evident in species like Datura stramonium, where elevated tropane levels correlate with resistance to specialist herbivores.^[30]^[31]

Enzymatic pathway

The biosynthesis of tropinone in plants occurs primarily through a polyketide-based pathway in the Solanaceae family, such as Atropa belladonna and Datura species, where it serves as a central intermediate in tropane alkaloid production. The pathway initiates with the condensation of malonyl-CoA and the iminium ion N-methyl-Δ¹-pyrrolinium, derived from the amino acid ornithine via putrescine N-methyltransferase (PMT) and N-methylputrescine oxidase (MPO), to form a linear β-keto acid precursor catalyzed by an atypical type III polyketide synthase (PKS), such as AbPYKS in A. belladonna. This is followed by oxidative cyclization mediated by a cytochrome P450 enzyme, CYP82M3, to yield the bicyclic tropinone core. Key enzymes beyond the initial steps include two NADPH-dependent tropinone reductases that diverge the pathway post-tropinone formation: tropinone reductase I (TRI), which stereospecifically reduces tropinone to the 3α-hydroxy derivative tropine, and tropinone reductase II (TRII), which produces the 3β-hydroxy isomer pseudotropine. These reductases represent a branch point, with TRI leading toward hyoscyamine and scopolamine in many Solanaceae, while TRII directs toward other tropanes. In the distantly related Erythroxylaceae, such as Erythroxylum coca, tropinone biosynthesis converges on a similar bicyclic structure but via a distinct route starting from spermidine-derived N-methyl-Δ¹-pyrrolinium, involving a type III PKS (EcOGAS) and a CYP81AN15 P450 for cyclization, highlighting independent evolution of the pathway. The genetic basis for tropinone biosynthesis involves conserved gene clusters identified in Datura stramonium and A. belladonna genomes, encompassing modules with PMT, MPO, PYKS, CYP82M3, and TRI genes, often co-expressed in root tissues. Similar clusters have been delineated in E. coca through transcriptomic and microbial engineering approaches, featuring spermidine synthase/methyltransferase and CYP81AN15. These clusters facilitate coordinated expression, with duplications in Solanaceae enhancing pathway flux. Natural yields of tropinone and related tropane alkaloids remain low, typically in the range of 0.2–8 mg/g dry weight in root tissues of Datura and Atropa, limiting commercial extraction. Biosynthesis is tightly regulated, with genes predominantly expressed in roots and upregulated by abiotic stresses or elicitors such as methyl jasmonate, which activates jasmonate signaling to boost transcript levels of PMT, TRI, and downstream enzymes by up to several-fold.

Chemical synthesis

Early methods

The earliest synthetic approaches to tropinone relied on multi-step routes, often involving complex transformations due to the bridged bicyclic structure. In 1901, Richard Willstätter achieved the first total synthesis starting from cycloheptanone through a series of oxidative degradations and ring contractions to construct the tropane skeleton. This involved initial bromination and dehydrohalogenation to form tropidine, addition to yield pseudotropine, and oxidation to tropinone, with potassium permanganate employed in key oxidative steps for ring modification and functional group introduction.^[10] The process spanned about 15 steps with a low overall yield of 0.75%, highlighting the complexity of building the bridged bicyclic structure from a seven-membered ring.^[32] These methods faced significant challenges, including low efficiency from multiple low-yielding transformations and heavy dependence on processing large quantities of starting materials, necessitating kilograms for gram-scale tropinone production.^[11] Despite their impracticality for large-scale preparation, they demonstrated tropinone's central role as a versatile intermediate in tropane alkaloid synthesis and paved the way for more efficient biomimetic approaches, such as Robinson's 1917 improvement.^[33]

Robinson synthesis and mechanism

The Robinson synthesis of tropinone, reported in 1917, is a pioneering one-pot reaction that condenses succinaldehyde, methylamine, and acetonedicarboxylic acid through a double Mannich process to produce tropinone, carbon dioxide, and water.^[34] The overall transformation can be summarized by the equation:

\ce{(CHOCH2CH2CHO) + CH3NH2 + (HO2CCH2COCH2CO2H) -> tropinone + CO2 + H2O}

This biomimetic approach mimics the formation of tropane alkaloids in nature by assembling the bicyclic framework from simple precursors in a five-step sequence under mild conditions.^[12] The initial yield was 17%, but careful optimization of reaction conditions, such as pH control and use of protected forms of the dicarboxylic acid, has elevated it to over 90%.^[10] The mechanism begins with imine formation, where methylamine adds nucleophilically to one carbonyl of succinaldehyde, followed by dehydration to generate an iminium ion.^[34] Next, the enol tautomer of acetonedicarboxylic acid performs a Michael addition to this iminium electrophile, establishing the initial carbon-carbon bond and positioning the dicarboxymethyl group for cyclization. Subsequent intramolecular Mannich reactions then occur: the first involves the enamine-like intermediate attacking the remaining aldehyde to close the pyrrolidine ring, and the second features the ketone enol adding to a newly formed iminium, forging the piperidone ring. Finally, thermal decarboxylation eliminates the two carboxylic acid groups, yielding the tropinone core.^[34]

Reactions and applications

Key transformations

Tropinone undergoes stereospecific reduction of its C3 carbonyl group, primarily catalyzed by two NADPH-dependent enzymes: tropinone reductase I (TRI), which yields the endo (3α-hydroxy) alcohol tropine, and tropinone reductase II (TRII), which produces the exo (3β-hydroxy) alcohol pseudotropine. These reductions represent a key branch point in tropane alkaloid metabolism. The enzymatic reactions are as follows:

\text{Tropinone} + \text{NADPH} + \text{H}^{+} \xrightarrow{\text{TRI}} \text{Tropine} + \text{NADP}^{+}

\text{Tropinone} + \text{NADPH} + \text{H}^{+} \xrightarrow{\text{TRII}} \text{Pseudotropine} + \text{NADP}^{+}

^[35]^[36] In plants of the Solanaceae family, such as those producing medicinal tropane alkaloids, the reduction favors tropine via TRI, thereby directing flux toward the atropine (hyoscyamine and scopolamine) biosynthetic pathway.^[27] Other notable transformations include α-bromination at the C2 position adjacent to the C3 carbonyl, facilitated by bromine in acetic acid to form 2-bromotropinone, which enables subsequent nucleophilic substitution for introducing diverse functional groups at that site.^[37] Tropinone can also undergo N-oxidation with hydrogen peroxide or m-chloroperbenzoic acid to generate tropinone N-oxide, a versatile intermediate for rearrangements like the Polonovski reaction.^[38] Tropinone's synthetic utility is evident in its role as a precursor for cocaine analogs, where reduction (often to 2-carbomethoxytropinone derivatives) followed by esterification with carboxylic acids such as tropic or benzoic acid affords tropane esters with cocaine-like structures.^[11]

Pharmaceutical and research uses

Tropinone functions as a crucial intermediate in the pharmaceutical synthesis of tropane alkaloids, notably atropine and scopolamine, which exhibit anticholinergic properties for treating conditions like bradycardia, gastrointestinal spasms, and motion sickness. Atropine, derived from tropinone via reduction to tropine and esterification with tropic acid, serves as a mydriatic agent to dilate pupils during ophthalmic examinations and as an antidote for organophosphate poisoning. Scopolamine, similarly synthesized, is employed for its sedative and antiemetic effects in preoperative medication and vertigo management. Cocaine analogs, built from tropinone scaffolds, have been developed as local anesthetics and analgesics, offering reduced cardiotoxicity compared to natural cocaine while retaining efficacy in pain relief applications.^[25]^[39]^[2] In research, tropinone is widely utilized as a model compound for elucidating tropane alkaloid biosynthesis pathways, particularly the enzymatic reduction by tropinone reductases I and II to form tropine or pseudotropine, which directs flux toward hyoscyamine or calystegines. Metabolic engineering efforts leverage tropinone's central role to enhance production of medicinal tropane alkaloids; for instance, overexpression of tropinone reductase I in Atropa belladonna hairy roots has increased hyoscyamine yields by up to 3-fold, while co-expression of putrescine N-methyltransferase and hyoscyamine 6β-hydroxylase in Scopolia parviflora roots boosted scopolamine levels significantly. These strategies extend to microbial platforms, where tropinone pathway genes introduced into Escherichia coli or yeast enable de novo synthesis of scopolamine precursors, addressing supply limitations from plant sources.^[40]^[41]^[42] Tropinone's role in toxicology stems from its position as a precursor to potent toxic alkaloids like those in Datura species, contributing to anticholinergic poisoning syndromes characterized by delirium and mydriasis; calystegine derivatives, formed via tropinone, exhibit glycohydrolase inhibition leading to vacuolar disruptions in mammals.^[25]^[43] In pesticide development, tropinone and its derivatives demonstrate insecticidal potential, with tropinone achieving 83% mortality against Thrips palmi larvae at 1000 µg/mL, acting as a stomach poison through stable binding to target proteins like cathepsin B, positioning it as an eco-friendly alternative to synthetic insecticides.^[25]^[43] Modern developments include synthetic tropinone's application in veterinary medicine, where derived atropine treats colic and bradycardia in horses and cattle, providing rapid anticholinergic relief. Additionally, tropane analogs from tropinone serve as probes for neurotransmitter receptors, such as serotonin transporters (SERT), with compounds like UCD0820 exhibiting potent inhibition (EC50 62.4 nM) and efflux activity, aiding studies on depression and addiction mechanisms.^[22]^[44]^[45]