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Sympathomimetic & Parasympathomimetic Drugs - Medicinal Chemistry
Source: Katzung's Basic & Clinical Pharmacology, 16th Ed. | Goodman & Gilman's Pharmacological Basis of Therapeutics
PART 1: SYMPATHOMIMETIC DRUGS
1. Parent Structure & Basic Chemistry
Phenylethylamine is the parent compound from which all sympathomimetic drugs are derived. It consists of a benzene ring with a β-CH₂-α-CH₂-NH₂ side chain.
Catecholamines are produced by adding -OH groups at positions 3 and 4 of the benzene ring (the "catechol" nucleus). Key endogenous catecholamines are dopamine, norepinephrine (NE), and epinephrine (Epi).
2. Structure-Activity Relationships (SAR) - Medicinal Chemistry Core
Three sites of chemical modification determine receptor selectivity and pharmacokinetics:
A. Substitution on the Benzene Ring
| Modification | Effect |
|---|---|
| -OH at positions 3 & 4 (catechol) | Maximum α and β activity |
| Loss of one -OH | 100-fold reduction in potency; remaining activity becomes more selective (e.g., phenylephrine = α₁ selective) |
| -OCH₃ in place of -OH (methoxamine) | Resistant to COMT; longer duration |
| No ring -OH groups (ephedrine, amphetamine) | Orally active, longer duration, CNS penetration |
The catechol -OH groups make drugs susceptible to inactivation by COMT (catechol-O-methyltransferase), so catecholamines are NOT orally active. Removal of one or both ring -OHs makes the drug orally bioavailable and CNS-penetrant.
B. Substitution on the Alpha (α) Carbon
| Modification | Effect |
|---|---|
| α-methyl group (e.g., ephedrine, amphetamine) | Blocks MAO metabolism → prolonged action; indirect-acting (releases stored NE) |
| No α-methyl (catecholamines) | Rapid metabolism by MAO |
C. Substitution on the Nitrogen (Amino Group)
| N-substituent | Receptor Preference |
|---|---|
| H (primary amine, norepinephrine) | α₁ = α₂; β₁ >> β₂ |
| -CH₃ (epinephrine) | α and β equally |
| -CH(CH₃)₂ isopropyl (isoproterenol) | β₁ = β₂ (pure β agonist) |
| Larger bulky groups (terbutaline, albuterol) | β₂ selective |
Key rule: Increasing the size of N-alkyl substituents progressively shifts selectivity from α toward β, and from β₁ toward β₂.
3. Chemical Structures of Key Sympathomimetics
Catecholamines (endogenous):
Non-catecholamine sympathomimetics:
β₂-Selective analogs (for asthma):
4. Synthesis Basics
Epinephrine (Adrenaline) - Biosynthetic pathway (also used in semi-synthesis):
Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine
Chemically:
- Start with pyrocatechol (3,4-dihydroxybenzene)
- Condense with chloroacetaldehyde to give the glycol → aminoketone → reduce to give norepinephrine
- N-methylation of norepinephrine (using formaldehyde/reduction or methyl iodide) → epinephrine
Albuterol (Salbutamol) synthesis key point:
- The catechol ring is modified by replacing the 4-OH with a -CH₂OH group (saligenin moiety)
- This resists COMT degradation → longer duration, β₂-selective
Amphetamine from ephedrine:
- Simple dehydroxylation of ephedrine (removal of the β-OH group and the 3,4-ring OH groups)
- This also explains why pseudoephedrine sales are restricted (precursor to methamphetamine)
5. Mechanism of Action & Receptor Selectivity
| Drug | Receptor | Mechanism |
|---|---|---|
| Epinephrine | α₁, α₂, β₁, β₂ | Direct agonist |
| Norepinephrine | α₁, α₂, β₁ | Direct agonist |
| Isoproterenol | β₁ = β₂ | Direct agonist |
| Albuterol/Salbutamol | β₂ selective | Direct agonist |
| Salmeterol | β₂ (long-acting) | Direct agonist |
| Phenylephrine | α₁ selective | Direct agonist |
| Clonidine | α₂ selective | Direct agonist |
| Ephedrine | α + β | Mixed (direct + indirect - releases NE) |
| Amphetamine | α + β | Indirect (releases NE from vesicles) |
| Cocaine | Blocks NE reuptake | Indirect |
6. Therapeutic Uses of Sympathomimetics
| Drug | Use |
|---|---|
| Epinephrine | Anaphylaxis, cardiac arrest, bronchospasm |
| Norepinephrine | Cardiogenic/septic shock (vasopressor) |
| Dopamine | Shock, acute heart failure (dose-dependent: DA₁ at low dose → renal dilation; β₁ at moderate; α₁ at high) |
| Dobutamine | Cardiac inotrope (β₁ selective), heart failure |
| Albuterol/Salbutamol | Bronchodilator - acute asthma (β₂ agonist, inhaled) |
| Salmeterol/Formoterol | Long-acting bronchodilator - COPD/asthma prophylaxis |
| Phenylephrine | Nasal decongestant, mydriasis, BP support |
| Clonidine | Hypertension, ADHD, opioid withdrawal |
| Midodrine | Orthostatic hypotension (prodrug → desglymidodrine) |
| Ephedrine | Nasal decongestant, hypotension under anaesthesia |
| Amphetamine/dextroamphetamine | ADHD, narcolepsy |
PART 2: PARASYMPATHOMIMETIC (CHOLINOMIMETIC) DRUGS
1. Classification
Parasympathomimetics
├── Direct-acting (bind cholinoceptors directly)
│ ├── Choline esters: ACh, Methacholine, Carbachol, Bethanechol
│ └── Alkaloids: Muscarine, Pilocarpine, Nicotine, Lobeline
└── Indirect-acting (AChE inhibitors - increase endogenous ACh)
├── Reversible: Physostigmine, Neostigmine, Pyridostigmine, Edrophonium
└── Irreversible: Organophosphates (echothiophate, sarin, parathion)
2. Chemical Structure of Choline Esters
All choline esters share the quaternary ammonium structure:
- Acetylcholine (ACh): CH₃-CO-O-CH₂-CH₂-N⁺(CH₃)₃
- Acetic acid ester of choline
- Rapidly hydrolyzed by AChE; very short action
- Methacholine: β-methyl group added to choline moiety
- More resistant to AChE hydrolysis
- Selective muscarinic agonist (β-methyl reduces nicotinic activity)
- Carbachol: Carbamic acid ester of choline (replaces acetyl with carbamoyl -NH-CO-)
- Completely resistant to AChE hydrolysis
- Activates both muscarinic and nicotinic receptors
- Bethanechol: Carbamic acid ester of β-methyl choline
- Resistant to hydrolysis + muscarinic selective (β-methyl group)
- Most clinically used choline ester
The muscarinic receptor shows strict stereoselectivity: (S)-bethanechol is ~1000× more potent than (R)-bethanechol.
3. Alkaloids as Parasympathomimetics
| Alkaloid | Source | Main Receptor | Key Feature |
|---|---|---|---|
| Muscarine | Amanita muscaria mushroom | Muscarinic | Prototype; no clinical use |
| Pilocarpine | Pilocarpus jaborandi | Muscarinic (M₃) | Tertiary amine → CNS penetrant; used in glaucoma |
| Nicotine | Tobacco | Nicotinic (NM, NN) | Low dose = stimulate; high dose = depolarizing block |
| Lobeline | Lobelia inflata | Nicotinic | Weak nicotinic agonist |
4. Indirect-Acting Agents: Anticholinesterases
Mechanism: Inhibit acetylcholinesterase (AChE) → ACh accumulates at all cholinergic synapses
Reversible AChE inhibitors:
| Drug | Structure/Type | Duration | Use |
|---|---|---|---|
| Edrophonium | Quaternary; ionic bond only | Very short (5-15 min) | Diagnosis of myasthenia gravis |
| Neostigmine | Quaternary; carbamic ester | Short | Myasthenia gravis, reverse neuromuscular block, postoperative ileus |
| Pyridostigmine | Quaternary; carbamic ester | Medium | Myasthenia gravis (preferred) |
| Physostigmine | Tertiary amine; carbamic ester | Short | Glaucoma, anticholinergic overdose (CNS penetrant) |
| Donepezil/Rivastigmine/Galantamine | Tertiary; CNS-selective | Long | Alzheimer's disease |
Irreversible AChE inhibitors (Organophosphates):
- Echothiophate (ophthalmic - glaucoma)
- Agricultural: Parathion, malathion
- Chemical warfare: Sarin, soman, VX
- Bond covalently to serine -OH of AChE ("aging" makes bond permanent)
- Treated with pralidoxime (2-PAM) if given before aging + atropine
5. Receptor Subtypes Targeted
| Receptor | Location | Effect when activated |
|---|---|---|
| M₁ | Nerves, CNS | IP₃/DAG cascade; gastric acid secretion |
| M₂ | Heart, nerve terminals | ↓ HR, ↓ AV conduction (Gᵢ) |
| M₃ | Glands, smooth muscle, endothelium | Secretion, smooth muscle contraction, NO-mediated vasodilation |
| M₄ | CNS | Inhibition of cAMP |
| Nicotinic (Nm) | Neuromuscular junction | Skeletal muscle contraction |
| Nicotinic (Nn) | Autonomic ganglia, adrenal medulla | Ganglionic transmission |
6. Therapeutic Uses of Parasympathomimetics
| Drug | Use |
|---|---|
| Bethanechol | Urinary retention, postoperative ileus (M₃ agonist) |
| Pilocarpine | Glaucoma (miosis, ↑ trabecular outflow); dry mouth (Sjogren's) |
| Carbachol | Glaucoma (ophthalmic), intraocular surgery (miosis) |
| Echothiophate | Refractory glaucoma (long-acting AChE inhibitor) |
| Neostigmine | Myasthenia gravis; reverse neuromuscular blockade; Ogilvie syndrome |
| Pyridostigmine | Myasthenia gravis (treatment of choice) |
| Edrophonium | Tensilon test for myasthenia gravis diagnosis |
| Physostigmine | Atropine/anticholinergic poisoning antidote |
| Donepezil | Alzheimer's disease (CNS AChE inhibitor) |
| Cevimeline | Sjogren's syndrome (selective M₃ agonist) |
Summary Comparison Table
| Feature | Sympathomimetics | Parasympathomimetics |
|---|---|---|
| Mimics | Sympathetic (adrenergic) NS | Parasympathetic (cholinergic) NS |
| Transmitter mimic | Norepinephrine / Epinephrine | Acetylcholine |
| Receptors targeted | α₁, α₂, β₁, β₂, DA₁, DA₂ | M₁-M₅ (muscarinic), Nm, Nn (nicotinic) |
| Parent structure | Phenylethylamine | Choline ester / Alkaloid |
| Key SAR site | N-substitution (α→β selectivity) | β-methyl reduces nicotinic activity |
| Clinical category | Vasopressors, bronchodilators, decongestants | Glaucoma, myasthenia, Alzheimer's, ileus |
- Katzung's Basic and Clinical Pharmacology, 16th Edition (Chapters 7 & 9)
- Goodman & Gilman's Pharmacological Basis of Therapeutics
- Barash's Clinical Anesthesia, 9th Edition
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