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Analgesics

Narcotic and Non Narcotic Analgesics

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Page 1: Narcotic and Non Narcotic Analgesics

Analgesics

Page 2: Narcotic and Non Narcotic Analgesics

Nociception and PainNociception is the mechanism whereby noxious peripheral stimuli

are transmitted to the central nervous system . Pain is a subjective experience and difficult to quantify, is very

important protective phenomenon that accompanies many pathological conditions and is not always associated with nociception (or) an unpleasant sensory and emotional experience

with actual or potential tissue damage. Superficial:

- Stimulation of skin & mucous membranes - Fast response

Deep: - Arises from muscles, joints, tendons, heart ..etc .

- Slow responseAnalgesia – absence of pain

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Pain receptors in our bodies are nerves that transmit pain. These are free nerve endings located in various body tissues that respond to thermal, mechanical and chemical stimuli.

When stimulated, these pain receptors generate an impulse. The pain results of various impulses arriving at the spinal cord and the brain.

When tissues become injured, they release chemicals called prostaglandins and leukotrienes that make the pain receptors more sensitive and thus causing pain.

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Acute Vs chronicAcute pain Chronic painSudden onsetTemporary (disappears once stimulus is removed) can be somatic, visceral, referredAssociated anxietyPhysiological responses to acute pain include increased RR, HR, BP and reduction in gastric motility – sympathetic response)

Persistent – usually lasting more than six monthsCause unknown – may be due to neural stimulation or a decrease in endorphinsPhysiological responses are less obvious especially with adaptation.Psychological responses may include depression

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Nociceptive Vs Neuropathic

Nociceptive pains result from activation of nociceptors (Pain receptors)Neuropathic pains result from direct injury to nerves in the peripheral nervous system

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Somatic Vs VisceralSomatic painSuperficial: stimulation of receptors in skinDeep: stimulation of receptors in muscles, joints and tendons

Visceral painStimulation of receptors in internal organs, abdomen and skeletonOften poorly localised as fewer receptors located in visceraVisceral pain can be referred.

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Referred painPain experienced at a point distant to its point of originArea of referred pain is supplied by same spinal segment as actual site of painBrain misinterprets signals as coming from somatic regionsKnowledge of different types of referred pain is important in clinical diagnosis because in many visceral ailments the only clinical signs is referred pain.

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Somatogenic Vs Psychogenic

Somatogenic pain is a pain originating from an actual physical cause e.g. trauma, ischaemia etc

Psychogenic pain is pain for which there is no physical cause. It is not however imaginary pain and can be as intense as somatic pain.

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Pain pathwayThere are four processes in the pain pathway

TransductionNoxious stimuli translated into electrical activity at sensory nerve endingsTransmissionPropagation of impulses along spinothalamic pathway .

ModulationTransmission is modifiedPerceptionAffective / motivational aspect

Each of these processes present a potential target for analgesic therapy

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Cell Membrane Phospholipids

Arachidonic Acid

Endoperoxides

Thromboxane

Prostaglandins Prostacyclin

Toxic Oxygen Radicals

Cyclo-oxygenaseCOX

Phospholipase

Tissue Trauma

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Action of cyclo-oxygenase

COX – 1 enzyme

Constitutive pathway (stable conc)

phospholipid

Arachidonic acid

Prostaglandins associated with normal body functions

e.g. prostaglandin E2 (for kidney function), prostaglandin I2 (for stomach protection)

COX-2 enzyme

Induced pathway

phospholipid

Arachidonic acid

Inflammatory prostaglandins

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NSAIDS: Mode of actionNSAIDS block both COX-1 and COX-2This accounts for most of the side effects of NSAIDSDifferent types of NSAIDS have different specificities for COX-1 and COX-2This contributes to differences in side effects between the NSAIDS.

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COX-1

Gastric ulcers

Bleeding

Acute renal failure

COX-2

Reduce inflammation

Reduce pain

Reduce fever

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Analgesics / Pain killers are common pain relievers acting centrally to elevate pain threshold without disturbing consciousness or altering other sensory modalities.

Many analgesics also have antipyretic properties as well. They can be used to reduce fever

Some analgesics are also anti-inflammatory and anti-clotting drugs as wellClassification:

)i (Opioid analgesics / Narcotic analgesics / Morphine like substances – predominantly acts on CNS

)ii (Non-opioid analgesics / NSAID – predominantly acts on peripheral nerves

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Aspirin Heroin

Codeine

OpiumIbuprofen

Acetaminophen

MildAnalgesics / OTC

StrongAnalgesics

Analgesics

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Mild analgesicsThey work by blocking the enzyme-controlled synthesis of prostaglandins.

The main effects of prostaglandins are:The constriction of blood vessels, which helps increase the body temperature.

Direct effect on the body’s heat regulating centre, hypothalamus, which produces fever.

Increase of the permeability of capillaries which allows water to pass to the tissue and cause pain and swelling.

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Natural Painkillers

They are produced naturally in the body.

Endorphins and Enkephalins are the natural opiates found in the part of the brain and the

spinal cord that transmit pain impulses .

They are able to bind to neuro-receptors in the brain and produce relief from pain.

The temporary loss of pain immediately after an injury is associated with the production of these chemicals.

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Strong analgesicsThey temporarily bind to the opiate receptor sites in the brain preventing the transition of pain impulses

-The opium alkaloids :Opiate: it is a natural or synthetic drug that exerts actions on the body similar to those induced by morphine.Narcotic: is a term generally used for drugs that have both a narcotic and analgesicNarcotic analgesics mainly acts by opioid receptor (GPCR),

)i (Mu – responsible for most of the analgesic effects of opioids, and for some major unwanted effects like respiratory depression, euphoria, sedation and dependence

)ii (Kappa – Contribute to analgesia at the spinal level, and may elicit sedation and dysphoria, but produce relatively few unwanted effects, and do not contribute to dependence

)iii (Delta- important in the periphery, but may also contribute to analgesia

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)iv( σ-receptors - not true opioid receptors, but are the site of action of certain psychotomimetic drugs, with which some opioids interact Note:

All opioid receptors are linked through G-proteins to inhibition of adenylate cyclase .

They also facilitate opening of K+ channels (causing hyperpolarisation), and inhibit opening of Ca2+ channels (inhibiting transmitter release) .

These membrane effects are linked to the decrease in cAMP formation.

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Pyrrole acetic acid Derivatives

R Name

CH3 Tolmetin

Cl Zomepirac

Aryl aceticacid derivatives

Aryl Propionicacid derivatives

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Heteroaryl acetic / Propionic acid derivatives

Oxicams

Selective COX-2 inhibitors

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Miscellaneous

Newer agents

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(i )Synthesis of Salicylates

a( Synthesis of Aspirin

b( Synthesis of Salsalate

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c( Synthesis of Sulphasalazine

(ii )Synthesis of P-amino phenol derivativesa( Synthesis of Phenacetin

b( Synthesis of paracetamol

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(iii )Synthesis of 3,5-pyrazolidine diones

a( Synthesis of phenylbutazone

(iv )Synthesis of Anthranilicacid derivatives

a( Synthesis of Flufenamic acid

b( Synthesis of Mefenamic acidUllman condensation is type of Aromatic amination reaction between aryl halogen acids and

aryl amines

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(v )Synthesis of Indole acetic acid derivatives

a( Synthesis of Indomethacin

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(vi )Synthesis of Indene acetic acid derivativesa( Synthesis of Sulindac

Reformatski is an organic reaction, which condenses aldehydes or ketones with α-halo esters using a metallic zinc to form β-hydroxy-esters.

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Vii( Synthesis of Pyrrole acetic acid Derivativesa( Synthesis of Tolmetin Sodium

b( Synthesis of Zomepirac

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Viii( Synthesis of Aryl acetic acid derivativesa( Synthesis of Ibufenac

b( Synthesis of Diclofenac

Wolf-kishner reduction is a chemical rxn that fully reduces a ketone or aldehyde to an alkane. It involved heating the hydrazine with Na-ethoxide in a sealed vessel

at about 180 °C. Diethylene glycol (DEG) is usually used as solvent.

Willgerodt oxidation is is an organic rxn converting an aryl alkyl ketone to the corresponding amide and carboxylic acid as side rxn product by reaction with Sulphur ,

Con. NH4OH and pyridine.

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ix( Synthesis of Aryl propionic acid derivativesa( Synthesis of Ibuprofen

b( Synthesis of Fenoprofen

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X( Synthesis of Heteroaryl acetic / propionic acid derivativesa( Synthesis of Caprofen

Xi) Synthesis of Oxicamsa( Synthesis of Piroxicam

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Xii( Synthesis of COX-2 inhibitorsa( Synthesis of Celecoxib

Xiii( Synthesis of Miscellaneous agentsa( Synthesis of Nimesulide

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SAR of Salicylates

-a) Subst. on either the –COOH or –OH grp may affect the potency and toxicity.-b) Reducing the acidity of –COOH grp retains the analgesic action but is devoid of

-Anti-inflammatory property. E.g. Salicylamide-c) Placing the –OH grp meta / para to –COOH grp totally abolishes the analgesic activity.

d) Subst. with halogens on the aromatic ring will increases the potency as well as- toxicity.

e) Subst. of aromatic ring at the 5th posn of salicylic acid increases the anti-inflammatory activity. E.g. Diflunisal

SAR of p-amino phenol derivatives

a) Esterification of the phenolic grp with methyl or propyl grps produces derivatives with

greater side effects than ethyl derivatives.b) Subst. on the N2 atom which reduces the basicity, and also reduce activity unless the

subst. is metabolically labile. e.g. acetylc) Amides derived from –COOH are less active or inactive. e.g. N-phenyl benzamides.

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SAR of 3,5-pyrazolidinediones

a) Pharmacological activity is related to the acidic H at 4th posn. Thus, presence of dicarbonyl grp at 3rd & 5th posn increases the acidity of H atom at the 4th posn.

b) Decreasing or eliminating the acidity by removing the acidic H at the 4th posn may completely abolishes anti-inflammatory activity. e.g. 4,4-dialkyl derivatives.c) If acidity is increased too much, anti-inflammatory & Na retaining activity decreases,

while other property such as the uricosuric effect increases. d) A single alkyl grp at the 4th posn increases the anti-inflammatory activity. Although n-butyl group increases the activity more.e) Presence of keto group in the γ-posn of the butyl side chain produces the active compound with better anti-inflammatory activity.

f) Presence of γ-OH-n-butyl derivative possesses pronounced uricosuric activity but gives lesser anti-inflammatory activity.

g) Subst. of 2-phenyl thio ethyl group at the 4th posn produces anti-gout activity e.g. Sulphin pyrazone.i) Presence of both phenyl group is essential for both anti-inflammatory & analgesic activity.g) m-Subst. in one of the aryl rings gives inactive compounds, but p-Subst. with –CH3, -Cl ,

NO2 or OH in one of the phenyl rings retains the activity.h) Replacement of one of the N2 atom with an O2 atom yields Isoxazole analogs which are as

active as pyrazolidinediones.

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SAR of Anthranilic acid derivatives

•a) The position of the –COOH group is more important for the activity whereas the m & p-amino benzoic acid analogs are not active.

b) Replacement of –COOH group with the Isosteric tetrazole results in the retention of anti-inflammatory activity.

c) Subst. on the anthranilic acid ring generally decreases the activity.

d) Subst. on the N-aryl ring can leads to conflicting results. (i.e.) In the UV erythema assay for anti-inflammatory, the order of activity was, 3’> 2’> 4’ for monosubst. With CF3 grp (Flufenamic acid) being particularly potent.

e) The opposite order of activity was observed in Rat Paw oedema assay, that is 2’-Cl > 3’-Cl analogs.

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f) In disubst. derivatives, the nature of two subst. is the same, 2’,3’-disubst. appears to be the most effective. E.g. Mefenemic acid

g) The –NH group of anthranilic acid is essential for the activity, since replacement of –NH group with O, CH2, S, SO2, N-CH3 or NCOCH3 functionality significantly reduces the activity.

SAR of Aryl alkanoic acid derivatives

a) All the agents posses a centre of acidity, which can be represented by –COOH, an enol, hydroxamic acid, sulphonamide or tetrazole.

b) The centre of acidity is generally located one carbon atom adjacent to a flat surface represented by an aromatic / heteroaromatic ring.

c) The distance between these centres is critical because increasing this distance to 2 or 3 carbons generally decreases the activity.

d) Susbt. of –CH3 group on the carbon atom separating the aromatic ring tends to increase the anti-inflammatory activity.

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SAR of Indole acetic acid derivatives

a) Replacement of the –COOH group at 3rd posn with other acidic functionalities decreases the activity. Amide analogs are inactive.

b) Subst. at R1, useful for increasing anti-inflammatory activity are ranked as C6H4CH2 > CH3 > Hc) Acylation of the Indole Nitrogen with aryl / alkyl carboxylic acids results in the decreasing of activity.

d) The N-benzoyl derivatives subst. in the Para position with F, Cl, CF3 & S-CH3 groups are the most active.

e) At the 5th position, X-subst. activity are ranked as OCH3 > F > N(CH3)2 > CH3 > COCH3 > H than the unsubstituted analogs.

f) Presence of Indole ring nitrogen is not essential for activity because the corresponding 1-benzylidenylindene analogs (i.e. Sulindac) is also active.g) CH3 group at 2nd position are more active than aryl subst. analogs.

h) Subst. of CH3 group at the α-position of the acetic acid side chain leads to equally active analogs.

i) Anti-inflammatory activity is displayed only by the dextrorotatory enantiomer (25 times more active than phenylbutazone)

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SAR of Pyrrole aceticacid derivatives

a) Replacement of p-Tolyl group with p-chloro benzoyl group produces little effect on activity.

b) Introduction of –CH3 group in the 4th position and p-chloro benzoyl analog (i.e. Zomepirac) was 4 times as potent as Tolmetin.

SAR of Oxicams

a) Most active analogs have subst. –CH3 group on the N2 and electron withdrawing subst. like Cl, CF3 on the anilide phenyl group.

b) Introduction of heterocyclic ring in the amide chain significantly increases the anti-inflammatory activity. (e.g.) Sudoxicam (2-thiazolyl ring) is potent than Indomethacin .

c) Interchanging of benzene ring with Thiophene gives biologically active compounds. (e.g.) Tenoxicam.

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SAR of COX-2 Inhibitors

-Diaryl heterocycle with Cis-Stilbene moiety and changes in the Para position of one of the aryl rings play an important role in the COX-2 selectivity .

)e.g (Celecoxib – SO2NH2 grpParecoxib – SO2NHCOCH3 grp (Prodrug for Valdecoxib)Rofecoxib and Etoricoxib – SO2CH3 grp

-The oxidation state on the sulphur is important for selectivity. -Sulfones and Sulfonamides are selective for COX-2 but Sulfoxides and Sulfides are not.

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Narcotic Analgesics / Opioid analgesicsThese are naturally occurring, semi-synthetic and synthetic drugs, which have morphine like action (i. e) relief from pain and depression of CNS associated with drug dependence (Mental and Physical) and withdrawal side effects.

Therapeutic Uses -Management of pain (Acute / Chronic / Severe)

-Cough Suppression -Treatment of Diaarhoea.

-Management of acute pulmonary oedema & acute MI -Pre-operative medication & Intra-operative adjunctive agents in anaesthesia

Side effects -Respiratory depression

-Physical dependency & addiction -Nausea, Vomiting & Constipation

Classification I) Morphine analogs

R’ R NameH H Morphine

H C2H5 Ethyl morphine

H CH3 Codeine

COCH3 COCH3 Heroine

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Ia( Hydromorphone derivatives

R’ R Name

H OH Hydromorphone

OH H Oxymorphone

H -OCH3 Hydrocodone

OH -OCH3 Oxycodone

Ib( Dihydromorphine derivatives

R Name

H Dihydromorphine

-CH3 Dihydrocodeine

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Ic( Morphinan derivatives

R’’ R’ R Name

-CH3 H H Levorphanol

-CH2-CH=CH2 H H Levallorphan

OH H Buterphanol

-CH3 H -CH3 Dextromethorphan

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R4 R3 R2 R1 Name

H COOC2H5 CH3 Meperidine

H COOC2H5 CH3 Bemidone

H COOCH(CH3)2 CH3 Properidone

H COC2H5 CH3 Ketobemidone

CH3 COOC2H5 CH3 Alphaprodine

H COOC2H5 Anileridine

H H Fentanyl

II( Meperidine / Phenyl )ethyl( Piperidines

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R4 R3 R2 R1 Name

CH3 COOCH3 Lofentanil

H CH2OCH3 Sufentanil

H CH2OCH3 Alfentanil

H COOC2H5 Diphenoxylate

H OH Loperamide

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III( Methadone / Diphenyl heptanones

R4 R3 R2 Name

-CH2-CH(CH3)N(CH3)2 COC2H5 C6H5 Methadone

-CH(CH3)CH2N(CH3)2 COC2H5 C6H5 Isomethadone

-CH2-CH2N(CH3)2 COC2H5 C6H5 Normethadone

COC2H5 C6H5 Dipanone

-CH2-CH(CH3)N(CH3)2 C6H5 α-Acetyl Methdone

C6H5 Dextromoramide

COC2H5 C6H5 Phenadoxone

-CH(CH3)-CH2N(CH3)2 OCOC2H5 CH2 C6H5 Propoxyphene or Dextropropoxyphene

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R1 R Name

CH3 CH2-CH=C(CH3)2 Pentazocine

CH3 CH2-CH2-C6H5 Phenazocine

CH3 Cyclazocine

C=O Ketazocine

CH3 CH3 Metazocine

IV( Benzazocine / Benzomorphan analogs

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V( Miscellaneous

Vi( Newer Drugs

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VII( Narcotic Antagonist

R2 R1 R Name

H -CH2-CH=CH2 OH Nalorphine

OH -CH2-CH=CH2 -C=O Naloxone (Single = Bond)

OH -C=O Naltrexone (Single = Bond)

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Synthesis of Morphine Analogs

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Bischler-Napaieralski reaction is an intramolecular electrophilic aromatic substitution reaction that allows for the cyclization of β-arylethylamides or β-Arylethylcarbamates. The reaction is most notably used in the synthesis of isoquinolines in presence of POCl3.

Birch reduction is the organic reduction of aromatic rings in liquid ammonia with sodium, lithium or potassium and an alcohol, such as ethanol and tert-butanol. This reaction is quite unlike catalytic hydrogenation, which usually reduces the aromatic ring all the way to a cyclohexane. It converts aromatic compounds having a benzenoid ring into a product, 1,4-cyclohexadienes, in which two hydrogen atoms have been attached on opposite ends of the molecule.

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Synthesis of Levallorphan & Levorphanol

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Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group (aldehyde or ketone) followed by a dehydration reaction in which a molecule of water is eliminated (hence condensation). The product is often an alpha, beta conjugated enone. The catalyst is usually a weakly

basic amine

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Synthesis of Meperidines / Phenyl)ethyl(PiperidinesSynthesis of Meperidine / Pethidine

Synthesis of Ketobemidone

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Synthesis of Anileridine

Synthesis of Fentanyl Citrate

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Synthesis of Alfentanil

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Synthesis of Lofentanil

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Synthesis of Methadone

Synthesis of Dextromoramide

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Synthesis of Benzazocin / Benzomorphan analogs

Synthesis of Pentazocine

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Synthesis of Miscellaneous agents

Synthesis of Buprenorphine

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Synthesis of Narcotic Antagonists Synthesis of Nalorphine or N-allyl morphine

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Synthesis of Narcotic AntagonistsSynthesis of Naloxone and Naltrexone

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SAR of Morphine

(i )Modification on alicyclic ring:-The alcoholic OH group at C-6 when methylated, esterified, oxidized, removed or

replaced by halogen means analgesic activity and toxicity increases .-Saturation of the double bond at C-7 results in more potent compound. (e.g) .

Dihydromorphine, Dihydro codeine -The 14-β-OH group generally increases μ-agonistic property and decreases the antitussive

activity .-Bridging of rings through ethylene linkage gives potent derivatives. (e.g) Buprenorphine,

Etorphine (1000 times more potent than morphine as μ-agonist) (ii )Modification on phenyl ring:

-Aromatic phenyl ring is essential for activity .-Modification on phenolic –OH group decreases the activity .-Any other substitution on phenyl ring decreases the activity .

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(iii )Modification of 30 Nitrogen atom: -Presence of 30 Nitrogen atom is necessary for good opioid activity. -The size of the N-substitution only determines the compounds potency, it’s agonistic and it’s antagonistic property. (e.g) N-CH3 leads to good agonistic property.Increased the size of substitution by 3-5 carbon atoms lead to antagonistic property. Still larger substitution on N-atom lead to agonistic property. N-allyl and N-cyclo alkyl group lead to antagonistic property

(iv )Modification of Epoxide / Ether bridge:-Removal of 4,5-epoxide bridge in morphine results in morphinans .

-Only levo isomer of morphinan possess opioid activity (e.g) levorphanol – more potent than morphine. Dextro isomer possess anti-tussive activity .

.

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-

SAR of Phenyl )ethyl(piperidines / Meperidine analogs: -Replacement of C-4 phenyl group of meperidine by H, alkyl, aryl, aralkyl and heterocyclic group decreases the analgesic activity. -Introduction of m-OH group on the phenyl ring increases the activity. -Presence of phenyl and ester group at 4th position of 1-methyl piperidine results in optimum activity. -Replacement of the carbethoxy group in Mepiridine by acyloxy group gave better analgesic as well as spasmolytic. -Replacement of phenyl group by phenylethyl derivative is seems to be 3-times as active as Mepiridine. The amino congener is 4 times more active (Anileridine)Enlargement of piperidine ring to 7-membered hexahydro azepine is less active but has low incidence of side effects. (e.g) Proheptazine -Contraction of piperidine ring to pyrrolidine gives more active compound but causes abuse liability (e.g) Alphaprodine and Procilidine. -In fentanyl, the phenyl and acyl groups are separated by Nitrogen. It is 50 times stronger than morphine with minimal side effects. It’s short duration of action makes it well suited for use

in anaesthesia .-The C-3 methyl analog with an ester group at C-4 like lofentanil is 8400 times more potent

than Meperidine as an analgesic. P-Chloro analog loperamide cannot penetrate BBB sufficiently to produce analgesia.-Diphenoxylate, a structural hybrid of Meperidine and Methadone is devoid of analgesic

activity. It is effective in the treatment of diarrhoea .

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SAR of Diphenyl Heptanone / Methadone analogs :-The levo Isomer of Methadone and Isomethadone are twice as effective as their racemic

mixture .-Removal of any one of the phenyl group sharply decreases the activity .

-Placement of m-OH group in any of the phenyl ring decreases the analgesic activity .-Replacement of terminal dimethylamino group at R4 by piperidine group decreases the

activity .Replacement of propionyl group at R4 by H, OH or acetyloxy group leads to decreased activity, where as amide, pyrrolidinoyl, morpholino group increased the activity by several times (e.g) Dextromoramide

SAR of Benzomorphan / Benzazocine analogs: -Trimethyl compound (R1=R2= CH3) is more active than dimethyl (R1=H, R2= CH3)

compound .-Placement of N-phenyl ethyl results in more activity than N-Methyl compound .

-Placement of –OH group at C-9, decreases the activity .-N-allyl or N-cyclo propyl methyl group confers antagonistic activity (e.g) Levorphanol,

Naloxone and Naltrexone .