Exhaustive Study Notes on Heterocyclic Compounds (Pyridine, Quinoline, Isoquinoline, Indole, and more)

PHARMACEUTICAL ORGANIC CHEMISTRY-III: PYRIDINE

• Introduction to Pyridine ($C_5H_5N$):     * Pyridine is an important six-membered heterocyclic compound containing one nitrogen atom.     * Natural Occurrence: It is present in natural products such as pyridoxine, nicotinic acid, nicotine, and piperine. It is also found in the light oil fraction of coal tar and bone oil.     * Nature: It is basic in nature and reacts with acids to form salts. Illustration: $C_5H_5N + HCl \rightarrow C_5H_5N.HCl$ (Pyridine Hydrochloride).     * Amine Classification: It is neither a primary nor a secondary amine. Evidence for this includes its lack of reactivity with nitrous acid ($HNO_2$) and acetyl chloride.

• Synthesis of Pyridine and Derivatives:     1. From Acetylene: A mixture of acetylene and hydrogen cyanide passed through a red hot tube producing pyridine: $2CH \equiv CH + HC \equiv N \rightarrow C_5H_5N$.     2. From Pyrrole: Heating pyrrole with methylene dichloride ($CH_2Cl_2$) in the presence of sodium ethoxide ($C_2H_5ONa$).     3. From Pentamethylene Diamine Hydrochloride: Heating this compound causes cyclization into piperidine, which is then oxidized using concentrated sulphuric acid to form pyridine.     4. From Tetrahydrofurfuryl alcohol: Reacting the alcohol with ammonia over $Al_2O_3$ at $773\,K$.     5. From Picolines ($\alpha, \beta, \gamma$): Oxidation of picolines with $K_2Cr_2O_7 / H_2SO_4$ yields picolinic acid, which undergoes decarboxylation with calcium oxide ($CaO$) to produce pyridine.     6. Hantzsch Synthesis: Condensation of a $\beta$-Ketoester (2 moles) with an aldehyde and ammonia (e.g., yielding 2,4,6-Trimethyl Pyridine).     7. Diels-Alder Reaction: Addition of cyanonitriles to dienes at $450\,^{\circ}C$; for example, 1,3-Butadiene and CN giving 2-Cyano Pyridine.     8. Guareschi Reaction: Condensation of a $\beta$-diketone (like 2,4-Pentanedione) with the sodium derivative of cyanoacetamide.

• Chemical Properties and Reactions of Pyridine:     * Physical State: Colorless hygroscopic liquid (b.p. $388\,K$) with an unpleasant odor. Miscible with water and organic solvents.     * Basic Character: Acts as a base due to the lone pair on the nitrogen atom. Forms salts with acids (Pyridine hydrochloride) and quaternary salts with alkyl halides (Pyridine methiodide).     * Basicity Comparison:         * Weakness vs. Aliphatic Tertiary Amines: Pyridine is weaker because the nitrogen lone pair is in an $sp^2$ hybrid orbital, whereas in aliphatic amines, it is in an $sp^3$ orbital. Electrons in $sp^2$ are closer to the nucleus and less available for sharing.         * Strength vs. Pyrrole: Pyridine is more basic than pyrrole because the nitrogen lone pair in pyrrole is involved in the aromatic sextet, whereas in pyridine, it remains available.     * Electrophilic Substitution Reactions:         * Pyridine is very unreactive. It requires drastic conditions for nitration, sulphonation, and halogenation. It does not undergo Friedel-Crafts acylation.         * Position: Substitution occurs primarily at the $\beta$-position (3-position).         * Reason for Low Reactivity: Nitrogen is more electronegative than carbon, withdrawing electron density from the ring. In acidic conditions, the formation of the pyridinium ion (positive charge on nitrogen) further deactivates the ring.         * Specific Reactions:             * Nitration: $KNO_3, H_2SO_4$ at $573\,K$ yields 3-Nitro pyridine.             * Bromination: $Br_2$ at $573\,K$ yields 3-Bromo pyridine and 3,5-Dibromo pyridine.             * Sulphonation: $H_2SO_4$ at $623\,K$ yields Pyridine-3-sulphonic acid.

• Nucleophilic Substitution Reactions:     * More reactive toward nucleophiles than electrophiles. Substitution occurs at positions 2 and 4 (position 2 is preferred).     * Amination (Chichibabin Reaction): Reaction with sodium amide ($NaNH_2$) at $375\,K$ yields 2-Amino pyridine.     * Other Nucleophilic Reactions:         * With $KOH$ at $300\,^{\circ}C$: yields 2-Pyridone.         * With lithium reagents ($LiC_6H_5$ at $383\,K$): yields 2-Phenyl Pyridine.         * With n-butyl lithium at $383\,K$: yields 2-Butyl Pyridine.

• Reduction and Oxidation:     * Reduction:         1. With $Na/C_2H_5OH$ or $H_2/Pt$: yields Piperidine.         2. With $HI$ and Red P at $573\,K$: Ring opens to form n-pentane.         3. Birch Reduction ($Na, NH_3, C_2H_5OH$): yields 1,4-dihydropyridine.         4. With $LiAlH_4$: yields 1,2-dihydropyridine.     * Oxidation: Use of peracids (like perbenzoic acid) yields Pyridine N-oxide.

• Free Radical Reactions:     * Reaction with benzoyl peroxide: yields a mixture of 2-phenyl (54%), 3-phenyl (32%), and 4-phenyl (14%) pyridine.     * In the presence of acid: Benzoyl peroxide preferentially yields 2-phenyl pyridine (80%).

• Medicinal Uses of Pyridine:     1. Antihistamines: Mepyramine, Pheniramine, Chlorpheniramine.     2. Anticholinergics: Tropicamide (used in ophthalmology as a mydriatic).     3. Antacids (Proton Pump Inhibitors): Omeprazole, Lansoprazole, Pantoprazole.     4. Antihyperlipidemics: Nicotinic acid (Niacin) and Nicotinamide.     5. Antituberculars: Isoniazid, Ethionamide.     6. Antifungals: Ciclopirox.     7. Anti-HIV: Indinavir.     8. Antidiabetics: Rosiglitazone, Pioglitazone.     9. NSAIDs: Piroxicam, Tenoxicam, Etoricoxib.     10. CNS/Respiratory Stimulants: Nikethamide.     11. Antihypertensives (Calcium Channel Blockers): Nifedipine, Amlodipine, Felodipine.     12. Anti-Arrhythmics: Disopyramide phosphate.

QUINOLINE

• Introduction to Quinoline ($C_9H_7N$):     * Six-membered heterocyclic system fused with a benzene ring. Colorless hygroscopic liquid; darkens on light exposure. High boiling solvent.     * Sources: First obtained from alkaline decomposition of quinine; present in coal tar and bone oil.

• Synthesis of Quinoline:     1. Skraup Synthesis: Heating aniline, glycerol, nitrobenzene (oxidizing agent), and concentrated sulphuric acid in the presence of $FeSO_4$.     2. Friendlander Synthesis: Alkaline condensation of o-aminobenzaldehyde with acetaldehyde.     3. Ring Enlargement: Disubstituted indenone treated with sodium azide ($NaN_3$) and $H_2SO_4$ gives quinolinones.     4. From Indole: Treatment with chloromethylene and methyl lithium.     5. Dobner Synthesis: Modified Skraup using simple aldehydes/ketones as precursors.     6. Pfitzinger Synthesis: Condensation of aldehydes/ketones with isatin in basic media.

• Chemical Reactions of Quinoline:     * Aromaticity: Has 10 $\pi$-electrons; resonance energy of $264.9\,kJ\,mol^{-1}$.     * Basicity: Slightly more basic than aniline but less than pyridine.     * Electrophilic Substitution: Occurs at positions 5 and 8 of the benzene ring.         * Nitration ($HNO_3 + H_2SO_4$): yields 5-nitro and 8-nitro quinoline mixture.         * Nitration (Nitric acid + Acetic anhydride): yields 3-nitro quinoline.         * Sulphonation: yields Quinoline-5-sulphonic acid (major) and 8-sulphonic acid (minor).         * Bromination: At 5 and 8 positions in acidic conditions; in vapor phase, gives 3-bromo; at $773\,K$, gives 2-bromo.     * Nucleophilic Substitution: Occurs at position 2 (e.g., Tschitschibabin reaction with $NaNH_2$ yields 2-aminoquinoline).     * Oxidation: With $KMnO_4$ yields quinolinic acid, which decarboxylates to nicotinic acid.     * Reduction:         1. $LiAlH_4$ or $Na$ in liq. $NH_3$: yields 1,2-dihydroquinoline.         2. $Sn/HCl$ or $H_2/Ni$: yields 1,2,3,4-tetrahydroquinoline.         3. $H_2/Pt$ in $CH_3COOH$: yields decahydroquinoline.

• Medicinal Uses of Quinoline:     1. Antimalarials: Quinine, Cinchonine, Primaquine.     2. Antiprotozoals: Quiniodochlor, Iodoquinol.     3. Anesthetics: Cinchocaine (local/spinal).     4. Anthelmintics: Oxaminiquine, Praziquantel.     5. Anti-HIV: Saquinavir (Reverse transcriptase inhibitor).     6. UTI Infections: Oxolinic acid, Norfloxacin.

ISOQUINOLINE

• Introduction: Degradation product of alkaloids like papaverine and narcotine; found in coal tar.

• Synthesis:     1. Bischler-Napieralski Reaction: Cyclodehydration of $\beta$-phenylethylamine with $POCl_3$ followed by dehydrogenation ($Pd-C$).     2. Pictet-Spengler Synthesis: Condensation of $\beta$-arylethylamine and an aldehyde with excess $HCl$ at $373\,K$.     3. Pomeranz-Fritsch Synthesis: Condensation of aromatic aldehyde with aminoacetal followed by sulphuric acid cyclization.

• Reactions:     * Basicity: Stronger base than quinoline because the nitrogen is not directly attached to the benzene ring, making the lone pair more available for protonation.     * Electrophilic Substitution: Occurs at position 5 (major) and position 8 (minor). Bromination/mercuration leads to 4-derivatives.     * Nucleophilic Substitution: Occurs mainly at position 1 (e.g., 1-Aminoisoquinoline via Tschitschibabin).     * Oxidation: Alkaline $KMnO_4$ yields an equimolar mixture of phthalic acid and cinchomeronic acid.     * Reduction: Pyridine ring reduces more readily ($Sn/HCl$ yields 1,2,3,4-tetrahydro-isoquinoline; catalytic hydrogenation yields octahydroisoquinoline).

• Medicinal Uses: Antihypertensives (Quinapril, Debrisoquine), Anesthetics (Dimethisoquin), Disinfectants (N-laurylisoquinolinium bromide), Vasodilators (Papaverine).

INDOLE (BENZOPYRROLE)

• Introduction: Fused heterocyclic ring of benzene and pyrrole; IUPAC name 1H-Benzo[b]pyrrole. Found in coal tar, jasmin flowers, and the amino acid L-tryptophan.

• Synthesis:     1. Fischer Indole Synthesis: Heating phenylhydrazone of an aldehyde/ketone with acid catalysts ($PPA, H_2SO_4, ZnCl_2$).     2. Madelung Synthesis: Intramolecular cyclization of N-acylated o-alkylanilines with strong base at high temps.     3. Reissert Synthesis: Condensation of o-nitrotoluene with diethyl oxalate followed by reduction ($Zn/CH_3COOH$).     4. Bischler Synthesis: Heating phenacyl bromide with excess aniline.

• Reactions:     * Electrophilic Substitution: Maximum electron density at position 3. Substitutions include nitration (3-nitro), sulfonation (indole-2-sulphonic acid is an exceptional case mentioned in the diagram as 2-position), chlorination (3-chloro), and Mannich reaction.     * Reduction:         1. $Sn/HCl$: yields 2,3-Dihydroindole (Indoline).         2. Birch Reduction: yields 4,7-Dihydroindole.         3. $H_2, Raney\,Ni$: yields Octahydroindole.

• Medicinal Uses: Tryptophan (essential amino acid), Serotonin (neurotransmitter). Drugs: Indomethacin (anti-inflammatory), Vincristine/Vinblastine (anticancer), Sumatriptan (migraine).

ACRIDINE

• Introduction: Alkaloid from anthracene; present in coal tar. Molecular formula $C_{13}H_9N$. Shows blue fluorescence.

• Preparation:     1. Ullmann Synthesis: Condensation of primary amines with aromatic aldehydes/carboxylic acids with cyclic dehydrogenation.     2. Berntsen Synthesis: Reaction of diphenylamine with carboxylic acid in the presence of $ZnCl_2$.

• Reactions:     * Electrophilic Substitution: At C-2 and C-7.     * Nucleophilic Reaction: With $NaNH_2$ gives 9-aminoacridine.     * Oxidation: With dichromate gives acridone; with permanganate gives quinoline 2,3-dicarboxylic acid.

• Medicinal Uses: Antiseptic (Acriflavine, Proflavine), Antimalarial (Quinacrine), Anticancer (Nitracrine), Anesthesia (Bucricaine).

PURINES

• Structure: Fusion of a pyrimidine ring and an imidazole ring.
• Synthesis:     1. Fischer Method: Uric acid converted to 2,6,8-trichloropurine with $POCl_3$, followed by reduction.     2. Traube's Method: Condensation of 4,5-diaminopyrimidine with formic acid.
• Medicinal Purines: Adenine and Guanine. Uses include Anti-Leishmanial (Allopurinol), Antiviral (Acyclovir), and Cancer treatment (Mercaptopurine, Thioguanine).

PYRIMIDINE (1,3-DIAZINE)

• Synthesis:     1. Gabriel Synthesis: Urea and malonic acid yields barbituric acid; reduction with $Zn$ yields pyrimidine.     2. Whittaker Synthesis: Reduction of 2,6-dichloropyrimidine with $Pd$ catalyst.

• N-Bases Synthesis:     * Uracil: Fischer and Roeder Method (Urea + Ethylacetate) or Wheeler and Liddle (Thiourea + dioformyl acetic ester).     * Thymine: Wheeler and Liddle synthesis.     * Cytosine: Tariso synthesis or Wheeler and Johnson method.

• Medicinal Uses: Antineoplastics (5-Fluorouracil, Gemcitabine), Antihyperthyroidism (Propylthiouracil), Sulpha drugs (Sulphadiazine), Anti-AIDS (Zidovudine/AZT, Lamivudine), Antifungals (Flucytosine), Sedatives (Barbiturates like Pentobarbital).

PYRAZOLE, THIAZOLE, OXAZOLE, IMIDAZOLE, AND AZEPINES

• Pyrazole: Five-membered ring with 2 nitrogens at 1,2 positions. Synthesized via Knorr synthesis. Uses: Phenylbutazone (analgesic), Celecoxib (COX-2 inhibitor).
• Thiazole: Sulphur and Nitrogen at 1,3 positions. Synthesized via Hantzsch synthesis (alpha-haloketones + thioamides). Uses: Vitamin $B_1$ (Thiamine), Penicillins, Sulphathiazole.
• Oxazole: Oxygen and Nitrogen at 1,3 positions. Synthesized via Robinson-Gabriel synthesis. Uses: Sulfamoxol (antibacterial), Aleglitazar (diabetic).
• Imidazole: Two nitrogens at 1,3 positions. Found in Histidine and Histamine. Synthesized via Debus method. Uses: Ketoconazole (antifungal), Cimetidine (anti-ulcer).
• Azepines: Seven-membered nitrogen heterocycle. Synthesis from benzene/phenylazide. Uses: Nylon-6 production (Caprolactam), antidepressants (Imipramine), antiepileptic (Carbamazepine), Benzodiazepines (Diazepam).

QUESTIONS & DISCUSSION

• Q: Why is pyridine more basic than pyrrole?

• A: In pyridine, the lone pair on nitrogen is in an $sp^2$ orbital and does not participate in the aromatic sextet, making it available for sharing with acids. In pyrrole, the lone pair is part of the aromatic system and protonation destroys the aromaticity.

• Q: Why does electrophilic substitution of quinoline take place at the benzene ring rather than the pyridine ring?

• A: The benzene ring is more electron-rich, whereas the nitrogen-containing ring is electron-deficient due to the electron-withdrawing effect of nitrogen. Substitution occurs at positions 5 and 8 of the benzene ring.

• Q: Why is Pyridine a weaker base than aliphatic tertiary amines?

• A: Because in pyridine, the lone pair is in an $sp^2$ orbital (held more tightly by the nucleus), while in aliphatic amines, it is in an $sp^3$ orbital, making it more available for sharing with acids.

• Q: What happens when oxazole is treated with base?

• A: It gets deprotonated at the C-2 position, accompanied by ring opening, and produces an isonitrile.

• Q: Is pyridine aromatic?

• A: Yes, it is cyclic, planar, and has 6 delocalized $\pi$ electrons fulfilling Huckel's rule ($4n+2$). The lone pair on nitrogen is not part of the aromatic sextet.