Alkaloids Phytochemistry Lecture Notes PDF

‫ﺗﺎﻟﺗﮫ ﻓﺎرم دي‬ ‫ﻣﺣﺎﺿره ‪ 7‬ﻓﺎﯾﺗ و‬ ‫(‬ ‫رﻗم اﻟﺣﻔظ )‬ ‫‪ 2‬ﺟﻧﯾﮫ وﻧص‬ ‫‪ALKALOIDS‬‬  Phytochemistry of Natural Products Overall aims of the course: Provides the students with a comprehensive knowledge of the various structural classes of natural products, methods of ext...

‫ﺗﺎﻟﺗﮫ ﻓﺎرم دي‬ ‫ﻣﺣﺎﺿره ‪ 7‬ﻓﺎﯾﺗ و‬ ‫(‬ ‫رﻗم اﻟﺣﻔظ )‬ ‫‪ 2‬ﺟﻧﯾﮫ وﻧص‬ ‫‪ALKALOIDS‬‬  Phytochemistry of Natural Products Overall aims of the course: Provides the students with a comprehensive knowledge of the various structural classes of natural products, methods of extraction, isolation and estimation (Qualitatively and Quantitatively) of these natural products.  Alkaloids The term “alkaloid” (alkali-like) is commonly used to designate basic heterocyclic nitrogenous compounds of plant origin that are physiologically active. Definition: Typical alkaloids are basic organic nitrogenous compounds of plant origin possessing physiological activity. They have complex structures and are of limited occurrence. The basicity of alkaloids is attributed to nitrogen. They have potent physiological effects and therefore, are considered as important therapeutic agents, e,g. atropine, morphine, quinine, etc …. Notes and Deviations from Definition: All alkaloids contain nitrogen, but not all compounds which contain nitrogen are classified as alkaloids. Basicity: Some alkaloids are not basic (e.g. Colchicine, Piperine, Quaternary alkaloids). Some are amphoteric (e.g. cephaline and psychotrine) or even acidic (e.g. Recinine). Nitrogen: The nitrogen in some alkaloids is not in a heterocyclic ring e.g. Ephedrine, Colchicine, Mescaline. Plant origin: Some alkaloids are derived from Bacteria, Fungi (e.g. Ergot alkaloids), Insects, Frogs, marine organisms.  CLASSIFICATION According to both the type of nitrogen and the biochemical origin, three main types of alkaloids are distinguished: 1) True (Typical) alkaloids: - Basic nitrogenous compounds, toxic with a wide range of physiological activities. - They are derived from amino acids and have nitrogen in a heterocyclic ring. - To be soluble in cell sap, they occur in plants as salts of organic acids (exception: colchicine, aristolochic acid which are not basic and have no heterocyclic ring, and the quaternary alkaloids). - e.g. Atropine. 2) Protoalkaloids (Biological amines): - This group includes simple amines in which the nitrogen is not in a heterocyclic ring. - They are derived from amino acids and basic in characters. - e.g. Ephedrine and Mescaline. 3) Pseudo alkaloids: - They are not derived from amino acid precursors but have nitrogen in a heterocyclic ring. - e.g. Steroidal alkaloids (e.g. Solanine) and purine base alkaloids (e.g. Caffeine).  Item True alk. Proto alk. Pseudo alk. NOT derived Derived from Derived from Origin from amino amino acids amino acids acids In a hetero- NOT in a hetero- In a hetero- Nitrogen cyclic ring cyclic ring cyclic ring Very weak Basicity Basic Basic bases Example Atropine Ephedrine Caffeine  Occurrence of Alkaloids Leguminosae Amaryllidaceae Loganiaceae Ranunculaceae Liliaceae Occurrence Rubiaceae Papaveraceae of alkaloids Rutaceae Lauraceae Asteraceae Apocyanaceae Solanaceae - Alkaloids are rare in lower plants. - Dicots are more rich in alkaloids than Monocots. - Some families are free from alkaloids e.g. Rosaceae and Labiatae. - Rarely an alkaloid may occur in botanically unrelated plants (Nicotine is an exception due to its simple biosynthesis).  Distribution of Alkaloids 1) In a particular organ: e.g. Reserpine in Rauwolfia roots e.g. Quinine in Cinchona bark e.g. Strychnine in Nux vomica seeds e.g. Opium alkaloids in latex of fruits of Papaver somniferum but seeds are free from alkaloids. 4) The amounts of 2) In different organs alkaloids differ at the of the same plant: Distribution different stages of e.g. Hyoscyamine of alkaloids growth and with different organs of the plant. 3) May be formed in an organ and translocated into another organ: e.g. Datura and Nicotiana alkaloids are produced in roots and translocated to leaves  Forms of Alkaloids Alkaloids are present in plants in the following forms: Free bases. Salts with organic acids e.g. oxalic and acetic acids. Salts with inorganic acids e.g. HCl and H2SO4. Salts with specific organic acids: e.g. Opium alkaloids with meconic acid. e.g. Cinchona alkaloids with cinchotannic and quinic acids. In combination with sugars (Glycosidal form) e.g. Solanine and solasonine in Solanum. N-oxides: N+ O-  Functions of Alkaloids in Plants They may be protective against insects and herbivores due to their bitterness and toxicity. They are end products of detoxification reactions to remove toxic harmful compounds. They, sometimes, act as growth regulators in certain metabolic systems. A reserve source of nitrogen in case of nitrogen deficiency. Alkaloids containing sugar moieties may supply energy. As waste products of biosynthetic processes.  Nomenclature of Alkaloids They should end with the suffix "ine“, Exception: capsaicin. Their names may refer to: The plant genus e.g. Atropine from Atropa belladonna. The plant species e.g. Cocaine from Erythroxylum coca. The common name of the drug e.g. Ergotamine from Ergot. Name of the discoverer e.g. Pelletierine discovered by Pelletier. The physiological action e.g. Emetine which is emetic. A prominent physical character e.g. Hygrine which is hygroscopic.  Prefixes: "Nor-": indicates N-demethylation or N-demethoxylation e.g. nor-pseudoephedrine and nor-nicotine. "Apo-": indicates dehydration e.g. apoatropine and apomorphine. "Iso-, pseudo-, neo-, epi-": indicate different types of isomers e.g. ephedrine and pseudo-ephedrine, vittatine and epivittatine.  Suffixes: “-dine": indicates isomerism and is used to differentiate between related alkaloids e.g. Cinchona alkaloids: quinine and quinidine, cinchonine and cinchonidine. “-ine": is used to show the less active isolysergic acid series of Ergot alkaloids e.g. ergotaminine and ergometrinine from the more potent lysergic acid series of e.g. ergotamine and ergometrine.  General Characters I- Physical Properties 1. Condition 2. Color 3. Solubility 4. Optical activity and isomerism 1. Condition: Most alkaloids are crystalline solids. Few alkaloids are amorphous solids e.g. emetine. Some are liquids that are either: Volatile (without oxygen) e.g. nicotine and coniine Non-volatile e.g. pilocarpine, pelletierine and hyoscine 2. Color: The majority of alkaloids are colorless but some are colored (those having complex highly aromatic structures and high degree of unsaturation) e.g. Colchicine and berberine are yellow. Canadine is orange. Betanine is red. 3. Solubility: Both alkaloidal bases and their salts are soluble in alcohol. Generally, alkaloidal bases are soluble in organic solvents (e.g. CHCl3 and ether) and insoluble in water. *Exceptions: Bases soluble in water: caffeine, ephedrine, codeine, colchicine, pilocarpine and quaternary bases e.g. d-tubocurarine. Bases insoluble in certain organic solvents: - Morphine is sparingly soluble in ether. - Theobromine and theophylline are insoluble in benzene.  Generally, alkaloidal salts are usually soluble in water and insoluble or sparingly soluble in organic solvents. *Exceptions: Salts soluble in organic solvents: lobeline HCl and apoatropine HCl are soluble in chloroform (CHCl3). Salts insoluble in water: quinine monosulphate. Colchicine is a weak base. It is soluble as its base or HCl salt in water and in chloroform. 4. Optical activity and isomerism: Many alkaloids contain one or more chiral (asymmetric) carbon centers in the molecule so they show optical activity. Normally, one of the optical isomers is found in nature, although in a few cases racemic mixtures are present. (−) or l-isomer (levorotatory): rotates the plane polarized light to the left. (+) or d-isomer (dextrorotatory): rotates the plane polarized light to the right.  The designations (l and d) differs from the designations (L and D) which refer to the absolute steric configuration, not to optical activity. Usually, the (−)-isomer (levorotatory) has greater pharmacological activity than the (+)-isomer (dextrorotatory) of the same alkaloid. e.g. l-ephedrine is 3.5 times more active than d-ephedrine. l-ergotamine is 3-4 times more active than d-ergotamine. *Exceptions: d-tubocurarine is more active than the corresponding l-form. Quinine (l-isomer) is anti-malarial and its d-isomer (quinidine) is anti- arrythmic. The racemic (optically inactive) dl-atropine is physiologically active.  General Characters II- Chemical Properties 1. Nitrogen and oxygen in alkaloids 2. Basicity and salt formation 3. Stability of alkaloids 4. Detection of alkaloids  General Characters II- Chemical Properties 1. Nitrogen and oxygen in the molecule: ❑Alkaloids must contain at least one nitrogen atom which imparts basicity. ❑The nitrogen may exist in: - 1ry amine form (R-NH2), rare: e.g. d-norpseudoephedrine. - 2ry amine form (R2-NH), few: e.g. l-ephedrine. - 3ry amine form (R3-N), common: e.g. atropine, nicotine, most alk. - Quaternary ammonium form (R4N+), few: e.g. d-tubocurarine chloride, berberine chloride. ❑Few alkaloids lack oxygen so occur as volatile distillable liquids e.g. coniine and nicotine. ❑Most alkaloids are oxygenated so occur as solids, except few exist as non- volatile liquids e.g. pilocarpine and pelletierine.  d-norpseudoephedrine l-ephedrine H OH H H C C CH3 C C CH3 OH HN CH3 H NH2 d-tubocurarine atropine O N + 7 1 2 O H HO O CH2 3 H3C N O C C CH2OH 6 5 4 H2C OH O N + O 2. Basicity and salt formation: The unshared pair of electrons on the nitrogen atom is responsible for alkaloidal basicity. Alkaloids are neutralized with acids to form salts which can be converted to the free base by alkalies (e.g. ammonia, sodium carbonate or calcium hydroxide) OH- RxN: + H+ Rx N+H (ionic salt) RxN: The degree of basicity of alkaloids depends on (is affected by): 1. Structure of the molecule. 2. Type of the amine. 3. Presence and location of the functional groups. 1. Structure of the molecule: Saturated cyclic amines is more basic than the corresponding aromatic amines. e.g. Piperidine alkaloids are more basic than pyridine alkaloids. > N N H N Coniine Nicotelline 2. Type of the amine: R2-NH > R-NH2 > R3-N in basicity [2ry > 1ry > 3ry]. 3. Presence and location of the functional groups: - Electron releasing (donating) groups adjacent to the nitrogen atom increase the availability of electrons on the nitrogen so increase the basicity e.g. alkyl groups. - Electron withdrawing groups adjacent to the nitrogen atom decreases the availability of electrons on the nitrogen so decreases the basicity e.g. cyano, carbonyl, amide groups,…etc. so the alkaloid will be neutral or slightly acidic e.g. Recinine. OCH3 C N O Recinine N Hippadine O N O O CH3 Steric hindrance: The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. So, the more the number of alkyl groups attached, lesser will be its basicity. Solvation of ions: When amines are dissolved in water, they form protonated amines and the number of possibilities of hydrogen bonding increases. So, the number of possible hydrogen bonds when primary amines are dissolved in water is the greatest, implying that they are the most stable species of amines, the least being the tertiary amines. According to basicity, Alkaloids are classified into: 1. Strong bases: e.g. Atropine 2. Weak bases (their salts are not stable): e.g. Caffeine, papverine. Papaverine 3. Amphoteric: O *Phenolic alkaloids (contain phenolic OH N O groups) e.g. Morphine Morphine OCH3 C O *Alkaloids with carboxylic groups OCH3 COOH e.g. Narceine C N OCH3 4. Neutral or very slightly acidic alkaloids N O OCH3 (can not form salts): Narceine CH3 e.g. Colchicine, Recinine, Theobromine Recinine Important Notes: Strong basic alkaloids can form salts even with very weak acids. Weakly basic alkaloids can not form salts with very weak acids but needs more acidic medium (i.e. strong acids). Neutral or very slightly acidic alkaloids can not form salts with acids. Salts of weakly basic alkaloids are not stable to alkalies. Alkaloidal salts are more stable than their free bases. So, most alkaloids are commercially available as their salts. Quaternary bases (R4N+X-) e.g. d-tubocurarine Cl have four organic groups covalently bonded to nitrogen. The quaternary ammonium ion (R4N+) has no proton to give up, so is not affected by alkalies. So, quaternary bases have different chemical properties than other amines. 3. Stability of alkaloids: Heat Exposure to light, heat, oxygen, acids and alkalis should be avoided. In general, alkaloids are less stable in solution than in the dry state Effect of heat: Alkaloids are generally decomposed by heat, while Some of them sublime e.g. Strychnine and caffeine.  Effect of acids: A) Alkaloids react with dil. acids to form salts (Alkaloidal salts must be prepared by cold dil. mineral acids). Most alkaloids behave as monoacidic bases. Few alkaloids behave as biacidic bases and from two series of salts (neutral and acidic salts) e.g. Cinchona alkaloids (e.g. quinine sulphate and quinine bisulphate). B) Conc. acids or heating with dil. acids (drastic conditions) cause remarkable changes such as: 1. Dehydration: [alkaloid apoalkaloid (anhydrous form)] e.g. morphine gives apomorphine e.g. atropine gives apoatropine 2. Demethoxylation: [alkaloid nor-alkaloid] e.g. quinine, narcotine, papaverine, codeine. 3. Hydrolysis: e.g. Ester alkaloids (e.g. atropine, cocaine, physostigmine) acid + alcohol (or phenol). e.g. Gluco-alkaloids (e.g. solanine) secondary alkaloid + sugar [Solanine solanidine + rhamnose + glucose + galactose]  Effect of alkalies: A) Dil. alkalies (e.g. NH4OH): ▪ Liberate alkaloidal bases from their salts. ▪ Form salts with alkaloids containing –COOH groups e.g. narceine ▪ May cause isomerization (racemization) of some alkaloids e.g. conversion of l-hyoscyamine to atropine (dl-hyoscyamine). B) Strong alkalies (e.g. NaOH, KOH): ▪ Form salts with alkaloids containing phenolic groups e.g. morphine and cephaline. ▪ Hydrolysis of ester alkaloids e.g. atropine, cocaine, physostigmine. ▪ Opening of lactone ring e.g. pilocarpine gives pilocarpic acid (leads to loss of activity).  Effect of light and oxygen: Some alkaloids are unstable when exposed to light and oxygen: Oxygen Eserine Rubreserine Alkaline solutions Reserpine Decomposition Ergot Alkaloids Lumi Alkaloids (Inactive) 4. Detection of alkaloids (qualitative tests for alkaloids): Alkaloids can be detected by two groups of reagents: 1- Alkaloidal Precipitants They give amorphous or crystalline precipitates. They are used to: 1- Indicate the absence or presence of alkaloids. 2- Test for complete extraction of alkaloids from the plant material. 3- Purification of certain types of alkaloids e.g. d-tubocurarine. Disadv. They may give ppt. with other plant constituents e.g. tannins, proteins, coumarins, lactones and some flavonoids (false positive reaction). So, it is necessary to get rid of such constituents when testing for the presence of alkaloids. 2- Alkaloidal color reagents They give characteristic colors. 1- Alkaloidal Precipitants 2- Alkaloidal Color Reagents a) Mayer’s Reagent a) Erdmann’s Reagent b) Marmé Reagent b) Marqui’s Reagent c) Dragendorff’s Reagent c) Mandalin’s Reagent d) Wagner’s Reagent d) Froehd’s Reagent e) Hager’s Reagent e) Mecke’s Reagent f) Tannic acid f) Shaer’s Reagent g) Gold chloride g) Liebermann’s Reagent h) Platinic chloride i) Phosphomolybdic acid j) Phosphotungestic acid k) Silicotungestic acid I- Alkaloidal Precipitants: 1) Reagents that form double salts: Most alkaloids are precipitated from neutral or acidic solutions by a number of reagents containing heavy metals e.g. Hg, Cd, Pt, Bi, Au by forming double salts with them. a) Mayer’s Reagent: (potassium mercuric iodide) White (creamy) ppt. Exceptions: Caffeine, Recinine, dil. Ephedrine -ve with Mayer’s Conc. Ephedrine +ve b) Marmé Reagent: (potassium cadmium iodide) Yellow ppt. c) Dragendorff’s Reagent: (potassium bismuth iodide) Orange to reddish brown ppt. d) Gold chloride. e) Platinic chloride. 2) Reagents containing halogens: a) Wagner’s Reagent: Iodine/ Potassium Iodide Reddish brown ppt. 3) Organic acids: a) Hager’s Reagent: Sat. aq. soln. of picric acid Yellow ppt. b) Tannic acid (5%). 4) Oxygenated high molecular weight acids: a- Phosphomolybdic acid b- Phosphotungestic acid c- Silicotungestic acid N.B. These reagents can be used to test for alkaloids in test tubes or on slides (micro-crystallization tests) II- Alkaloidal Color Reagents: 1) General color reagents: Most of these reagents contain H2SO4 and are applied to the alkaloids themselves (not to their solutions). a) Erdmann’s Reagent: (conc. H2SO4 + HNO3). b) Marqui’s Reagent: (conc. H2SO4 + HCHO). c) Mandalin’s Reagent: (conc. H2SO4 + ammonium vanadate). d) Froehd’s Reagent: (conc. H2SO4 + ammonium molybdate). e) Mecke’s Reagent: (conc. H2SO4 + selenious acid). f) Shaer’s Reagent: (conc. H2SO4 + H2O2). g) Liebermann’s Reagent: (conc. H2SO4 + NaNO2). 2) Specific color reagents: e.g. p-Dimethylaminobenzaldehyde reagent Blue color with Ergot alkaloids. e.g. Vitali’s reagent: (HNO3 + alc. KOH) Violet color with Solanaceous alkaloids