1. Chemistry of natural products
Islamic University in Madinah
Department of Chemistry
CH-2
Chemistry of natural products
Prepared By
Dr. Khalid Ahmad Shadid
Chemistry Department – Islamic University in Madinah

2. STEROIDS
Lipids are substances soluble in organic solvents and insoluble in water. The extraction of plant and animal tissues with organic solvents such as ether, chloroform or benzene yields lipids. The lipids so obtained are of two main categories:
The first: are converted into water-soluble matter on saponification (alkaline hydrolysis).
The second : are do not get converted into water-soluble matter on saponification like ‘Steroids’
Steroids (Greek stereos = solid) are compounds possessing a characteristic tetracyclic carbon skeleton, the perhydrocyclopentano phenanthrene nucleus.
the above molecule include a wide range of naturally occurring compounds like the sterols, the bile acids, the sex hormones, the adrenocortical

3. STEROIDS
Steroids may be recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring.
The four rings are designated A, B, C & D as noted below. The substituents designated by R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at the C:D fusion is usually methyl. The substituent at C- 17 varies considerably, and is usually larger than methyl if it is not a functional group.
The most common locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic.
Carbon numbering of steroid skeleton

4. Any compound which gives Diels hydrocarbon with Se distillation is defined as steroid. When steroids are distilled at 420oC yields mainly chrysene and a little picene.
chrysene
picene
Diels hydrocarbon

5. Classification of Steroids
1- Sterols
Such as cholesterol, the characteristic steroid of higher animals, ergosterol which is converted to vitamin D by irradiation as well as the common phytosterols: -sitosterol and stigmasterol.
(mycosterol)
(phytosterol)
(phytosterol)
(zoosterol)

6. Cholesterol
Isolation: the spinal cord and brain of cattle constitute the most important raw material for the industrial production of cholesterol. Its obtained by solvent extraction and purity by recrystallization as the 5-6-dibromide derivative. Debromonation is achieved by treatment with zinc-acetic acid or sodium iodide-ethanol.
1. Molecular formula: C27H46O, [α]D ͦ (chloroform)
2. Presence of double bond and hydroxyl group: a. The conversion of cholesterol into cholestanol shows the presence of double bond.
H2 Pt
cholesterol
cholestanol

7. b. The oxidation of cholestanol with chromic acid into cholestanone shows the presence of a secondary alcoholic group in cholesterol.
C. The Clemmenson’s reduction of cholestanone yields a saturated hydrocarbon called cholestane.
Cholestanone
Cholestanol
Cholestanone
Cholestane

8. Cholesterol
Presence of a ring system: The molecular formula of cholestane corresponds to the general formula CnH2n-6, of a tetracyclic system. On distillation with Se at 360oC, cholesterol yields Diel’s hydrocarbon.
The reaction shows the presence of a cyclopentenophenanthrene nucleus in cholesterol and thus cholesterol is a steroid.

9. Position of the Hydroxyl Group:
The position of hydroxyl group at C-3 is finally proved by Kon et al. The formation of 3’,7-dimethyl cyclopentenophenanthrene from cholesterol by the following steps is possible only if –OH group is considered at position C-3

10. The Utilization of Cholesterol
Cholesterol is transported in the plasma predominantly as cholesteryl esters associated with lipoprotein.
Dietary cholesterol is transported from the small intestine to the liver within chylomicrons.
Cholesterol synthesized by the liver, as well as any dietary cholesterol in the liver that exceeds hepatic needs, is transported in the serum within Low Density Lipoproteins (LDLs).
The liver synthesizes very Low Density Lipoproteins (VLDLs) and these are converted to LDLs through the action of endothelial cell-associated lipoprotein lipase.
Cholesterol found in plasma membranes can be extracted by High Density-Lipoproteins (HDLs) and esterified by the HDL-associated enzyme Lecithin cholesterol acyltransferase(LCAT).
The cholesterol acquired from peripheral tissues by HDLs can then be transferred to VLDLs and LDLs via the action of cholesteryl ester transfer protein Apolipoprotein D (apo-D) which is associated with HDLs. Reverse cholesterol transport allows peripheral cholesterol to be returned to the liver in LDLs.
Ultimately, cholesterol is excreted in the bile as free cholesterol or as bile salts following conversion to bile acids in the liver.

11. 2- Vitamin D group
They are about seven compounds (vitamin D1-D7) with the ring B being opened. Vitamin D2 (or calciferol) is formed from ergosterol by the sunlight irradiation.

12. 3- Bile acids
As their names implies, are isolated from the bile of various animals, as well as humans. Their chief functions to facilitate the digestion of fats.

13. 4- Sex hormones These could be divided into:
(A) Oestrogens: Characterized by ring A being aromatic, and hence without the C-19 methyl group. They are responsible for development and maintenance of the female secondary sex organs. An example is -Oestradiol, main female sex hormone.

14. (B) Androgens: Are hormones which control the development of the male genital organs and secondary sex characteristics. An example is testosterone (male sex hormone).
(C) Gestogens: Are hormones which are responsible for the maintenance of pregnancy. An example is progesterone.

15. 5- Adrenocortical hormones
Produced by the cortex of the adrenal glands. A lack of these hormones leads to multiple symptoms e.g. muscular weakness, change in carbohydrate and protein metabolism, disturbance of electrolyte balance, etc. and eventually death. An example is cortisone.

16. 6- Cardenolides
Are plant steroid which occur as glycosides. Cardiac glycosides have powerful cardiotonic activity and can be used for treatment of heart failure. These compounds are characterized by having a lactone group. An example is strophanthidin.

17. 7- Sapogenins
Are the aglycones of saponins. The first source of steroidal saponins was Digitalis purpurea, more important as a source of the cardiac glycosides. Sapogenins are characterized structurally by the presence of both furan and pyran rings and spiro carbon atom (named spirostane). An example is diosgenin.

18. Mycosterol Ergosterol: C28H44O
This occurs in yeast, Ergosterol forms an esters e.g. an acetate with acetic anhydride, thus there is a hydroxyl group present in ergosterol.
Catalytic hydrogenation (platinum) of ergosterol produces ergostanol C28H50O, hence there are three double bonds in ergosterol.
Ozonolysis of ergosterol gives, among other products, methyl isopropyl acetaldehyde which proves a methyl group at C24 and C22  unsaturation.

19. Mycosterol Ergosterol: C28H44O
The side chain must contain only one double bond, since if more than one were present, more than one fragment would have been removed on ozonolysis, when heated with maleic anhydride at 135°C, ergosterol forms an adduct, and so it follows that the two double bonds (in the nucleus) are conjugated. Now ergosterol has an absorption maximum at 282 nm. Conjugated acyclic dienes absorb in the region of nm, but if the diene is in a ring system, then the absorption is shifted to the region nm. Thus the two double bonds in the nucleus of ergosterol are in one of the rings.
The ultraviolet light effect on ergosterol resulted in the isolation of vitamin D2 or ergocalciferol.

20. Ergocalciferol (Vitamin D2)
Is a crystalline solid, m.p °C, []D +130°. Its molecular formula is C28H44O, and since it forms esters, the oxygen is present as a hydroxyl group. Furthermore, since ergocalciferol gives a ketone on oxidation, this hydroxyl group is a secondary alcoholic group. Ozonolysis of ergocalciferol produces, among other products, methyl isopropyl acetaldehyde. Thus the side-chain in ergocalciferol is the same as that in ergosterol.
Catalytic hydrogenation converts ergocalciferol into the fully saturated compound octahydro ergocalciferol, C28H52O. This shows that there are four double bonds present and since one is in the side-chain, three are therefore in the nucleus.

21. Ergocalciferol (Vitamin D2)
The parent hydrocarbon of ergocalciferol is C28H52 and since this corresponds to the general formula, CnH2n-4, the molecule therefore is tricyclic ( /2 = 3) therefore three rings are present.
Furthermore, ergocalciferol does not gives Diel’s hydrocarbon when distilled with selenium, these fact indicate that ergocalciferol does not contain the four-ring system of ergosterol.
Ozonolysis of vitamin D2 gave a keto acid C13H20O3 and formaldehyde, while careful oxidation with chromium trioxide or permanganate gave the aldehyde C21H34O.

22. Ergocalciferol (Vitamin D2)
As a result of the addition of vitamin D to milk, deficiencies in this vitamin are rare in this country. The main symptom of vitamin D deficiency in children is rickets and in adults is osteomalacia. Rickets is characterized improper mineralization during the development of the bones resulting in soft bones. Osteomalacia is characterized by demineralization of previously formed bone leading to increased softness and susceptibility to fracture.

23. Steroid hormones Androgens Sex hormones:
The sex hormones are of three types: the androgens (male hormones), the oestrogens (female hormones) and gestogens (the corpus luteum hormones). The sex hormones are responsible for the sexual processes, and for the secondary characteristics which differentiate males from females.
Androgens
Androsterone: C19H30O2, m.p. 183°C, []D +94°
It was first isolated by Butenandt et al. (1931) from male urine (about 15 mg from litres of urine).
Androsterone behaves as a saturated compound, and since it forms mono-esters, one oxygen atom is present as a hydroxyl group. The functional nature of the other oxygen atom was shown to be oxo, since androsterone forms an oxime, etc.

24. Androgens
The parent hydrocarbon of androsterone, C19H30O2, is therefore C19H32, and since this corresponds to the general formula CnH2n-6, the molecules is tetracyclic (D.B.E. of C19H30O2 = – 30/2 = 5; 1 double bond due to C=O, and so there are four rings). This led to the suggestion that androsterone probably contains the steroid nucleus, and since it is a hydroxyketone, it was thought that it is possibly related to oestrone. Butenandt (1932) therefore proposed a structure which was proved correct by Ruzicka (1934) as follows.
oestrone

25. Androgens
Ruzicka oxidized 5-cholestanyl 3-acetate with chromium trioxide in acetic acid to epiandrosterone, a hydroxyketone with the structure proposed for androsterone by Butenandt. When, however, 5-cholestanyl 3-acetate was oxidized, the product was androsterone. Thus the configuration of the hydroxyl group at C-3 is  and not  as Butenandt suggested. Epiandrosterone (formerly known as isoandrosterone), m.p. 174°C, []D +88°, has about 1/8th of the activity of androsterone.

26. Testosterone
Testosterone: C19H28O2, m.p. 155°C, []D +109°, λmax 240 nm
Oxidation: In 1935 K. David oxidized testosterone to a diketone namely androst-4-ene-3,17-dione and its structure is well known. This diketone also obtained during the Oppenauer oxidation of dehydroepiandrosterone.
The formation of diketone can be explained if the shown structure is assigned to testosterone.

27. Testosterone
Testosterone was prepared from dehydroepiandrosterone which was available from natural product. The dehydroepiandrosterone is the most important and useful intermediate of sex hormones.
Testosterone was isolated by E. Laquer et. al. in 1935 from testes. About 10 g of testosterone is obtained from 100 Kg of testes.
Synthesis of Dehydroepiandrosterone: Ruzicka & Butenandt (1935)

28. Synthesis of Testosterone Ruzicka & Butenandt (1935)

29. Adrenocortical Hormones
In the adrenal glands (of mammals) there are two regions, the medulla which produces adrenaline, and the cortex which produces steroid hormones.
The production of corticoids is controlled by the hormone produced in the anterior lobe of the pituitary, the so-called adrenocorticortrophic hormone, ACTH. The corticoids have many physiological functions, but their main functions are the control of carbohydrate and protein metabolism and the control of the balance of water and electrolytes.

30. Cortisone Cortisone: mp. 215°C, []D +209°
Cortisone is a steroid that prevents the release of substances in the body that cause inflammation. Cortisone is used to treat many different conditions such as allergic disorders, skin conditions, ulcerative colitis, arthritis, lupus, psoriasis, or breathing disorders.
Partial Synthesis starts from 3,21-diacetoxypregnane-11,20-dione (Sarett, 1948).

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