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![In the fully saturated nucleus, there
are six chiral centres and hence there
will be 64 n=6 optically active forms.
[ theoratically possible]](https://image.slidesharecdn.com/3-170223051044/75/3-stereochemistry-of-steroids-12-2048.jpg)
Uploaded byDR.Gopinathan Narasimhan
3. stereochemistry of steroids
The document discusses stereo chemistry of steroid rings. It notes that there are six asymmetric carbon atoms in the steroid nucleus, allowing for 64 possible optical isomers. The rings prefer a chair conformation to minimize angle strain. Hydrogen and groups on the beta side are denoted by solid lines, and alpha side by dotted lines. Most natural steroids have A/B, B/C, and C/D rings in a trans fusion. The exact stereo chemistry is important for biological activity. NMR spectroscopy can provide information about angular methyl group couplings and configurations.
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Introduction of Dr. N. Gopinathan and background on stereochemistry in pharmaceutical chemistry.
Stereochemistry of steroids affects biological activity; 6 asymmetric carbons result in 64 optically active forms.
Cyclohexane rings can exist in chair or boat forms; chair is more stable due to less angle strain.
Functional groups on β side are shown with solid lines, and α side with dotted lines.
A/B trans configuration described; C-5 has α configuration opposite to C-19 angular methyl group.
Trans configurations in rings B/C and C/D; discusses cis and trans relationships.
5α Cholestane features trans-anti-trans-anti-trans backbone, defining anti relationships of rings.
5β Cholestane has Cis-Syn-trans-anti-trans backbone, denoting cis relationships between A/B rings.
256 possible optical isomers with 8 chiral centers; including C-20 increases count to 512.
6 chiral centers yield 64 optically active forms theoretically possible in saturated nucleus.
Only a few optical isomers exist due to steric limitations; oxidative degradation method provides evidence.
X-ray reveals trans fusion of rings B and C; natural steroids typically exhibit this fusion.
Majority of steroids have trans C/D fusion; exceptions noted in certain cardiac glycosides.
NMR spectroscopy aids in understanding methyl group couplings in steroid chemistry.
Larger peak width for trans fused isomers compared to cis fused isomers in NMR analysis.
Hydroxyl group at C-3 configuration; β series indicates above the plane, α below.
Closing remarks and expression of gratitude.
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3. stereochemistry of steroids
- 1. Stereo chemistry Prepared by Dr.N.GOPINATHAN ASSISTANT PROFESSOR DEPARTMENT OF PHARMACEUTICAL CHEMISTRY FACULTY OF PHARMACY SRI RAMACHANDRA UNIVERSITY CHENNAI-116 TAMILNADU INDIA
- 2. The stereo chemistryof the rings markedly affects biological activity of a given class of steroids There are six asymmetric carbon atoms 5,8,9,10,13,14 in the nucleus. Therefore 64 optically active forms are possible
- 3. • It canexist in two conformations namely chair form and boat form. • Chair conformation is more stable than boat conformation due to less angle strain. • Hence all cyclohexane rings • exist in chair form
- 4. • Hydrogen orfunctional groups on the β side of the molecule are denoted by solid lines and the α side is denoted by dotted lines.
- 5. A/B trans • Thehydrogen atom at C-5 has α configuration, which is opposite from the c-19 angular methyl group β, making the A/B ring juncture trans
- 6. Rings B/C andC/D - trans • The configuration of 8 β and 9 α hydrogens and 14 α hydrogen and C-18 angular methyl group denotes trans fusion for rings B/C and C/D. • Cis and trans relationship of the four rings may be expressed in terms of the back bone.
- 7. 5 α cholestanehave trans- anti –trans –anti-trans back bone Anti denotes orientation of ring that are connected to each other and have trans type relationship
- 8. 5 β cholestanehas Cis –Syn- trans-anti- trans back bone in which the A/B rings are fused Cis. The term Syn is used in a similar fashion as anti to define a Cis type relationship.
- 11. • The numberof possible optical isomer is 256. n= 8 • If the chiral centre in the side chain C-20 is also included the number of possible optical isomer will be 512. n= 9 • Two types • Deal with fused rings • Configuration of substituent at C3 & C17
- 12. In the fullysaturated nucleus, there are six chiral centres and hence there will be 64 n=6 optically active forms. [ theoratically possible]
- 13. It is ofworth mentioning that only few exist actually while many cannot exist due to steric limitation. Oxidative degradation method gives a great deal of evidence for the stereo chemistry of the nucleus. In A/B fusion both cis and trans were found in the natural steroids.
- 14. X-ray analysis revealthat the fusion of the rings B and C was trans. Steroid is a flat which can be explained only if the fusion ring B and C has occurred together in trans manner. Only trans B/C fusion takes place in natural steroid. Ring C/D posses trans fusion.
- 15. Mostly steroids havetrans C/D fusion exception in cardiac glycoside and toad poison in which the fusion is cis. Hydrogen at C-9 is trans to methyl group at C-10.
- 16. • The methylgroups at C-10 & C-13 are Cis • NMR spectroscopy is quite useful in steroid chemistry. • Angular methyl groups undergo coupling with certain protons on the steroid nucleus.
- 17. It is alsofound that peak width of angular methyl group half height for trans fused isomer is larger than that for cis fused isomer
- 18. Configuration of hydroxylgroup at C-3 • β series found in all natural steroids. • The prefix β indicates that it lies above the plane of molecule. • If hydroxyl group lies below the plane it gives rise to α series.
- 19.