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.

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 chemistry of 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 can exist 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 or functional
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
• The hydrogen 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 and C/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 cholestane have 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 cholestane has 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 number of 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 fully saturated nucleus, there
are six chiral centres and hence there
will be 64 n=6 optically active forms.
[ theoratically possible]

13. It is of worth 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 reveal that 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 have trans 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 methyl groups 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 also found that peak width of angular
methyl group half height for trans fused isomer
is larger than that for cis fused isomer

18. Configuration of hydroxyl group 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. Thank you