650
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Gossypol is a biphenyl system with ortho substituents, so there is restricted rotation about the bond joining
the aromatic rings, creating torsional asymmetry and the existence of enantiomers without a chiral centre. We are
only asked about the shape, and this will have the planar aromatic systems at right angles to minimize interaction
between the ortho substituents.
CHO
OH
OH
OH
CHO
HO
HO
OH
gossypol
CHO
HO
HO
OH
OHC
OH
OH
HO
planes of aromatic
rings at right angles
OHC
OH
OH
HO
CHO
HO
HO
OH
this would be the alternative
enantiomeric form
Problem 16.16 (level 2; chapters 3 and 10)
Treatment with acid converts steroid (A) into its isomer (B).
H
H
O
O
H
H
O
O
H
+
(A)
(B)
Use a stereodrawing to indicate the molecular shape of (A).
Give a mechanistic explanation for the acid-catalysed isomerization of (A) into (B).
Answer 16.16
The shape of fused ring structures can be deduced readily by extrapolation from a standard ring system. The
most useful starting point is the all-trans-fused steroid system. The approach is to modify this according to
requirements.