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HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Answer 16.15
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.
H
H
H
H
H
H
H
H
all-trans
≡
A B
C D
A B
C D
Chenodeoxycholic acid typifies a steroid with A/B cis-fusion; the remaining rings are the same as the
all-trans model.
H
H
H
HO
H
OH
CO 2 H
chenodeoxycholic acid
H
H
H
H
cleave off ring A,
leaving residual bonds
all-trans
H
H
H
use residual bonds to
form basis of new ring
H
H
H
H
H
CO 2 H
H
OH
H
OH
H
A/B cis
H
H
H
H
A/B cis
put in substituents
A B
A B
A B
A B
Forskolin is not a steroid; however, if we disregard substituents and that one ring is a heterocycle, then all
we have is the first three rings A, B, and C of an all-trans steroid.
H
OH
O
O
HO
OH
OAc
forskolin
H
H
H
H
all-trans steroid
cleave off ring D, leaving
residual bonds
H
H
H
H
HO
O
HO
OAc
OH
O
put in substituents
and heteroatom
A B
C D
C D
Strophanthidin is a steroid with rings A/B cis-fused, and rings C/D also cis-fused. The latter feature is
approached in the same way as the A/B cis-fusion shown with chenodeoxycholic acid above.