WORKED PROBLEMS
649
OHC
OH
OH
O
O
HO
H
H
strophanthidin
H
H
H
H
H
H
use residual bonds to
form basis of new rings
cleave off appropriate ring,
leaving residual bonds
all-trans
A/B cis, C/D cis
OHC
HO
H
H
OH
O
O
H
HO
H
H
H
H
H
A/B cis, C/D cis
put in substituents
A B
C D
A B
C D
A B
C D
A B
C D
Artemisinic acid is a cis-fused decalin; we can still deduce its shape from the steroid approach. Note that
both hydrogens at the ring fusion are down, whereas we have been looking at systems where the ring fusion
substituents are up. We shall need to change ring B rather than ring A.
H
H
HO 2 C
artemisinic acid
cis-fused
H
H
H
H
H
H
H
H
H
HO 2 C
H
put in substituents
double bond distorts
left hand ring
cis-fused;
hydrogens up
cis-fused;
hydrogens down
trans-fused
cleave off ring B,
leaving residual bonds
use residual
bonds to form
basis of new
rings
A B
A B
A B
A B
A B
Cineole provides an example of a bridged ring system. The essence of this problem is to spot that we cannot
bridge the opposite carbons of a cyclohexane chair conformation; it is necessary to have the cyclohexane in a
boat form. Then the answer is readily obtained.
O
cineole
boat
chair
2-carbon bridge
need to
bridge
can now bridge
put in substituents
O
H