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easy epoxide opening. However, this quinone 96 could be obtained only in
low yield by oxidation of the phenol 98 (
Scheme 19.25
).
19.4.3 C
OMPOUNDS
W
HERE THE
E
NEDIYNE
D
OUBLE
B
OND IS
F
ORMED BY A
S
UITABLE
R
EACTION
Only few examples of bridged polycyclic enediynes belonging to this family
are known and they cannot be assimilated to specific natural derivatives.
However, they have the same safety catch, that is the absence of the double
bond conjugated with the two triple bonds. In order to prevent cycloaro-
matization the double bond had to be introduced as the last step by
a suitable reaction. This can be done by the action of a base, causing an
elimination reaction as in compound 99 (when R
2
SO
2
Me)
101
or by a
DDQ-mediated benzylic oxidation as in compound 100
31
(Scheme 19.26).
The latter compound cycloaromatizes when heated at 80
C and shows some
cytotoxicity with respect to rat embryo fibroblast cells.
ΒΌ
SCHEME 19.26
19.4.4 M
ISCELLANEOUS
A dynemicin A analog resembling also the calicheamicin structure has
been synthesized by Magnus et al.
102
In this case the epoxide is not present
and the role of safety catch is played by the carbonyl on the shortest bridge.
The synthesis of 101 (Scheme 19.27) is achieved exploiting the reaction
SCHEME 19.27
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