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diisobutylaluminum hydride to give 107 (with an estimated half life time of
the diol equipped with the enediyne moiety of around 40 min at 24.5
C).
In addition, enediyne macrocycles, belonging to the family of cyclo-
phanes, have been developed. Usually, they have been designed for diff-
erent applications, such as the use as intermediates for fullerenes synthesis.
However, studies of their propensity to undergo cycloaromatization have
been performed for compounds 108 and 109 (Scheme 19.29). The first one is
stable towards Bergman cycloaromatization up to 100
C and decomposes
beyond this temperature, confirming molecular dynamics predictions.
104
The second one undergoes the same reaction at 237
C, but, when treated
with Hg(CF
3
CO
2
)
2
, the same reaction started at 145
C: this is probably due
to the intervention of a double coordination of the two pyridine nitrogens
with mercury(II) which causes important conformational changes in the
macrocycle.
105
SCHEME 19.29
19.5 ACYCLIC ENEDIYNES
The thermal Bergman rearrangement of the unconstrained (Z)-hex-3-ene-
1,5-diyne system to give a 1,4-didehydrobenzene diradical usually occurs
above 400K and therefore cannot take place in biological enviroments.
Many research groups have devoted their attention to studying the factors
influencing the cycloaromatization process of acyclic enediyne systems, in
order to synthesize molecules that can cycloaromatize at physiological
temperatures and ultimately find pharmacological applications. The final
goal is the discovery of a blockbuster drug: the main advantage of acyclic
systems over their mono- or policyclic counterparts would be, of course,
a more easily achievable synthesis, especially on large scale. The two main
approaches that have been followed in this field are (a) the substitution
of the terminal sp carbons of simple enediyne systems with opportune
functional groups; and (b) the coordination of the enediyne molecule with
metal ions.
106
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