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MeO
OMe
MeO
OMe
MeO
Cu(OAc) 2
+
MeCN/pyridine
rt, 3 h
MeO
OMe
MeO
20
MeO
OMe
21
2.5%
MeO
OMe
22
9.1%
Fig. 14 Hopf's synthesis of putative CPPA precursors 21 and 22 [ 28 ]
CO 2 Me
CO 2 H
CO 2 Me
HO 2 C
Cl
MeO 2 C
Cl
LiAlH 4 , Et 2 O
hv, DCE, -70˚C
27%
Cl
Cl
Cl
89%
Cl
Cl
Cl
25
CO 2 Me
24
23
1,1'-dimethoxycyclohexane
TsOH, benzene
68%
t - BuOK
THF
O
O
O
+
O
O
O
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
28
35%
27
45%
26
Fig. 15 Synthesis of Dewar benzene derivatives [ 29 ]
dimerization, and Glaser-Ellington macrocyclization afforded 31 which was quan-
titatively converted by photoirradiation to dodecasilyl [6 2 ]CPPA comprising the
first synthesis of a substituted [ n 2 ]CPPA (Fig. 16 )[ 30 ].
While this remains the most unsaturated CPPA ever isolated (stable on the order
of days), evidence of the formation of cyclocarbon-like [2 6 ]CPPA was reported
“by Tobe”. [4.3.2]Propelatriene units are known to undergo photoinduced retro
[2+2] addition to yield indane and the corresponding alkyne [ 34 ]. Having employed
a similar strategy en route to cyclocarbon, these masked alkynes were chosen to
provide curvature to the CPPA macrocyclic precursors 35 and 36 (Fig. 17 ).
Fragments 33 and 34 were synthesized by Sonogashira coupling with 1,4-
diiodobenzene and 1,4-diiodo-2,3,5,6-tetrachlorobenzene, respectively, followed
by deprotection and dilute oxidative coupling to offer 35 and 36. Under mass
spectroscopic conditions, irradiation of the macrocycle generated the highly unsta-
ble [2 6 ]CPPA [ 31 ]. Interestingly, when using furan as solvent, 37 was obtained as a
quickly decomposing orange solid.
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