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To avoid lithiation, Ohta has shown that 2-bromoimidazoles can undergo palladium
catalyzed coupling with trimethylsilylazide (Scheme-1.10.12). Reduction again
provides the 2-aminoimidazole. This strategy has been implemented by Ohta’s group
to access several Leucetta alkaloids discussed below. Also, noteworthy, Ohta has
shown that during reduction of the azide it can be directly protected as its
benzylidenimine. 101-102
N
N
Br
TMSN 3
PdCl 2(PPH 3) 2
80 o C N
N
N 3
H2, Pd-C
Scheme-1.10.12 Ohta’s palladium catalyzed synthesis.
1.10.4 Heterocyclic Exchange Reactions
The transformation of 3-amino-1,2,4-oxadiazoles to 2-aminoimidazoles can be carried
out by reaction with fluorinated ß-dicarbonyls to give the ß-hemiaminal (Scheme-
1.10.13). 103 This can undergo subsequent conversion to the 2-amidoimidazole through
a Boulton-Katritzky Rearrangement. This intermediate can then be reduced to give
Intermediate. The benzamide can also be removed via acid catalyzed hydrolysis to
give the trifluoromethyl amino-imidazole. An interesting reaction, however the
synthesis of the intermediate ß-enaminocarbonyl is low yielding and initial results
suggest limited substituent scope, as the initial condensation requires the presence of
the trifluoromethyl ketone.
Ph
O
O
N
NH 2
N
O
CF 3
R
Montmorillonite K-10,
PhMe, 100 o C;
Ph
F3C HN
OH O
N
N
O
R
Ph
O
N
H
Ph
N
F3C HN
OH OH
N
N
O
R
NaBH 4
THF
reflux
Scheme-1.10.13 Boulton-Katritzky rearrangment of aminooxadiazoles.
N
H
CF 3
OH
R
N
N
Ph
NH 2
O
N
H
N
N
H N
2N
H
N
H
CF 3
HCl
EtOH
reflux
CF 3
O
R
O
R