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