Download (8Mb) - Etheses - Saurashtra University
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2.1 Introduction<br />
As described in chapter-1 the class of 2-aminoimidazoles has recently been given<br />
particular interest due to various biological properties of these compounds. 2-<br />
Aminoimidazole alkaloids and their metabolites, isolated from the marine sponges<br />
Hymeniacidon sp., have been described as potent antagonists of serotonergic 1 and<br />
histaminergic 2 receptors. Naamine and isonaamine alkaloids from the marine sponges<br />
Leucetta sp. exhibit antiviral and anticancer activity. 3,4 Because of these interesting<br />
biological activities, numerous synthetic routes to 1-substituted and 1-unsubstituted 2aminoimidazoles<br />
have been reported. Modern synthetic methods for accessing 1unsubstituted<br />
2-amino-1H-imidazoles can be roughly classified as heterocyclization<br />
of substituted or protected guanidines with 1,2-dielectrophiles, 5a-c heteroaromatic<br />
nucleophilic substitution 5c,6 and recyclization of 2-aminooxazoles. 7 Although different<br />
substituted guanidines are readily available and can be prepared in situ (e.g. from<br />
cyanamines 8 ), the high basicity of guanidines together with non-regioselectivity often<br />
leads to multiple products. 9 Protection by acetyl 5a and Boc-groups 5c requires, in turn,<br />
acidic deprotection conditions. Another procedure is the cyclocondensation of<br />
aldehydes and guanidine nitrate using sodium cyanide and supported aluminum oxide,<br />
which provides symmetric 2-aminoimidazoles. 10<br />
Recently, we described two new approaches to the synthesis of substituted 2-amino-<br />
1H-imidazoles. The first approach is based on the cleavage of imidazo[1,2a]pyrimidines<br />
11 with hydrazine 12 In the second approach we reported microwave<br />
assisted one pot synthesis.(scheme-3) 13