Formation of Oligomeric
and Macrocyclic Ureas Based
on 2,6-Diaminopyridine
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Abstract
The conversion of 1,3-bis-(6-amino-pyridin-2-yl)-urea
(<b>1</b>) with <i>N</i>,<i>N</i>′-carbonyldiimidazole
at high temperatures in DMSO yielded a mixture of defined cyclic trimers
and tetramers. On the basis of model reactions, exchange reactions
were evidenced, which convert the cyclic tetramer into a stable cyclic
trimer. Linear even numbered oligomers were obtained in acetone under
reflux where side reactions were suppressed. The pronounced tendency
of cyclization is attributed to a preferred folded conformation of
the urea bond between two pyridyl units