Synthesis and structural investigation of C4- and C2-symmetric molecular scaffolds based on imidazole peptides.
The syntheses of C4- and a C2-sym. scaffolds I (X = Y = NH; X = O, Y = NH; X = Y =O) are presented. The synthesis of the key building block for both systems has been fundamentally improved. Starting from these scaffolds, various platforms possessing two or four arms can easily be prepd. The structures of the scaffolds, together with that of a C4-sym. oxazole scaffold, were investigated in the solid state and in the gas phase. It was found that these 24-membered cyclic peptides exist as two different structure types. In one case (type I), the nitrogen atoms of the azole units pointing into the interior of the macrocycle form a square, whereas in the second case (type II), they form a parallelogram. The type of mol. structure does not depend on the symmetry of the system, but on the type of azole used. Cyclic systems composed of four imidazole units are present in the type I structure, whereas systems consisting of four oxazole units exhibit the type II nature. Structure type II is also present in cycles composed of two oxazole units and two imidazole units. The existence of the different structure types can be explained in terms of the different orbital energies of the imidazole and oxazole systems.
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