Three-ring circus: An expedient route to tricyclic fused 2-trimethylsilyl-3-aminopyridines exhibiting unprecedented skeletons is described. The key step is a very efficient cobalt-catalyzed [2+2+2] cycloaddition of a polyunsaturated compound displaying an ynamide, an alkyne, and a nitrile functionality (see picture). The first [2+2+2] cocyclizations between ynamides, nitriles, and alkynes are reported. They open a new access to unprecedented nitrogen-containing heterocycles of type 2-trimethylsilyl-3-aminopyridines. Such frameworks, which can be found in various compounds of biological interest, are very difficult to prepare by conventional methods. However, using [CpCo(C(2)H(4))(2)] (Cp=cyclopentadienyl) as catalyst, the intramolecular cyclizations could be achieved in up to 100 % yield. The presence of the trimethylsilyl group allowed a rare type of Hiyama cross-coupling: one of the silylated pyridines could be coupled with p-iodoanisole to give a new type of biaryl system.
Unprecedented nitrogen polyheterocycles have been prepared by means of intramolecular Co-catalyzed [2+2+2] cycloaddition of two alkynes to one nitrile. They exhibit two nitrogencontaining rings fused in an angular fashion to one pyridine unit. Several relative positions of the nitrogen atoms have been studied, giving rise to eight different new scaffolds. In order to allow selective functionalization of the two amino groups, orthogonal protecting groups (PG 1 and PG 2 ) were introduced prior to cyclization. Eleven combinations of seven different protecting groups (Bn, COCF 3 , Cbz, Boc, Ts, SO 2 -2-py, Ns) were tested, most of them being perfectly tolerated under the cyclization conditions.
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