Expansion of a 2 + 2 macrocycle into a 6 + 6 macrocycle: template effect of cadmium(II)

The reaction of trans-1,2-diaminocyclopentane with 2,6-diformylpyridine results in formation of 2 + 2, 3 + 3, and 4 + 4 Schiff base macrocycles as well as trace amounts of 6 + 6 and 8 + 8 macrocycles. In contrast, the 6 + 6 Schiff base macrocycle is a dominant product of the reaction of the isolated 2 + 2 macrocycle with excess of cadmium(II) chloride. The X-ray crystal structure of the protonated amine derivative of the 6 + 6 macrocycle reveals an unusual container-like conformation with the S6 axis.