Homocalix arenes

Whereas caiixarenes (I) are based on a [l ]metacyclophane skeleton, a//-homocalix-arenes (H) contain an expanded (c.f Sessier "Expanded Porphyrins" [5]) cavity, which is achieved by synthesis of [2 ]metacyclophanes, and yieids one additional CH2-group compared to calixarenes in all Aeir bridges (Fig. 1). In addition, homocalixarenes allow use of pyridines as aromatic units in their ring skeleton Homocalixpyridines (HI). 

Ikcda, A. L dzu, H. Yoshimura, M. Shinkai, S. Inclusion of [bOJfullerene in a self-assembled homocalix[3]arene-based dimeric capsule constructed by a Pd(ll)-pyridine interaction. The Li" binding to the lower rims can improve the inclusion ability. Tetrahedron 2000,56 (13), 1825-1832. 

Yamato, T. Synthesis, conformations and inclusion properties of homocalix[3]arenes. J. Inch Phenom. Mol. Rec. Chem. 1998. 32 (2-3). 195-207. 

Fig. 10. Crystal structures of a near to planar a//-homocalix[6]arene ...

Pentahydroxy-a//-homocalix[5]arene 8 discriminates alkaline earth metals Ca , Sr , Ba with high selectivity for alkali metal ions [11] (Fig. 13) [21]. Octaphenol 4 is also selective towards alkaline earth ions, and, in view of its increased number of dissociated OH-groups, is more efficient in the extraction process. From a mixture of calcium, strontium and barium, 4 extracts barium selectively by formation of a 1 1 complex. 

Fig. 2. Structural features of an a//-homocalix[4]pyridine (left side) and an all-bishomocalix[4]arene (right side).

Fig. 13. Crystal structures of 37 (left side), and the a//-homocalix[4]arene substituted with OCHa groups (right side).

Fig. 14. Crystal structure of the intra- and extraannular methoxy substituted all-homocalix[5]arene (left side) compared to 38 (right side).

Pentahydroxy-a//-homocalix[5]arene 44 discriminates strongly in favour of the alkaline earth metal ions Ca, Sr + and Ba " " over the alkali metal ions, even when the latter are present in as much as a 1000-fold excess [56] (Figure 18). 

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