Conformational structure 1,3-altemate conformation

Figure 12 Molecular structure and stable conformers of p /ert-butycalix14 arcnc (a) cone (b) partial-cone (c) 1,3-alternate (d) 1,2-altemate.

The crystal structure of the calix[4Jindole 26a was elucidated and it was found that the molecule adopts a 1,3-alternate conformation in the solid state forming a "square" channel (Figure 29) that is analogous to what is seen in the 1,3-altemate conformation of calixarenes. ... 

Figure 4 (a) Simplified possible conformations of calix[4]arene (b) structure of caUxcrown, cone conformation (c) 1,3-altemate... 

A series of tetrahomodioxacalix[4]-biscrowns 77-81 with crown ether units varying from crown-2 to crown-6 were obtained in different conformations, according to the NMR spectra and X-ray crystal structures [75, 76]. The results from the two-phase picrate extraction method indicated that 1,3-altemate biscrown-4 (79a) showed preference for K, while both 1,3-altemate biscrown-5 (80) and biscrown-6 (81a) showed preference for Cs. The cone biscrown-2 (77) and the 1,4-altemate conformers 78, 79b and 81b displayed no affinity for any of the cations studied. 

The corresponding X-ray crystal structures of these triptycene-derived calixarenes and analogues were also obtained. As shown in Fig. 18.2a, b, macrocycle la adopted a typical cone conformation with a highly symmetric structure, while trans-isomer 2a adopted a chair conformation with two opposite phenol rings in the same plane [11a, b]. Similar stmctural features were also found in the crystals of their derivatives 5 and 6 [11c]. In the case of 7a, b and e [1 Id, e] (Fig. 18.2c) and 8a [12] (Fig. 18.2d), the crystal structures showed that they adopted cone conformations with fixed cavities while 7g [lie] was in a 1,2-altemate conformation. 

X-ray crystal structural analysis showed that 10a, 12a and 13a were c T-isomers with a 1,3-altemate conformation (Fig. 18.6a) while 10b and 13b were trans-... 

Fig. 33.16 L molecular structure of 15 center, two geometry-optimized molecular mechanics structures corresponding to the cone (top) and altemate-OH (bottom) conformations Right optimized molecular mechanics structure for the (15)2 C6o (CFC) complex (Figure adapted with permission from Ref. [23], Copyright 2012, American Chemical Society)...

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