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Optimal convergence is, of course, not only a graphical problem but also one of yield (57)! 5. Steps making use of systems containing sites that are more reactive than those to be condensed introduce (protection)-(protective group removal) operations. -M. Lehn 6. Paths having the last step of highest yield are preferable. This may, for example, make path a (Figure 4) better suited than c or conversely. 7. Paths containing the smallest number of cyclization steps (which may require special conditions, see below) are preferable, provided only one ring is formed per step.

On the other hand 38, which gives a more lipophilic (Table 12) and much less stable complex, is quite an efficient K + carrier. The larger macrobicyclic ligands 31 and 33 induce K + transport through a chloroform barrier with increasing efficiency. They have opposite transport selectivities for Na + and K +, the order being Na+ > K+ for 31 and K+ 3> Na + for 33. It thus appears that the macrobicyclic ligands display efficient carrier properties for those cations which form complexes with stability constants of about 105 in methanol.

By adding bridges to 19), would show a gain both in stability (by about the energy difference of the free and complexed conformations of 19) and in selectivity, especially for the K+]Rb +, Cs+ couples, since the increased rigidity of the ligand would oppose the inclusion of cations larger than K +. On the other hand the cation exchange rates would probably be decreased (see below). 3. The K+/Na+ selectivity of the known natural ligands except vahnomycin, is much lower than that of several synthetic ligands especially 19 and 80--88.

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Alkali Metal Complexes with Organic Ligands by J. D. Dunitz

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