Sapphyrin carboxylic acid

In addition to the capped sapphyrins discussed above, a variety of oligomeric sapphyrin systems are now known. The first of these to be presented, system 5.130, was prepared by treating two equivalents of the activated form of sapphyrin carboxylic acid 5.109 with one equivalent of 1,3-diaminopropane (Scheme 5.6.6). In a similar manner, the trimer 5.131 and tetramer 5.132 were prepared by reacting a 30% excess of sapphyrin 5.109 (after activation) with the corresponding polyamine. In each case, reaction yields were reported to be on the order of 50-80%. 

In addition to phosphate and halide anion binding, carboxylate chelation by sapphyrin macrocycles has been the subject of recent investigation. To date, two crystal structures have been solved. A 2 1 complex formed between diprotonated sapphyrin 3 and trifluoroacetic acid shows that the oxyanions are chelated above and below the sapphyrin plane (Figure 10). ° Greater complexity of organi-... 

As expected, system 13 did in fact bind and transport zwitterionic a-amino acids through a model membrane barrier with good selectivity under conditions where the porphyrin-derived control system (14), lacking the carboxylate anion chelation ability inherent in 13, would not. Specifically, it was found that at neutral pH compound 13 acts as a very efficient carrier for the through model membrane (H2O-CH2CI2-H2O) transport of phenylalanine and tryptophan. Further, in direct competition experiments, L-phenylalanine was found to be transported four times faster than L-tryptophan and 1000 times faster than L-tyrosine. As implied above, little or no transport was observed when a porphyrin control (14) was used. Nor was significant transport observed when a mixture of sapphyrin and lasalocid was used. 

In order to support the contention that the protonated forms of sapphyrin have a role to play as anion receptors, particularly for physiologically important species such as phosphates and carboxylates, efforts were made to crystallize complexes containing these anions. In the case of phosphate, a number of such structures have in fact been obtained. Two are considered particularly informative. The first involves the 1 1 complex formed between monobasic phosphoric acid and diprotonated sapphyrin 10.6 (Figure 10.5.15) and the second, the mixed salt of diprotonated sapphyrin 10.29, chloride anion, and cyclic-AMP (Figure 10.5.16). ... 

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