Sapphyrins aromaticity

The approach makes use of a bipyrroledicarbaldehyde 54 and a tripyrranedicarboxylic acid 55 which are condensed with subsequent decarboxylation in the presence of acid and oxygen. The presence of oxygen, as in many other syntheses of porphinoid macrocycles, is necessary to adjust the oxidation level of the chromophore which is in sapphyrins a 2271-aromatic cyclically conjugated system. Many sapphyrins 56 with different substitution patterns have been synthesized according to this general scheme. As in all McDonald-type condensations at least one of the components has to be symmetric because otherwise mixtures of constitutional isomers would be formed. 

The [22]pentapyrrolic(1.1.1.1.0), sapphyrins (1966MI), are aromatic and show intense blue-green color in organic solvents. Serendipitous isolation of sapphyrins by Woodward (1966MI) led to the development of synthetic... 

Sessler, J. L., Andrievsky, A., Sapphyrin-Lasalocid conjugate A novel carrier for aromatic amino acid transport. Chem. Commun. 1996, 1119-1120. 

In the case of sapphyrin-based carboxylate receptors, the relative binding affinities were found to depend on the nature of the substrate. In particular, aromatic carboxylates were found to be bound with higher affinity to protonated sapphyrins than aliphatic ones. Indeed, in the case of the decaalkyl sapphyrin 3, affinity constants measured in dichloromethane, were found to be ca. 103 M-1 for aromatic substrates but less than 102 M-1 for aliphatic ones. 

To date, the poor stability of the smaragdyrins 226, 229, and 230 has unfortunately precluded a complete study of their chemistry. However, the unique nature of these unusual aromatic materials suggests that further efforts may be warranted. Just as a leinv gation of the sapphyrin system has provided a wealth of new chemistry [157], a sinular study of the synthesis and properties of smaragdyrin and related compounds could uncover further interesting observations including, perhaps, stabilized forms of this class of expanded porphyrins. 

As implied above, the first synthesis of a sapphyrin was purely accidental and came about as the result of the reaction of the linear tetrapyrrolic precursor 5.1 with formic acid and HBr. Following oxidation with iodine, instead of obtaining the desired tetrapyrrolic corrole macrocycle, a different macrocycle was obtained (Scheme 5.1.1). This unexpected by-product was best formulated as being the deca-alkyl-substituted aromatic pentapyrrolic macrocycle 5.2. The unexpected formation... 

As would be expected for an aromatic macrocycle, pentaphyrins display exceptionally strong absorbance bands in the visible portion of the electronic spectrum. For instance, pentaphyrin 6.35 displays a sharp Soret-like absorbance band at 458 nm (CH2CI2, = 238 000 cm ), and two Q-like absorbances at 642 and 695 nm (e = 5990 and 3540M cm , respectively). They thus resemble spectroscopically both the sapphyrins and smaragdyrins, as well as the tetrapyrrolic porphyrins from which they are conceptually derived. Like these various aromatic cousins , the... 

In 1996 Sessler and coworkers reported the synthesis of a new Lasalocid modified sapphyrin that acts as an efficient carrier for the transport of aromatic amino acids in their zwitterionic form. Again, a standard H2O-CH2CI2-H2O U-tube transport model was used to determine the transport efficiency of Compound 31. This receptor showed not only a high selectivity for phenylalanine and tryptophan for L-Phe), but was also... 

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