Porphyrin open chain tetrapyrrolic intermediate

Porphyrin Synthesis from Open-Chain Tetrapyrrolic Intermediates... 

PORPHYRIN SYNTHESIS FROM OPEN-CHAIN TETRAPYRROLIC INTERMEDIATES... 

Porphyrins from open chain tetrapyrrolic intermediates... 

There is only one way to build up a porphyrin in a completely general way, and that is through an open-chain tetrapyrrolic intermediate, which is usually separated out prior to the final cyclisation step. Different types of tetrap5UTolic intermediates have been used for example, bilanes, bilenes, oxobilanes, and biladienes. The fact that they are open-chain tetrapyrroles is inherent in the name all these compounds refer in there own way to the bile pigments, the naturally occurring products of porphyrin catabolism. 

In terms of synthetic strategy, approaches to porphyrins from open-chain tetrapyrroles are the only truly general routes. The principle is to construct, in a stepwise manner, an open-chain tetrapyrrole bearing a pre-determined arrangement of peripheral substituents. Cyclization to produce a porphyrin, or an immediate precursor, under mild conditions which do not cause redistribution of the pyrrole rings, should be accomplished after full characterization of the open-chain intermediate. 

Once the porphyrin ring is open, the metal is deposited on the catalyst. Open chain tetrapyrrolic structures (M-biliverdenates) are unstable under catalytic hydrogenation conditions (Subramanian and Fuhrhop, 1978). The ease of this sequence with Ni-etioporphyrin (Ni-etio) is apparent by the lack of stable metal intermediates other than the chlorin (M-PH2). 

Anodic oxidation in DMF of the open-chain tetrapyrrole, 1, 8 -dimethyl-a,c-bila-diene, yielded porphyrin one methyl group is lost from a cyclic intermediate with four conjugated pyrrole subunits by attack of a nucleophile [32]. 

A number of improvements have also been described in the other methods used for synthesizing unsymmetrical porphyrins from open chain tetrapyrroles. However, the direct coupling of two dipyrrolic precursors to give porphyrins is still often the method of choice as a convergent procedure it minimizes the number of stages involved and is usually only limited by the need for one of the dipyrrolic intermediates to be symmetrical (to avoid the formation of more than one porphyrin). 

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