Porphyrins constitutional isomers

The cyclotetramerization of suitable monopyrroles is the most simple approach to prepare porphyrins and it has been used in the synthesis of numerous porphyrin structures. But this method can only be used satisfactorily if the pyrroles 1 and 2 bear identical /i-substituents (R = R), otherwise the cyclotetramerization leads to a statistical mixture of four possible constitutional isomers. 

Although one of the two building blocks has to be symmetric to avoid constitutional isomers, the symmetry constraints differ from those of the 2 + 2 approach and this allows the synthesis of structures that would be difficult to obtain by other synthetic strategies. Consequently, the 3 + 1 strategy has been accepted and is an increasingly used method for porphyrin synthesis.49... 

The preparations of chlorins described so far by attack of different reagents at a peripheral C — C double bond of the porphyrin macrocycle are all restricted to highly symmetric porphyrins because otherwise complex mixtures of constitutional isomers can be formed. The problem of... 

Uroporphyrinogen I (16c), a constitutional isomer of uroporphyrinogen III, also plays no direct role in porphyrin and corrin biosynthesis, but this unnatural substrate is methylated to give 17c10c f in the presence of SAM by the methyl transferase of some bacteria. A constitutional type I dihydroisobacteriochlorin can be obtained by methylation of uroporphyrinogen I with methylase Ml. Methyltransferase M1 is able to methylate the unnatural precorrin once more to give the trimethylpyrrocorphin type I.IOc 1... 

The tetramerization of suitable monopyrroles is one of the simplest and most effective approaches to prepare porphyrins (see Section 1.1.1.1.). This approach, which is best carried out with a-(hydroxymethyl)- or ot-(aminomethyl)pyrroles, can be designated as a biomimetic synthesis because nature also uses the x-(aminomethyl)pyrrole porphobilinogen to produce uroporphyrinogen III. the key intermediate in the biosynthesis of all kinds of naturally occurring porphyrins, hydroporphyrins and corrins. The only restriction of this tetramerization method is the fact that tnonopyrroles with different -substituents form a mixture of four constitutionally isomeric porphyrins named as porphyrins I, II, III, and IV. In the porphyrin biosynthesis starting from porphobilinogen, which has an acetic acid and a propionic acid side chain in the y6-positions, this tetramerization is enzymatically controlled so that only the type III constitutional isomer is formed. 

In order to increase the n-orbital overlap between carotenoid and porphyrin we designed 26, a constitutional isomer of 24 [79]. The proton... 

Figure 8.1 Constitutional isomers of porphyrin. The names are given for the compounds which have been synthesized to date.

The structural skeleton of I. (tetrahydroporphyrins) is formally derived from porphine by reduction of two peripheral double bonds in neighboring pyrrole rings of the porphyrin skeleton. I. are constitutional isomers of the bacteriochlorins, from which their name is derived. Natural members of this structural type are heme d, siroheme, and sirohydrochlorin. 

Isobacteriochlorins (8) (2,3,7,8-tetrahydroporphyrin) with two adjacent saturated pyrrole rings are the constitutional isomers of bacterio-chlorins. Whereas in porphyrins, chlorins, and bacteriochlorins the 187T-perimeter includes two nitrogens excluding their lone pairs, the 187T-system of the isobacteriochlorin includes three nitrogens, one with its lone pair as part of the aromatic 7r-system. It is therefore less stabilized. 

Although uroporphyrinogen I (20), a constitutional isomer of uroporphyrinogen III (14), plays no direct role in porphyrin biosynthesis, this unnatural substrate is methylated in the presence of SAM by the methyl transferases of some bacteria. 

One of the widely studied, stable constitutional porphyrin isomers encompassing 18jt-electrons in a cyclic conjugated pathway is porphycene 38 (Scheme 17). Other isomers such as NCPs are much less stable and are not discussed here. Porphycenes consist of two 2,2 -bipyrrole subunits linked by two double bonds and are thus named as [18] porphyrin( 2.0.2.0). ... 

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