Porphinoids

In connection with this development it is worth mentioning that uroporphyrinogen III (8), the reduced form of uroporphyrin III (9), is the key building block of all naturally occurring porphinoid structures known at present. 

The enzymatic conversion of a-(aminomethyl)pyrroles is also used by nature to produce porphyrinogens like uroporphyrinogen III (see introduction, compound 8), which is the key building block in the biosynthesis of all known porphinoid natural products. This biomimetic method is a powerful tool for the synthesis of different porphyrins, e.g. for the preparation of JV,Af, V ,Ar"-tetramethylporphyrin-2,3,7,8,12,13,17,18-octaacetic acid dibromide 12.36... 

These tautomerization experiments have also been performed with uroporphyrinogens,25e 24 which are important intermediates in the biosynthesis of natural porphinoids and corrinoids. This has led to a deeper insight into biosynthetic pathways and a possible prebiotic origin of... 

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. 

In marked contrast to what is usually observed with porphinoid macrocycles under these oxidation conditions and therefore surprisingly, the rosarins are obtained as 2471 cyclically conjugated, but nonaromatic species, an observation which was also made in the amethyrin series vide supra). 

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

Methyl coenzyme M reductase plays a key role in the production of methane in archaea. It catalyzes the reduction of methyl-coenzyme M with coenzyme B to produce methane and the heterodisulfide (Figure 3.35). The enzyme is an a2P2Y2 hexamer, embedded between two molecules of the nickel-porphinoid F jg and the reaction sequence has been delineated (Ermler et al. 1997). The heterodisulfide is reduced to the sulfides HS-CoB and HS-CoM by a reductase that has been characterized in Methanosarcina thermoph-ila, and involves low-potential hemes, [Fe4S4] clusters, and a membrane-bound metha-nophenazine that contains an isoprenoid chain linked by an ether bond to phenazine (Murakami et al. 2001). 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

This cofactor is a rare, highly saturated Ni-containing porphinoid system called corphin, meaning a hybrid of corrinoid and porphinoid tetrapyrrolic macrocycle. It is a most reduced... 

Several authors indicated that the microbial transformation of CT is considered to be closely related to the presence of microbial cofactors, such as porphinoids (cofactor F430) and corrinoids (vitamin B12) [60-70,238,386]. In vitro, abiotic degradation of CT, mediated by these cofactors under reducing conditions, has been widely reported. Such cofactors can serve as electron carriers passing electrons from a donor to reduce CT, as follows ... 

The cyclization reaction developed by Scheffold, and shown in Scheme 65, makes use of the addition of an amidine to an iminium salt, and produces a product of reduced porphinoid character.196 197... 

SYNTHESIS AND SOME REACTIONS OF PHTHALOCYANINE, PORPHINOID AND CORRINOID 192 MACROCYCLES... 

A consideration of the synthesis and reactions of phthalocyanines, porphinoids, corrinoids and related macrocycles will follow in the second section. Discussion in this section will be restricted because these topics will receive consideration in other chapters. 

More direct attempts by Eschenmoser and his group to build up corrins at the desired oxidation level have unearthed a wealth of kinetic template cyclization processes. These have in turn developed into techniques which can now be applied to porphinoid compounds as well. A brief summary of the types of template cyclization reactions is relevant here. 

Diazaporphyrins - Like the phthalocyanines, a 10,20-diaza-porphinoid palladium complex was composed by cyclooligomerization. The condensation of two suitable bispyrrole halves yielded the tetrapyrrole complex, cis [1,11-dimethoxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl- 10,20-diaza-decahydro-porphyrinato]palladium(II) [124]. 

Muller PM (1973) Corphin ein neuartiges porphinoid-corrinoides Ligandsystem. Dissertation, ETH Zurich... 

An increasing number of porphinoid macrocyclic pigments have been found to be related to important biochemical processes. As a result, much of the pertinent chemistry has been thoroughly studied. 

The construction of abbreviations for a particular porphinoid (e.g., OEC or TMPc) is self-evident. 

Delocalization of the chromophore is as indicated in XVII. Porphyrins and porphinoids closely resemble the 18-membered 18 7t-electron [18]an-nulenes, avoiding energetically unfavorable conjugation via pyrrolic nitrogen lone pairs. If interactions of this type are necessary for structural reasons [as in... 

As a consequence of the tendency of porphinoids to form an 18 membered 18 7i-electron delocalization circle, NH-groups will be found preferentially at pyrrolic rings trying to avoid the nitrogens of hydrogenated pyrroline rings. 

The natural corrinoids are made exclusively by microorganisms, which are also exploited on a large scale for the industrial production of vitamin B12. The biosynthesis of the corrin ligand branches off from that of the other porphinoids at their common tetrapyrrolic intermediate, uroporphyrinogen III, and has been studied both in aerobic and in anaerobic... 

X-ray crystallographic studies reveal that MCR is composed of three different subunits in an aifiiyi arrangement containing tightly bound 2 mol of the nickel porphinoid... 

The final step in methanogenesis is the reductive demethylation of CH3-S-CoM to CH4. This reduction involves two reactions CH3-S-C0M is reduced with jV-7-mercaptoheptanoylthreonine phosphate (H-S-HTP) (Fig. 2B) as electron donor to yield CH4 and a heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) (Reaction 7, Table 2). This reaction is catalyzed by CH3-S-C0M reductase [70-72] which contains a nickel porphinoid, factorF430, as prosthetic group (Fig. 2C) (for a recent review see Friedmann et al. [73]). The subsequent reduction of the heterodisulfide with H2 to yield H-S-HTP and H-S—CoM (Reaction 8, Table 2) is catalyzed by CoM-S-S-HTP-dependent heterodisulfide reductase. The enzyme is an iron-sulfur protein containing FAD as prosthetic group [74]. The physiological electron donor for the heterodisulfide reductase is not known. 

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