Tetrapyrrol macrocycles

Corrole is a tetrapyrrolic macrocycle 1-Norbomyl is a bicyclo[2.2.1]hept-l-yl... 

From these stractural features it is interesting to note that each molecule of chlorophylls a and b consists of a hydrophilic part (tetrapyrrole macrocycle) and a hydrophobic portion (long terpenoid chain of phytol esterifying the acid group at C-17). Figure 2.1.2 shows the structures and nomenclature of chlorophylls a and b and their major breakdown derivatives. 

The biosynthesis of the tetrapyrrole macrocycle and its branches leading to haem and chlorophylls has been covered in detail in several reviews - - and will be concisely described in this section. Tetrapyrrole biosynthesis occurs entirely in the plastids and is composed of several enzymatic steps starting from 5-aminolevulinic acid (ALA), which is the key precursor of porphyrins and the source of their carbon and nitrogen. 

Most of the subsequent steps of tetrapyrrole synthesis are identical in plants, animals, and bacteria. The pathway includes synthesis of the monopyrrole porphobilinogen from two molecules of ALA by the action of ALA dehydratase with the elimination of two molecules of water, followed by the assembling of a linear tetrapyrrole hydroxymethylbilane from fonr molecnles of porphobilinogen, ring closure and two modification reactions of side chains. This produces the first tetrapyrrole macrocycle, uroporphyrinogen HI. Therefore, eight molecules of ALA are necessary to form one tetrapyrrole. 

During recent years, cross metathesis has found a wide range of applications in total synthesis. CM has been the key step in the syntheses of (-)-lasubine 11 [134], (+)-7a-ept-7-deoxycasuarine [135], and melithiazole C [136] to name just a few examples. It has been used for the modification of tetrapyrrolic macrocycles [137] as well as erythromycin derivatives [138], the dimerisation of steroids [139] and the synthesis of prostaglandin analogues [140]. 

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... 

P. Maillard, J.-L. Guerquin-Kern, M. Momenteau, and S. Gaspard, Glycocon-jugated tetrapyrrolic macrocycles, J. Am. Chem. Soc., Ill (1989) 9125-9127. 

A simple and rapid synthesis of tetrapyrrolic macrocycle has been achieved under dry media conditions with microwave activation. Pyrrole and benzaldehyde adsorbed on silica gel afford tetraphenylporphyrin within 10 min (Scheme 8.26), whereas with conventional methods (e. g. acetic acid in the presence of pyridine) 24 h were necessary. 

As a class, metal-ion derivatives of tetrapyrrole macrocyclic rings, such as the corrins or porphyrins (see Chapter 1 for the parent ring structures), are of major biological importance. 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

Abbreviations for anions of tetrapyrrole macrocycles Pc phthalocyanine (dianion)... 

A ruthenium(VI) nitrido complex containing a tetrapyrrolic macrocycle ligand, [Ru (N)(L)] (82) (H4L = mcio-octamethylporphyrinogen), has been synthesized by the reaction of diphewldiazomethane with [Ru"(L)] , which is prepared from [Ru(cod)(Cl)2] and Na4(L) 4THF. X-ray crystal studies reveal that [Ru (N)(L)] has a Cjv symmetry, and it has the usual saddle conformation, with the metal atom displaced 0.482 A out of the N4 mean plane. The reactivities and redox chemistry of this complex are summarized in Scheme 12 (see also Section 5.6.5.2). 

The last step of methanogenesis by archaebacteria is catalyzed by coenzyme F430 which contains nickel coordinated to the most reduced tetrapyrrolic macrocycle... 

One of the structures present in many enzymes is the corrinoid structure. It consists of a tetrapyrrolic macrocycle, where a direct link between two pyrrole rings exists. Such a direct link and the different degrees of unsaturation that can be introduced into the macrocycle modulate its chelating properties towards metal ions and its reactivity. 

The very rich chemistry of porphyrins has created a great deal of interest in other tetrapyrrolic macrocycles. 

Despite the similarities existing among natural tetrapyrrolic macrocycles, e.g. they are all tetradentate equatorial N4 ligands, structural variations cause a fine modulation of the reactivity of the central metal atom. Thus it is most probably the corrin skeleton that enables the cobalt atom of vitamin B12 to carry out reactions impossible for a similar cobalt porphyrinate. 

Biologically essential tetrapyrrolic macrocycles of two main types exist the porphyrins, the prosthetic group of heme proteins, and the corrins, most representative among which is the coenzyme of Vitamin B12. 

It is now known [5] that the common intermediate to hemes, chlorophyll and vitamin B12 is uroporphyinogen III (Fig. 1). However, a very interesting outcome of Johnson s research is that a new tetrapyrrolic macrocycle Corrole (Fig. 2) was synthesized for the first time. 

Corrole is a tetrapyrrolic macrocycle with a direct link between two pyrrole rings. Lacking a C-20 meso carbon bridge it has a corrin like skeleton with double bonds involving porphyrin-like conjugation. It has an 18 electron rt system and hence aromatic character. The direct link between the A and D... 

Figure 8-12. The use of a sulfide nucleophile to form a new tetrapyrrole macrocycle by the displacement of bromide from co-ordinated bromopyrroles.

Corroles 8 (Figure 2) are aromatic tetrapyrrole macrocycles bearing a direct... 

Tetrapyrrole macrocycles such as haems and corrin exhibit a number of special features that make... 

To give an example of how a coenzyme functions we will look at the example of coenzyme B]2 which has proved a popular target for model studies over many years. Coenzyme B12 and its derivatives such as vitamin B12 are also based on tetrapyrrole macrocycles, Figure 2.10. The structures of these... 

Explain why tetrapyrrole macrocycles feature highly as cation binding hosts in biological systems (Chapter 2) while expanded porphyrins such as (4.14) are more suited to anion binding. 

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