Tetradehydrocorrin

Presumably the biosynthetic pathways of the porphyrins and corrins are related to some extent, and the direct template cyclization of a tetrapyrrole has now been described to give a mixture of 10 diastereoisomers of tetradehydrocorrin complexes (equation 54).271 In the pursuit of chemical and biosynthetic similarities and differences between corrins and porphyrins, it is necessary to... 

The name tetradehydrocorrin has also been used by the group of Johnson and Murakami for octadehydrocorrin following the IUPAC recommendations the latter name will be used here in order to avoid confusion. 

C,D-Tetradehydrocorrins have been prepared during the studies on the synthesis of isobacteriochlorins [74], They represent the only example of reduced corrins isolated as metal-free ligands. 

The synthetic pathways leading to tetradehydrocorrins and isobacteriochlorins are very similar and it is just by fine variations of the reaction conditions that the preparation is driven towards specific tetrapyrrolic rings. The linear precursors have been synthesized using the sulfide contraction method (also indicated by other authors as sulfur extrusion ). The corrin skeleton is formed by alkaline hydrolysis of the cyano protecting group present at the 19 position and subsequent acid catalyzed coupling of pyrroles A and D, as described in Fig. 26. 

The reaction affords a complex mixture of products containing the dia-stereoisomers of a Ni(II) C,D-tetradehydrocorrinate. In addition to this major product, other nickel corrinoids have been isolated from the reaction mixture containing Ni(AODC), B,C,D-Ni(AXDC) and A,C,D-Ni(AXDC) which surprisingly are derived from dehydrogenation of the parent complex, despite the anaerobic conditions of the reaction. 

B,C,D-Ni(AXDC) fills the last gap in the corrin family and its preparation has been reported by Monforts [76, 77]. This compound has been obtained by cyclization of a Ni complex of the linear tetrapyrrole reported in Fig. 28. The synthetic pathway is similar to that reported for tetradehydrocorrins a cyano group is utilized to protect the 1-position and then removed by alkaline hydrolysis. Cyclization may be acid catalyzed at room temperature, or accomplished thermally at 240-260 °C, with similar yields. 

Fig.31a-c. The structural formulae of the cobalt complexes of a) cobinamide, b) l-I9-dimethyl-A,D-tetradehydrocorrin, and c) 1,19-di-methyloctadehydrocorrin... 

Additional double bonds have been inserted at the peripheral positions of the macroring in tetradehydrocorrin (A2TDC)- (Fig. 31b) unsaturations are present at A and D pyrroles, while it is the (3 positions of octadehydrocorrin (A2ODC)- (Fig. 31c) which are fully unsaturated. Murakami has used the name bidehydrocorrin for tetradehydrocorrin and tetradehydrocorrin for octadehydrocorrin. In order to avoid confusion the latter names will be used here as described in Sect. 2. 

The stronger diamagnetic shielding of the resonances of the meso-protons in the NMR spectrum of [Co(CN)2(A2ODC)] with respect to the corresponding tetradehydrocorrin complex is consistent with the existence of an extended it conjugation. 

Nature has furnished the corrin ring of a conjugation system which enables the cobalt atom to carry on metabolic reactions tetradehydrocorrin, having a similar k system, is therefore a better model than octadehydrocorrin. 

Table 22. Stability constants concerning to the coordination of pyridine bases to Co(A2TDC)C104 in dichloro-methane. (A2TDC)- represents the anion of 8,12-diethyl-1,2,3,7,8,12,13,17,18,19-octamethy I-tetradehydrocorrin. According to Ref. [2]...

The general electronic structure and the coordination and reaction behaviors of the bisdehydrocorrin complexes are discussed here in reference to those of the corrinoid and tetradehydrocorrin complexes. 

The 7r-conjugation effects in corrin, bisdehydrocorrin, and tetradehydrocorrin rings are reflected in the electronic spectra of their... 

The use of metal ions as templates for macrocycle synthesis has an obvious relevance to the understanding of how biological molecules are formed in vivo. The early synthesis of phthalocyanins from phthalonitrile in the presence of metal salts (89) has been followed by the use of Cu(II) salts as templates in the synthesis of copper complexes of etioporphyrin-I (32), tetraethoxycarbonylporphyrin (26), etioporphyrin-II (78), and coproporphyrin-II (81). Metal ions have also been used as templates in the synthesis of corrins, e.g., nickel and cobalt ions in the synthesis of tetradehydrocorrin complexes (64) and nickel ions to hold the two halves of a corrin ring system while cycliza-tion was effected (51), and other biological molecules (67, 76, 77). 

Examples of template syntheses of other synthetic corrins, as well as of related synthetic corroles and tetradehydrocorrins, have been considered in detail elsewhere [231]. 

Dicyanocobyrinic acid heptamethyl ester (52) undergoes oxidation at the a-bridge by molecular oxygen in the presence of ascorbic acid to form the wew-hydroxy-lactone (53) the dicyanocobyrinic acid lactone (54a) can be transformed into a variety of /S-meso-substituted derivatives, e.g. (54b), and reduction of the bromo- and iodo-lactones (54b R = Br or I) led to dicyanocobyrinic acid heptamethyl ester (52)/ Nickel complexes of 5-methyl derivatives of 1,19-dimethyl-octadehydrocorrins have been prepared, and the synthesis of l,19-diethoxycarbonyl-octamethylplatinum(ll) tetradehydrocorrin bromide by oxidative ring closure of the corresponding 1,19-diethoxycarbonyl-bilatriene in the presence of platinum chloride has been reported. 

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