Corrins demetallation

Both of the above approaches employed a metal ion as a template about which the corrin cyclization was performed, but the nickel or cobalt ions could not subsequently be removed. In order to obtain metal-free corrins, a new route was therefore devised (67AG865) which employed the novel principle of sulfide contraction (Scheme 22). Thus the sodium salt of the precorrin (284) (Scheme 23) was transformed into the thiolactam (285), and loose complexation with zinc(II) ions caused cyclization to give (286), which was treated with benzoyl peroxide and acid to give the ring-expanded compound (287). Contraction with TFA/DMF gave the corrins (288) and (289), and the major of these (289) was desulfurized with triphenylphosphine and acid to give (288). Finally, demetallation with TFA gave the required metal-free corrin (290), a source for a whole variety of metal derivatives. 

The chemistry outlined in Scheme 24 was then put into effect catalytic hydrogenation of the tris-isoxazole (302) and recyclization with triethylamine gave a tricyclic ligand which was chelated with nickel ions to give (303). Introduction of the fourth nitrogen atom was accomplished by treatment of (303) with ammonium acetate, giving (304). Treatment with cyanide removed the nickel ion which was then replaced with zinc(II) to give (305). The reasons for this transmetallation step were two-fold firstly, zinc(II) corrins, as shown by Eschenmoser, can be readily demetallated, and this fact opens up many options later in the synthesis, but secondly, and more importantly, Eschenmoser s photochemical cyclization of seco-corrins (see Section 3.07.3.4.2.3) does not proceed with nickel complexes of seco-corrins, whereas zinc(II) seco-corrins can be cyclized in almost quantitative yield... 

Coproporphyrin I synthesis, 816 Coronands classification, 919 metal ion complexes, 928,938 Corphins, 855 Coninoids, 983 Corrins, 871-888 demetallation, 882 deuteration, 879 electrophilic reactions, 879 metallation, 882 NMR, 878 nucleophilicity, 886 nucleophilic reactions, 879 oxidation, 879 oxidative lactamization, 880 oxidative lactonization, 880 photochemistry, 887 reactions, 879 at metal, 885 rearrangements, 879 redox chemistry, 888 spectra, 877 synthesis, 878 Corroles, 871-888 demetallation, 874 deuteration, 872 hydrogenation, 872 metallation, 874 reactions, 872 at metal, 875 redox chemistry, 876 synthesis, 871 Corticotropin zinc complexes medical use, 966 Cotton effect anils, 717... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

In 1982, Krautler and Hilpert reported on the preparation of (D-didehydrocorrinate)(dicyano)Co(III) by non-oxidative photocyclization of 19-cyanosecocorrin (Fig. 24a (R = = CH2, R = R" = CN) to the corresponding 1-methyl-19-cyanocorrinate Cd2+ [73]. Demetallation, subsequent insertion of cobalt and careful thermal elimination of the 19-cyano group affords the didehydrocorrinato cobalt complex as illustrated in Fig. 25. The complex has been converted into the corresponding corrinate by treatment with zinc in acetic acid. 

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