Corrins rearrangements

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

Vitamin Bjj (8.50, cobalamin) is an extremely complex molecule consisting of a corrin ring system similar to heme. The central metal atom is cobalt, coordinated with a ribofuranosyl-dimethylbenzimidazole. Vitamin Bjj occurs in liver, but is also produced by many bacteria and is therefore obtained commercially by fermentation. The vitamin is a catalyst for the rearrangement of methylmalonyl-CoA to the succinyl derivative in the degradation of some amino acids and the oxidation of fatty acids with an odd number of carbon atoms. It is also necessary for the methylation of homocysteine to methionine. 

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... 

On the other hand, Hisaeda and coworkers [504] found that the vitamin Bi2-mediated electroreductive optical resolution was accompanied by rearrangement Electroreduction of racemic 3-bromo-2-methoxy-2-phenylpropionate using a hydrophobic vitamin Bi2 mediator afforded ethyl (S)-2-methoxy-2-phenylpropionate in 55% ee, while a novel hydrophobic vitamin Bj2 modified by introducing a 1,3-phenylene diacetyl moiety into the peripheral site around the corrin s B ring, a so-called strapped hydrophobic vitamin B12, provided the corresponding R-enantiomer in 26% ee. 

Precorrin-4 acts as the substrate for another class III methyl transferase (Figure 19), CobM, which methylates at Cl 1 to generate precorrin-5 (Figure 20). It was a surprise when this intermediate was first identified as the corrin macrocycle does not contain a methyl group at C11 but does have one at C12. In fact, as will be described later, the methyl group added to Cll is subsequently rearranged to Cl2. The reason for methylation at Cll is to allow the decarboxylation of the acetate side chain attached to C12 prior to its methyaltion but it also plays a role in permitting the function of CobF (see Section 7.13.11.5). 

Figure 22 The structures and mechanism of CobH and CbiC. (a) The structures of CobH and CbiC are shown as indicated. The enzyme cataiyzes the migration of a methyl group from Cl 1 of the corrin ring to C12. The substrate-bound (hydrogenobyrinic acid - stick representation) structure of CobH indicates that an invariant histidine residue, located above ring C of the substrate, plays an important role in this process, (b) Proposed mechanism for the 1,5-sigmatropic rearrangement.

Mixed isomers (80%) can be obtained from tetrasubstituted pyrroles, the 3H compound sometimes predominating.8 The reaction has been used successfully with substituted alkyl halides in a search for routes to corrin intermediates, giving products such as 11 and 12, and in the preparation of 2//-pyrroles for use in mechanistic studies on the thermal and photochemical rearrangements of N-substituted pyrroles.18-20 Thus, the reaction of... 

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

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