Secocorrin

Scheme 1 outlines the retrosynthetic analysis of the Woodward-Eschenmoser A-B variant of the vitamin B12 (1) synthesis. The analysis begins with cobyric acid (4) because it was demonstrated in 1960 that this compound can be smoothly converted to vitamin B12.5 In two exploratory corrin model syntheses to both approaches to the synthesis of cobyric acid,6 the ability of secocorrinoid structures (e. g. 5) to bind metal atoms was found to be central to the success of the macrocyclization reaction to give intact corrinoid structures. In the Woodward-Eschenmoser synthesis of cobyric acid, the cobalt atom situated in the center of intermediate 5 organizes the structure of the secocorrin, and promotes the cyclization... 

Schlosser s super base 615 f. Schmidt trichloroacetimidate glyco-sidation 528,531,537,556 Schwartz s reagent 606, 616 scopadulcic acid A 571 scopadulcic acid diterpenes 569 secocorrin 100, 122, 126 f., 130... 

In what is one of the few examples of utilization of a higher order sigmatropic hydrogen shift in the synthesis of complex molecules, Eschenmoser, in studies directed toward the synthesis of Vitamin B12, found that an antarafacial [1,16] H-shift could be utilized to effect closure of secocorrin 63 to corrin 65 (Scheme 16)31. An intermediate biradical... 

Another route to corrin via 4,5-seco intermediates has been established (Scheme 88).250,255 It was shown by X-ray structure analyses of the Ni-4,5-secocorrin, and its cyclization product, that the pyrrolic A and B rings are properly oriented for the reaction.250 A sulfur extrusion method was employed in the B12 synthesis of Woodward and Eschenmoser.253... 

Saxitoxin occurrence, 5, 604 Saxitoxiri, hydroxy-occurrence, 5, 604 Scholtz synthesis indolizines, 4, 466 Scillaren A occurrence, 3, 883 Scutellarein chemistry, 3, 697 Secobarbital, 3, 150 1,19-Secocorrin nomenclature, 1, 30 A/D-Secocorrin complexes cyclization, 4, 429 A/D-Secocorrin, Al8-dehydro-synthesis, 4, 429 A/D-Secocorrin, 1-oxo-synthesis, 4, 429 Secocorrins... 

The secocorrin nickel complex (101) can also be induced to undergo an electrochemical cyclization to corrin (100) but only in low yield. The experimental conditions involve a one-electron oxidation, followed by a one-electron reduction.269 The same secocorrin complex (101) is the starting material for two cyclization sequences leading to a didehydrocorrin complex (107) with a chromophore of seven double bonds (Scheme 67).269,270 The most interesting feature of these sequences is the remarkably easy acid-catalyzed ring closure of the secocorrinoid complex (106) to corrin (107). 

When we consider a proper linear precursor for a corrin, which is a highly reduced structure, a different parent tetrapyrrole may be used this is named secocorrin and its shown in Fig. 5. 

However, the name secocorrin has been used for compounds in different degrees of hydrogenation in the literature in order to eliminate confusion it should therefore be avoided. 

Eschenmoser conceived a synthesis of corrins which could be extended to biological compounds. In this method a Cd2+ complex of the secocorrin which formula is shown in Fig. 24a (R = =CH2, R = H, R" = CN) is converted... 

Such photochemical cyclization may be also accomplished when the secocorrin is coordinated to Li+, Mg2+, Zn2+, Pd2+ or Pt2+, but fails with Cu2+, Ni2+, Co2+ or Mn2+. The metal-free ligand is also unable to cyclize [66, 67],... 

A direct synthesis of a Ni complex of corrin may be carried out thermally when the secocorrin shown in Fig. 24 a (R =CH3, R = formyl or acetyl, R" = CN) is heated in the presence of Ni salts [68, 69]. The corresponding 1-methyl-19-unsubstituted corrinate Ni2+ is then achieved by base-induced removal of the formyl or acetyl group. 

The synthetic conditions are unsatisfactory with respect to the intended role of the reaction as a potentially biomimetic model, but demonstrate that it is possible to achieve interconversion between the structures of hydroporphyrin, secocorrin and corrin. 

One of the earliest, and certainly one of the more impressive examples of apparent cation radical sigmatropic shifts involves the corrin series [97], Electrochemical oxidation of a nickel(II)-A/D-secocorrinate in acetonitrile containing a trace of water was found to provide an almost quantitative yield of a secocoriinoxide in which a [1,16] hydrogen shift, from the methylene group at C19 in the D ring to the methylidene carbon in the A ring, had occurred in the intermediate cation radical. 

For the ring closure of macrocycles, the use of templates is a common tool to enhance the yield and selectivity. Substrates using metal ions as templates were successfully converted to the macrocyclic product. The templating metal center remains in the product. The nickel complex 20 is oxidized on platinum, undergoes hydrogen shift, and is trapped by water providing nickel secocorrinate 21 [36] (Scheme 8). 

The synthesis of cobyric acid is the culminating achievement in corrin chemistry. The key step in cobyric acid synthesis is transformation of the open-chain organic system into the corresponding macrocyclic corrin by means of chemically joining cycles A and B (the A/B route) as well as by secocorrin/corrin cyclisation between rings A and D (the A/D route). 

Photoinduced cycloisomerisation can also be carried out in the presence of other metal ions serving as templates. For example, secocorrin coordinated to metal ions such as lithiiun(I), magnesirun(II), zinc(II) or cadmium(II) readily cyclises in the absence of oxygen in almost quantitative yield at room temperature [441, 444]. Platinum(II) and palladium(II) complexes (quantum )deld 0.008 in chloroform at 20°C) also cyclise, but more slowly. However, nickel(II), cobalt(III) and copper(II) secocorrinates do not undergo such a transformation [441, 444]. If, however, the nickel(II) l-methylidene-l,19-secocorrinate is subjected to one-electron electrochemical oxidation, followed by direct reduction of the isomerised cation-radical, then such a transformation is realisable [450]. 

Another model system, nickel(Il) A -dehydro-A/D-secocorrinate [Ni(L605)]C104, is cyclised to corrin [Ni(L606)]C104 in 50-60% yield by electrochemical reduction (Eq. 2.233) [451]. 

Thermal decarboxylation of nickel(ll) l-methylidene-2,2,7,7,12,12-hexamethyl-15-cyano-19-carboxy-l,19-secocorrinate [Ni(L607)]C104 in a non-polar solvent also leads to formation of a corrin system, [Ni(L608)]C104 (Eq. 2.234) [452]. 

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