Secocorrin complexes

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). 

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... 

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... 

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. 

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). 

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]. 

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