DISCOVERY AND SYNTHESIS OF LESS COMMON NATURAL HYDROPORPHYRINS

The Chart 6 shows the nickel-containing frameworks of all the highly reduced hydroporphins synthesized75e [formula omitted] to date. The tautomeric hexahydroporphins 189a-d have the same oxidation level as uroporphyrinogen III (153), the key building block in the biosynthesis of all currently known, naturally occurring porphinoid structures. They can be formed under controlled experimental conditions from the porphyrinogens by tautomerization. The chemistry of natural hydroporphinoid compounds and their importance for the biosynthesis of vitamin B12 were discussed in section V. Naturally occurring chlorins, bacteriochlorins and isobacteriochlorins, which have oxidation levels in between the hexahydroporphins and the completely unsaturated porphins, were dealt with in the sections II–IV. The upper limit in the spectrum of hydroporphinoid structures, which are characterized by different oxidation levels, is currently represented by the dodecahydroporphin chromophore 152 of factor F 430 (150). It remains to be seen whether nature has been able to produce more highly reduced porphyrins or the missing links in the spectrum of hydroporphinoid structures, octahydroporphin (188) and decahydroporphin (187). © 1994, American Chemical Society. All rights reserved.