C. RIMINGTON 



(see Figure 1). Although no experimental support has been adduced 

 in favour of this scheme, it remains an attractive piece of chemical 

 speculation. 



Me S Me S Me S 



II II II 

 CO H 2 C C— C C C 



II II II II I 



CO OC — 2H 2 HO.C C.OH + 2NH 3 C C 



I I > I I -» / \nh/ \ 



CH 2 H 2 C HC C= -4H 2 HC _ C= 



I I II I \ / N \ / 

 CO OC HOC CO C C 



II II II 

 CH 2 OC C=C C C 



S Me S Me S Me 



Figure 1. Suggested biosynthesis ofhaem pigments* 



Other authors have generally assumed that pyrrolic precursors or 

 building stones are first built up from some such likely raw material 

 as proline or pyrrolidone carboxylic acid and may have postulated 

 certain arrangements of combination in order to explain the appear- 

 ance in nature of the I and HI series but not the II or IV series of 

 pigments. Efforts to explain the non-appearance of the latter on 

 structural grounds lead into bewildering complexity without any 

 really acceptable solution. 



K. Dobriner and C. P. Rhoads 3 pictured synthesis of the porphyrin 

 ring from dipyrromethenes A and B together with formaldehyde 

 {Figure 2) but even so, were unable to explain the non-appearance of 

 the theoretically possible type II isomer. 



A somewhat different scheme was proposed by W. J. Turner 4 in 

 1940 on the basis of the study by A. H. Corwin and J. S. Andrews 5 ' 6 

 of the aldehyde synthesis of dipyrrylmethenes. These authors showed 

 that a tripyrrylmethene is almost certainly formed as an intermediate 

 but that by fission, which could occur in either of two ways, 

 dipyrrylmethenes are the final result. Turner visualized a possible 

 porphyrin synthesis as shown in Figure 3. A fact of much significance 

 relative to this hypothesis is the known occurrence of a compound 

 of similar structure — the bacterial pigment prodigiosin {Figure 4). 

 Nevertheless Turner's hypothesis would again imply the possible 

 synthesis of porphyrins of isomeric type II. The production of the 

 simple pyrrole units might occur, according to Turner, by degradation 

 of tryptophan {Figure 5). 



244 



