Corrinoids metal-free

A second synthesis of cobyric acid (14) involves photochemical ring closure of an A—D secocorrinoid. Thus, the Diels-Alder reaction between butadiene and /n j -3-methyl-4-oxopentenoic acid was used as starting point for all four ring A—D synthons (15—18). These were combined in the order B + C — BC + D — BCD + A — ABCD. The resultant cadmium complex (19) was photocyclized in buffered acetic acid to give the metal-free corrinoid (20). A number of steps were involved in converting this material to cobyric acid (14). 

Metal-free corrinoids are produced by photosynthetic bacteria.863 This allows the synthesis of analogues of vitamin B12 having other metals present, such as zinc, copper or rhodium. The... 

The presence of a metal salt is generally necessary to achieve the macrocycle some corrinoids have been isolated only as metal complexes since the metal-free ligand is not stable enough to be isolated. The reasons for such instability have not been clarified, but it is noted to increase with the unsaturation level of the macrocycle. 

However, the early cobalt incorporation during the anaerobic pathway could not be essential for ring contraction since metal free corrinoids have been found in anaerobic bacteria 94) like Chromatium vinosum and Rhodospirillum rubrum in the absence of cobalt. Moreover, metal free corrinoids are produced by strictly anaerobic methanogenic bacteria in the absence of cobalt. Thus further biosynthetic explorations are necessary to discover the primitive anaerobic corrin formation in Archaebacteria. 

---