Vitamin B12. structure

One of the most unusual features of the vitamin B12 structure is that cobalt is bonded directly to carbon. In heme, iron is bonded to five nitrogen atoms, but the sixth position is vacant until oxygen attaches. In vitamin B12, the sixth position is that bonded to the group indicated by R in the structure... 

It would be interesting to know whether this enzyme system works with cobalt only when the latter is held in the vitamin B12 structure, or whether it is possible to produce cobalt-carbon compounds from other cobalt complexes as well. 

Chemical Name 5,6-Dimethylbenzimidazocyl cyanocobamide Common Name Vitamin B12 Structural Formula ... 

The first natural product in which the presence of the metal cobalt (Co) was shown is Vitamin B12 (cobal-aminum). It was isolated from the liver simultaneously by K. Fokers (United States) and Lester Smith (England). The Vitamin B12 structure was established by Todd in 1955. The center of the molecule contains the metal Co(III) with a coordination number 6. It is connected with four mutually bound pyrol cycles. Vitamin Bi2 is used in the treatment of pernicious anemia. 

All of the naturally occurring isobacteriochlorins contain the geminally dialkylated structural parts in the saturated pyrrole rings, which require special approaches for their synthesis. Until the discovery of siroheme 24 and sirohydrochlorins 25, 26 this structural element was only known from the vitamin B12 structure. 

Although all of these steps and their elucidation are extremely complex, there is growing evidence from attempts to s)mthesise the vitamin B12 structure chemically, that most of the structural elements in the molecule (even... 

Figure 5. Final Superstructure menu with Vitamin B12 structure.

Firefly lucifenn is an example of an azole that contains a benzene ring fused to the five membered ring Such structures are fairly common Another example is benzimidazole present as a structural unit m vitamin B12 Some compounds related to benzimidazole include purine and its ammo substituted derivative adenine one of the so called het erocychc bases found m DNA and RNA (Chapter 28)... 

Naturally occurring compounds with carbon-metal bonds are very rare The best example of such an organometallic compound is coenzyme Bi2 which has a carbon-cobalt ct bond (Figure 14 4) Pernicious anemia results from a coenzyme B12 deficiency and can be treated by adding sources of cobalt to the diet One source of cobalt IS vitamin B12 a compound structurally related to but not identical with coen zyme B12... 

The structure of the diamagnetic, cherry-red vitamin B12 is shown in Fig. 26.6 and it can be seen that the coordination sphere of the cobalt has many similarities with that of iron in haem (see Fig. 25.7). In both cases the metal is coordinated to 4 nitrogen atoms of an unsaturated macrocycle (in this case part of a corrin ring which is less symmetrical and not so unsaturated as the porphyrin in haem) with an imidazole nitrogen in the fifth position. A major... 

Macrocyclic polyamines may be viewed as an extended form of linear polyamines 13-161 with one less degree of saturation. Polyamines with lower degrees of saturation, have important biological functions. Compounds such as macrocyclic polyimines 171 and porphyrines 18), function as 02 carriers and activators, promote photosynthesis, form the basic structure of vitamine B12, etc., and for these reasons have been subjects of intense investigation. 

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... 

Although phthalocyanines are structurally similar to porphyrins such as hemoglobin, chlorophyll a and vitamin B12, they do not occur in nature. Formally, phthalocyanines can be regarded as tetrabenzotetraazaporphyrins and as the condensation products of four iso-indoline units. 

Vitamin B12. Figure 1 Structure of the different vitamin B12 vitamers. 

Much research focuses on the structures, properties, and uses of the complexes formed between d-metal ions acting as Lewis acids and a variety of Lewis bases, partly because they participate in many biological reactions. Hemoglobin and vitamin B12, for example, are both complexes—the former of iron and the latter of cobalt (Box 16.1). Complexes of the d-metals are often brightly colored and magnetic and are used in chemistry for analysis, to dissolve ions (Section 11.13), in the... 

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