Vitamin model compounds

Sawyer, D. T., Chiericato, Jr., G., Tsuchiya, T, (1982). Oxidation of ascorbic acid and dehy-droascorbic acid by superoxide ion in aprotic media, J. Am. Chem. Soc., 104 6273. Schrauzer, G. H. (1968). Organocobalt chemistry of vitamin model compounds (cobaloximes), Acc. Chem. Res., 1 97. 

New developments in the field of vitamin B12 reactions of the cobalt atom in corrins and vitamin model compounds. G. N. Schrauzer, Angew. Chem., Int. Ed. Engl., 1976,15, 417-426 (67). 

Schrauzer GN (1968) Organocobalt chemistry of vitamin model compounds (cobaloximes). Acc Chem Res 1 97-103 Schwartz AC (1973) Anaerobiosis and oxygen consumption of some strains of... 

One development involves the use of vitamin B 2 to cataly2e chemical, in addition to biochemical processes. Vitamin B 2 derivatives and B 2 model compounds (41,42) cataly2e the electrochemical reduction of alkyl haUdes and formation of C—C bonds (43,44), as well as the 2inc—acetic acid-promoted reduction of nitriles (45), alpha, beta-unsaturated nitriles (46), alpha, beta-unsaturated carbonyl derivatives and esters (47,48), and olefins (49). It is assumed that these reactions proceed through intermediates containing a Co—C bond which is then reductively cleaved. 

Figure 26.7 Model vitamin Bn compounds (a) a Schiff base derivative, and (b) a cobal-oxime, in this case derived from dimethylglyoxime.

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

Matsuo, M. Matsumoto, S. Iitaka, Y. Niki, E. Radical-scavenging reactions of vitamin E and its model compound 2 2,5,7,8-pentamethylchroman-6-ol, in a tert-butylperoxyl radical generating system. J. Am. Chem. Soc. 1989, 111, 7179-7185. 

Suarna, C. Southwell-Keely, P. T. Effects of alcohols on the oxidation of the vitamin E model compound 2,2,5,7 8-pentamethyl-6-chromanol. Lipids 1989, 24, 56-60. 

Vitamin B12 derivatives and their model compounds have recently been used as recyclable electrocatalysts for the reduction of alkyl halides since low-valent Co species are good nucleophiles toward organic substrates [367-369]. Examples of such elec-trocatalysts are the vitamin B12 derivatives aquocobalamin (230), dibromo[l-hydr-oxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl-10,20-diazaoctahydropor-phinato]cobalt(III) (231), and cobaloxim (232). The above Co(I) complexes can be... 

A new series of vitamin Bj2 model compounds has been prepared by treating the cobalt(ii) clielate (100) with BH in the presence of RX (R = Me, Et, Pr", Bu", n-pentyl, n-hexyl, n-heptyl, n-octyl, or n-decyl). All the complexes RCoL have pseudo-octahedral structures in the solid state and, in poorly... 

Oxidation of chromans has been extensively studied, especially of those which resemble the tocopherols which act as antioxidants in nature and in food. The behaviour of a-tocopherol (vitamin E, 547) and many simpler model compounds under oxidizing conditions varies with the oxidizing agent. Among the many products formed are the o- and p-quinones (609) and (610), the spiran (611) and dimer (612). A detailed account is available (81HC(36)59). 

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

Since the interpretation of NMR parameters in complex biological systems is considerably facilitated by the use of model compounds it is evident that a proper understanding of the metal-ligand interactions in the synthetic counterparts of vitamin B12, which are the subject of this review, can yield additional details and help clarify the role of corrinoids in Nature. 

Further work on the structural requirements for the antioxidant effect has been performed with micelles and liposomes [ 104]. This group concluded that the antioxidant properties of vitamin E and its model compound without the phytyl side-chain were similar within micelles, liposomes and homogeneous solution but that the phytyl side-chain enhances the retention of vitamin E in liposomes and suppresses the transfer of the vitamin between liposomal membranes. 

Pro-oxidant conditions are favoured in infant formulae by the presence of iron and of vitamin C and can lead to oxidative damage to tryptophan residues, which here is of particular importance, tryptophan often being the limiting amino acid. Using a-lactalbumin as a model compound, as it is high in tryptophan, Puscasu and Birlouez-Aragon484 studied the loss of fluorescence due to tryptophan (Aex=290/Aem=340 nm) on incubation with lactose, preformed early and advanced MRP (from proteose-peptone, because it is low in tryptophan), H202/Fe2+, or ascorbate/Fe3+. In each case, after 3 h, there was an appreciable loss of tryptophan from the pH 4.6-soluble protein of about 28%. The MRPs, both formed and preformed, exhibited fluorescence at Aex = 350/Aem = 435-440 (major) and Aex = 330/Aem = 420 nm. 

Studies using model compounds can be useful in structure determinations. For example, the presence of a 1,4-naphthoquinone chromophore in vitamin K was suggested by the similarity of its UV spectrum to those of simpler compounds such as 2-tert-butyl-1,4-naphthoquinone ... 

Finally, there are several very interesting papers currently in press. First, work by Rupee and Freeman [129] has demonstrated the possibility of obtaining natural abundance correlation spectra using vitamin B-12 as a model compound for... 

Stoichiometric Processes. In 1964, Schrauzer published the first of many papers on the synthesis and properties of alkyl cobaloximes. This work led to the development of cobaloximes and related compounds as vitamin B12 model compounds, e.g. (l)-(4), summarized in a review in 1976. By the mid 1970 s, many of the fundamental reactions of vitamin B12 and model complexes were well established. This work is summarized in several reviews. ... 

The role of transition metals in the 23 chemical synthesis of corrins (22) Organocobalt chemistry of vitamin 7 Bi2 model compounds (cobal- (40) oximes)... 

The complexity of vitamin B12 led to the preparation and investigation of model compounds, of which derivatives of bis(dimethylglyoximato)-cobalt(III), termed cobaloximes, have been the most extensively studied (29, 235-237). These derivatives, especially methylaquocobaloxime, have helped cast considerable light on the mechanisms involved in the biological action of vitamin B g. 

Interest in the antioxidant properties of the naturally occurring para-alkoxyphenol a-tocopherol (vitamin E) led to cyclic voltammetry (CV) studies on the radical cations (XXXV) and dications (XXXVI) of model compounds (XXXIV). Both the dication and phenoxonium ion (XXXVII) were found to possess surprising stability when prepared in MeCN [43,44] ... 

The pathway of the book as a whole leads from less reactive, chiral molecules, which are useful as membrane components (lipids steroids, carbohydrates), to molecules that react reversibly with light and electrons and are helpful in energy conversion (carotenes, porphyrins, redoxactive vitamins), and finally to helical and reactive biopolymers (nucleic acids, proteins), which are used as frameworks for molecular machinery. Natural compounds that do not form important supramolecular assemblies or have not been used extensively as model compounds (e.g., alkaloids, antibiotics, metabolites) are not treated in separate chapters, but appear occasionally. 

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