Vitamin addition reactions

All industrial vitamin A syntheses use p-ionone as the starting compound (36, see page 14) 3S). This monocyclic C13 ketone can be obtained either completely synthetically from acetone and acetylene by consequent use of the C2 and C3 addition reaction, or via citral (59, see page 14) obtainable from natural sources (lemongrass oil). 

Interest in the study of the oxidative addition reactions of low-spin cobalt complexes is enhanced by parallels with corresponding reactions of vitamin B,2 derivatives and hence by their possible relevance as Bi2-model systems . 

The most useful, and thus far successful, examples have involved irreversible reactions of nucleophilic functions of an enzyme s reactive site with an enzymatically activated Kcat inhibitor of a Michael-type addition reaction. The activation invariably requires participation of the enzyme s prosthetic group (e.g., flavin of monoamine oxidase) or coenzymes such as pyridoxal (vitamin B) as its phosphate, which is associated with several enzymes (e.g., threonine dehydrase, ornithine decarboxylase, a-ketoglutarate transaminase). 

Table I is a summary of all of the reactions known to date which require vitamin B12 coenzyme (i). The reaction catalyzed by glutamate mutase, the first reaction in which vitamin B12 was known to be involved, was discovered by Barker and his coworkers. The second reaction is the only one of these which also occurs in mammals all other reactions occur in bacteria. Much of the experimental data described here will be derived from the enzyme called dioldehydrase. An additional reaction which is not shown is the conversion of nucleotides to deoxynucleotides. The conversion of —CHOH— to — CH2— is in some way very similar to the reaction catalyzed by dioldehydrase. Vitamin B12 coenzyme-depend-...

Thiamin plays an essential role in different foods as a water soluble vitamin. Additionally, its function as a flavor precursor in heated foods, e.g. meat, should not be neglected. But certainly, this aspect depends very much on its amoimt and the specific conditions in the food system. Another important field in which thiamin plays a remarkable role is the application of flavorings. Along with carbohydrates, amino acids, ribonucleotides, and other constituents, thiamin is widely used as a flavor precursor. This fact is clearly demonstrated by many patented reaction or processed flavors. 

Free radicals such as ROO- (and RO-, OH- etc. which can be formed by extra side-reactions) can react at random by hydrogen abstration and a variety of addition reactions to damage proteins, other lipids and vitamins (particularly vitamin A). Compounds which react rapidly with free radicals can be useful in slowing peroxidation damage. Thus, naturally-occurring compounds such as vitamin E are powerful anti-oxidants and tissues which are deficient in such compounds may be prone to peroxidation damage (section 8.11). Formation of lipid hydroperoxides can be readily detected by a number of methods of which the absorption of conjugated hydroperoxides at 235 nm is particularly useful. 

Covalent synthesis of complex molecules involves the reactive assembly of many atoms into subunits with aid of reagents and estabUshed as well as innovative reaction pathways. These subunits are then subjected to various reactions that will assemble the target molecule. These reaction schemes involve the protection of certain sensitive parts of the molecule while other parts are being reacted. Very complex molecules can be synthesized in this manner. A prime example of the success of this approach is the total synthesis of palytoxin, a poisonous substance found in marine soft corals (35). Other complex molecules synthesized by sequential addition of atoms and blocks of atoms include vitamin potentially anticancer KH-1 adenocarcinoma antigen,... 

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. 

When the enamine is in conjugation with a carbonyl function, as in a-aminomethylene aldehydes (528,529), ketones (530), or esters (531), a Michael addition is found in vinylogous analogy to the reactions of amides. An application to syntheses in the vitamin A series employed a vinyl lithium compound (532). 

Stepl of Figure 29.11 Addition of Thiamin Diphosphate The conversion of pyruvate to acetyl CoA begins by reaction of pyruvate with thiamin diphosphate, a derivative of vitamin B(. Formerly called thiamin pyrophosphate, thiamin diphosphate is usually abbreviated as TPP. The spelling thiamine is also correct and frequently used. 

A common problem associated with the administration of the bile acid sequestrants is constipation. Constipation may be severe and may occasionally result in fecal impaction. Hemorrhoids may be aggravated. Additional adverse reactions include vitamin A and D deficiencies, bleeding tendencies (including gastrointestinal bleeding) caused by a depletion of vitamin K, nausea, abdominal pain, and distention. 

Mitochondrial pyruvate carboxylase catalyzes the cat-boxylation of pymvate to oxaloacetate, an ATP-tequit-ing reaction in which the vitamin biotin is the coenzyme. Biotin binds CO2 from bicatbonate as carboxybiotin ptiot to the addition of the COj to pym-... 

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

Addition of such a-lithiosulfinyl carbanions to aldehydes could proceed with asymmetric induction at the newly formed carbinol functionality. One study of this process, including variation of solvent, reaction temperature, base used for deprotonation, structure of aldehyde, and various metal salts additives (e.g., MgBrj, AlMej, ZnClj, Cul), has shown only about 20-25% asymmetric induction (equation 22) . Another study, however, has been much more successful Solladie and Moine obtain the highly diastereocontrolled aldol-type condensation as shown in equation 23, in which dias-tereomer 24 is the only observed product, isolated in 75% yield This intermediate is then transformed stereospecifically via a sulfoxide-assisted intramolecular 8, 2 process into formylchromene 25, which is a valuable chiron precursor to enantiomerically pure a-Tocopherol (Vitamin E, 26). 

Routine antioxidant vitamin supplementation, e.g. with vitamins C and/or E, of the diabetic diet should be considered. Vitamin C depletion is present in all diabetics irrespective of the presence of vascular disease. A recent study demonstrated no significant difference between the dietary intake of vitamin C (the main determinant of plasma ascorbate) in patients with diabetes and age-matched controls, confirming the view that ascorbate depletion is secondary to the diabetic process and su esting that diabetic patients require additional intakes of the vitamin to maintain optimal levels (Sinclair et /., 1994). Antioxidant supplementation may have additive beneficial effects on a wide variety of processes involved in diabetic vascular damage including blood pressure, immune function, inflammatory reactions. 

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