A-meso-Hydroxylation

There does exist one prominent exception to the generalization 95> that the dl pinacol heavily predominates in reductions carried out in alkali. When the aromatic ring bears a phenolic hydroxyl, reduction in alkali affords the meso pinacol exclusively. Thus reduction of 93 produces a mixture of dl and meso pinacols, but 94 affords the pure meso pinacol90). Other aromatic car-... 

Another proof of the configuration of D-mannitol and also of D-manno-n-manno-octitol (XVI), which is likewise dependent on the experimental proof of the equivalent symmetry of D-mannitol is the following. D-Mannose has been converted, by successive cyanohydrin syntheses, first to a mannoheptose and then to a mannooctose which on reduction yielded a mannooctitol whose octaacetyl derivative is optically inactive. (It was not possible to examine the octitol itself because of its very low solubility in water.)87 The meso character of the octaacetate shows that the mannooctitol must possess a meso configuration, with a plane of symmetry between carbon atoms 4 and 5. To write its formula, the hydroxyl at carbon atom 7 is placed on the... 

Fig. 21. Detailed mechanism for HO-1 catalysis. In 1, oxygenation and electron transfer forms the ferric (Fe +)-peroxy complex. Steric factors and H-bonding help to bend the peroxide toward the a-meso-heme position for regio-selective hydroxylation. One proposed mode of forming verdoheme is shown in part 2. A key part of step 2 is the resonance structures between Fe + and Fe +/radical, which enable the porph3rrin ring to be oxygenated. Although the mechanism shown does not require any reducing equivalents (176), there remain experimental inconsistencies on the requirement of an additional electron in step 2. However, reduction of the verdoheme iron is necessary to prepare the substrate for step 3, verdoheme to bihverdin.

If the a-meso regioselectivity of hydroxylation is not inherent to the heme group, as indicated by the fact that all four isomers are obtained by coupled heme oxidation (40), how is the regiochemistry of heme oxidation influenced by electronic factors One possibility is that the puckering or ruffling of the porphyrin caused by steric clashes between the meso and flanking substituents in the meso-substituted heme probes... 

However, in contrast to the human His25Ala HO-l heme complex, which has no detectable activity in the absence of imidazole (78), the His20Ala Hmu O rheme complex in the presence of NAD PH and NADPH-cytochrome P450 reductase was foimd to catalyze the initial meso-hydroxylation of the heme (151). The product of the reaction was Fe verdoheme, as judged by the electronic absorption spectrum and the detection of carbon monoxide as a product of the reaction. Hydrolytic conversion of the verdoheme product to biliverdin and subsequent HPLC analysis confirmed that the oxidative cleavage of the porphyrin macrocycle was specific for the a-meso-carbon. 

Only three of the compounds shown have chiral centres these are indicated. These three compounds could, therefore, exist in optically active form. None of the compounds shown is a meso compound. Note that we have to consider part of the ring system as a group attached to the centre in question. Follow this around until a decision can be made. The last two are not exactly trick questions, but require care. One has a plane of symmetry, and the carbon carrying the hydroxyl has two of its attached groups the same the benzene ring is planar, so none of its carbons has the potential to be chiral. 

Both are optically active. Had the C-4 hydroxyl group been to the left, one of the aldaric acids would have been a meso form. 

The stereochemistry of the product of a reaction will be influenced by the structures of the reagent and substrate and the mechanisms by which they react. For example, the hydroxylation of but-2-ene by osmium tetroxide and water yields a racemate whilst bromination of the same compound with bromine produces a meso compound (Figure 10.5). Flowever, a stereoselective reaction is most likely to occur when steric hindrance at the reaction centre restricts the approach of the reagent to one direction (Figure 10.6). Furthermore, the action of both enzyme and non-enzyme catalysts may also be used to introduce specific stereochemical centres into a molecule. 

Gels transform more or less gradually into a meso/microporous glass. A logarithmic plot of the weight loss vs. the inverse of the temperature allows the onset determination of water and hydroxyl departures, and hence the measurement... 

Finally, reaction E in Fig. 4 illustrates a stereoselective synthesis that proceeds by differentiation of two enantiotopically related groups of a meso compound. Here, one hydroxyl group of d.s-1,2-cyclohexanediol is preferentially benzoylated in the presence of one molar equivalent of an enantiomerically pure diamine [26]. These desymmetrization reactions (that have many biological versions) are also called meso-tricks , and are currently receiving a great deal of attention for the preparation of new chiral building blocks [27]. 

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