Conversion pathway outlining

Many dioxolanes. particularly those where the diisopropylidene unit is used as a protecting group for diols, have already been discussed in Sections 4.2. and 4.3. (derivatives of tartaric acid and carbohydrates). In addition, vinylketene acetals containing the 1,2-dioxolane moiety, e.g., 1, have been prepared from chiral diols, such as l,2-diphenyl-l,2-ethanediol (see Section 4.1. for access to the starting material) by the pathway outlined. This involves selective conversion of the (/ ,/ )-diol to (1 / ,2S )-2-chloro-l, 2-diphenylethanol (with inversion of the configuration)1, followed by esterification with 3-meihyl-2-propenoic acid and base-induced rearrangement2. Such dioxolanes have been used for diastereoselective Diels-Alder reactions (Section D. 1.6.1.1.1.1.4.4.). 

The proposed pathway for the conversion is outlined in Eq. 2. The acid-catalyzed elimination of water from the alcohols 1 affording the carbenium ion is followed by a migration of one trimethylsilyl group from the central silicon atom to the neighboring carbon atom and attack of X", the conjugate base of the acid used as the catalyst, at the electrophilic silicon. The hydrolysis of the intermediates 4 gives the silanols 3 [2]. 

At 25 °C the density of mercury, the only metal that is liquid at this temperature, is 13.5 g/mL. Suppose we want to know the volume, in mL, of 1.000 kg of mercury at 25 °C. We proceed by (1) identifying the known information 1.000 kg of mercury and d = 13.5 g/mL (at 25 °C) (2) noting what we are trying to determine—a volume in milliliters (which we designate mL mercury) and (3) looking for the relevant conversion factors. Outlining the conversion pathway will help us find these conversion factors ... 

Example 2-9 is perhaps the most representative use of the mole concept. Here it is part of a larger problem that requires other unrelated conversion factors as well. One approach is to outline a conversion pathway to get from the given to the desired information. 

Metabolic pathways containing dioxygenases in wild-type strains are usually related to detoxification processes upon conversion of aromatic xenobiotics to phenols and catechols, which are more readily excreted. Within such pathways, the intermediate chiral cis-diol is rearomatized by a dihydrodiol-dehydrogenase. While this mild route to catechols is also exploited synthetically [221], the chirality is lost. In the context of asymmetric synthesis, such further biotransformations have to be prevented, which was initially realized by using mutant strains deficient in enzymes responsible for the rearomatization. Today, several dioxygenases with complementary substrate profiles are available, as outlined in Table 9.6. Considering the delicate architecture of these enzyme complexes, recombinant whole-cell-mediated biotransformations are the only option for such conversions. E. coli is preferably used as host and fermentation protocols have been optimized [222,223]. 

Ring transposition processes are well established in six-membered heteroaromatic systems. Recent studies have centered on perfluoro systems in which bicyclic and tricyclic intermediates are sufficiently stable to permit isolation or at least detection. Thus, on irradiation in CF2C1CFC12, the perfluoropyridine 207 is converted into the azabicyclo[2.2.0]hexa-2,5-diene 208 and the two azaprismanes 209 and 210.154 An azabicyclo[2.2.0]hexa-2,5-diene has also been shown to be an intermediate in the photorearrangement of substituted 2-methylpyridines to o-substituted anilines.155 Diaza-bicyclo[2.2.0]hexa-2,5-dienes have similarly been shown to be intermediates in the conversion of fluorinated pyridazines (211) into the corresponding pyrazines (212)156 the proposed pathway is outlined in Scheme 7. Photoproducts which are formally dimers of intermediate azetes have been obtained when analogous reactions are carried out in a flow system.157... 

The second stretch of lignification, the conversion of the first non-sugar substance into the aromatic monomers ready for polymerization, has been examined more thoroughly by hgnin biochemists. The pathways followed here by the plant are outline in Fig. 2. Excellent reviews of the enzymes known to be involved at each step here as well as in the polymerization at the third stretch have recently appeared 28 a, 82). The processes encountered in higher plants are in essence the same as those known to be in operation in the aromatization of aliphatic precursors in microorganisms following the work of Davis and Sprinson with Escherichia coli mutants 32, 101). 

Outline the metabolic pathways that are utilized by acetic acid-producing bacteria (acetogens) in the stoichiometric conversion of one molecule of glucose into three molecules of acetic acid. Indicate briefly the nature of any unusual coenzymes or metalloproteins that are required. 

Outline possible pathways of metabolism of dietary fats. Consider digestion, transport of fatty acids, storage, conversion to prostaglandins, steroids, etc. Will any of the fat be converted into glucose ... 

Figure 24-23 Outline of the biosynthetic pathways for conversion of protoporphyrin IX into the chlorophylls and bacteriochlorophylls. After Bollivar et al.i17...

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

Outline of the biosynthesis of the 20 amino acids found in proteins. The de novo biosynthesis of amino acids starts with carbon compounds found in the central metabolic pathways. The central metabolic pathways are drawn in black, and the additional pathways are drawn in red. Some key intermediates are illustrated, and the number of steps in each pathway is indicated alongside the conversion arrow. All common amino acids are emphasized by boxes. Dashed arrows from pyruvate to both diaminopimelate and isoleucine reflect the fact that pyruvate contributes some of the side-chain carbon atoms for each of these amino acids. Note that lysine is unique in that two completely different pathways exist for its biosynthesis. The six amino acid families are screened. 

It is not impossible that this fascinating biosynthetic pathway still hides a few surprises, but its main outline stands on a solid basis. Future research will be devoted to the identification and characterization of the individual enzymes that catalyse the conversion of urogen-III to the corrin nucleus. 

In any case, the conversion of methyl or ethyl chlorides to methane and ethane, respectively, precludes the allylic hydrogen donation pathway for these systems and lends support to a mechanism such as outlined in schemes 7 and 8. 

The electron-transfer initiated photoaddition of allylsilanes to iminium salts has been examined in detail. Irradiation of l-methyl-2-phenylpyrrolinium perchlorate (177) and the allylsilanes (178) in methanol, for example, gave the 2-allylpyrrolidines (179) " the proposed pathway is outlined in Scheme 11. Photoaddition is initiated by electron transfer and driven to completion by desilylation of the allylsilane-derived cation radical. Analogous conversions have been reported in the allyl iminium perchlorates (180), " and cyclization to the spiro-compounds (181) occurs on irradiation of the /3-enaminone-derived allyliminium salts (182). " A photocyclization of this type has been... 

MisceUaneous Reactions.—Details of the formation of nitrous oxide by photofragmentation of methyl nitrite have been reported.Photorearrangement is observed, however, on irradiation of the nitrites (183) derived from 6-methyl- and 4,4,6-trimethyl-cholest-5-en-3-ol to give the cyclic hydroxamic acids (184). There is ample precedent for these transformations, the likely pathway for which is outlined in Scheme 12. An alkoxyl radical (185) is also thought to be involved in the photochemicaUy induced conversion of O-nitrobenzoin (186) into benzaldehyde (187) and 2-phenylbenzo[b]furan (188). Reductive photoelimination of vicinal dinitro-groups takes place by... 

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