Glycolate pathway scheme

Oxidative degradation of a ketose may proceed by two pathways (Scheme IX). Hexuloses by addition of a peroxide anion form a ketose peroxide (13). Cleavage of this intermediate at the C-2-C-3 bond gives glycolic acid and an aldo-tetrose. The aldo-tetrose is smoothly converted to four moles of formic acid. Alternatively, the ketose peroxide (13)... 

Major pathways for the synthesis and interconversion of glycine and serine in plants are outlined in the scheme in Fig. 1. The most studied pathway, the glycolate pathway, is that associated with photosynthesis and responsible for photorespiration (see Tolbert, this series, Vol. 2, Chapter 12). Flux through this pathway is rapid especially in the leaves of C3 plants. Alternatives to the glycolate pathway exist in green and nongreen tissues but because of slower turnover rates evidence for them from experiments using isotopic tracers is less clear and the enzymes concerned are more difficult to study because of their lower concentration in the tissues. 

In the presence of Au/C catalyst, the reaction pathway was studied concluding that glycerate/tartronate amounts represents the probe of path a and glycolate of path b [41c] (Scheme 1). The overall selectivity of the reaction is dictated by the balance of path a and b and represents the most valuable parameter to be considered for evaluating the effectiveness of a catalyst. 

Our interest in polyethylene glycols centered on a simple scheme to immobilize these materials onto metal oxide surfaces. The surface of silica gel contains both silanol-OH groups and -0-strained siloxane groups(29). A simple synthetic pathway to produce covalently bonded glycols was proposed where reaction(30) would occur between the OH group of the glycol and the surface of a refractory oxide. 

Figure 21-4 Biosynthesis of triacylglycerols, glycol ipids, and major phospholipids that are formed both in prokaryotes and eukaryotes. More complete schemes of phospholipid synthesis are shown in Figs. 21-3 and 21-5. Green arrow pathway occurring only in eukaryotes.

The mechanistic aspects of LTA oxidation are more complicated and the results indicate several pathways dependent on the steric environment of the glycols. In cases where geometry is favorable, oxidative scission via a cyclic intermediate (11) proceeds by a two-electron transfer (pad) a, Scheme 7). With trans-diols possessing antiperiplanar hydroxy groups, which for steric reasons cannot form the lead(IV) cyclic intermediate (11), an alternative cyclic pathway consisting of an intramolecular proton transfer in... 

Much of the early work described in the literature was centered around vinyl chloroformate classically made by the gas phase pyrolysis of ethylene glycol bis-chloroforma-te at 460-480°C (Ref. Ill, 112,113). However, this route proved to be industrially and economically impracticable because of low yields (11-44%) and formation of large amount of chlorinated side products and tars. Scheme 87 presents the decomposition pathways of ethylene glycol bis-chloroformate. 

The Corey mechanistic proposal is founded on the ability of the alkene substrate to bind between the two quinohne ring walls which are spaced parallel with a separation of 7.2 A. When the substrate binds in this elongated cleft, the alkene complexes to the osmium center in the W-complexated osmium tetroxide through a donor-acceptor (d-Jt) interaction (Scheme 10). The interaction between the alkene and osmium tetroxide is also complemented with favorable van der Waals interactions between the alkene and the binding cleft. These interactions are implied by the Michaelis-Menten kinetics [72] observed in the process. The observation of Michaelis-Menten kinetics in the AD process has however been questioned as an experimental artifact by the Sharpless group [73]. The (3+2) addition takes place between the axial and one of the equatorial oxygen atoms in osmium tetroxide which are in close proximity with the alkene. This represents the minimum motion pathway in the formation of the pentacoordinate os-mium(VI) glycolate ester. In the Corey model the rate acceleration observed in... 

The reaction pathway can lead either to the expected Diels-Alder cycloadducts A or the monoadduct B or bisadduct C resulting from a Michael-type addition (Scheme 10.22), In the case of catalysis, with the exception of LPDE and Znl2, the acidic character of Yb(OTf)3 or BiCh diverts the reaction along both pathways or favors the exlusive formation of Michael-type products. Such chemical behavior is not uncommon in catalyzed furan reactions [106]. At variance with this is the uncatalyzed high pressure cycloaddition and the reaction carried out in solvophobic media at atmospheric pressure which are particularly selective and afford the Diels-Alder cycloadduct A in nearly similar yields. Interestingly, the reaction also proceeds chemoselectively in water-like solvents at ambient pressure but not in hydrocarbon solvents and methanol. In water-like solvents the reactivity cannot be ascribed to polarity effects only, since methanol and glycol have similar values. Solvophobic interactions are very probably mostly responsible for the enhanced reactivity. This is supported by the similar values of the endoiexo ratio. 

Cyclic sulfites and cyclic sulfates of ethanediol undergo hydrolysis with acids to furnish glycol. The substituted cyclic sulfate, such as tetramethyl-1,3,2-dioxathiolane 2-dioxide, may undergo a pinacol type of rearrangement under acidic conditions to furnish pinacolone in good yield (74JOC3415). The mechanistic pathway is rationalized in Scheme 17. 

Transannular sulfur participation occurs in certain solvolytic rearrangement reactions. Hence under pinacol rearrangement conditions either glycol (53) or (54) is converted to the bicyclic derivative (55). A possible mechanism for the formation of (55) from (53) is shown in Scheme 1. There appears to be no simple pathway to (55) from (54). Three possible pathways leading to the necessary ring expansion are shown in Scheme 1. 

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