Multistep chain oxidation

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Chain termination during the oxidation of hydrocarbons usually is a result of the interaction of two peroxyl radicals by a multistep mechanism. The mechanism of dispropotionation is... 

Cycloaddition of pyridine Ar-oxides (see Section 11.13.2) led to careful examination of thermodynamic aspects, though no experimental measurement was provided. Thermodynamic profile for the ring-chain isomerization of [l,2,3]triazolo[l,5- ]pyridines via a ring-opening pathway (Equation 2) was calculated. Based on this computational study, a multistep mechanism was proposed <20050BC3905>. 

Experiments with chloroplasts showed an apparent inhibition of fatty acid synthesis by PAN (at 72 ppm for 10 min). The result is difficult to interpret the inhibition could be attributed to inactivation of one of the enzymes of the multistep system or to oxidation of the reductant (reduced NADP, or NADPH) required in the chain elongation process. 

A unique approach to the stereochemical complexities of erythronolide A was developed by Deslongchamps as outlined in Scheme 2,19. The methyl ester of erythronolide A seco acid (212) was dehydrated to form the cyclic ketal 213. A multistep oxidation of the side chain then gave aldehyde 214 which, when condensed with the zirconium enolate of methyl propionate, afforded a 10 1 ratio of aldol diastereomers, the major being 213. Furthermore, aldehyde 214 could easily be converted into the y-lactone 215. 

In other fields of kinetics, the principle is particularly useful in two situations for pathways with repetitive, consecutive addition reactions such as multiple ethoxylation or paraffin oxidation (see examples in Section 5.5) and for networks with coupled parallel steps, as in olefin reactions such as hydrogenation, hydroformylation, and hydrocyanation that involve double-bond migration in concert with conversion to products [17]. Consider as a prototype the network 12.13 of n-heptene hydroformylation, keeping in mind that the arrows represent multistep pathways and that the reactions of higher straight-chain olefins involve still more... 

A similar incremental effect of porphyrin-quinone separation was observed with the systems shown in Scheme 36 which were prepared by Wittig condensation of the meso-substituted porphyrin 116 (as the nickel complex) with the phosphorus ylide 117 Demethylation, reduction of the double bonds and then oxidation furnished the free base porphyrins 118 and 119a, b. The rate of photoinduced electron transfer in such systems showed an inverse exponential dependence on the length of the chain In order to demonstrate a multistep electron transfer the bis-quinone porphyrin 120 was prepared in which the pair of quinone rings provide a redox potential gradient and may thus stabilize charge separation. Comparison with the mono-quinone etioporphyrin 119a... 

The ability of A -heterocyclic carbenes to activate a,p-unsaturated carbonyl compounds via the formation of the corresponding Breslow intermediate, which plays the role of a homoenolate nucleophile, has also been applied to a cascade process involving a formal intramolecular Michael reaction/oxidation/ lactonization, leading to the formation of complex tricyclic carbon frameworks starting from a bifunctional substrate containing an enone and an a,p-unsa-turated aldehyde side chain linked to each other via a benzene tether (Scheme 7.82). The reaction involved a complex multistep mechanism which started with the activation of the enal by the catalyst, forming the Breslow intermediate, which subsequently underwent intramolecular Michael reaction and next the generated enol-type intermediate reacted intramolecularly with the... 

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