Radicals 1,2-dihydroxyethyl

Racemic pipecolic acid (6) is obtained by ring closure of TV-alkylglycines by ionic 203 or radical 204 mechanisms. It also may be obtained by conversion of suitable substituents at the C2 of piperidine into the 2-carboxy group, e.g. hydrolysis of a nitrile group 205 or oxidation of a 1,2-dihydroxyethyl group. 206 Resolution of the racemic mixture can be carried out by fractional crystallization. 207-209 Enzymatic resolution of racemic pipecolic acid 210-213 or of synthetic intermediates 214 has been reported. 

Alternative mechanisms for the OH transfer process in enzyme-coenzyme B12-catalysed dehydration of 1,2-dihydroxyethane, to give acetaldehyde and water, have been explored using ah initio MO calculations.75 Transfer within an (HOCH— CH2OH) radical was ruled out because the activation energy is too high, and no intermediate bridge structure could be found to facilitate conversion of 1,2-dihydroxyethyl cation (if it could be formed from the radical) to 2,2-dihydroxyethyl cation. The radical cation (HOCH—CH20H2)+ transformed rapidly to a stable... 

The free radical chemistry of ethylene glycol is somewhat complicated by the water elimination reaction from the 1,2-dihydroxyethyl radical (reaction 20) formed by hydrogen abstraction from the substrate. This reaction was first observed in 1966 in ESR experiments by Livingston and Zeldes (152), ... 

AU the nitroxyl adducts I-V were generated [1 ] pulseradiolytically in aqueous solution from nitrobenzenes and the corresponding a-hydroxyethyl, o.Jl-dihydrox-yethyl, 6-methyluracil-6-yl, 6-methyl-/-cytosinc-6-yl and 6-methyldihydrouracil-6-yl radicals. Production of a-hydroxyethyl, a,3-dihydroxyethyl, 6-rncthyluracil-6-yl, 6-methyl-/-cytosine-6-yl and 6-methyldihydrouracil-6-yl radicals were described elsewhere [1—4]. The heterolysis rate constants, and all activation free energy changes were taken from those references. The LFER plot for the heterolysis reaction of the nitroxyl adducts 1 and V is linear (Fig. 1) (r = 0.99). The slope of this plot, which should be equal to the ratio of the Hammett p values for the heterolysis of the two, adducts 1 and V, results as 1.0, in agreement with the individually determined p values (both 1.5). Also the solvent isotope effect (SIE) on of adduct V, that is... 

AG values of adduct III and TV with that of adduct II, sufficient data for adduct II is not available. Yet the similar solvent KIE is a strong evidence to say that the a,p-dihydroxyethyl radical (I lO CI ICH,OH) is a good model for the 6-methyluracil-6-yl and 6-methyl-i-cytosine-6-yl radices when their adducts III and IV lead to similar transition states on their way to the products. And adducts II, III and IV differ with adducts I and V where former set have an OH group in p position as shown in their structures above. The strong difference in KIE between the two sets of adducts I and... 

So in conclusion the two sets of adducts (I and V), and (II and III) behave differently and one can conclude that the a-hydroxyethyl radical is a good model for 6-methyldihydrouracil-6-yl radical and a,P-dihydroxyethyl radical is a good model for 6-methyluracil-6-yl and 6-methyl-/-cytosine-6-yl radicals. 

Miscellaneous. The photoreduction of benzoquinone, deuterobenzo-quinone and benzohydroquinone has been investigated in ethylene glycol by laser flash TR EPR by Jager et By hydrogen transfer from the solvent, semiquinone radical and the 1,2-dihydroxyethyl radical both emissive polarized by the triplet mechanism are generated. No alkyl radicals were observed in solutions of benzoquinone with excess benzohydroquinone. Analysis of the polarization patterns confirmed that the reaction of benzoquinone triplet with the benzohydroquinone yields two semiquinone radicals. 

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