Methylene groups stereospecific

The active enzyme abstracts a hydrogen atom stereospecifically from the intervening methylene group of a PUFA in a rate-limiting step, with the iron being reduced to Fe(II). The enzyme-alkyl radical complex is then oxidized by molecular oxygen to an enzyme-peroxy radical complex under aerobic conditions, before the electron is transferred from the ferrous atom to the peroxy group. Protonation and dissociation from... 

Applying the procedure just described, Rouzaud and Sinay (65) described a stereospecific synthesis of a C-disaccharide in which a methylene group takes the place of the interunit oxygen atom. Starting from 51a and the dibromoalkene 59, the hemiacetal 60 was obtained. Reduction of 60, followed by catalytic hydrogenolysis, gave the /J-(l — 6)- C-disaccharide 61. 

The stereochemistry of the reaction has been examined both theoretically92 and experimentally.93 It has been found to be stereospecific, with disrotatory motion of the methylene groups. Cyclopropyl anions also undergo thermal rearrangements to allyl anions.94... 

The observation of the silacyclohex-3-ene 83 was rationalized by a [2+2] cycloaddition of ethylene to give unusual 2-vinylcyclobutane 536. A facile ring expansion via a 1,3-silyl shift to the terminal methylene group yields 83. Surprisingly, the relative product ratio 82/83 suggests that Afor the forbidden [2 + 2] reaction is slightly smaller than for the stereospecific allowed Diels-Alder path54. 

Oxidation of a,p-enals. Sodium chlorite is superior to other reagents for oxidation of oc,/l-unsaturated aldehydes to the corresponding carboxylic acid, particularly of aldehydes containing an ct-methylene group. In addition, the oxidation is stereospecific. 2-Methyl-2-butene2 was used as the chlorine scavenger. 

The hydrogen atom is always stereospecifically introduced by the enzyme, so that, at a methylene group, only one of the two hydrogen atoms becomes labeled. Thus, D-glucose 6-phosphate ketol isomerase (E.C. 5.3.1.9) incorporates only one deuterium atom from deuterium oxide,33 or one tritium atom34 from water-1, giving D-fructose-1 (R)-t 6-phosphate (11). The isomeric D-fructose-l(S)-f 6-phosphate (12) can be obtained from D-glucose-l-t in water,35 or,... 

Stereospecific reductive ring opening of bis-epoxides separated by two methylene groups to 2-alkylidene tetrahydrofurans has been brought about by elimination-cyclization of l-iodomethyl-l,5-bis-epoxides with zinc dust (Equation 76) <20030L1931>. 

A curious reaction was observed when epoxy derivatives of certain exo-methylene groups e.g. i) were treated with BFs or perchloric acid [34,35]. The products are believed to be five-membered cycHc ethers 2.). The exact mechanism of the ring expansion process is uncertain, but it has been represented as epoxide cleavage concerted with two stereospecific hydride... 

However, this concept cannot be transferred to the polymerization of 1-alkenes. Recently, critical remarks on the carbene mechanism of ethylene insertion into the Co—CHj bond were published Zambelli et al. performed an elegant study on the discrimination of the carbene mechanism for the stereospecific polymerization of propylene. According to the carbene mechanism the insertion of the first C3H6 molecule into the Mt—bond have to result in the formation of chain with an isobutyl end group i) enriched by in the methyl and methylene groups (scheme (17 a) >) ii) enriched by in the methyl group with threo- or erithro-configuration (see structures (12a) and (12b)). 

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