Fragmentation-recombination mechanism

Furthermore, the oxy-Cope rearrangement of allenic cycloheptane alcohol 47 (NaOEt, THF, 20 °C, 12 h, 80%) gave rise only to ring-enlarged product 48 without transannular cyclization (equation 15)29. The above transformations can be rationalized either by the fragmentation-recombination mechanism or by a concerted oxy-Cope mechanism29. 

The Skraup-Doebner-Von Miller synthesis of quinolines - involving condensation of an aniline with an ,/i-unsalurated ketone - has been investigated using 13C-labelled ketones in cross-over experiments a complex fragmentation-recombination mechanism involving imine intermediates is indicated.17... 

III. Mechanism of Formation op Saccharinic Acids 1. The Fragment-recombination Mechanism of Kiliani and Windaus... 

Figure 29 Active site contacts and a fragmentation recombination mechanism for GM. (a) Saiient features of the active site inciude the binding of giutamate through hydrogen-bonded eiectrostatic contacts with three arginine residues.

A few selected types of such relationships between different structures will be presented in this paragraph and in the following ones ( 3.9 and 3.10). Examples will be considered of possible (virtual and/or real) transformation mechanisms from one structure type to another (degenerate, derivative structures) examples will also be discussed of groups of structures interrelated because based on the assembly of similar blocks and fragments (recombination structures). 

A maltol-ammonia browning reaction produced thirteen pyrazines, two pyrroles, two oxazoles, and one pyridine (12). The major products of this system were 2-ethyl-3-hydroxy-6-methylpyridine and 2-ethyl-3,6-dimethylpyrazine. It is difficult to construct possible formation mechanisms for these compounds from maltol and ammonia. All the carbon atoms must come from maltol. It is possible, then, that maltol degrades into smaller carbon units and that these fragments recombine to form larger carbon units, producing these compounds. Recently, the formation of thiophenones and thiophenes from the reaction of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and cysteine or cystine was reported (13. 14). All these reaction mixtures were reported to possess a cooked meat-like flavor. 

The first is the fragmentation-recombination pathway (see mechanism a. Scheme 4), with acrylate and an acrylate-derived radical as the intermediate state (Scheme 6). This possibility has been suggested only recently, and is based on the reported inhibition of 2-methyleneglutarate mutase by acrylate [47], The second suggested mechanism is the addition-elimination pathway (see reaction c. Scheme 4), with a substituted cyclopropylcarbinyl radical as the intermediate (Scheme 6) [6, 26],... 

The presence of a C=C double bond in the migrating group of the but-3-enyl radical introduces the possibility of the addition-elimination mechanism (path b, Scheme 7), where the appropriate intermediate is the previously discussed cyclopropylcarbinyl radical (5). We find a significant preference ca 100 kJ mof ) for the addition-elimination pathway compared with the fragmentation-recombination pathway. Thus it is more favorable, in the gas phase at least, for the migrating HC=CH2 group to stay bonded to the remaining framework rather than to become detached from it. The cyclopropylcarbinyl radical intermediate involved in the addition-elimination mechanism is predicted to lie in a well of depth 30 kJ mof. ... 

In contrast to the above formation of metasaccharinic acid, fragment recombination appears to be a predominant feature in the formation of the branched-chain a -D-glucosaccharinic acid from D-mannose-l-C plus lime-water. In this case, the radioactivity originally present in Cl of the hexose was found to have become distributed almost entirely between the methyl carbon atom and the tertiary carbon atom of the saccharinic acid, with the latter atom more heavily labeled than the former. In contrast to these observations, the Isbell mechanism, in the absence of compli-... 

Dimethyl ether was reacted over a radical initiator, pre-dried dibenzoyl peroxide supported a celite, at various temperatures (Table 2). Although dibenzoyl peroxide was short lived under these conditions sufficient was reactive in the timescale of the experiments, typically 5 min. Under all conditions only dimethoxyethane was observed as a dimerisation product showing that the methoxymethyl radical was generated under these conditions. The absence of other products, e.g. alkenes or ethanol, discounts the methoxymethyl radical fragmentation/recombination pathway (mechanism 1, Fig. 1). 

Another feature common to all hexoses is their fragmentation to trioses and related products (such as lactic acid, l-hydroxy-2-propanone, and pyruvaldehyde). l-Hydroxy-2-propanone is formed by heating a solution of D-fructose in potassium acid phosphate buffer solution of pH 6.7 it was proposed that an intermediary 3,4-enediol (s) rearranges to a /3-diketone, which yields (t) by fragmentation of the molecule. Studies by Wolfrom and Schumacher and by Blair and Sowden showed that this mechanism, followed by recombination and aldolization, accounts for some of the observed products. The identification of L-a 2/io-hexulose and DL-a yZo-hexulose was conclusive in this respect, but deuterium studies by Sowden and Thompson proved the minor role of this recombination mechanism in the transformations. 

---