Side-chain Reactivities

Side chain reactivity is also enhanced and is typified by the difference in reactivity of 2-methylpyrazine and 2-methylpyrazine 1,4-dioxide towards anion formation and subsequent condensation reactions. 2-Methylpyrazine undergoes condensation with benzal-dehyde at 180 °C, with zinc chloride catalysis, to yield the styrylpyrazine (58), whereas the corresponding reaction of 2-methylpyrazine 1,4-dioxide proceeds at 25 °C under base catalysis (67KGS419). 

The large success of the Hammett equation in relating and systematizing the side-chain reactivities of meta- and poro-substituted benzenes has stimulated similar studies in the thiophene series. 

The greatest difference observed between the side-chain reactivities of selenophene and thiophene derivatives was in the reaction of aldehydes with aniline (cf. Eq. 13), a ratio of 4.2. 

As well as the previous chapter on the Hammett equation,1 the importance of the approach has found expression in specific areas in other contributions to these volumes side chain reactivities of thiophenes 17 tautomerism 18 reactions of azines with nucleophiles 19,20 formation of benzofuroxans by decomposition of o-nitrophenyl azides 21 acid dis-... 

P-Values for Side-Chain Reactivities of Five-Membered Heterocycles... 

In Table II the pK s of the 2- and 3-carboxylic acids of thiophene, furan, and pyrrole are reported. While pyrrole carboxylic acids, like the alkoxy- and amino-substituted benzoic acids, are weaker, thiophene- and furan carboxylic acids, like the chloro-and bromobenzoic acids, are stronger than unsubstituted benzoic acid. This behavior is confirmed by other side-chain reactivity data.41... 

Some side-chain reactivity data substantiate the above statement. Thus, the rates of hydrolysis of compounds 28 and 29 are, respectively,... 

Enzymes that perform oxidation-reduction catalysis generally require some type of cofactor to store electrons (and protons) during the catalytic cycle. The cofactors are usually low-molecular weight species that reversibly bind to the enzyme, but in some cases are intrinsic elements of the protein structure. For example, quinoenzymes contain covalently bound quinones derived from tyrosine or tryptophan residues in the protein. These quinocofactors represent a striking extension of protein side-chain reactivity. " ... 

Thermal reaction of halothiophenes with thiyl radicals 90UK1338. Side-chain reactivity of thiophenes 86HC(44,3)975. 

Duewel, H. S., Daub, E., Robinson, V. and Honek, J. F. (2001) Elucidation of solvent exposure, side-chain reactivity, and steric demands of the trifluoromethionine residue in a recombinant protein. Biochemistry, 40(44), 13167-13176. 

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