Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

Abstract

A number of lumazines and 5-deazalumazines containing a methyl group at C-7 have been prepared, their pKBH+ values determined, and measurements made of the rates at which the hydrogen atoms of their 7-methyl groups undergo isotopic exchange in aqueous sulfuric acid. The presence of an alkyl group at N-8 in the protonated forms of these compounds activates the neighbouring methyl group at C-7; the effect is considerably larger than that previously observed for a methyl group at C-6, which is the other neighbouring position. The comparison of methyl and hydrogen at N-8 can be made only for the acid-catalyzed reaction because the structures of the neutral compounds, which take part in the base-catalyzed reaction, are not analogous.Ross STEWART et S. J. GUMBLEY. Can. J. Chem. 63, 3290 (1985) On a prkpark un certain nombre de lumazines et de dCaza-5 lumazines contenant un groupement mkthyle en position C-7; de plus, on a determink les valeurs de leurs pKBH+ et, opkrant dans de I'acide sulfurique aqueux, on a mesure les vitesses d'kchange isotopique des atomes d'hydrogenes de leurs groupements mkthyles en C-7. La prksence d'un groupement alkyle dans la position N-8 des formes protonkes de ces composCs active le groupement mCthyle voisin en C-7; cet effet est beaucoup plus important que celui observk antkrieurement pour un groupement mCthyle en C-6, I'autre position adjacente. On ne peut faire une cornparaison entre les effets des groupements mkthyles et ceux des atomes d'hydrogenes en N-8 que pour les rkactions acido-catalysCes puisque les structures des composks neutres, qui prennent part 3 la rkaction catalyske par les bases, ne sont pas semblables.[Traduit par le journal]

Arylphosphonic acids. II. General acid and general base catalysis of acetone enolization

“…[13]- [15]). This is in sharp contrast to the effect that neighbouring groups produce in reactions [I] and [2], where significant positive deviations from Bronsted plots are observed. We conclude that proton transfer from acetone to the ion ArP032-is normal in the sense that the effect of nearby substituents on the transition state and on the product is similar.…”

“…The latter reactions are the deprotonations of 1,2,3-trimethylpyrazinium ion and methylcreatininium ion by carboxylate ions (eqs. [l] and [2]), in which the more hindered bases, e.g. ortho-substituted benzoate ions, exhibit marked positive deviations from the Bronsted line drawn for the least hindered carboxylate ions (see also ref.…”

“…We have shown previously that proton removal from activated methyl groups in a number of cyclic conjugated systems is catalyzed by general acids and bases and exhibits steric activation. Activation by adjacent alkyl groups in the substrate has been demonstrated in many cases (1)(2)(3)(4)(5) and in two instances (involving general base catalysis) by adjacent groups in the base (6,7). The latter reactions are the deprotonations of 1,2,3-trimethylpyrazinium ion and methylcreatininium ion by carboxylate ions (eqs.…”

Steric activation in prototropic reactions of pyrazine derivatives. II. The Brønsted relation

“…The role of steric effects in proton transfer reactions has been the subject of numerous publications from this laboratory (1)(2)(3)(4)(5). In particular, the accompanying paper deals with the increases in rate of proton removal from the activated methyl group of pyrazinium ions (eq.…”