Purines. XVII. : Kinetic Studies of the Base-Catalyzed Conversion of 1-Alkyladenosines into N-Alkyladenosines : Effect of Substituents on the Rearrangement Rate

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The rates of the Dimroth rearrangements of 1-alkyladenosines (Ie, f) and 1-alkyl-9-methyladenines (Ia-d) at various pH's and ionic strength 1.0 at 40° have been measured. It has been shown that all reactions obey good pseudo-first-order kinetics and follow the rate law given by k⁽⁰⁾_obsd=k⁽⁰⁾_ionic [I·H⁺] [OH^-]+k⁽⁰⁾_neut [I] [OH^-] where k⁽⁰⁾_obsd is the observed limiting specific rate for zero buffer concentration ; [I·H⁺] is the fraction of the base protonated at each pH ; [I], the fraction present as free base. Comparison between the individual second-order rate constants thus obtained (Table III) has revealed that attack of hydroxide ion on the protonated species is faster than on the neutral species by a factor of 90-1100 and that the former is affected by the electronic property of a substituent at the 1-position, whereas the latter is influenced by a steric factor. The rate of the rearrangement is enhanced by the benzyl group at the 1-position at near neutrality and by the β-D-ribofuranosyl group at the 9-position at all pH's examined.