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Model Studies on the Mechanism of Biotin-Dependent Carboxylations. 2. Site of Protonation vs. CO2 Transfer

Three irreversibly acidified model compounds (6-8) of N'-carboxybiotin (2) have been prepared to access the importance of proir protonation of the biotin ring system of the CO2-transfer potential of the N'-carboxy group.Substrates 6 and 7 can be considered model compounds of N'-carboxybiotin (2) in which protonation has occurred at the ureido carbonyl oxygen atom.Conversely, compound 8 was synthesized to evaluate the CO2-transfer potential of the N'-carboxy group, if protonation occurred at the N'-nitrogen atom.The reactivity of each substrate with nucleophiles has been evaluated.Of these three compounds, only 8 led to efficient transfer to the carbomethoxy group upon treatment with nitrogen-containing nucleophiles (morpholine, cyclohexylamine, and diisopropylamine).With smaller nucleophiles (i.e, water, methanol) reaction was centered at the ring C-2 position.Correspondingly, treatment of compound 6 with nucleophiles (i.e, alcohols, amines) led to products which can be explained in terms of two competing reactions.One pathway involves initial attack of the nucleophile at the C-2 position of the imidazolinium cation (an AAC2 process) to give a tetrahedral intermediate which then undergoes bond cleavage in either of two directions.The competing pathway observed was an irreversible SN2 displacement reactions (an AAL2 process) at the methylene position of the O-alkyl side chain.Factors are presented which account for the overall product distribution obtained from these reactions.Finally, the products obtained from the treatment of compound 7 with nucleophiles (i.e., alcohols, amines) could be accounted for solely by reactions which occurred at the C-2 position of the ring (an AAC2 process).The corresponding SN2 pathway is not a viably route for this substrate.The significance of these results to the mechanism of action of biotin is discussed.