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4-Hydroxyphenacyl Ammonium Salts: A Photoremovable Protecting Group for Amines in Aqueous Solutions


BOWNIK Iwona Božena ŠEBEJ Peter LITERÁK Jaromír HEGER Dominik ŠIMEK Zdeněk GIVENS Richard S. KLÁN Petr

Druh Článek v odborném periodiku
Časopis / Zdroj The Journal of Organic Chemistry
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Obor Organická chemie
Klíčová slova photochemistry; ammonium salts; photoremovable protecting group
Popis Irradiation of N-protected p-hydroxyphenacyl (pHP) ammonium caged derivatives at 313 nm releases primary and secondary amines or ammonia in nearly quantitative yields via the photo-Favorskii reaction when conducted in acidic or neutral aqueous buffered media. The reaction efficiencies are strongly dependent on the pH with the most efficient and highest yields obtained when the pH of the media maintains the ammonium and p-hydroxyl groups as their conjugate acids. For example, the overall quantum yields of simple secondary amines release are 0.5 at acidic pH from 3.9 to 6.6 dropping to 0.1 at neutral pH 7.0 and 0.01 at pH 8.4. Speciation studies provide an acid-base profile that helps define the scope and limitations of the reaction. When the pK(a) of the ammonium group is lower than that of the phenolic hydroxyl group, as is the case for the alpha-amino-protected amino acids, the more acidic ammonium ion deprotonates as the media pH is changed from acidic toward neutral or basic, thus diminishing the leaving group ability of the amino group. This, in turn, lowers the propensity for the photo-Favorskii rearrangement reaction to occur and opens the reaction pathway to alternative competing photoreduction process.
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