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

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• 作者：Iwona Bownik ; Peter 艩ebej ; Jarom铆r Liter谩k ; Dominik Heger ; Zden臎k 艩imek ; Richard S. Givens ; Petr Kl谩n
• 刊名：Journal of Organic Chemistry
• 出版年：2015
• 出版时间：October 2, 2015
• 年：2015
• 卷：80
• 期：19
• 页码：9713-9721
• 全文大小：515K
• ISSN：1520-6904

文摘

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鈥揵ase 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 伪-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.