Utilization of Acidic α-Amino Acids as Acyl Donors: An Effective Stereo-Controllable Synthesis of Aryl-Keto α-Amino Acids and Their Derivatives

Abstract

Aryl-keto-containing α-amino acids are of great importance in organic chemistry and biochemistry. They are valuable intermediates for the construction of hydroxyl α-amino acids, nonproteinogenic α-amino acids, as well as other biofunctional components. Friedel-Crafts acylation is an effective method to prepare aryl-keto derivatives. In this review, we summarize the preparation of aryl-keto containing α-amino acids by Friedel-Crafts acylation using acidic α-amino acids as acyl-donors and Lewis acids or Brönsted acids as catalysts.

Trifluoromethanesulfonic Acid as Acylation Catalyst: Special Feature for C- and/or O-Acylation Reactions

“…This reaction is known to be catalyzed mainly by TfOH (either added or released at the onset of the reactions); therefore, a metalfree reaction system utilizing neat TfOH is still of interest. The synthesis of aryl keto α-amino acids through straightforward and convenient methods has attracted more attention [12] because these Metal triflates can promote the acylation of benzene, chlorobenzene or fluorobenzene using benzoyl chloride [38] and alcohol using benzoic anhydride [39]. This reaction is known to be catalyzed mainly by TfOH (either added or released at the onset of the reactions); therefore, a metal-free reaction system utilizing neat TfOH is still of interest.…”

“…Similarly, no reaction was observed when the conventional catalyst AlCl3 was replaced with TiCl4, sulfuric acid, or trifluoromethanesulfonic anhydride (Tf2O) [45]. In contrast to other catalysts for the Friedel-Crafts acylation, TfOH can behave both as a catalyst and solvent because of its high dissolving capacity [12]. Aspartic anhydride derivatives 60b (1 equiv.)…”

“…This reaction is known to be catalyzed mainly by TfOH (either added or released at the onset of the reactions); therefore, a metalfree reaction system utilizing neat TfOH is still of interest. The synthesis of aryl keto α-amino acids through straightforward and convenient methods has attracted more attention [12] because these The development of an efficient intramolecular Friedel-Crafts reaction of 1-(2-isocyanatoethyl) benzene derivatives 58 using TfOH as catalyst (Scheme 4) [37] demonstrates the vast applications of the catalytic system based on neat TfOH. In contrast to neat TfOH utilization, other catalysts such as AlCl 3 , ZnCl 2 , TiCl 4 , or sulfuric acid require the use of an organic solvent for this intramolecular reaction.…”

“…This reaction is known to be catalyzed mainly by TfOH (either added or released at the onset of the reactions); therefore, a metal-free reaction system utilizing neat TfOH is still of interest. The synthesis of aryl keto α-amino acids through straightforward and convenient methods has attracted more attention [12] because these compounds are convenient intermediates that can be directly converted into the desired α-amino acids. C-acylation involving the aromatic compounds and side chains of carboxylic derivatives is a potential pathway for the synthesis of aryl keto α-amino acids that obviates the requirement of special reagents or precursors.…”

“…The synthesis of this acid was first reported in 1954 [8,10]. Since then, the novel properties of TfOH have been developed for modern chemical applications [11]; thus, comprehensive discussions of its utilization in various reactions and applications have been published [5,8,9,[11][12][13]. From the point of view of TfOH as a catalyst for C-or O-acylation, consideration of comprehensive substrates with the reaction condition are needed to achieve optimum conditions.…”