tert-Butyl esters of tripeptides based on Pro-Phe as organocatalysts for the asymmetric aldol reaction in aqueous or organic medium

Conjugating proline derivatives onto multi-walled carbon nanotubes: Preparation, characterization and catalytic activity in water

“…However, mainly due to its poor solubility in organic solvents, dimethylsulfoxide (DMSO) has to be employed and high catalyst loadings are required. In order to tackle this problem, a variety of proline derivatives containing bioisosteric groups like sulfonamides [8][9][10] and tetrazoles [11] as well as prolinamides [12,13] and substituted diaryl prolinols [14] have been synthesized and frequently employed in organocatalytic transformations [15]. On top of the aforementioned points, we recently reported that fullerene C 60 carrying a covalently attached proline unit efficiently catalyzes the typical asymmetric aldol reaction between acetone and 4-nitrobenzaldehyde, both in DMSO and water [16].…”

“Side‐Product Catalysis”: Substrate Autoxidation as an Overlooked Side Reaction Generating a Co‐Catalyst for Enhancing Asymmetric Aldol Reactions

“…Although the enantioselectivity of the reaction is determined in the aldol addition step, the rate‐determining step is the formation of the enamine ,. Nowadays, it is well accepted that organocatalysts combining a proline amide unit with functionalities able to act as hydrogen‐bond donors are among the most successful classes of catalysts employed in the AAR . The aldehyde is activated by hydrogen bonding with the amide and hydroxy groups of the catalyst, and a main feature of this model is that the transition state is stabilized through hydrogen bonds.…”

“…In many cases, optimal additives have turned out to be acids with moderate acidity, such as, acetic acid and benzoic acid (derivatives) ,,,,,. Even though the exact corresponding acid of the used benzaldehyde (or its derivatives) was identified to be a potent additive enhancing the AAR,,,,, surprisingly no connection has been drawn between the known oxidation lability of the aldehyde substrates and the observed additive effects. In the following we will present empirical data implying that in situ autoxidatively formed (substituted) benzoic acid does act as a Brønsted acid co‐catalyst that positively affects the reaction rate and stereochemical outcome of the direct aldol reaction, especially when performing the AAR in aqueous media with low catalyst loadings, as is today's state of the art.…”

Stereoselective Aldol and Conjugate Addition Reactions Mediated by Proline‐Based Catalysts and Its Analogues: A Concise Review

“…Psarra et al ., [46] solved the solubility problems faced with water or organic solvents to some extent by introducing tertiary butyl ester functionalities to the C‐terminal end of Pro−Phe−AA (AA‐any amino acid) 56 and obtained good to excellent yields (up to 99 %) for aldol reaction of 3 with 4 having excellent diastereoselectivity (up to 97 : 3 dr ) and enantioselectivities (up to 99 % ee ) (Scheme 7). Ketones such as tetrahydropyran‐4‐one, tetrahydrothiopyran‐4‐one achieved the best yields (92 % for both) and stereoselectivities ( dr : 92 : 8 and 96 : 4, ee : 85 % and 98 %, respectively) in aprotic conditions.…”