Brønsted Acid-Catalyzed Epimerization-Free Preparation of Dual-Protected Amino Acid Derivatives

Recent advances in asymmetric synthesis of chiral amides and peptides: racemization-free coupling reagents

Over past decades, chiral amides and peptides have emerged as powerful and versatile compounds due to their various biological activities and interesting molecular architectures. Although some chiral condensation reagents have been applied successfully for their synthesis, the introduction of racemization-free methods of amino acid activation have shown lots of advantages in terms of their low cost and low toxicity. In this review, advancements in amide and peptide synthesis using racemization-free coupling reagents over the last 10 years are summarized. Various racemization-free coupling reagents have been applied in the synthesis of enantioselective amides and peptides, including ynamides, allenones, HSi[OCH(CF3)2]3, Ta(OMe)5, Nb(OEt)5, Ta(OEt)5, TCFH-NMI, water-removable ynamides, DBAA, DATB, o-NosylOXY, TCBOXY, Boc-Oxyma, NDTP, 9-silafluorenyl dichlorides, the Mukaiyama reagent, EDC and T3P. The racemization-free reagents described in this review provide an alternative greener option for the asymmetric synthesis of chiral amides and peptides. We hope that this review will inspire further studies and developments in this field.

Inhibitory Potential of Synthetic Amino Acid Derivatives against Digestive Enzymes as Promising Hypoglycemic and Anti-Obesity Agents

Over the last decades, the increased incidence of metabolic disorders, such as type two diabetes and obesity, has motivated researchers to investigate new enzyme inhibitors. In this study, the inhibitory effects of synthetic amino acid derivatives (PPC80, PPC82, PPC84, PPC89, and PPC101) on the activity of digestive enzymes were assessed using in vitro assays. The inhibitory effect was determined by the inhibition percentage and the 50% inhibitory concentration (IC₅₀), and the mechanism of action was investigated using kinetic parameters and Lineweaver-Burk plots. PPC80, PPC82, and PPC84 inhibited pancreatic lipase (IC₅₀ of 167-1023 µM) via competitive or mixed mechanisms. The activity of pancreatic α-amylase was suppressed by PPC80, PPC82, PPC84, PPC89, and PPC101 (IC₅₀ of 162-519 µM), which acted as competitive or mixed inhibitors. Finally, PPC84, PPC89, and PPC101 also showed potent inhibitory effects on α-glucosidase (IC₅₀ of 51-353 µM) as competitive inhibitors. The results suggest that these synthetic amino acid derivatives have inhibitory potential against digestive enzymes and may be used as therapeutic agents to control metabolic disorders.

N-Acetyl-l-phenylalanine Racemization during TBTU Amidation: An In-Depth Study for the Synthesis of Anti-Inflammatory 2-(N-Acetyl)-l-phenylalanylamido-2-deoxy-d-glucose (NAPA)

A thorough study on the amidation conditions of N-acetyl-l-phenylalanine using TBTU and various bases is reported for the synthesis of 2-(N-acetyl)-l-phenylalanylamido-2-deoxy-d-glucose (NAPA), a promising drug for the treatment of joints diseases. TBTU-mediated diastereoselective amidation reaction with 1,3,4,6-tetra-O-acetyl-β-d-glucosamine always gave racemization of N-acetyl-l-phenylalanine. The stereochemical retention under amidation conditions was studied in detail in the presence of difference bases and via other control experiments, evidencing the possibility to reduce racemization using pyridine as base.

Synthesis of protected α-amino acids via decarboxylation amination from malonate derivatives

A general and efficient strategy for the synthesis of protected α-amino acids is reported. The method uses malonate derivatives as the starting materials and Cs2CO3 as a base at 60 degrees, giving α-amino acid derivatives in moderate yields by releasing CO2. This methodology shows broad substrate scope (primary and secondary acids), excellent functional group tolerance and high efficiency to give the desired products under mild reaction conditions. It also allows the construction of β and γ-amino acids and other unnatural products.

Recent Contributions of Nuclear Magnetic Resonance in Organocatalysis Mechanism Elucidation

Background:

Nuclear Magnetic Resonance (NMR) is one of the most employed techniques in structural elucidation of organic compounds. In addition to its use in structural characterization, it has been widely employed in the investigation of reaction mechanisms, especially those involving catalysis.

Objective:

In this review, we aim to provide recent examples of the interface of NMR and organocatalysis reaction mechanism.

Methods:

Selected examples of different approaches for mechanism elucidation will be presented, such as isotopic effect, catalyst labelling and online reaction monitoring. A discussion involving the use of solid-state NMR will also be disclosed.

Conclusion:

NMR consists of a non-destructive technique, extremely useful in the real-time identification of intermediates in crude reaction mixtures. With the advent of two-dimensional experiments and high field NMR spectrometers, the reports of studies involving mechanistic elucidation were greatly enhanced. In this context, nowadays NMR appears as a powerful tool for the comprehension of reaction mechanisms, including the possibility of the proposal of unknown reaction mechanisms within organocatalysis.

Theoretical Study on the Epimerization of Azlactone Rings: Keto-Enol Tautomerism or Base-Mediated Racemization?

Azlactones are versatile heterocycles employed in a diversity of transformations; the main drawback of these cycles consists in the epimerization of the α-carbonyl stereocenter during its preparation. We hereby present a theoretical study to explain how the racemization occurs. Two hypotheses were investigated: the keto-enol tautomerism and the base-mediated racemization, through an enolate intermediate. The results showed that the latter is consistent with the experimental data and can spontaneously occur at room temperature. The same pathway was evaluated for 2-alcoxy azlactone, showing a slower epimerization ratio, consistent with the literature data.