Pedro P. de Castro

Azlactone Reaction Developments

Azlactones (also known as oxazolones) are heterocycles usually employed in the stereoselective synthesis of α,α-amino acids, heterocycles and natural products. The versatility of the azlactone scaffold arises from the numerous reactive sites, allowing its application in a diversity of transformations. This review aims to cover classical and recent applications of oxazolones, especially those involving stereoselective processes. After a short introduction on their structures and intrinsic reactivities, dynamic kinetic resolution (DKR) processes as well as reactions involving stereoselective formation of a new σ C-C bond, such as alkylation/allylation/arylation, aldol, ene, Michael and Mannich reactions will be exposed. Additionally, cycloadditions, Steglich rearrangement and sulfenylation reactions will also be discussed. Recent developments of the well-known Erlenmeyer azlactones will be described. For the most examples, the proposed mechanism, activation modes and/or key reaction intermediates will be exposed to rationalize both the final product and the observed stereochemistry. Finally, this review gives an overview of the synthetic utility of oxazolones.

Synthesis and biological evaluation of a new series of N-acyldiamines as potential antibacterial and antifungal agents

In continuation of our efforts to find new antimicrobial compounds, series of fatty N-acyldiamines were prepared from fatty methyl esters and 1,2-ethylenediamine, 1,3-propanediamine or 1,4-butanediamine. The synthesized compounds were screened for their antibacterial activity against Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) and for their antifungal activity against four species of Candida (C. albicans, C. tropicalis, C. glabrata and C. parapsilosis). Compounds 5a (N-(2-aminoethyl)dodecanamide), 5b (N-(2-aminoethyl)tetracanamide) and 6d (N-(3-aminopropyl)oleamide) were the most active against Gram-positive bacteria, with MIC values ranging from 1 to 16μg/mL and were evaluated for their activity against 21 clinical isolates of methicillin-resistant S. aureus. All the compounds exhibited good to moderate antifungal activity. Compared to chloramphenicol, compound 6b displayed a similar activity (MIC50=16μg/mL). A positive correlation could be established between lipophilicity and biological activity.

Brønsted Acid-Catalyzed Dipeptides Functionalization through Azlactones

Azlactones are useful building blocks in the synthesis of functional amino acid derivatives, heterocycles and bioactive molecules. In this work, a protocol for the organocatalytic functionalization of dipeptides has been presented. 2-Alkyl-substituted azlactone intermediates in the presence of different amines and alcohols were combined in a ring opening reaction approach. The products were synthesized in moderate to excellent isolated yields, providing new insights in peptide transformations involving carbodiimide activation.

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

An organocatalytic protocol, employing the commercially available EDC as coupling agent, has been developed for the preparation of dual-protected amino acid derivatives without epimerization. This methodology was then applied to different Boc-amino acid and amine derivatives in moderate to excellent isolated yields. In addition, racemization-free Boc deprotection was also demonstrated. Mechanism investigation through electrospray ionization (+)-mass spectrometry/mass spectrometry revealed an acyclic intermediate (no azlactone formation) activated by the camphorsulfonic acid as an organocatalyst as a key step for the sequential attack of the nucleophile.

Old Drawback on Azlactone Formation Revealed by a Combination of Theoretical and Experimental Studies

New insights into the formation of azlactone heterocycles bearing different substituents are hereby presented. The sum of both kinetic and thermodynamic factors contributes for the formation of 2-alkyl or 2-aryl substituted azlactones, while the cyclization of 2-alcoxy azlactones is less favored. These results are in perfect accordance with experimental observations obtained by infrared (IR) and electrospray ionization mass spectrometry (ESI(+)-MS) of the crude reaction mixture.

Dual-protected amino acid derivatives as new antitubercular agents

Tuberculosis is an infectious disease with high incidence and growing drug‐resistant rates. In an attempt to develop new antitubercular agents, 35 compounds were synthesized, most of them bearing a carbamate and enantiopure amino acid moiety. These compounds had their activity evaluated toward a Mycobacterium tuberculosis strain (ATCC 27294) and cytotoxicity against fibroblast MRC‐5 cells (ATCC CCL‐171). Three of the prepared derivatives presented a good antimicrobial inhibition and two of them a moderate cytotoxicity. The lipophilicity seems to play a vital role in the cell growth activity, with best results for the derivatives with a higher logP.

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.

A Brønsted base-promoted diastereoselective dimerization of azlactones

A novel Brønsted base system for the diastereoselective dimerization of azlactones using trichloroacetate salts and acetonitrile has been developed. Desired products were obtained in good yields (60–93%) and with up to >19:1 dr after one hour of reaction. Additionally, the relative stereochemistry of the major dimer was assigned as being trans, by X-ray crystallographic analysis. The kinetic reaction profile was determined by using 1H NMR reaction monitoring and revealed a second order overall kinetic profile. Furthermore, by employing this methodology, a diastereoselective total synthesis of a functionalized analogue of streptopyrrolidine was accomplished in 65% overall yield.