Comparison of the reactivity of β-thiolactones and β-lactones toward ring-opening by thiols and amines

Thietanes and derivatives thereof in medicinal chemistry

Unlike the oxetane ring, which, as evidenced by numerous studies, is known to play an increasingly important role in medicinal chemistry, the thietane ring has thus far received comparatively limited attention. Nonetheless, a growing number of reports now indicate that this 4-membered ring heterocycle may provide opportunities in analog design. In the present review article, we discuss the possible use and utility of the thietane fragment in medicinal chemistry and provide an overview of its properties and recent applications with a focus on isosteric replacements.

Coupling of sterically demanding peptides by β-thiolactone-mediated native chemical ligation

The ligation of sterically demanding peptidyl sites such as those involving Val-Val and Val-Pro linkages has proven to be extremely challenging with conventional NCL methods that rely on exogenous thiol additives. Herein, we report an efficient β-thiolactone-mediated additive-free NCL protocol that enables the establishment of these connections in good yield. The rapid NCL was followed by in situ desulfurization. Reaction rates between β-thiolactones and conventional thioesters towards NCL were also investigated, and direct aminolysis was ruled out as a possible pathway. Finally, the potent cytotoxic cyclic-peptide axinastatin 1 has been prepared using the developed methodology.

Pd-Catalyzed Acyl C-O Bond Activation for Selective Ring-Opening of α-Methylene-β-lactones with Amines

A Pd-catalyzed ring-opening of β-lactones with various types of amines (primary, secondary, and aryl) to provide β-hydroxy amides with excellent selectivity toward acyl C-O bond cleavage is reported. The utility of this protocol is demonstrated in an asymmetric kinetic resolution providing enantioenriched α-methylene-β-lactones.

Kinetic and Theoretical Studies of Beta-Lactone Reactivity-A Quantitative Scale for Biological Application

Natural products comprise a rich source for bioactive molecules with medicinal relevance. Many of these contain electrophilic scaffolds that bind conserved enzyme active sites covalently. Prominent examples include beta-lactams and beta-lactones, which specifically acylate serine residues in diverse peptidases. Although these scaffolds appear similar, their bioactivities and corresponding protein targets vary. To quantify and dissect these differences in bioactivities, the kinetics of the reactions of beta-butyrolactone with a set of reference amines in buffered aqueous solution at 37 °C have been analyzed. Different product ratios of C1 versus C3 attack on the beta-butyrolactone have been observed, depending on the aliphatic or aromatic nature of the standard amine used. Quantum mechanics/molecular mechanics (QM/MM) calculations reveal that a H₃ O⁺ molecule has a crucial role in stabilizing C3 attack by aniline, through coordination of the lactone ring oxygen. In agreement with their weak proteome reactivity, monocyclic beta-lactams did not react with the set of standard nucleophiles studied herein. Bicyclic beta-lactams, however, exhibited a lower activation barrier, and thus, reacted with standard nucleophiles. This study represents a starting point for semiquantitative reactivity scales for natural products, which, in analogy to chemical reactivity scales, will provide predictions for electrophilic modifications in biological systems.

Chemistry of the β-thiolactones: substituent and solvent effects on thermal decomposition and comparison with the β-lactones

The synthesis of a series of di-, tri-, and tetraalkyl β-thiolactones and β-lactones is described as well as their thermal decomposition with extrusion of carbon oxysulfide and carbon dioxide in two solvents of opposite polarities. The β-thiolactones are considerably more thermally stable than the β-lactones and require higher temperatures for efficient decomposition in both solvents, whatever the degree of substitution. The results are interpreted in terms of a zwitterionic mechanism for fragmentation with a change in the rate-determining step between the two series.