A Sequential Acyl Thiol-Ene and Thiolactonization Approach for the Synthesis of δ-Thiolactones

A novel strategy for the synthesis of δ-thiolactones from inexpensive and readily available γ-unsaturated esters has been developed. This strategy incorporates a radical acyl thiol-ene reaction as the key C-S bond forming step. Cyclization is achieved via a Steglich-type thiolactonization of 5-mercaptopentanoic acids. We report the facile and scalable synthesis of δ-thiolactones in moderate to good yield under mild reaction conditions with tolerance for a range of functional groups.

Recent advances in photocatalytic C–S/P–S bond formation via the generation of sulfur centered radicals and functionalization

In this review, we have focused on the recent advances in photocatalytic C–S/P–S bond formation via the generation of thioyl/sulfonyl radicals and further functionalization.

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.

Amine-catalyzed tunable reactions of allenoates with dithioesters: formal [4+2] and [2+2] cycloadditions for the synthesis of 2,3-dihydro-1,4-oxathiines and enantioenriched thietanes

The chemoselective [4+2] vs. [2+2] cycloaddition between allenoates and dithioesters can be controlled by switching the nucleophilic amine catalyst. The two modes of cyclizations represent the first example of controllable and chemoselective annulations between allenoates and dienophiles catalyzed by amine. These cyclizations are useful in offering a divergent synthesis of sulfur-containing heterocycles. On the basis of this investigation, it can be realized that dithioesters with a vicinal electron-withdrawing group can react not only like a Michael acceptor but also as a ketone or imine.

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.

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

An investigation into the comparative reactivity of simple β-lactones and β-thiolactones toward a thiol and a primary amine is reported. A simple 3-mercaptomethyl-2-oxetanone is found to undergo rearrangement in the presence of aqueous base to give the corresponding thietane-3-carboxylic acid rather than the 3-hydroxymethyl-2-thietanone. Implications for the use of β-thiolactones in bioorganic and medicinal chemistry are discussed.