Total syntheses of spirocyclic PKS-NRPS-based fungal metabolites

Enantioselective total synthesis of (+)-rubrobramide, (+)-talaramide A, and (-)-berkeleyamide D by a skeletal diversification strategy

A unified synthesis of (+)-rubrobramide, (+)-talaramide A, and (-)-berkeleyamide D was achieved from the vinylogous esters by a skeletal diversification strategy based on regioselective 5-exo or 6-endo cyclization. This report describes the first enantioselective total synthesis of (+)-rubrobramide and (+)-talaramide A. Additionally, synthetic spirocyclic lactam compounds, including (-)-berkeleyamide D, showed moderate inhibitory activity against amyloid-β aggregation for the potential treatment of Alzheimer's disease.

Stereocontrolled Synthesis of Pseurotin A2

We report synthesis of two diastereomeric structures previously proposed for the complex secondary metabolite pseurotin A₂. Both structures were accessed from the same building blocks taking advantage of a stereodivergent nickel(II)-diamine-catalyzed 1,4-addition of a chiral 2-alkoxycarbonyl-3(2H)-furanone. Late-stage Csp-Csp³ cross-coupling of a highly functionalized bromoalkyne featured in the pseurotin A₂ side-chain assembly. The work supports the 2016 stereochemical revision of pseurotin A₂ and represents the first chemical synthesis of this natural product.

Synthesis of spiro[4.4]thiadiazole derivatives via double 1,3-dipolar cycloaddition of hydrazonyl chlorides with carbon disulfide

An operationally simple and convenient synthesis method toward a series of diverse spiro[4.4]thiadiazole derivatives via double [3 + 2] 1,3-dipolar cycloaddition of nitrilimines generated in situ from hydrazonyl chlorides with carbon disulfide has been achieved under mild reaction conditions.

Pseurotin D Inhibits the Activation of Human Lymphocytes

Background: Pseurotins, a family of secondary metabolites of different fungi characterized by an unusual spirocyclic furanone-lactam core, are suggested to have different biological activities including the modulation of immune response. Purpose: Complex characterization of the effects of pseurotin D on human lymphocyte activation in order to understand the potential of pseurotin to modulate immune response in humans. Methods: CD4+ and CD8+ T cells and CD19+ B cells isolated from human blood were activated by various activators simultaneously with pseurotin D treatment. The effects of pseurotin were tested on the basis of changes in cell viability, apoptosis, activation of signal transducers and activators of transcription (STAT) signaling pathways, production of tumor necrosis factor (TNF)-α by T cells, expression of activation markers CD69 and CD25 on T cells and Human Leukocyte Antigen–DR isotype (HLA-DR) on B cells, and the differentiation markers CD20, CD27, CD38, and immunoglobulin (Ig) D on B cells. Results: Pseurotin D significantly inhibited the activation of both CD4+ and CD8+ human T cells complemented by the inhibition of TNF-α production without significant acute toxic effects. The Pseurotin D-mediated inhibition of T-cell activation was accompanied by the induction of the apoptosis of T cells. This corresponded with the inhibited phosphorylation of STAT3 and STAT5. In human B cells, pseurotin D did not significantly inhibit their activation; however, it affected their differentiation. Conclusions: Our results advance the current mechanistic understanding of the pseurotin-induced inhibition of lymphocytes and suggest pseurotins as new attractive chemotypes for future research in the context of immune-modulatory drugs.

Natural pseurotins and analogs thereof inhibit activation of B-cells and differentiation into the plasma cells

BACKGROUND

The frequency of allergic diseases is constantly rising. Dysregulated production of isotype E immunoglobulins is one of the key factors behind allergic reactions and its modulation is therefore an important target for pharmacological intervention. Natural products of the pseurotin family were reported to be inhibitors of IgE production in B-cells. Mechanistic details underlying these effects are however not well understood.

PURPOSE

In the present study, we synthesized new analogs of natural pseurotins and extensively investigated their inhibitory effects on activation, proliferation and differentiation of B-cells, as well as on the production of IgE.

STUDY DESIGN

Effects of two natural pseurotins (pseurotins A and D) and a collection of fully synthetic pseurotin analogs were studied on mouse B-cells stimulated by the combination of IL-4 and E. coli lipopolysaccharide. The IgE production was determined along with cell viability and cell proliferation. The phosphorylation of selected members of the STAT transcription factor family was subsequently investigated. Finally, the in vivo effect of pseurotin D on the ovalbumin-induced delayed type hypersensitivity response was tested in mice.

RESULTS

We discovered that several fully synthetic pseurotin analogs were able to decrease the production of IgE in stimulated B-cells with potency comparable to that of pseurotins A and D. We found that the two natural pseurotins and the active synthetic analogs inhibited the phosphorylation of STAT3, STAT5 and STAT6 proteins in stimulated B-cells, resulting in the inhibition of B-cell proliferation and differentiation into the plasma cells. In vivo, pseurotin D decreased ovalbumin-induced foot pad edema.

CONCLUSION

Our results advance the current mechanistic understanding of the pseurotin-induced inhibition of IgE production in B-cells by linking the effect to STAT signaling, and associated modulation of B-cell proliferation and differentiation. Together with our finding that structurally simpler pseurotin analogs were able to reproduce the effects of natural pseurotins, the presented work has implications for the future research on these secondary metabolites in the context of allergic diseases.

Total Synthesis of (-)-FD-838 and (-)-Cephalimysin A

We completed a nine-step total synthesis of (-)-FD-838 and (-)-cephalimysin A. Our synthesis features a biogenetically guided assembly of the highly oxidized spirocyclic core by Snider-type tandem epoxidations of the chiral substrate derived from an amino acid derivative. Our synthetic approach provides a general and versatile solution to access spirocyclic PKS-NRPS-based secondary fungal metabolites.

Enantioselective Epoxypyrrolidines via a Tandem Cycloaddition/Autoxidation in Air and Mechanistic Studies

A tandem cycloaddition/autoxidation reaction between heterocyclic ketene aminals and diazoester in air is described for the enantioselective preparation of epoxypyrrolidines. Notably, the results of mechanistic studies suggest that epoxide was oxidized from an sp³ C-C single bond, which is of mechanistic and practical interest as this protocol may be suitable for constructing other bioactive heterocyclic epoxides.