Total Syntheses of Strasseriolide A and B, Antimalarial Macrolide Natural Products

Collective and Diverted Total Synthesis of the Strasseriolides: A Family of Macrolides Endowed with Potent Antiplasmodial and Antitrypanosomal Activity

The strasseriolide macrolides show promising in vitro and in vivo activities against P. falciparum and T. cruzi, the parasites causing malaria and Chagas disease, respectively. However, the as yet poor understanding of structure/activity relationships and the fact that one family member proved systemically toxic for unknown reasons render a more detailed assessment of these potential lead compounds difficult. To help overcome these issues, a collective total synthesis was devised. The key steps consisted of a ring closing alkyne metathesis (RCAM) reaction to forge a common macrocyclic intermediate followed by a hydroxy-directed ruthenium catalyzed trans-hydrostannation of the propargyl alcohol site thus formed. The resulting alkenyltin derivative served as the central node of the synthesis blueprint, which could be elaborated into the natural products themselves as well as into a set of non-natural analogues according to the concept of diverted total synthesis. The recorded biological data confirmed the potency of the compounds and showed the lack of any noticeable cytotoxicity. The "northern" allylic alcohol subunit was recognized as an integral part of the pharmacophore, yet it provides opportunities for chemical modification.

Total Synthesis of Antibacterial Macrolide Sorangiolide A

A convergent route for the asymmetric total synthesis of antibacterial macrolide sorangiolide A has been developed for the first time. The key feature of this synthesis includes Krische iridium-catalyzed anti-diastereoselective carbonyl crotylation, Crimmins acetate aldol, Yamaguchi esterification, Julia-Kocienski olefination, Horner-Wadsworth-Emmons olefination, and ring-closing metathesis. The origin of the low intensity of the 13C{1H} NMR signals of the C1 and C2 centers of the natural product has been investigated, disclosing possible forms of existence for the natural product in the solution phase.

1,2-Metallate Rearrangement as a Toolbox for the Synthesis of Allylic Alcohols

The development of new methods and protocols for the synthesis of biologically active substances remains one of the most important pillars in organic chemistry, and one of these privileged structural motifs are allylic alcohols. The method of choice to date for the synthesis of these is the Nozaki-Hiyama-Takai-Kishi reaction. We describe here a valuable alternative to the synthesis of allylic alcohols via 1,2-metallate rearrangement. In this work, various vinyl boronic esters with different functional groups have been applied in the Hoppe-Matteson-Aggarwal reaction. In addition, two monoterpenoids were constructed via this convergent synthetic strategy.

Strasseriolides display in vitro and in vivo activity against trypanosomal parasites and cause morphological and size defects in Trypanosoma cruzi

Neglected diseases caused by kinetoplastid parasites are a health burden in tropical and subtropical countries. The need to create safe and effective medicines to improve treatment remains a priority. Microbial natural products are a source of chemical diversity that provides a valuable approach for identifying new drug candidates. We recently reported the discovery and bioassay-guided isolation of a novel family of macrolides with antiplasmodial activity. The novel family of four potent antimalarial macrolides, strasseriolides A-D, was isolated from cultures of Strasseria geniculata CF-247251, a fungal strain obtained from plant tissues. In the present study, we analyze these strasseriolides for activity against kinetoplastid protozoan parasites, namely, Trypanosoma brucei brucei, Leishmania donovani and Trypanosoma cruzi. Compounds exhibited mostly low activities against T. b. brucei, yet notable growth inhibition and selectivity were observed for strasseriolides C and D in the clinically relevant intracellular T. cruzi and L. donovani amastigotes with EC50 values in the low micromolar range. Compound C is fast-acting and active against both intracellular and trypomastigote forms of T. cruzi. While cell cycle defects were not identified, prominent morphological changes were visualized by differential interference contrast microscopy and smaller and rounded parasites were visualized upon exposure to strasseriolide C. Moreover, compound C lowers parasitaemia in vivo in acute models of infection of Chagas disease. Hence, strasseriolide C is a novel natural product active against different forms of T. cruzi in vitro and in vivo. The study provides an avenue for blocking infection of new cells, a strategy that could additionally contribute to avoid treatment failure.

Palladium-Catalyzed Direct Esterification via C-H Bond Activation of Aldehydes

A concise and highly efficient synthesis method of direct esterification of aldehydes via Pd-catalyzed C-H bond activation of aldehyde group has been developed. The strategy avoids the preoxidation step of aldehyde or use of condensing agents in ester synthesis, which is not only applicable to various alcohols but also suitable for the esterification of phenolics which are usually difficult to be esterified. The methodology has the significant advantages of broad substrate scope, mild reaction conditions, and nonrequirement of additional oxidants.

Total Synthesis of Strasseriolide A

Stereoselective total synthesis of structurally intriguing antimalarial macrolide strasseriolide A has been accomplished by adopting a convergent approach. The salient features of this synthesis include Co(BH₄)₂-mediated selective reduction of conjugated olefin, Crimmins propionate aldol, Evans alkylation, intermolecular Horner-Wadsworth-Emmons olefination, Yamaguchi macrolactonization, and selective saponification of ester moiety in the presence of a lactone functionality. The ¹³C{¹H} NMR data of strasseriolide A were found to be very sensitive to its solution concentration.

New structural classes of antimalarials

Malaria remains a major vector borne disease claiming millions of lives worldwide due to infections caused by Plasmodium sp. Discovery and development of antimalarial drugs have previously been dominated majorly by single drug therapy. The malaria parasite has developed resistance against first line and second line antimalarial drugs used in the single drug therapy. This has drawn attention to find ways to alleviate the disease burden supplanted by combination therapy with multiple drugs to overcome drug resistance. Emergence of resistant strains even against the combination therapy has now mandated the revision of the current antimalarial pharmacotherapy. Research efforts of the past decade led to the discovery and identification of several new structural classes of antimalarial agents with improved biological attributes over the older ones. The following is a comprehensive review, addressed to the new structural classes of heterocyclic and natural compounds that have been identified during the last decade as antimalarial agents. Some of the classes included herein contain one or more pharmacophores amalgamated into a single bioactive scaffold as antimalarial agents, which act upon the conventional and novel targets.