Total Synthesis of Lipopeptide Bacilotetrin C: Discovery of Potent Anticancer Congeners Promoting Autophagy.

A convergent strategy for the first total synthesis of the lipopeptide bacilotetrin C has been developed. The key features of this synthesis include Crimmins acetate aldol, Steglich esterification, and macrolactamization. Twenty-nine variants of the natural product were prepared following a systematic structure-activity relationship study, where some of the designed analogues showed promising cytotoxic effects against multiple human carcinoma cell lines. The most potent analogue exhibited a ∼37-fold enhancement in cytotoxicity compared to bacilotetrin C in a triple-negative breast cancer (MDA-MB-231) cell line at submicromolar doses. The study further revealed that some of the analogues induced autophagy in cancer cells to the point of their demise at doses much lower than those of known autophagy-inducing peptides. The results demonstrated that the chemical synthesis of bacilotetrin C with suitable improvisation plays an important role in the development of novel anticancer chemotherapeutics, which would allow future rational design of novel autophagy inducers on this template.

Unveiling the anticancer potential of Pestalotioprolide E, an unexplored macrolide: Targeting TRXR1-TRX1-ASK1-P38 signaling cascade in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is highly aggressive and metastatic in nature. Existing treatment modalities for TNBC are associated with severe side effects. Thioredoxin reductase (TRXR), the pivotal component of the thioredoxin system, remains overexpressed in various cancer cells including TNBC; promotes cell growth, proliferation, and metastasis, and inhibits apoptosis. Pestalotioprolide E is one of the potent macrolides, a class of secondary metabolites derived from an endophytic fungus Pestalotiopsis microspora with relatively unexplored biological activities. Our study revealed increased expression and activity of TRXR1 in MDA-MB-231 cells compared to the non-cancerous cells. In silico docking analysis and in vitro activity assay demonstrated that Pestalotioprolide E directly interacts with TRXR1 and inhibits its enzymatic activity. This inhibition induces apoptosis via TRX1/ASK1/P38MAPK death signaling cascade and retards metastasis through modulating VEGF, MMP-2, MMP-9, E-cadherin, N-cadherin in MDA-MB-231 cells. Taken together present study establishes TRXR1 as a molecular target for Pestalotioprolide E and its anticancer effect can be attributed to the inhibition of TRXR1 activity in MDA-MB-231.

Structure-Activity Relationship Study of Biselyngbyolide B Reveals Mitochondrial Fission-Induced Cytotoxicity in Cancer.

A systematic structure-activity relationship study of the potent anticancer marine macrolide biselyngbyolide B has been accomplished. A total of 11 structural variants of the parent natural product, of which 2 are natural analogues, have been studied against a human colorectal carcinoma cell line. The requisite functional units of the parent molecule responsible for the cytotoxic activities have been disclosed. Biselyngbyolide C, one of the natural analogues of biselyngbyolide B, has been studied in depth to explore its molecular mechanism. Interestingly, the in vitro data demonstrated an induction of dynamin-related protein 1-mediated mitochondrial fission and reactive oxygen species production which led to activation of ASK1/P38/JNK-mediated apoptosis in colon cancer cells as an important pathway for biselyngbyolide B-mediated cytotoxicity. Notably, this study revealed that a macrolide participated in mitochondrial fission to promote apoptosis of cancer cells, providing new insight.

Stereoselective synthesis of thailandamide A methyl ester.

A convergent strategy for the stereoselective synthesis of the methyl ester of the structurally challenging and highly labile antibacterial polyene polyketide natural product thailandamide A has been developed. The key steps include the Zincke aldehyde reaction, Stille cross coupling, Negishi reaction, Julia-Kocienski olefination, cross metathesis, and the less explored Pd(I)-based Heck coupling to access different unsaturation bonds. Additionally, Urpi acetal aldol, Evans methylation, and Crimmins acetate aldol reactions were employed to construct four out of six asymmetric centers of the molecule.

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.

Total synthesis of the antibacterial polyketide natural product thailandamide lactone.

Stereoselective total synthesis of the structurally intriguing polyketide natural product thailandamide lactone was accomplished, and done so using a convergent approach for the first time to the best of our knowledge. The key features of this synthesis included use of a Crimmins acetate aldol reaction, Evans methylation, Urpi acetal aldol reaction, Sharpless asymmetric epoxidation and subsequent γ-lactonization for the installation of six asymmetric centers and the use of the Negishi reaction, Julia-Kocienski olefination, cross metathesis, HWE olefination and intermolecular Heck coupling for construction of a variety of unsaturated linkages. Pd(i)-based Heck coupling was introduced, for the first time to the best of our knowledge, quite efficiently to couple the major eastern and sensitive western segments of the molecule. The antibacterial activity of thailandamide lactone was also evaluated.

Late-Stage Functionalization: Total Synthesis of Beauveamide A and Its Congeners and Their Anticancer Activities.

Asymmetric total synthesis of cyclotetradepsipeptide beauveamide A has been achieved for the first time. A macrolactamization strategy involving two possible sites has been explored to find the most effective route for cyclization. A late-stage functionalization approach has been adopted for easy access of non-natural analogues of beauveamide A for further biological evaluation. Interestingly, the anticancer activity of one of the synthesized analogues was better than that of the parent natural product.

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.

Cananginone Abrogates EMT in Breast Cancer Cells through Hedgehog Signaling.

Cananginones, a family of linear acetogenins found as secondary metabolites in the plant kingdom, show cytotoxicity against several types of cancer cells. We aimed to investigate the efficacy of cananginone and its mechanism as an anti-cancer agent. Our initial screening of Cananginone against HepG2, PC3, A549, and MCF7 cells showed anti-cancer activities and is more potent against MCF7 cells, consistent with the previous report. Next, cell-based assays have revealed that cananginone abrogates cancer stem cell renewal as well as Epithelial-Mesenchymal Transition (EMT) and increased the ROS level beyond the threshold level thus reducing the viability of cancer cells. In the connection of Hh-Gli to EMT, our study indicated that cananginone inhibits Gli1 in a non-canonical pathway. Presumably, this is the first report on the inhibitory activity of cananginone in the Hh pathway and is different from Hh-antagonists cyclopamine and GANT 61 considering the mechanism.

Total synthesis and stereochemical assignment of bipolamide A acetate.

Asymmetric total synthesis of an acetate analogue of the endophytic unstable secondary metabolite bipolamide A has been achieved for the first time adopting a convergent approach. The key feature of this synthesis includes Evans's asymmetric ethylation, Wittig olefination, Takai olefination, stereoselective Grignard addition and intermolecular Heck coupling. This eventually developed a synthetic route of the rarely found branched amine bearing an acyloin moiety. Our synthesis finally established unambiguously the stereochemistry of the unassigned C-8 center of the naturally occurring unstable bipolamide A.

Asymmetric Total Synthesis of Amphirionin-2.

A convergent route for the asymmetric total synthesis of potent anticancer polyketide natural product amphirionin-2 has been developed. Our initial synthetic trials revealed that the proposed structures of amphirionin-2 need to be revised consistent with a recent report of Fuwa et al., where the actual structure of amphirionin-2 was established. The key features of our synthesis comprised Sharpless asymmetric dihydroxylation, followed by cycloetherification, Wittig olefination, Julia-Kocienski olefination, and Crimmins propionate aldol reaction.

Total synthesis: the structural confirmation of natural products.

This feature article highlights total synthesis as one of the reliable tools for the structural confirmation of natural products. Even though there has been substantial improvement in spectroscopic techniques that has opened an important avenue up to the synthetic community for discoveries, the structural misassignment of natural products remains a common occurrence. Herein, we have included a few case studies, along with some selected examples of natural products associated with our current research interests, where structures were revised in recent years using the art of chemical synthesis.

Biselyngbyolides A & C: their total synthesis and anticancer activities.

Convergent strategies for the first total synthesis of biselyngbyolide C and an alternative route for the total synthesis of biselyngbyolide A have been developed. The key strategic feature in this study is Heck macrocyclization. The use of intramolecular Heck coupling for biselyngbyolide B was demonstrated by us earlier; however such a strategy has not been explored further for the other members of this family of natural products, in particular, where sensitive skipped olefins are involved. The other highlights of this synthetic study include iterative Crimmins acetate aldol and Wittig olefination processes, followed by the less explored cobalt-hydride-based reduction of an activated olefin and Shiina esterification. Our synthetic study enabled us to amend the reported NMR data of biselyngbyolides A and C. An evaluation of the anticancer activities of both biselyngbyolides A and C revealed that the apoptosis generated in cancer cells followed an intrinsic pathway.

Total synthesis of nafuredin B.

Total synthesis of marine secondary metabolite nafuredin B has been achieved for the first time using a convergent strategy. Sharpless epoxidation followed by acid catalyzed epoxide opening were adopted to install the tetrasubstituted hydroxy center, whereas the iterative Julia-Kocienski olefination, Wittig olefination and HWE olefination afforded the olefin bonds. Ring closing metathesis in the presence of a free tetrasubstituted hydroxy group provided the unsaturated δ-lactone moiety. This synthetic study provided unambiguous structural confirmation of the isolated nafuredin B.

Total Synthesis and Stereochemical Assignment of Sunshinamide and Its Anticancer Activity.

Total synthesis of cyclodepsipeptide sunshinamide has been achieved for the first time using a convergent approach. The key features of this synthesis comprise Crimmins acetate aldol, Shiina esterification, amide coupling, macrolactamization, and an I2-mediated deprotection with concomitant disulfide-bridge formation. This synthetic study enabled the unambiguous determination of the stereochemistry of the unassigned stereocenter of the isolated sunshinamide. The cytotoxicity of sunshinamide and one of its analogues was evaluated against different cancerous and noncancerous human cell lines, which revealed their attractive and selective activities toward cancer cells at very low concentrations.

Total Synthesis and Stereochemical Assignment of Penicitide A.

Stereoselective total synthesis of marine secondary metabolite penicitide A has been accomplished for the first time following a convergent approach. The salient feature of this study includes Horner-Wadsworth-Emmons (HWE) olefination, Evans methylations, Crimmins acetate aldol reaction, and cross olefin metathesis. Our synthetic study established the stereochemistry of unassigned C-10 and C-12 centers and also disclosed the absolute configurations of C-3 and C-5 stereocenters.

Cyclodepsipeptide alveolaride C: total synthesis and structural assignment.

First stereoselective total synthesis of naturally occurring bioactive cyclodepsipeptide alveolaride C has been achieved using a convergent approach. This synthetic study enabled us to establish unambiguously the stereochemistry of three unassigned chiral centres embedded in the nonpeptidic segment as well as revised the stereochemistry of the proposed ß-phenylalanine counterpart of the molecule. The key strategic features of this synthesis include Sharpless asymmetric dihydroxylation for installing the vicinal diol moiety, Julia-Kocienski olefination for constructing the aliphatic side chain, the Shiina protocol for intermolecular esterification, amide coupling and macrolactamization for the ring formation.

Studies toward the synthesis of macrotermycin C: stereoselective construction of the acyclic skeleton of the aglycon.

The stereoselective construction of the acyclic skeleton of the aglycon of polyene macrolactam macrotermycin C has been achieved for the first time using a convergent strategy. The important features of this synthesis study include Evans methylation, Takai olefination, Sonogashira coupling followed by selective alkyne reduction, intermolecular Heck coupling, and Wittig olefination. Intramolecular Heck coupling has been tested for macrocyclization.

Studies toward the synthesis of strevertenes A and G: stereoselective construction of C1-C19 segments of the molecules.

Stereoselective synthesis of common C1-C19 skeletons of pentane macrolides strevertenes A and G possessing 10 stereogenic centers has been achieved using a flexible and convergent strategy. The salient features of this synthetic study include the Evans aldol reaction for the constructions of C2, C3, C13, and C14 centers, CBS reduction for the generation of a C7 center, Hoveyda-Grubbs cross olefin metathesis for the synthesis of C8-C9 bond, and Wittig olefination for the installation of C16-C19 conjugated olefins.

Total Synthesis of Pestalotioprolide E and Structural Revision of Pestalotioprolide F.

A short and convergent strategy for the first asymmetric total synthesis of cytotoxic macrolides pestalotioprolides E and F has been developed. The key features of this synthesis include Takai olefination, Sonogashira coupling, Ni-assisted partial hydrogenation of alkyne, modified Steglich reaction to generate the ester moiety, and intramolecular Horner-Wadsworth-Emmons (HWE) olefination to complete the macrocycle. This synthetic study revised the proposed structure of pesralotioprolide F.