Total synthesis of fostriecin: via a regio- and stereoselective polyene hydration, oxidation, and hydroboration sequence

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

Selective synthesis of boron-substituted enynes via a one-pot diboration/protodeboration sequence

An efficient and facile one-pot protocol to access enynylboronates via a Pt-catalyzed diboration/protodeboration strategy has been developed. The reaction is suitable for various silylsubstituted symmetrical and unsymmetrical 1,3-diynes, leading to π-conjugated organoboron compounds with excellent regio- and stereoselectivity.

[Pt(PPh3)4]-Catalyzed Selective Diboration of Symmetrical and Unsymmetrical 1,3-Diynes

A straightforward, efficient, and selective method for the preparation of novel boryl-functionalized enynes or dienes via [Pt(PPh₃)₄]-catalyzed diboration of a broad spectrum of symmetrical and unsymmetrical 1,3-diynes was developed. The catalytic cycle of diboration was proposed on the basis of low-temperature ³¹P NMR studies. An alternative isolation method via product condensation on a cold finger was developed, which, in contrast to previous literature reports, eliminates the need for the additional transformation of rapidly decomposing enynyl pinacol boronates to more stable silica-based column chromatography derivatives during the separation step. To prove the efficiency of this simple catalytic protocol, bisboryl-functionalized enynes were synthesized in a gram scale and tested as useful building blocks in advanced organic transformations, such as hydrosilylation and Suzuki and sila-Sonogashira couplings. The presence of silyl, boryl, as well as other functions like halogen or alkoxy in their structures builds a new class of multifunctionalized enynes that might be modified in various chemical reactions.

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Catalytic Enantioselective Allylation of Acetylenic Aldehydes by Chiral Phosphoric Acid/Transition Metal Cooperative Catalysis: Formal Synthesis of Fostriecin

An enantioselective allylation of silyl-substituted acetylenic aldehydes by chiral phosphoric acid (CPA)/transition metal cooperative catalysis was developed. Enantioenriched homoallylic propargyl alcohols were obtained in good yields with excellent enantioselectivities (>99% ee) under mild conditions. Moreover, the shortest formal synthesis of fostriecin was achieved by the present enantioselective allylation protocol as the key step. The known intermediate of fostriecin reported by McDonald and co-worker was synthesized in only nine steps in 39% total yield.

Controllable regio- and stereo-selective coupling reactions of homoallenylboronates

Unprecedented palladium-catalysed regio- and stereo-selective coupling reactions of homoallenylboronates with (hetero)aryl iodides, allyl bromides and alkynyl bromides in aqueous solution were successfully developed.

Total and Formal Syntheses of Fostriecin

Two formal syntheses and one total synthesis of fostriecin (1) have been achieved, as well as, the synthesis of its related congener dihydro-dephospho-fostriecin. All the routes use the Sharpless dihydroxylation to set the absolute stereochemistry at C-8/9 positions and a Leighton allylation to set the C-5 position of the natural product. In the formal syntheses a Noyori transfer hydrogenation of an ynone was used to set the C-11 position while the total synthesis employed a combination of asymmetric dihydroxylation and Pd-π-allyl reduction to set the C-11 position. Finally in the total synthesis, a trans-hydroboration of the C-12/13 alkyne was used in combination with a Suzuki cross coupling to establish the Z,Z,E-triene of fostriecin (1).

Total synthesis of elansolids B1 and B2

The elansolids A1-A3, B1, and B2 are secondary metabolites formed by the gliding bacterium Chitinophaga sancti. They show antibacterial activity against Gram-positive bacteria. A second generation total synthesis of the antibiotic elansolid B1 (2) and the first synthesis of elansolid B2 (3) are reported. In contrast to previous work, the (Z,E,Z)-triene at C10-C15 was assembled by using an optimized C-C cross-coupling sequence with a Suzuki cross-coupling reaction as key step.

Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Fostriecin and related natural products present a significant challenge for synthetic chemists due to their structural complexity and chemical sensitivity. This review will chronicle the successful efforts of synthetic chemists in the construction of these biologically active molecules. Key carbon-carbon bond forming reactions will be highlighted, as well as the methods used to install the numerous stereocenters present in this class of compounds.

A Highly Convergent Total Synthesis of Leustroducsin B

Leustroducsin B exhibits a large variety of biological activities and unique structural features. An efficient and highly convergent total synthesis of Leustroducsin B was achieved in 17 longest linear and 39 total steps by disconnecting the molecule into three fragments having similar levels of complexity. These pieces were connected via a highly efficient chelate-controlled addition of a vinyl zincate to an α-hydroxy ketone and a silicon-mediated cross-coupling. The stereochemistry of the central and western fragments was set catalytically in high yields and excellent de by a zinc-ProPhenol-catalyzed aldol reaction and a palladium-catalyzed asymmetric allylic alkylation.

Construction of the co-expression plasmids of fostriecin polyketide synthases and heterologous expression in Streptomyces

CONTEXT

Polyketides are bioactive natural products with diverse bioactivities, and heterologous production of polyketides in easily engineered microbial hosts is preferred for the production of structurally diverse and the therapeutically active polyketides.

OBJECTIVE

In this study, heterologous expression of the biosynthetic genes encoding type I polyketide synthases (PKS) involved in biosynthesis of fostriecin, a unique phosphate monoester polyketide antibiotic, was attempted.

MATERIALS AND METHODS

Fostriecin PKS (Fos-PKS) biosynthetic gene cluster in a total of 48.4 kb were cloned downstream of the act I promoter in two compatible Streptomyces vectors using Red/ET recombination. The co-expression plasmids were sequentially transferred into Streptomyces lividans and Streptomyces coelicolor. Active transcription of the polyketide genes was confirmed by reverse transcription PCR (RT-PCR) analysis, and the metabolites were detected using high-performance liquid chromatography (HPLC).

RESULTS

The recombinant strains S. lividans TK24/p6-fosAB-p4-fosCDEF and S. coelicolor M512/p6-fosAB-p4-fosCDEF were obtained for heterologous expression in Streptomyces. Pigmentation was observed in the recombinant strains, whereas the control strain with empty vector displayed no change in pigment production. Active transcription of the polyketide genes was confirmed by RT-PCR analysis and subsequent sequencing.

CONCLUSION

The present study is the first attempt to overexpress Fos-PKS biosynthetic gene cluster in Streptomyces. More studies on heterologous expression of the fostriecin biosynthetic gene cluster would be beneficial for further understanding the mechanisms of its structural as well as the potential pharmaceutically effect.

Catalytic Z-selective cross-metathesis in complex molecule synthesis: a convergent stereoselective route to disorazole C1

A convergent diastereo- and enantioselective total synthesis of anticancer and antifungal macrocyclic natural product disorazole C1 is reported. The central feature of the successful route is the application of catalytic Z-selective cross-metathesis (CM). Specifically, we illustrate that catalyst-controlled stereoselective CM can be performed to afford structurally complex Z-alkenyl-B(pin) as well as Z-alkenyl iodide compounds reliably, efficiently, and with high selectivity (pin = pinacolato). The resulting intermediates are then joined in a single-step operation through catalytic inter- and intramolecular cross-coupling to furnish the desired 30-membered ring macrocycle containing the critical (Z,Z,E)-triene moieties.

Synthesis of cyclic alkenylsiloxanes by semihydrogenation: a stereospecific route to (Z)-alkenyl polyenes

Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes-a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama-Denmark coupling to give (Z)-alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five-membered cyclic siloxanes also couple under fluoride-free conditions, whilst their six-membered homologues do not, enabling orthogonality within this structural motif.

Cryptocaryols A and B: total syntheses, stereochemical revision, structure elucidation, and structure-activity relationship

The first total syntheses and structural elucidation of cryptocaryol A and cryptocaryol B were achieved in 23 and 25 linear steps, respectively. The synthesis relied on the use of a key pseudo-Cs symmetric pentaol intermediate, which in a stereochemically divergent manner was converted into either enantiomer as well as diastereomers. This synthetic effort enabled the first structure-activity relationships of this class of PDCD4 stabilizing natural products.

Suppression of Ser/Thr phosphatase 4 (PP4C/PPP4C) mimics a novel post-mitotic action of fostriecin, producing mitotic slippage followed by tetraploid cell death

Fostriecin is a natural product purified from Sterptomyces extracts with antitumor activity sufficient to warrant human clinical trials. Unfortunately, difficulties associated with supply and stable drug formulation stalled further development. At a molecular level, fostriecin is known to act as a catalytic inhibitor of four PPP-family phosphatases, and reports describing the design of molecules in this class suggest derivatives targeting enzymes within the fostriecin-sensitive subfamily can be successful. However, it is not clear if the tumor-selective cytotoxicity of fostriecin results from the inhibition of a specific phosphatase, multiple phosphatases, or a limited subset of fostriecin sensitive phosphatases. How the inhibition of sensitive phosphatases contributes to tumor-selective cytotoxicity is also not clear. Here, high-content time-lapse imaging of live cells revealed novel insight into the cellular actions of fostriecin, showing that fostriecin-induced apoptosis is not simply induced following a sustained mitotic arrest. Rather, apoptosis occurred in an apparent second interphase produced when tetraploid cells undergo mitotic slippage. Comparison of the actions of fostriecin and antisense-oligonucleotides specifically targeting human fostriecin-sensitive phosphatases revealed that the suppression PP4C alone is sufficient to mimic many actions of fostriecin. Importantly, targeted suppression of PP4C induced apoptosis, with death occurring in tetraploid cells following mitotic slippage. This effect was not observed following the suppression of PP1C, PP2AC, or PP5C. These data clarify PP4C as a fostriecin-sensitive phosphatase and demonstrate that the suppression of PP4C triggers mitotic slippage/apoptosis.

IMPLICATIONS

Future development of fostriecin class inhibitors should consider PP4C as a potentially important target. Mol Cancer Res; 11(8); 845-55. ©2013 AACR.

Synthesis of Z-(pinacolato)allylboron and Z-(pinacolato)alkenylboron compounds through stereoselective catalytic cross-metathesis

The first examples of catalytic cross-metathesis (CM) reactions that furnish Z-(pinacolato)allylboron and Z-(pinacolato)alkenylboron compounds are disclosed. Products are generated with high Z selectivity by the use of a W-based monoaryloxide pyrrolide (MAP) complex (up to 91% yield and >98:2 Z:E). The more sterically demanding Z-alkenylboron species are obtained in the presence of Mo-based MAP complexes in up to 93% yield and 97% Z selectivity. Z-selective CM with 1,3-dienes and aryl olefins are reported for the first time. The utility of the approach, in combination with catalytic cross coupling, is demonstrated by a concise and stereoselective synthesis of anticancer agent combretastatin A-4.

Total synthesis of the potent androgen receptor antagonist (-)-arabilin: a strategic, biomimetic [1,7]-hydrogen shift

The first total synthesis of (-)-arabilin, a Streptomyces metabolite that inhibits hormone activation of the androgen receptor, has been completed. The key step, a [1,7]-hydrogen shift, establishes the enol ether-containing skipped-tetraene substructure. This nonenzymatic pericyclic reaction is considered to be biomimetic.

De novo synthesis of natural products via the asymmetric hydration of polyenes

For the last ten years our group has been working toward the development of an asymmetric hydration approach to polyketide natural products based on the regioselective hydration of di- and tri-enoates. Key to the success of this approach is the recognition that both high regiocontrol and asymmetric induction could be obtained by the use of a Sharpless asymmetric dihydroxylation reaction. Herein we describe the development of the method and its application to natural product total synthesis.

Highly diastereoselective chelation-controlled additions to α-silyloxy ketones

The polar Felkin-Anh, Cornforth-Evans, and Cram-chelation models predict that the addition of organometallic reagents to silyl-protected α-hydroxy ketones proceeds via a nonchelation pathway to give anti-diol addition products. This prediction has held true for the vast majority of additions reported in the literature, and few methods for chelation-controlled additions of organometallic reagents to silyl-protected α-hydroxy ketones have been introduced. Herein, we present a general and highly diastereoselective method for the addition of dialkylzincs and (E)-di-, (E)-tri-, and (Z)-disubstituted vinylzinc reagents to α-silyloxy ketones using alkyl zinc halide Lewis acids, RZnX, to give chelation-controlled products (dr ≥18:1). The compatibility of organozinc reagents with other functional groups makes this method potentially very useful in complex molecule synthesis.