Enantioselective total synthesis of borrelidin

Nitrile biosynthesis in nature: how and why?

Covering: up to the end of 2023Natural nitriles comprise a small set of secondary metabolites which however show intriguing chemical and functional diversity. Various patterns of nitrile biosynthesis can be seen in animals, plants, and microorganisms with the characteristics of both evolutionary divergence and convergence. These specialized compounds play important roles in nitrogen metabolism, chemical defense against herbivores, predators and pathogens, and inter- and/or intraspecies communications. Here we review the naturally occurring nitrile-forming pathways from a biochemical perspective and discuss the biological and ecological functions conferred by diversified nitrile biosyntheses in different organisms. Elucidation of the mechanisms and evolutionary trajectories of nitrile biosynthesis underpins better understandings of nitrile-related biology, chemistry, and ecology and will ultimately benefit the development of desirable nitrile-forming biocatalysts for practical applications.

Genome-based discovery and total synthesis of janustatins, potent cytotoxins from a plant-associated bacterium

Host-associated bacteria are increasingly being recognized as underexplored sources of bioactive natural products with unprecedented chemical scaffolds. A recently identified example is the plant-root-associated marine bacterium Gynuella sunshinyii of the chemically underexplored order Oceanospirillales. Its genome contains at least 22 biosynthetic gene clusters, suggesting a rich and mostly uncharacterized specialized metabolism. Here, in silico chemical prediction of a non-canonical polyketide synthase cluster has led to the discovery of janustatins, structurally unprecedented polyketide alkaloids with potent cytotoxicity that are produced in minute quantities. A combination of MS and two-dimensional NMR experiments, density functional theory calculations of 13C chemical shifts and semiquantitative interpretation of transverse rotating-frame Overhauser effect spectroscopy data were conducted to determine the relative configuration, which enabled the total synthesis of both enantiomers and assignment of the absolute configuration. Janustatins feature a previously unknown pyridodihydropyranone heterocycle and an unusual biological activity consisting of delayed, synchronized cell death at subnanomolar concentrations.

Natural products from mangrove sediments-derived microbes: Structural diversity, bioactivities, biosynthesis, and total synthesis

The mangrove forests are a complex ecosystem, and the microbial communities in mangrove sediments play a critical role in the biogeochemical cycles of mangrove ecosystems. Mangrove sediments-derived microbes (MSM), as a rich reservoir of natural product diversity, could be utilized in the exploration of new antibiotics or drugs. To understand the structural diversity and bioactivities of the metabolites of MSM, this review for the first time provides a comprehensive overview of 519 natural products isolated from MSM with their bioactivities, up to 2021. Most of the structural types of these compounds are alkaloids, lactones, xanthones, quinones, terpenoids, and steroids. Among them, 210 compounds are obtained from bacteria, most of which are from Streptomyces, while 309 compounds are from fungus, especially genus Aspergillus and Penicillium. The pharmacological mechanisms of some representative lead compounds are well studied, revealing that they have important medicinal potentials, such as piericidins with anti-renal cell cancer effects, azalomycins with anti-MRSA activities, and ophiobolins as antineoplastic agents. The biosynthetic pathways of representative natural products from MSM have also been summarized, especially ikarugamycin, piericidins, divergolides, and azalomycins. In addition, the total synthetic strategies of representative secondary metabolites from MSM are also reviewed, such as piericidin A and borrelidin. This review provides an important reference for the research status of natural products isolated from MSM and the lead compounds worthy of further development, and reveals that MSM have important medicinal values and are worthy of further development.

One-pot access to 2-oxazolines via a Castro-Stephens coupling and intramolecular cyclization

Aim:

Here, we have reported easy one-pot access to a series of oxazolines using a modified Castro-Stephens coupling protocol.

Background:

2-oxazolines have been shown to have significant biological activity and wide-ranging applications in organic chemistry. These properties make oxazolines as heterocyclic compounds of immense importance.

Objective:

The objective of this study is to synthesize oxazoline derivatives via an economical and one-pot protocol.

Method:

2-oxazoline has been synthesized through Cu-powder mediated Castro-Stephens coupling and intramolecular cyclization route. The mechanism involves a rearrangement in which one of the oxygen from the N-acylamino alcohol group is liberated as water and then transferred to alkyne functionality to form 2-oxazoline derivatives. The oxazolines were characterized by NMR, mass, and XRD studies.

Result:

The protocol is economically viable and uses readily available Cu-powder along with DMF for cross-coupling and cyclization steps.

Cis double bond formation in polyketide biosynthesis

Covering: up to 2020Polyketides form a large group of bioactive secondary metabolites that usually contain one or more double bonds. Although most of the double bonds found in polyketides are trans or E-configured, several cases are known in which cis or Z-configurations are observed. Double bond formation by polyketide synthases (PKSs) is widely recognised to be catalysed by ketoreduction and subsequent dehydration of the acyl carrier protein (ACP)-tethered 3-ketoacyl intermediate in the PKS biosynthetic assembly line with a specific stereochemical course in which the ketoreduction step determines the usual trans or more rare cis double bond configuration. Occasionally, other mechanisms for the installation of cis double bonds such as double bond formation during chain release or post-PKS modifications including, amongst others, isomerisations or double bond installations by oxidation are observed. This review discusses the peculiar mechanisms of cis double bond formation in polyketide biosynthesis.

Comprehensive optimization of precursor-directed production of BC194 by Streptomyces rochei MB037 derived from the marine sponge Dysidea arenaria

BC194, a derivative of borrelidin (BN) that features a lower cytotoxicity than that of BN due to an altered starter unit, trans-1,2-cyclobutanedicarboxylic acid (trans-1,2-CBDA), is a potent inhibitor of angiogenesis. However, BC194 production has only been reported to occur via mutasynthesis, which requires tedious, multistep genetic manipulation. In this study, we surveyed several factors contributing to the precursor-directed biosynthesis of BC194 and provided an alternative method for the production of BC194 that is directly applicable to other BN-producing strains. First, the precursor-directed biosynthesis of BC194 by a BN-producing strain, Streptomyces rochei MB037 derived from sponge Dysidea arenaria, was carried out in modified Radix astragali (RA) medium with 5 mM trans-1,2-CBDA. Next, possible inhibitors of BN starter unit trans-1,2-cyclopentanedicarboxylic acid (trans-1,2-CPDA) biosynthesis were investigated. It was found that potassium ferricyanide was a possible inhibitor of 3,4-dihydroxyphenylacetate 2,3-dioxygenase (DHPAO) and capable of suppressing the yield of BN and increasing the BC194 yield by 112.5% (from 5.2 ± 0.76 to 11.9 ± 0.59 mg/L). BC194 yield was further enhanced in the presence of 50 mM trans-1,2-CBDA, reaching 20.2 ± 0.62 mg/L. Furthermore, 3% macroporous adsorbent DA-201 resin was added to the fermentation broth, enabling a further 36.6% increase in BC194 production and reaching 27.59 ± 1.15 mg/L. Moreover, an efficient separation of BC194 with approximately 95% purity was developed by employing high-speed counter-current chromatography (HSCCC), achieving an improved recovery (approximately 93%).

Truncated borrelidin analogues: synthesis by sequential cross metathesis/olefination for the southern fragment and biological evaluation

The construction of novel borrelidin analogues is reported in which the northern fragment is truncated to a simple hydroxyundecanecarboxylate and the original cyclopentanecarboxylic acid in the southern fragment is replaced with different six-membered rings. The required precursors were prepared by cross metathesis of the appropriate carbocycle-based homoallylic alcohol with crotonaldehyde followed by HWE olefination of the resulting enal with bromocyanophosphonate. The key aldehyde for intramolecular cross coupling was accessible by oxidation of the hydroxy group of the linked undecanecarboxylate unit. Grignard mediated macrocyclization finally yielded the borrelidin related products. The investigation is complemented by SAR studies and quantum-chemical calculations.

Asymmetric Synthesis of Deoxypropionate Derivatives via Catalytic Hydrogenolysis of Enantioenriched Z-Ketene Heterodimers

A diastereoselective approach to deoxypropionate derivatives through Pd/C-catalyzed hydrogenolysis of enantioenriched ketene heterodimers is described. Catalytic hydrogenolysis of the Z-isomer of ketene heterodimers facilitates access to anti-deoxypropionate derivatives (10 examples with dr 7:1 to >20:1). Transfer of chirality from the Z-ketene heterodimer to an acid product was good to excellent in most cases (78-99% ee for 12 examples).

Anti-tuberculous activity of treponemycin produced by a streptomyces strain MS-6-6 isolated from Saudi Arabia

A Streptomyces strain MS-6-6 with promising anti-tuberculous activity was isolated from soil samples in Saudi Arabia. The nucleotide sequence of its 16S rRNA gene (1426 bp) evidenced a 100% similarity to Streptomyces mutabilis. Through an anti-tuberculous activity-guided approach, a polyketide macrolide was isolated and identified as treponemycin (TP). The structure of the isolated compound was determined by comprehensive analyses of its 1D and 2D NMR as well as HRESI-MS. In addition to the promising anti-tuberculous activity (MIC = 13.3 µg/mL), TP showed broad spectrum of activity against the Gram positive, Gram negative strains, and Candida albicans. Improvement of TP productivity (150%) was achieved through modification in liquid starch nitrate medium by replacing KNO3 with corn steep liquor and yeast extract or tryptone, and removing CaCO3 and K2HPO4. The follow up of TP percentage as well as its metabolites profile for each media was assessed by LC/DAD/MS.

Sonogashira coupling in natural product synthesis

This review will focus on selected applications of Sonogashira coupling and subsequent transformations as key steps in the total synthesis of natural products.

Uncovering the origin of Z-configured double bonds in polyketides: intermediate E-double bond formation during borrelidin biosynthesis

The dehydratase domain BorDH3 is assayed with a synthetic surrogate of the predicted tetraketide substrate and shown to be E-selective. Detailed NMR spectroscopic analysis of pre-borrelidin assigns the timing of the E-5 Z-isomerization to the very final steps of borrelidin biosynthesis.

Alkenenitrile Transmissive Olefination: Synthesis of the Putative Lignan "Morinol I"

Grignard reagents trigger an addition-elimination with α'-hydroxy acrylonitriles to selectively generate Z-alkenenitriles. The modular assembly of Z-alkenenitriles from a Grignard reagent, acrylonitrile, and an aldehyde is ideal for stereoselectively synthesizing alkenes as illustrated in the synthesis of the putative lignan "morinol I."

A general, asymmetric, and noniterative synthesis of trideoxypropionates. Straightforward syntheses of the pheromones (+)-vittatalactone and (+)-norvittatalactone

A novel, highly stereocontrolled, and very flexible synthetic access to biologically relevant trideoxypropionate building blocks in optically pure form has been developed. On the basis of a three-step sequence comprising a thermal oxy-Cope rearrangement, an iridium-catalyzed hydrogenation, and an auxiliary-controlled enolate methylation, trideoxypropionates with easily adjustable relative configuration were synthesized in excellent yields. In addition, the functionalized end groups allow for chemoselective manipulations and further elongation of the chain. The underlying strategy constitutes the first noniterative process for the assembly of polydeoxypropionates and has further been applied in total syntheses of the pheromones (+)-vittatalactone and (+)-norvittatalactone, which had been isolated from the striped cucumber beetle Acalymma vittatum.

Phorboxazole Synthetic Studies: Design, Synthesis and Biological Evaluation of Phorboxazole A and Hemi-Phorboxazole A Related Analogues

The design, synthesis and biological evaluation of a new phorboxazole analogue, comprising an acetal replacement for the C-ring tetrahdropyran of the natural product and carrying a potency-enhancing C(45-46) vinyl chloride side chain, is described. In addition, the synthesis of (+)-hemi-phorboxazole A and a series of related hemi-phorboxazole A analogues has been achieved. The new acetal ring replacement analogue displayed activity comparable to that of the parent natural product against HCT-116 (colon) cells (IC(50) 2.25 ng/mL). Equally important, the phorboxazole analogue and two related hemiphorboxazole A congeners exhibited significant antifungal activity when assayed against pathogenic Candida albicans strains.

A non-iterative, flexible, and highly stereoselective synthesis of polydeoxypropionates--synthesis of (+)-vittatalactone

A short sequence comprising an oxy-Cope rearrangement, iridium-catalyzed hydrogenation, and enolate methylation provides trideoxypropionates with excellent diastereocontrol. A straightforward synthesis of the cucumber beetle pheromone (+)-vittatalactone illustrates this new strategy.

Formal synthesis of the anti-angiogenic polyketide (-)-borrelidin under asymmetric catalytic control

Borrelidin (1) is a polyketide that possesses extremely potent anti-angiogenesis activity. This paper describes its formal total synthesis by the most efficient route to date. This modular approach takes optimal benefit of asymmetric catalysis and permits the synthesis of analogues; in addition, the high yields and selectivities obtained eliminate the need for separation of stereoisomers. The upper half of borrelidin has been accessed by iterative copper-catalysed asymmetric conjugate addition of methylmagnesium bromide, whereas synthesis of the lower half of the molecule was achieved by relying on asymmetric hydrogenation and cross-methathesis as key steps.

Generation of Nucleophilic Chromium Acetylides from gem-Trichloroalkanes and Chromium Chloride: Synthesis of Propargyl Alcohols

Nucleophilic mixed chromium(II) and chromium(III) acetylides are generated from the smooth reduction of primary 1,1,1-trichloroalkanes with chromium(II) chloride in the presence of an excess amount of triethylamine at room temperature. These species arise from chromium(III) vinylidene carbenoids. It has been demonstrated that uncommon low-valent Cr(II) acetylides are formed by C-H insertion of Cr(II)Cl(2) into terminal alkynes, formed in situ through the Fritsch- Buttenberg-Wiechell (FBW) rearrangement, whereas Cr(III) acetylides are concomitantly generated by HCl elimination from the chromium(III) vinylidene carbenoid. Both divergent pathways result, overall, in the formation of nucleophilic acetylides. In situ trapping with electrophilic aldehydes afforded propargyl alcohols. Furthermore, deuteration experiments and the use of deuterium labeled 1,1,1-trichloroalkane substrates demonstrated the prevalence of low-valent Cr(II) acetylides, potentially useful, yet highly elusive synthetic intermediates.

Hemi-phorboxazole a: structure confirmation, analogue design and biological evaluation

A synthesis providing totally synthetic (+)-hemi-phorboxazole A (1), proceeding in two steps (85% yield) from known vinyl iodide precursor (+)-2, has been achieved in conjunction with the design, synthesis, and biological evaluation of two hemi-phorboxazole analogues [(+)-3 and (-)-4] featuring ring replacements inscribed within the macrolide. Although hemi-phorboxazole A (1) displayed no activity when tested against Candida albicans and two human cancer cell lines, analogue (-)-4 exhibited significant tumor cell growth inhibitory activity in the nanomolar range against HCT-116 (colon) and SK-BR-3 (breast), while (+)-3 displayed promising antifungal activity against C. albicans.