Total Synthesis of Solandelactones A, B, E, and F Exploiting a Tandem Petasis−Claisen Lactonization Strategy

Gold(I)-Catalyzed Cascade Cyclization of Alkynyl Indoles for the Stereoselective Construction of the Quaternary Carbon Center of Akuammiline Alkaloids

A gold-catalyzed cyclization reaction of alkynyl-indoles has been developed for the stereoselective construction of the quaternary carbon center of fused indolines. This reaction efficiently produces fused indolines via diastereoselective 6-endo-dig cyclization controlled by a bulky TIPS group, followed by nucleophilic attack of the carboxy group on the resulting imine. The lactone moiety of the fused indoline can be reductively cleaved to produce a tricyclic indoline, which could be useful for the synthesis of akuammiline alkaloids.

Simmons-Smith Cyclopropanation: A Multifaceted Synthetic Protocol toward the Synthesis of Natural Products and Drugs: A Review

Simmons-Smith cyclopropanation is a widely used reaction in organic synthesis for stereospecific conversion of alkenes into cyclopropane. The utility of this reaction can be realized by the fact that the cyclopropane motif is a privileged synthetic intermediate and a core structural unit of many biologically active natural compounds such as terpenoids, alkaloids, nucleosides, amino acids, fatty acids, polyketides and drugs. The modified form of Simmons-Smith cyclopropanation involves the employment of Et2Zn and CH2I2 (Furukawa reagent) toward the total synthesis of a variety of structurally complex natural products that possess broad range of biological activities including anticancer, antimicrobial and antiviral activities. This review aims to provide an intriguing glimpse of the Furukawa-modified Simmons-Smith cyclopropanation, within the year range of 2005 to 2022.

The impact of lysyl-phosphatidylglycerol on the interaction of daptomycin with model membranes

Daptomycin is an important clinical antibiotic for which resistance is rising. Daptomycin resistant strains of S. aureus often have increased 1,2-diacyl-sn-glycero-3-[phospho-1-(3-lysyl(1-glycerol))] (lysyl-PG) and mutations to the proteins directly involved in the synthesis and translocation of lysyl-PG are implicated in resistance mechanisms. To study the interaction of daptomycin with lysyl-DMPG-containing model membranes a new stereospecific and regioselective synthesis of lysyl-DMPG was developed. Studies on model membranes containing lysyl-DMPG demonstrate that: (1) daptomycin is not significantly repelled by the cationic charge of lysyl-DMPG; (2) daptomycin binds less avidly to lysyl-DMPG compared to DMPG; (3) the presence of lysyl-DMPG does not impact the membrane bound backbone conformation of daptomycin in a significant way; (4) lysyl-DMPG increases oligomer formation; (5) lysyl-DMPG does not impact model membrane fluidity at lysyl-PG : PG ratios that are relevant to daptomycin resistance. The results of these studies suggest that increased lysyl-PG content does not confer resistance to daptomycin by altering membrane fluidity or reducing membrane affinity but may confer resistance by altering the structure of daptomycin oligomers.

An amide mimic of desTHPdactylolide: Total synthesis and antiproliferative evaluation

(-)-Zampanolide is a unique microtubule stabilizing agent (MSA) with covalent-binding mechanism and low nanomolar anitproliferative potency towards multi-drug resistant cancer cells. MSAs have a special connection with prostate cancer by inhibiting androgen receptor nuclear translocation. Zampanolide and the structurally related dactylolide have thus been sought after by us as lead compounds for development of anti-prostate cancer agents. DesTHPdactylolide is a simplified mimic of dactylolide and has previously been synthesized by us in both configurations, with the (17R) configuration being more potent in suppressing prostate cancer cell proliferation. The current study aims to synthesize an amide mimic of (17R) desTHPdactylolide that was anticipated to be metabolically more stable than (17R) desTHPdactylolide. To this end, the amide mimic has been successfully synthesized through a 26-step transformation from 2-butyn-1-ol. Our WST-1 cell proliferation assay in five human prostate cancer cell models indicated that the lactam moiety can serve as a bioisostere for the lactone in desTHPdactylolide.

Total Synthesis of Chlorinated Oxylipin Eiseniachloride B

Eiseniachloride B is a marine chlorinated oxylipin isolated from the brown alga Eisenia bicyclis. This natural product contains cyclopentane, chlorohydrin, and 14-membered lactone systems that incorporate five stereogenic centers. In this paper, we report on the total synthesis of structurally unique oxylipin eiseniachloride B from optically active lactol via ecklonialactone B in a linear sequence comprising 11 steps with a 12.1% overall yield.

Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from Vinyl Cyclic Carbonates

Cyclic carbonates have long been considered relatively inert molecules acting as protecting groups in complex multistep synthetic routes. This study shows that a concise, yet modular synthesis of indolizidine and quinolizidine alkaloids can be developed from vinyl-substituted cyclic carbonate (VCC) intermediates. Through a highly stereoselective palladium-catalyzed allylic alkylation reaction, these alkaloid motifs can be assembled in four synthetic and only two column purification steps. The combined results help to further advance functionalized cyclic carbonates as useful and reactive intermediates in natural product synthesis.

Synthesis of solandelactone F, constanolactone A and an advanced intermediate towards solandelactone E from a common synthetic intermediate

The stereoselective synthesis of solandelactone F, constanolactone A and an advanced intermediate towards solandelactone E, from a common synthetic intermediate, is disclosed. The propargylic sulfide stereocenter is created stereoselectively via carbon-carbon bond formation in the reaction of α-chloro sulfides with alkynylzinc reagents via 1,2-asymmetric induction by a β-siloxy group. The characteristic 1,4-diol motif of the natural products is introduced by a [2,3] sigmatropic rearrangement of an allylic sulfoxide or by the Mislow-Evans-Braverman rearrangement of a propargylic sulfoxide followed by stereoselective reduction of the ensuing α,β-unsaturated ketone. Unlike earlier reports, the C11/C9 carbinol center is created with excellent stereocontrol and derivatives of natural products differing at C14/C12 can be readily obtained. Catalytic asymmetric protocols and substrate-controlled asymmetric induction are utilized for the efficient introduction of the stereogenic centers.

Optimized synthesis and antiproliferative activity of desTHPdactylolides

Dactylolide and certain analogues are attractive targets for study due to their structural resemblance to zampanolide, a very promising anticancer lead compound and a unique covalent-binding microtubule stabilizing agent. The primary goal of this project is identification and synthesis of simplified analogues of dactylolide that would be easier to prepare and could be investigated for antiproliferative activity in comparison with zampanolide. Extension of Almann's concept of a simplified zampanolide analogue to dactylolide in the form of desTHPdactylolide was attractive not only for reasons of synthetic simplification but also for the prospect that analogues of dactylolide could be prepared in both (17S) and (17R) configurations. Since Altmann's overall yield for the six-step procedure leading to the C9-C18 fragment of desTHPdactylolide was only 8.7%, a study focused on optimized synthesis and antiproliferative evaluation of each enantiomer of desTHPdactylolide was initiated using Altmann's route as a framework. To this end, two optimized approaches to this fragment C9-C18 were successfully developed by us using allyl iodide or allyl tosylate as the starting material for a critical Williamson ether synthesis. Both (17S) and (17R) desTHPdactylolides were readily synthesized in our laboratory using optimized methods in yields of 37-43%. Antiproliferative activity of the pair of enantiomeric desTHPdactylolides, together with their analogues, was evaluated in three docetaxel-sensitive and two docetaxel-resistant prostate cancer cell models using a WST-1 cell proliferation assay. Surprisingly, (17R) desTHPdactylolide was identified as the eutomer in the prostate cancer cell models. It was found that (17S) and (17R) desTHPdactylolide exhibit equivalent antiproliferative potency towards both docetaxel-sensitive (PC-3 and DU145) and docetaxel-resistant prostate cancer cell lines (PC-3/DTX and DU145/DTX).

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.

Total Synthesis of Solandelactone I

Since the marine natural products solandelactones A-I were isolated from the hydroid Solanderia secunda and investigated by Seo et al. in 1996, considerable synthetic efforts toward these marine oxylipins followed. However, the structure elucidation of solandelactone I remained incomplete, and no synthesis has been reported. On the basis of our retrosynthetic analysis, the key building blocks were combined in a Horner-Wadsworth-Emmons reaction to create two common intermediates for the stereodivergent synthesis of all four diastereomers 1-4 matching the proposed structure of solandelactone I. Comparison of the published analytical data of natural product solandelactone I and data obtained from the synthetic endeavor toward diastereomers 1-4 enabled the structure assignment of isomer 3; the proposed biosynthetic pathway for marine oxylipins also supports the result.

Total synthesis of (-)-goniotrionin

A stereoselective total synthesis of the reported structure of goniotrionin (4) has been accomplished. The key steps involved the opening of a chiral epoxide, a highly diastereoselective Mukaiyama aerobic oxidative cyclization, a selective 1,2-syn Mukaiyama aldol reaction, and a Noyori reduction.

Stereocontrolled asymmetric synthesis of syn-E-1,4-diol-2-enes using allyl boronates and its application in the total synthesis of solandelactone F

The solandelactones A-H comprise a novel class of oxygenated fatty acids bearing an eight-membered lactone, trans cyclopropane, and a 2-ene-1,4-diol subunit. The relative stereochemistry of the 1,4-diol subunit is anti in solandelactones A, C, E & G, and syn in solandelactones B, D, F & H. Having prepared one member of the solandelactones bearing anti stereochemistry (solandelactone E), we have targeted the syn series and developed methodology for the synthesis of enantioenriched syn-2-ene-1,4-diols. The methodology comprises asymmetric deprotonation of an alkyl 2,4,6-triisopropylbenzoate using sBuLi/sparteine, followed by addition of the α-lithiobenzoate to β-silyl vinyl boronic acid ethylene glycol ester. The boron-ate complex generated undergoes a 1,2-metallate rearrangement furnishing an intermediate allyl boronic ester which is trapped by an aldehyde in the presence of MgBr(2) to furnish anti-β-hydroxy E-allylsilanes in good yields, high diastereoselectivity and high enantioselectivity. These sensitive products were oxidized using mCPBA to the corresponding epoxides and subsequently treated with acid to furnish syn-E-2-ene-1,4-diols (~4:1 d.r.). Application of the methodology to appropriately functionalized aldehyde and ω-alkenyl 2,4,6-triisopropylbenzoate coupling partners, led to a short, highly selective route to solandelactone F (bearing a syn-E-2-ene-1,4-diol).

Total synthesis and bioactivities of two proposed structures of maresin

Maresin is a potent anti-inflammatory lipid mediator derived from docosahexaenoic acid (DHA). A highly convergent total synthesis of two proposed structures of C7-epimeric maresins from the four known fragments was achieved in 17 steps. The three key coupling reactions were the BF(3)-mediated alkyne attack on the epoxide, chiral titanium complex-promoted enantioselective alkyne addition to the aldehyde, and a Julia-Kocienski olefination. The two synthesized diastereomers were found to be comparably active in blocking neutrophil infiltration in the acute peritonitis model.

Palladium-catalyzed direct oxidation of alkenes with molecular oxygen: general and practical methods for the preparation of 1,2-diols, aldehydes, and ketones

1,2-Diols, aldehydes, and ketones are important intermediates in chemical synthesis, and alkenes are possible precursors for 1,2-diols, aldehydes, and ketones. Herein, novel and environmentally benign methods for palladium-catalyzed dihydroxylation and oxidative cleavage of olefins with oxygen as sole oxidant are presented. The cleavage reactions were performed with acid as additive in aqueous solution, whereas 1,2-diols were formed in the presence of base. A broad substrate scope has been demonstrated allowing monosubstituted aromatic and aliphatic terminal alkenes, 1,2-disubstituted, and 1,1-disubstituted olefins. The cleavage reactions of dioxo-Pd(II) complexes implicate 1,2-diol might act as a key intermediate of olefin cleavage.

Allylic and allenic halide synthesis via NbCl(5)- and NbBr(5)-mediated alkoxide rearrangements

Addition of NbCl(5) or NbBr(5) to a series of magnesium, lithium, or potassium allylic or propargylic alkoxides directly provides allylic or allenic halides. Halogenation formally occurs through a metalla-halo-[3,3] rearrangement, although concerted, ionic, and direct displacement mechanisms appear to operate competitively. Transposition of the olefin is equally effective for allylic alkoxides prepared by nucleophilic addition, deprotonation, or reduction. Experimentally, the niobium pentahalide halogenations are rapid, afford essentially pure (E)-allylic or -allenic halides after extraction, and are applicable to a range of aliphatic and aromatic alcohols, aldehydes, and ketones.

[3,3]-Sigmatropic rearrangements: recent applications in the total synthesis of natural products

Among the fundamental chemical transformations in organic synthesis, the [3,3]-sigmatropic rearrangement occupies a unique position as a powerful, reliable, and well-defined method for the stereoselective construction of carbon-carbon or carbon-heteroatom bonds. While many other reactions can unite two subunits and create a new bond, the strengths of sigmatropic rearrangements derive from their ability to enable structural reorganization with unmatched build-up of complexity. Recent applications that illustrate [3,3]-sigmatropic processes as a key concept in the synthesis of complex natural products are described in this tutorial review, covering literature from about 2001 through early 2009.