Asymmetric total synthesis of the epoxykinamycin FL-120 B'

Stereocontrolled Synthesis of the Fully Glycosylated Monomeric Unit of Lomaiviticin A

We describe a stereocontrolled synthesis of 3, the fully glycosylated monomeric unit of the dimeric cytotoxic bacterial metabolite (-)-lomaiviticin A (2). A novel strategy involving convergent, site- and stereoselective coupling of the β,γ-unsaturated ketone 6 and the naphthyl bromide 7 (92%, 15:1 diastereomeric ratio (dr)), followed by radical-based annulation and silyl ether cleavage, provided the tetracycle 5 (57% overall), which contains the carbon skeleton of the aglycon of 3. The β-linked 2,4,6-trideoxy-4-aminoglycoside l-pyrrolosamine was installed in 73% yield and with 15:1 β:α selectivity using a modified Koenigs-Knorr glycosylation. The diazo substituent was introduced via direct diazo transfer to an electron-rich benzoindene (4 → 27). The α-linked 2,6-dideoxyglycoside l-oleandrose was introduced by gold-catalyzed activation of an o-alkynyl glycosylbenzoate (75%, >20:1 α:β selectivity). A carefully orchestrated endgame sequence then provided efficient access to 3. Cell viability studies indicated that monomer 3 is not cytotoxic at concentrations up to 1 μM, providing conclusive evidence that the dimeric structure of (-)-lomaiviticin A (2) is required for cytotoxic effects. The preparation of 3 provides a foundation to complete the synthesis of (-)-lomaiviticin A (2) itself.

Metric-Based Analysis of Convergence in Complex Molecule Synthesis

Convergent syntheses are characterized by the coupling of two or more synthetic intermediates of similar complexity, often late in a pathway. At its limit, a fully convergent synthesis is achieved when commercial or otherwise readily available intermediates are coupled to form the final target in a single step. Of course, in all but exceptional circumstances this level of convergence is purely hypothetical; in practice, additional steps are typically required to progress from fragment coupling to the target. Additionally, the length of the sequence required to access each target is a primary consideration in synthetic design.In this Account, we provide an overview of alkaloid, polyketide, and diterpene metabolites synthesized in our laboratory and present parameters that may be used to put the degree of convergence of each synthesis on quantitative footing. We begin with our syntheses of the antiproliferative, antimicrobial bacterial metabolite (-)-kinamycin F (1) and related dimeric structure (-)-lomaiviticin aglycon (2). These synthetic routes featured a three-step sequence to construct a complex diazocyclopentadiene found in both targets and an oxidative dimerization to unite the two halves of (-)-lomaiviticin aglycon (2). We then follow with our synthesis of the antineurodegenerative alkaloid (-)-huperzine A (3). Our route to (-)-huperzine A (3) employed a diastereoselective three-component coupling reaction, followed by the intramolecular α-arylation of a β-ketonitrile intermediate, to form the carbon skeleton of the target. We then present our syntheses of the hasubanan alkaloids (-)-hasubanonine (4), (-)-delavayine (5), (-)-runanine (6), (+)-periglaucine B (7), and (-)-acutumine (8). These alkaloids bear a 7-azatricyclo[4.3.3.0^1,6]dodecane (propellane) core and a highly oxidized cyclohexenone ring. The propellane structure was assembled by the addition of an aryl acetylide to a complex iminium ion, followed by intramolecular 1,4-addition. We then present our synthesis of the guanidinium alkaloid (+)-batzelladine B (9), which contains two complex polycyclic guanidine residues united by an ester linkage. This target was logically disconnected by an esterification to allow for the independent synthesis of each guanidine residue. A carefully orchestrated cascade reaction provided (+)-batzelladine B (9) in a single step following fragment coupling by esterification. We then discuss our synthesis of the diterpene fungal metabolite (+)-pleuromutilin (10). The synthesis of (+)-pleuromutilin (10) proceeded via a fragment coupling involving two neopentylic reagents and employed a nickel-catalyzed reductive cyclization reaction to close the eight-membered ring, ultimately providing access to (+)-pleuromutilin (10), (+)-12-epi-pleuromutilin (11), and (+)-12-epi-mutilin (12). Finally, we discuss our synthesis of (-)-myrocin G (13), a tricyclic pimarane diterpene that was assembled by a convergent annulation.In the final section of this Account, we present several paramaters to analyze and quantitatively assess the degree of convergence of each synthesis. These parameters include: (1) the number of steps required following the point of convergence, (2) the difference in the number of steps required to prepare each coupling partner, (3) the percentage of carbons (or, more broadly, atoms) present at the point of convergence, and (4) the complexity generated in the fragment coupling step. While not an exhaustive list, these parameters bring the strengths and weaknesses each synthetic strategy to light, emphasizing the key contributors to the degree of convergence of each route while also highlighting the nuances of these analyses.

Synthesis and Biological Evaluation of (2S,2'S)-Lomaiviticin A

(-)-Lomaiviticin A (1) is a genotoxic C2-symmetric metabolite that arises from the formal dimerization of two bis(glycosylated) diazotetrahydrobenzo[b]fluorenes. Here we present a synthesis of the monomer 17 and its coupling to form (2S,2'S)-lomaiviticin A (4), an unnatural diastereomer of 1. (2S,2'S)-Lomaiviticin A (4) is significantly less genotoxic, a result we attribute to changes in the orientation of the diazofluorene and carbohydrate residues, relative to 1. These data bring the importance of the configuration of the conjoining bond to light and place the total synthesis of 1 itself within reach.

Synthesis of the bis(cyclohexenone) core of (-)-lomaiviticin A

(-)-Lomaiviticin A is a complex C ₂-symmetric bacterial metabolite comprising two diazotetrahydrobenzo[b]fluorene (diazofluorene) residues and four 2,6-dideoxy glycosides, α-l-oleandrose and N,N-dimethyl-β-l-pyrrolosamine. The two halves of lomaiviticin A are linked by a single carbon-carbon bond oriented syn with respect to the oleandrose residues. While many advances toward the synthesis of lomaiviticin A have been reported, including synthesis of the aglycon, a route to the bis(cyclohexenone) core bearing any of the carbohydrate residues has not been disclosed. Here we describe a short route to a core structure of lomaiviticin A bearing two α-l-oleandrose residues. The synthetic route features a Stille coupling to form the conjoining carbon-carbon bond of the target and a double reductive transposition to establish the correct stereochemistry at this bond. Two synthetic routes were developed to elaborate the reductive transposition product to the bis(cyclohexenone) target. The more efficient pathway features an interrupted Barton vinyl iodide synthesis followed by oxidative elimination of iodide to efficiently establish the enone functionalities in the target. The bis(cyclohexenone) product may find use in a synthesis of lomaiviticin A itself.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Polyphosphazenes for the Stille reaction: a new type of recyclable stannyl reagent

A random phosphazene copolymer {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}n (2) and a block copolyphosphazene {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}n (3) and {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (6, An = p-MeOC6H4). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with ArI, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without a significant release of tin, but its efficiency decreased after three consecutive cycles. This effect was explained by studying the self-assembly of the polymer under the same conditions used for the catalytic experiments, which evidenced the progressive coalescence of the polymeric vesicles (polymersomes) leading to stable and bigger core-shell aggregates by the attraction of the [N = P(O2C12H8)] rich membranes, thus decreasing the accessibility of the tin active centers.

A photo-induced C–C bond formation methodology to construct tetrahydrofluorenones and their related structures

A metal-free, photo-induced C–C bond formation methodology was developed to construct tetrahydrofluorenones and their related structures.

Development of enantioselective synthetic routes to (-)-kinamycin F and (-)-lomaiviticin aglycon

The development of enantioselective synthetic routes to (-)-kinamycin F (9) and (-)-lomaiviticin aglycon (6) are described. The diazotetrahydrobenzo[b]fluorene (diazofluorene) functional group of the targets was prepared by fluoride-mediated coupling of a β-trimethylsilylmethyl-α,β-unsaturated ketone (38) with an oxidized naphthoquinone (19), palladium-catalyzed cyclization (39→37), and diazo transfer (37→53). The D-ring precursors 60 and 68 were prepared from m-cresol and 3-ethylphenol, respectively. Coupling of the β-trimethylsilylmethyl-α,β-unsaturated ketone 60 with the juglone derivative 61, cyclization, and diazo transfer provided the advanced diazofluorene 63, which was elaborated to (-)-kinamycin F (9) in three steps. The diazofluorene 87 was converted to the C(2)-symmetric lomaiviticin aglycon precursor 91 by enoxysilane formation and oxidative dimerization with manganese tris(hexafluoroacetylacetonate) (94, 26%). The stereochemical outcome in the coupling is attributed to the steric bias engendered by the mesityl acetal of 87 and contact ion pairing of the intermediates. The coupling product 91 was deprotected (tert-butylhydrogen peroxide, trifluoroacetic acid-dichloromethane) to form mixtures of the chain isomer of lomaiviticin aglycon 98 and the ring isomer 6. These mixtures converged on purification or standing to the ring isomer 6 (39-41% overall). The scope of the fluoride-mediated coupling process is delineated (nine products, average yield = 72%); a related enoxysilane quinonylation reaction is also described (10 products, average yield = 77%). We establish that dimeric diazofluorenes undergo hydrodediazotization 2-fold faster than related monomeric diazofluorenes. This enhanced reactivity may underlie the cytotoxic effects of (-)-lomaiviticin A (1). The simple diazofluorene 103 is a potent inhibitor of ovarian cancer stem cells (IC(50) = 500 nM).

Studies toward the synthesis of the epoxykinamycin FL-120B': discovery of a decarbonylative photocyclization

Photo-Friedel-Crafts acylation of a naphthoquinone was attempted in an effort to access a diazobenzofluorenone en route to the epoxykinamycin natural product FL-120B'. Photoirradiation of the naphthoquinone substrate which resulted in the unexpected formation of a tetracyclic naphthofuran via a decarbonylative photocyclization process is described.