Total Synthesis of (−)-Kendomycin Exploiting a Petasis−Ferrier Rearrangement/Ring-Closing Olefin Metathesis Synthetic Strategy

Deoxygenation of Oxiranes by λ3 σ3 -Phosphorus Reagents: A Computational, Mechanistic, and Stereochemical Study

The deoxygenation of parent and substituted oxiranes by λ3 σ3 -phosphorus reagents has been explored in detail, therefore unveiling mechanistic aspects as well as regio- and stereochemical consequences. Attack to a ring C atom is almost always preferred over one-step deoxygenation by direct P-to-O attack. In most cases a carbene transfer occurs as first step, leading to a phosphorane and a carbonyl unit that thereafter react in the usual Wittig fashion via the corresponding λ5 σ5 -1,2-oxaphosphetane intermediate. Betaines rarely constitute true minima after the first C-attack to oxiranes, at least in the gas-phase. Use of the heavier derivatives AsMe3 and SbMe3 as oxirane deoxygenating reagents was also mechanistically studied. The thermodynamic tendency of λ3 σ3 -phosphorus reagents to act as oxygen (O-attack) or carbene acceptors (C-attack) was theoretically studied by means of the thermodynamic oxygen-transfer potential (TOP) and the newly defined thermodynamic carbene-transfer potential (TCP) parameters, that were explored in a wider context together with many other acceptor centres.

Recent Trends in the Petasis Reaction: A Review of Novel Catalytic Synthetic Approaches with Applications of the Petasis Reaction

The Petasis reaction, also called the Petasis Borono-Mannich reaction, is a multicomponent reaction that couples a carbonyl derivative, an amine and boronic acids to yield substituted amines. The reaction proceeds efficiently in the presence or absence of a specific catalyst and solvent. By employing this reaction, a diverse range of chiral derivatives can easily be obtained, including α-amino acids. A broad substrate scope, high yields, distinct functional group tolerance and the availability of diverse catalytic systems constitute key features of this reaction. In this review article, attention has been drawn toward the recently reported methodologies for executing the Petasis reaction to produce structurally simple to complex aryl/allyl amino scaffolds.

E- and Z-trisubstituted macrocyclic alkenes for natural product synthesis and skeletal editing

Many therapeutic agents are macrocyclic trisubstituted alkenes but preparation of these structures is typically inefficient and non-selective. A possible solution would entail catalytic macrocyclic ring-closing metathesis, but these transformations require high catalyst loading, conformationally rigid precursors and are often low yielding and/or non-stereoselective. Here we introduce a ring-closing metathesis strategy for synthesis of trisubstituted macrocyclic olefins in either stereoisomeric form, regardless of the level of entropic assistance. The goal was achieved by addressing several unexpected difficulties, including complications arising from pre-ring-closing metathesis alkene isomerization. The power of the method is highlighted by two examples. The first is the near-complete reversal of substrate-controlled selectivity in the formation of a macrolactam related to an antifungal natural product. The other is a late-stage stereoselective generation of an E-trisubstituted alkene in a 24-membered ring, en route to the cytotoxic natural product dolabelide C.

Stereoselective Synthesis of Oxacycles via Ruthenium-Catalyzed Atom-Economic Coupling of Propargyl Alcohols and Michael Acceptors

Synthesis of β-hydroxyenones and its application toward development of tetrahydro-4H-pyran-4-one in an atom-economic fashion is limited. This manuscript describes a ruthenium-catalyzed atom-economic coupling of pent-2-yne-1,5-diols and Michael acceptors as an efficient route for the synthesis of β-hydroxyenones with excellent yields and high regioselectivity. The β-hydroxyenones further undergo a 6-endo trig cyclization under acid-catalyzed conditions to deliver the tetrahydro-4H-pyran-4-ones with high diastereoselectivity. An intramolecular aldol condensation under mild basic conditions and palladium-catalyzed oxidative aromatization was developed for the synthesis of hexahydro-6H-isochromen-6-ones and isochromanols, respectively, from highly substituted tetrahydro-4H-pyran-4-ones with excellent yield and diastereoselectivity. Overall, this work demonstrates the synthetic potential toward the synthesis of oxacycles like tetrahydro-4H-pyran-4-ones, hexahydro-6H-isochromen-6-ones, and isochromanols via an atom-economic catalysis.

Carbonylative, Catalytic Deoxygenation of 2,3-Disubstituted Epoxides with Inversion of Stereochemistry: An Alternative Alkene Isomerization Method

Reactions facilitating inversion of alkene stereochemistry are rare, sought-after transformations in the field of modern organic synthesis. Although a number of isomerization reactions exist, most methods require specific, highly activated substrates to achieve appreciable conversion without side product formation. Motivated by stereoinvertive epoxide carbonylation reactions, we developed a two-step epoxidation/deoxygenation process that results in overall inversion of alkene stereochemistry. Unlike most deoxygenation systems, carbon monoxide was used as the terminal reductant, preventing difficult postreaction separations, given the gaseous nature of the resulting carbon dioxide byproduct. Various alkyl-substituted cis- and trans-epoxides can be reduced to trans- and cis-alkenes, respectively, in >99:1 stereospecificity and up to 95% yield, providing an alternative to traditional, direct isomerization approaches.

The Evans Aldol–Prins cyclization: a general and stereoselective method for the synthesis of 2,3,4,5,6-pentasubstituted tetrahydropyrans

A method for the synthesis of 2,3,4,5,6-pentasubstituted THPs with good yields and diastereoselectivities through an Evans Aldol–Prins strategy is described.

Formation of highly substituted tetrahydropyranones: application to the total synthesis of cyanolide A

A new tetrahydropyranone synthesis has been developed that leads to cis-2,6-disubstituted 3,3-dimethyltetrahydropyran-4-one rings by condensation of an aldehyde and a hydroxy silyl enol ether. The reaction works with a variety of aldehydes to produce the tetrahydropyranone products in moderate to high yields. This new method was applied to the enantioselective synthesis of cyanolide A and its aglycone.

DFT study on the mechanism and stereochemistry of the Petasis-Ferrier rearrangements

The Petasis-Ferrier rearrangement is a very important and useful reaction for the synthesis of multifunctional tetrahydrofurans and tetrahydropyrans from easily synthesized enol acetals. Here we report our DFT investigation of the detailed reaction mechanism of the Petasis-Ferrier rearrangement, proposing that the active promoting species in this reaction is the cationic aluminum species, instead of the usually considered neutral Lewis acid (this will give very high activation energies and cannot explain why the Petasis-Ferrier rearrangements usually take place at low temperature or under mild conditions). Calculations indicated that the mechanisms of the Petasis-Ferrier rearrangements for the formations of five- and six-membered rings are different. Formation of five-membered tetrahydrofuranone is stepwise with C-O bond cleavage to generate an oxocarbenium enolate intermediate, which then undergoes an aldol-type reaction to give the desired cyclized oxacycle. In contrast, the formation of six-membered tetrahydropyranone is a concerted and asynchronous process with the C-O bond breakage and aldol-type C-C bond formation occurring simultaneously. A DFT understanding of why the catalytic versions of the Petasis-Ferrier rearrangements cannot be realized when using R2Al(+) as the active promoting species has also been discussed. In addition, DFT calculations were used to reveal the origins of the stereochemistry observed in the Petasis-Ferrier rearrangements.

Concise formal synthesis of (+)-neopeltolide

A concise formal synthesis of (+)-neopeltolide (1) has been accomplished. The synthesis demonstrated high atom efficiency employing only one step of functional group protection. Key steps involved iridium-catalyzed double asymmetric carbonyl allylation, palladium-catalyzed intramolecular alkoxycarbonylation, ruthenium-catalyzed olefin isomerization, and ring-closing metathesis.

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.

Ring-closing metathesis and photo-fries reaction for the construction of the ansamycin antibiotic kendomycin: development of a protecting group free oxidative endgame

Two convergent total syntheses of the ansa-polyketide (-)-kendomycin (1) are described. The syntheses benefit from the use of readily available and cheap starting materials. Highly complex diastereoselective Claisen-Ireland rearrangements were used to introduce the (E)-double bond and the C16-Me group. The ring closure of the strained ansa macrocycle was achieved by ring-closing metathesis and a highly efficient combination of macrolactonization and photo-Fries reaction. A protecting group free endgame via an unstable o-quinone is presented. Additionally some unsuccessful synthetic efforts towards the total synthesis of 1 are described.

Evolution of the total synthesis of (-)-okilactomycin exploiting a tandem oxy-cope rearrangement/oxidation, a Petasis-Ferrier union/rearrangement, and ring-closing metathesis

An effective, asymmetric total synthesis of the antitumor antibiotic (-)-okilactomycin (1), as well as assignment of the absolute configuration, has been achieved exploiting a convergent strategy. Highlights of the synthesis include a diastereoselective oxy-Cope rearrangement/oxidation sequence to install the C(1) and C(13) stereogenic centers, a Petasis-Ferrier union/rearrangement to construct the highly functionalized tetrahydropyranone inscribed within the 13-membered macrocycle ring, employing for the first time a sterically demanding acetal, an intramolecular chemoselective acylation to access an embedded bicyclic lactone, and an efficient ring-closing metathesis (RCM) reaction to generate the macrocyclic ring.

Total synthesis of (-)-kendomycin

An enantioselective synthesis of (-)-kendomycin is described and is based on the application of the organosilane-based [4 + 2]-annulation strategy for the assembly of the C1a-C10 fragment. An underutilized samarium(II) iodide-assisted cyclization (intramolecular Barbier-type reaction) is employed to afford the protected macrocycle.

Evolution of the Petasis-Ferrier union/rearrangement tactic: construction of architecturally complex natural products possessing the ubiquitous cis-2,6-substituted tetrahydropyran structural element

The frequent low abundance of architecturally complex natural products possessing significant bioregulatory properties mandates the development of rapid, efficient, and stereocontrolled synthetic tactics, not only to provide access to the biologically rare target but also to enable elaboration of analogues for the development of new therapeutic agents with improved activities and/or pharmacokinetic properties. In this Account, the genesis and evolution of the Petasis-Ferrier union/rearrangement tactic, in the context of natural product total syntheses, is described. The reaction sequence comprises a powerful tactic for the construction of the 2,6- cis-substituted tetrahydropyran ring system, a ubiquitous structural element often found in complex natural products possessing significant bioactivities. The three-step sequence, developed in our laboratory, extends two independent methods introduced by Ferrier and Petasis and now comprises: condensation between a chiral, nonracemic beta-hydroxy acid and an aldehyde to furnish a dioxanone; carbonyl olefination; and Lewis-acid-induced rearrangement of the resultant enol acetal to generate the 2,6- cis-substituted tetrahydropyranone system in a highly stereocontrolled fashion. To demonstrate the envisioned versatility and robustness of the Petasis-Ferrier union/rearrangement tactic in complex molecule synthesis, we exploited the method as the cornerstone in our now successful total syntheses of (+)-phorboxazole A, (+)-zampanolide, (+)-dactylolide, (+)-spongistatins 1 and 2, (-)-kendomycin, (-)-clavosolide A, and most recently, (-)-okilactomycin. Although each target comprises a number of synthetic challenges, this Account focuses on the motivation, excitement, and frustrations associated with the evolution and implementation of the Petasis-Ferrier union/rearrangement tactic. For example, during our (+)-phorboxazole A endeavor, we recognized and exploited the inherent pseudo symmetry of the 2,6- cis-substituted tetrahydropyranone product to overcome the inherent chelation bias of an adjacent oxazolidine ring during the Lewis-acid-promoted rearrangement. In addition, we discovered that a more concentrated solution of Cp2TiMe2 (0.7 versus 0.5 M in THF) with the addition of ethyl pivalate dramatically improves the yield in the Petasis-Tebbe olefination. During the (+)-zampanolide and (+)-dactylolide programs, we observed that the addition of trifluoromethanesulfonic acid (TfOH), especially on a preparative scale, was crucial to the efficiency of the initial condensation/union reaction, while our efforts toward (-)-kendomycin led to the improved implementation of a modified Kurihara condensation of the beta-hydroxy acid and aldehyde involving i-PrOTMS and TMSOTf. Finally, the successful deployment of the Petasis-Ferrier tactic in our synthesis of (-)-clavosolide A validated the viability of this tactic with a system possessing the highly acid-labile cyclopropylcarbinyl moiety, while the challenges en route to (-)-okilactomycin demonstrated that a neighboring alkene functionality can participate in an intramolecular Prins cyclization during the TMSOTf-promoted union process, unless suitably protected.

The [1, 3] O-to-C rearrangement: opportunities for stereoselective synthesis

The relay of stereochemistry of a breaking C-O bond into a forming C-C bond is well-known in the context of [3, 3] sigmatropic shifts; however, this useful strategy is less well-known in other types of molecular rearrangements. Though the first successful example of a [1, 3] O-to-C rearrangement was reported more than 100 years ago, this class of reactions has received less attention than its [3, 3] counterpart. This perspective analyzes the various methods used for the activation and [1, 3] rearrangement of vinyl ethers with an emphasis on mechanism and applications to stereoselective synthesis. We also highlight our own contributions to this area.

Synthesis of bicyclic alkene-/alkane-bridged nisin mimics by ring-closing metathesis and their biochemical evaluation as lipid II binders: toward the design of potential novel antibiotics

This report describes the design, synthesis, and biochemical evaluation of alkene- and alkane-bridged AB(C)-ring mimics of the lantibiotic nisin. Nisin belongs to a class of natural antimicrobial peptides, and has a unique mode of action: its AB(C)-ring system binds to the pyrophosphate moiety of lipid II. This mode of action was the rationale for the design of smaller nisin-derived peptides to obtain novel potential antibiotics. As a conformational constraint the thioether bridge was mimicked by an alkene- or alkane isostere. The peptides of the linear individual ring precursors were synthesized on solid support or in solution, and cyclized by ring-closing metathesis in solution with overall yields of between 36 and 89 %. The individual alkene-bridged macrocycles were assembled in solution by using carbodiimide-based synthesis protocols for the corresponding AB(C)-ring mimics. These compounds were tested for their binding affinity toward lipid II by evaluation of their potency to inhibit nisin-induced carboxyfluorescein release from large unilamellar vesicles. It was found that these AB(C)-ring mimics were not able to induce membrane leakage; however, they acted by inhibiting nisin-induced carboxyfluorescein release; this indicates their affinity toward lipid II. These results imply that an alkene or alkane moiety is a suitable thioether bridge mimic.