Ruthenium-catalyzed enyne cycloisomerizations. Effect of allylic silyl ether on regioselectivity

Propargylic Dialkyl Effect for Cyclobutene Formation through Ir(III)-Catalyzed Cycloisomerization of 1,6-Enynes

The propargylic dialkyl effect (PDAE) has a significant impact on the cyclization reaction of enynes, partly reflected in changing the types of products. Herein, we described the influence of the propargylic dialkyl effect on the Ir(III)-catalyzed cycloisomerization of 1,6-enynes to provide strained cyclobutenes. A series of substituted 1,6-enynes were proved to be excellent substrate candidates in the presence of [Cp*IrCl2]2 in toluene. Mechanistic investigation, based on deuterium labeling experiments and control experiments, indicated that the propargylic dialkyl effect might boost C(sp)-H activation by preventing the coordination of active iridium species to the C(sp)≡C(sp) bond of enynes. This finding contributes to the fundamental understanding of enyne cyclization reactions and offers valuable insight into the propargylic dialkyl effect.

Rh-Catalyzed Borylative Cyclization of Acrylate-Containing 1,6-Enynes

A Rh-catalyzed regioselective, stereoselective carbocyclization/borylation of acrylate-containing 1,6-enynes was described, which offers a general and practical method for constructing versatile and densely functionalized pyrrolidines with the (Z) geometry or the uncommon (E) geometry at the double bond with different substituents of the alkyne, with sterically hindered and conjugative aryl groups favoring the latter, featuring a tetrasubstituted vinyl boronate, containing an all-carbon quaternary stereocenter.

Highly Enantioselective Rhodium(I)-Catalyzed Alder-ene-type Cycloisomerization of 1,7-Enynes

The transition-metal-catalyzed asymmetric cycloisomerization of 1,7-enynes is regarded as a formidable challenge due to the poor ability of 1,7-enynes to serve as bidentate ligands to metal. In this Letter, a highly enantioselective rhodium(I)-catalyzed Alder-ene-type cycloisomerization of 1,7-enynes is disclosed, offering an efficient method for the synthesis of a wide range of fused six-membered cyclic compounds. Furthermore, a high turnover frequency experiment and deuterium-labeling experiment were performed to give insight into this transformation.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Utilization of the Thorpe-Ingold Effect in the Synthesis of Cyclooctanoid Ring Systems via Anionic 6-exo-dig Cyclization/Claisen Rearrangement Sequence

We demonstrate a facile approach for the synthesis of gem-disubstituted cyclooctanoids, a motif found in several biologically active compounds. Appropriately substituted 1-alkenyl-5-pentyn-1-ols bearing gem-dialkyl substituents at either the C2, C3, or C4 position serve as useful precursors to a number of cyclooct-4-enone derivatives via a tandem, microwave-assisted oxyanionic 6-exo-dig cyclization/Claisen rearrangement reaction. gem-Dialkyl activation is necessary for these reactions to occur, as unactivated 1-alkenyl-5-pentyn-1-ols fail to undergo 6-exo-dig cyclization under the conditions employed. Further application of the methodology to the corresponding gem-dialkoxy system was also explored to facilitate access to more complex carbocycles.

Metal-catalyzed enyne cycloisomerization in natural product total synthesis

Enyne cycloisomerization has become a powerful and attractive strategy for the construction of cyclic compounds, thus possessing great potential for applications in total synthesis of natural products and pharmaceuticals.

Enantioselective Rhodium-Catalyzed Cycloisomerization of (E)-1,6-Enynes

An enantioselective rhodium(I)-catalyzed cycloisomerization reaction of challenging (E)-1,6-enynes is reported. This novel process enables (E)-1,6-enynes with a wide range of functionalities, including nitrogen-, oxygen-, and carbon-tethered (E)-1,6-enynes, to undergo cycloisomerization with excellent enantioselectivity, in a high-yielding and operationally simple manner. Moreover, this Rh(I) -diphosphane catalytic system also exhibited superior reactivity and enantioselectivity for (Z)-1,6-enynes. A rationale for the striking reactivity difference between (E)- and (Z)-1,6-enynes using Rh(I) -BINAP and Rh(I) -TangPhos is outlined using DFT studies to provide the necessary insight for the design of new catalyst systems and the application to synthesis.

A Review on Platensimycin: A Selective FabF Inhibitor

Emerging resistance to existing antibiotics is an inevitable matter of concern in the treatment of bacterial infection. Naturally occurring unique class of natural antibiotic, platensimycin, a secondary metabolite from Streptomyces platensis, is an excellent breakthrough in recent antibiotic research with unique structural pattern and significant antibacterial activity. β-Ketoacyl-(acyl-carrier-protein (ACP)) synthase (FabF) whose Gram-positive bacteria need to biosynthesize cell membranes is the target of inhibition of platensimycin. So, isolation, retrosynthetic analysis, synthesis of platensimycin, and analogues of platensimycin synthesized till today are the objectives of this review which may be helpful to further investigate and to reveal untouched area on this molecule and to obtain a potential antibacterial lead with enhanced significant antibacterial activity.

[Cp*Ru]-catalyzed selective coupling/hydrogenation

Selective coupling and hydrogenation catalyzed by [Cp*Ru] have been achieved affording valuable polyfunctionalized cyclic enamide derivatives.

Catalytic asymmetric synthesis of (+)-anthecotulide using enyne and Meyer-Schuster rearrangements

The bioactive sesquiterpene lactone (+)-anthecotulide (1) is synthesized for the first time, in a six-step sequence devoid of protecting groups. The key transformations are a novel Rh(I)-catalyzed asymmetric enyne rearrangement of a terminal alkynyl ester (4), to form the α-methylene-γ-butyrolactone core, and a final-step mild Au(I)-catalyzed Meyer-Schuster rearrangement.

Discovery and syntheses of "superbug challengers"-platensimycin and platencin

Bacteria have developed resistance to almost all existing antibiotics known today and this has been a major issue over the last few decades. The search for a new class of antibiotics with a new mode of action to fight these multiply-drug-resistant strains, or "superbugs", allowed a team of scientists at Merck to discover two novel antibiotics, platensimycin and platencin using advanced screening strategies, as inhibitors of bacterial fatty acid biosynthesis, which is essential for the survival of bacteria. Though both these antibiotics are structurally related, they work by slightly different mechanisms and target different enzymes conserved in the bacterial fatty acid biosynthesis. This Focus Review summarizes the synthetic and biological aspects of these natural products and their analogues and congeners.

Total synthesis of platensimycin and related natural products

Platensimycin is the flagship member of a new and growing class of antibiotics with promising antibacterial properties against drug-resistant bacteria. The total syntheses of platensimycin and its congeners, platensimycins B(1) and B(3), platensic acid, methyl platensinoate, platensimide A, homoplatensimide A, and homoplatensimide A methyl ester, are described. The convergent strategy developed toward these target molecules involved construction of their cage-like core followed by attachment of the various side chains through amide bond formation. In addition to a racemic synthesis, two asymmetric routes to the core structure are described: one exploiting a rhodium-catalyzed asymmetric cycloisomerization, and another employing a hypervalent iodine-mediated de-aromatizing cyclization of an enantiopure substrate. The final two bonds of the core structure were forged through a samarium diiodide-mediated ketyl radical cyclization and an acid-catalyzed etherification. The rhodium-catalyzed asymmetric reaction involving a terminal acetylene was developed as a general method for the asymmetric cycloisomerization of terminal enynes.

Recent advances in the chemistry and biology of naturally occurring antibiotics

Ever since the world-shaping discovery of penicillin, nature's molecular diversity has been extensively screened for new medications and lead compounds in drug discovery. The search for agents intended to combat infectious diseases has been of particular interest and has enjoyed a high degree of success. Indeed, the history of antibiotics is marked with impressive discoveries and drug-development stories, the overwhelming majority of which have their origin in natural products. Chemistry, and in particular chemical synthesis, has played a major role in bringing naturally occurring antibiotics and their derivatives to the clinic, and no doubt these disciplines will continue to be key enabling technologies. In this review article, we highlight a number of recent discoveries and advances in the chemistry, biology, and medicine of naturally occurring antibiotics, with particular emphasis on total synthesis, analogue design, and biological evaluation of molecules with novel mechanisms of action.

Selective Ruthenium-Catalyzed Transformations of Enynes with Diazoalkanes into Alkenylbicyclo[3.1.0]hexanes

Reaction of a variety of CCH bond-containing 1,6-enynes with N2CHSiMe3 in the presence of RuCl(COD)Cp* as catalyst precursor leads, at room temperature, to the general formation of alkenylbicyclo[3.1.0]hexanes with high Z-stereoselectivity of the alkenyl group and cis arrangement of the alkenyl group and an initial double-bond substituent, for an E-configuration of this double bond. The stereochemistry is established by determining the X-ray structures of three bicyclic products. The same reaction with 1,6-enynes bearing an R substituent on the C1 carbon of the triple bond results in either cyclopropanation of the double bond with bulky R groups (SiMe3, Ph) or formation of alkylidene-alkenyl five-membered heterocycles, resulting from a beta elimination process, with less bulky R groups (R = Me, CH2CH=CH2). The reaction can be applied to in situ desilylation in methanol and direct formation of vinylbicyclo[3.1.0]hexanes and to the formation of some alkenylbicyclo[4.1.0]heptanes from 1,7-enynes. The catalytic formation of alkenylbicyclo[3.1.0]hexanes also takes place with enynes and N2CHCO2Et or N2CHPh. The reaction can be understood to proceed by an initial [2+2] addition of the Ru=CHSiMe3 bond with the enyne CCH bond, successively leading to an alkenylruthenium-carbene and a key alkenyl bicyclic ruthenacyclobutane, which promotes the cyclopropanation, rather than metathesis, into bicyclo[3.1.0]hexanes. Density functional theory calculations performed starting from the model system Ru(HCCH)(CH2=CH2)Cl(C5H5) show that the transformation into a ruthenacyclobutane intermediate occurs with a temporary eta3-coordination of the cyclopentadienyl ligand. This step is followed by coordination of the alkenyl group, which leads to a mixed alkyl-allyl ligand. Because of the non-equivalence of the terminal allylic carbon atoms, their coupling favors cyclopropanation rather than the expected metathesis process. A direct comparison of the energy profiles with respect to those involving the Grubbs catalyst is presented, showing that cyclopropanation is favored with respect to enyne metathesis.

Gold-catalyzed cyclo-isomerization of 1,6-diyne-4-en-3-ols to form naphthyl ketone derivatives

We report a new efficient gold-catalyzed cyclization of 1,6-diyne-4-en-3-ols to give naphthyl ketone derivatives under ambient conditions. The value of this cyclization is reflected by its applicability to a wide range of alcohol substrates.

Ruthenium-catalyzed reactions--a treasure trove of atom-economic transformations

The demand for new chemicals spanning the fields of health care to materials science combined with the pressure to produce these substances in an environmentally benign fashion pose great challenges to the synthetic chemical community. The maximization of synthetic efficiency by the conversion of simple building blocks into complex targets remains a fundamental goal. In this context, ruthenium complexes catalyze a number of non-metathesis conversions and allow the rapid assembly of complex molecules with high selectivity and atom economy. These complexes often exhibit unusual reactivity. Careful consideration of the mechanistic underpinnings of the transformations can lead to the design of new reactions and the discovery of new reactivity.

Sequential Ru-Pd catalysis: a two-catalyst one-pot protocol for the synthesis of N- and O-heterocycles

An atom economic, selective, and highly practical two-metal one-pot synthesis of heterocycles has been developed that efficiently affords enantio- and diastereopure N- and O-heterocyclic products. Furthermore, use of a chiral catalyst in the two-metal procedure allows formation of all possible diastereomers, even those that are traditionally difficult to access via cyclization routes due to thermodynamics. Interestingly, the nature of the enantiodiscriminating event differs between the use of amine versus alcohol nucleophiles. The method also affords heterocyclic products that are synthetically useful intermediates. Through the Z-vinylsilane a variety of stereodefined trisubstituted olefin products can be accessed including several all-carbon motifs. Finally, the utility of these heterocyclic products in total synthesis is demonstrated through concise syntheses of a kainoid intermediate, a constituent of oil of rose, and the ring B portion of bryostatin, a potent chemotherapeutic.

Catalytic Cycloisomerization of Enynes by Using a Nickel-Zinc-Acid System

Catalytic cycloisomerization of enynes has been accomplished in the presence of an Ni0-PPh3-Zn-carboxylic acid or -ZnCl2 system. A nickel(I)-hydride complex, thought to be generated by reduction of the protonated nickel(II) complex with Zn, is proposed as the catalytic species. This cycloisomerization shows reactivity behavior that is different from that of a conventional metal-catalyzed reaction. In particular, in the reaction with (E)-enynes, the catalytic system has a selectivity that favors the formation of the 1,3-diene over the 1,4-diene. In addition, this catalytic system has been applied to the domino cyclization of dienynes for the construction of tricyclic compounds.