Regiochemical control in the metal-catalyzed transposition of allylic silyl ethers

Iridium-Catalyzed 1,3-Rearrangement of Allylic Ethers

The 1,3-rearrangement of allylic derivatives has rarely been reported, except for allylic alcohols. Herein, we describe an iridium-catalyzed 1,3-rearrangement of readily available allylic ethers to access the difficultly prepared allylic ethers with a large steric hindrance. The developed method shows a broad substrate scope and could be used in the late-stage modification of several natural products. In addition, a possible reaction pathway is also provided on the basis of the control experiments.

Stereoselective syntheses of 2-methyl-1,3-diol acetals via Re-catalyzed [1,3]-allylic alcohol transposition

Rhenium-catalyzed stereoselective transposition of allylic alcohols is reported. In the presence of 1 mol% of Re2O7, (E)- or (Z)-δ-hydroxymethyl-anti-homoallylic alcohols were converted into the acetals of 2-methyl-1,3-syn-diols with excellent diastereoselectivities. 1,3-syn-Diol acetals can also be synthesized from (E)-δ-hydroxymethyl-syn-homoallylic alcohols.

Re2O7/HReO4 Mediated Intramolecular Hydroacyloxylation of Unactivated Alkenes: A Dual Hydrogen-Bonding Effect

This publication describes the application of Re₂O₇ in hexafluoroisopropanol (HFIP) for the activation of inert as well as electronically deactivated olefins to facilitate a challenging intramolecular hydroacyloxylation reaction. Both HFIP and an internal carboxy group have been proven to be crucial for the successful implementation of this transformation; these are proposed to assist the formation and stabilization of the key cationic intermediate via hydrogen-bonding interactions with perrhenate anion (ReO₄^-).

Iridium-Catalyzed 1,3-Rearrangement of Allylic Alcohols

The allylic alcohol structural motif is prevalent in many important molecules and valuable building blocks. The rearrangement reaction is one of the most important transformations, however there are only a few reports for the 1,3-rearrangement of allylic alcohols. Herein, a 1,3-rearrangement of allylic alcohols catalyzed by an Ir(III) dihydride complex is described. This reaction could provide the corresponding less accessible allylic alcohols regio- and stereoselectively from readily available E/Z mixtures of the substrates. Furthermore, a tandem alkene isomerization followed by 1,3-rearrangement of homoallylic alcohols was also realized. In addition, this rearrangement reaction could be used to synthesize the natural product Navenone B. Mechanistic investigation indicated that the reaction pathway involved a π-allyl-Ir(V) intermediate and that the dihydride in the iridium catalyst acts as a hydrogen switch to modulate the valence of the iridium center.

Palladium-Catalyzed Borylative Cyclizations of α-(2-Bromoaryl) Ketones to Form 1,2-Benzoxaborinines

Herein, we report that palladium catalyzes the borylative cyclization of α-(2-bromoaryl) ketones to afford 1,2-benzoxaborinines. The developed system is compatible with a variety of functionalities (Me, t-Bu, OMe, NMe₂, F, Cl, CN, CF₃, CO₂Me, and heteroaryl groups) and is applicable to the synthesis of B-O-containing tri- and tetracyclic fused-ring compounds.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

Synthesis and reactivity of alkynyl boron compounds

Over the last century, there have been considerable developments in organoboron chemistry due to the stability, non-toxicity, and easy commercial availability of various boronic esters. Several organoboron reagents have emerged and play an increasingly important role in everyday organic synthesis. Among them, alkynyl boron compounds have attracted significant attention due to their easy synthesis and diverse reactivity. In this review, we summarize the advancement of research on alkynyl boron compounds, highlighting their importance in the synthesis of valuable compounds.

Perrhenate Esters as Intermediates in Molecular Complexity-Increasing Reactions

Allylic alcohols form perrhenate esters upon reaction with Re2O7 or HOReO3. These species undergo nonstereospecific and nonregiospecific alcohol-transposition reactions through cationic intermediates. Sequencing these nonselective processes with reversible trapping by electrophiles results in cyclization reactions where regio- and stereocontrol are dictated by thermodynamics. The cationic intermediates can also be utilized as electrophiles in intra- or intermolecular dehydrative reactions with nucleophiles. These processes serve as the basis for applications in catalytic syntheses of a wide range of heterocyclic and carbocyclic structures that often show considerable increases in molecular complexity. This Account describes a sequence of events that started from a need to solve a problem for the completion of a natural product synthesis and evolved into a central element in the design of numerous new transformations that proceed under mild conditions from readily accessible substrates.

1 Introduction

2 Exploratory Studies

3 Application to Spiroketal Synthesis

4 Reactions with Epoxides as Trapping Agents

5 Development of Dehydrative Cyclizations

6 Bimolecular Reactions

7 Spirocyclic Ether Formation

8 Conclusions

A nickel(ii)-catalyzed asymmetric intramolecular Alder-ene reaction of 1,7-dienes

A highly diastereo- and enantioselective intramolecular Alder-ene reaction with an alkene as the enophile has been developed by using a chiral N,N'-dioxide/nickel(ii) complex as the catalyst. This protocol provides a facile route towards the synthesis of diverse 3,4-disubstituted chroman, tetrahydroquinoline, piperidine and thiochroman derivatives in high yields with good to excellent diastereo- and enantioselectivities.

Diarylmethane synthesis through Re2O7-catalyzed bimolecular dehydrative Friedel-Crafts reactions

This manuscript describes the application of Re2O7 to the syntheses of diarylmethanes from benzylic alcohols through solvolysis followed by Friedel-Crafts alkylation. The reactions are characterized by broad substrate scope, low catalyst loadings, high chemical yields, and minimal waste generation. The intermediate perrhenate esters are superior leaving groups to chlorides and bromides in these reactions. The polarity and water sequestering capacity of hexafluoroisopropyl alcohol are critical to the success of these processes. Re2O7 is a precatalyst for HOReO3, which serves as a less costly and easily handled promoter for these reactions. Oxorhenium catalysts selectively activate alcohols in the presence of similarly substituted acetates, indicating a unique chemoselectivity and mechanism in comparison to Brønsted acid catalysis.

Selective Cleavage of C-O Bonds in Lignin Catalyzed by Rhenium(VII) Oxide (Re2 O7 )

The selective cleavage of C-O bonds in typical model lignin β-O-4 compounds and deconstruction of a realistic lignin feedstock catalyzed by Re₂ O₇ is described. High yields of C-O cleavage products (up to 97.8 %) from model compounds and oils (76.3 %) from organosolv pinewood lignin were obtained under mild conditions. Evidence for the pathway of this catalytic process is also provided.

Re2 O7 -Mediated Dehydrative Cyclization Reactions: Total Synthesis of Herboxidiene and Its 12-Desmethyl Analogue

Re₂ O₇ catalysis effects efficient and stereoselective dehydrative cyclization reactions from monoallylic diols, with stereocontrol arising from thermodynamic equilibration. This method was applied to a rapid synthesis of the spliceosome inhibitor herboxidiene. The route was also utilized for the synthesis of an analogue that highlights the importance of a single methyl group in biasing the conformation in the acyclic region of the molecule.

Enantioselective Synthesis of anti-1,2-Oxaborinan-3-enes from Aldehydes and 1,1-Di(boryl)alk-3-enes Using Ruthenium and Chiral Phosphoric Acid Catalysts

A cationic ruthenium(II) complex catalyzes double-bond transposition of 1,1-di(boryl)alk-3-enes to generate in situ 1,1-di(boryl)alk-2-enes, which then undergo chiral phosphoric acid catalyzed allylation of aldehydes producing homoallylic alcohols with a (Z)-vinylboronate moiety. 1,2-Anti stereochemistry is installed in an enantioselective manner. The (Z)-geometry forged in the products allows their isolation in a form of 1,2-oxaborinan-3-enes, upon which further synthetic transformations are operated.

Distortion–interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes

The reactivity of uncatalyzed Alder-ene type reactions of hetero-substituted propylene is interpreted by distortion–interaction analysis of both the transition states and the complete reaction pathways.

Zampanolide and dactylolide: cytotoxic tubulin-assembly agents and promising anticancer leads

Zampanolide is a marine natural macrolide and a recent addition to the family of microtubule-stabilizing cytotoxic agents. Zampanolide exhibits unique effects on tubulin assembly and is more potent than paclitaxel against several multi-drug resistant cancer cell lines. A high-resolution crystal structure of αβ-tubulin in complex with zampanolide explains how taxane-site microtubule-stabilizing agents promote microtubule assemble and stability. This review provides an overview of current developments of zampanolide and its related but less potent analogue dactylolide, covering their natural sources and isolation, structure and conformation, cytotoxic potential, structure-activity studies, mechanism of action, and syntheses.

Gold-catalyzed 1,3-transposition of ynones

An efficient, regioselective gold-catalyzed 1,3-transposition reaction of ynones and diynones has been developed. It was found that stereoelectronic (interrupted conjugation) and electronic (extended conjugation) factors could efficiently govern the regioselectivity of this thermodynamically controlled transformation. The produced conjugated diynones were efficiently transformed into diverse alkyne-substituted five- and six-membered heterocycles.

Recent developments of direct rhenium-catalyzed [1,3]-transpositions of allylic alcohols and their silyl ethers

The direct metal-catalyzed [1,3]-transposition of allylic alcohols and allylic silyl ethers is a synthetically useful isomerization process that occurs via [3,3]-sigmatropic rearrangement induced by high oxidation state oxometal complexes. The isomerization requires only a catalytic amount of promoter, and high chirality transfer can be achieved. Thus, it bears a significant potential to become a powerful tool in multistep synthesis. Although [1,3]-transposition of allylic alcohols has been known since the late 1960s, the development of synthetically useful protocols that allow for a high level of regio- and stereoselectivity control and their synthetic applications have emerged only recently. This tutorial review summarizes recently developed regioselective [1,3]-transpositions of allylic alcohols and silyl ethers and their applications to natural product synthesis.

Heterocycle synthesis based on allylic alcohol transposition using traceless trapping groups

Allylic alcohols undergo transposition reactions in the presence of Re2 O7 whereby the equilibrium can be dictated by trapping one isomer with a pendent electrophile. Additional ionization can occur when the trapping group is an aldehyde or ketone, thus leading to cyclic oxocarbenium ion formation. Terminating the process through bimolecular nucleophilic addition into the intermediate provides a versatile method for the synthesis of diverse oxygen-containing heterocycles. Understanding the relative rates of the steps in the sequence leads to the design of reactions which create multiple stereocenters with good to excellent levels of control.

Asymmetric total synthesis of (-)-amphidinolide V through effective combinations of catalytic transformations

An asymmetric total synthesis of (-)-amphidinolide V was accomplished. The synthesis features a base-catalyzed alkynyl silane alcoholysis/ring-closing enyne metathesis sequence for facile construction of a 1,3-diene motif. A diene RCM followed by a ring-contractive allylic transposition of cyclic silyl ethers was incorporated for the stereoselective installation of a functionalized 1,5-diene subunit. An efficient proline-mediated direct cross-aldol condensation of two advanced aldehyde intermediates was utilized for the construction of a key α,β-unsaturated epoxyaldehyde. This total synthesis demonstrates the prowess of metal-catalyzed transformations in complex molecule synthesis.

Subtle electronic effects in metal-free rearrangement of allenic alcohols

A general and stereoselective rearrangement of allenic alcohols to (E,E)-1,3-dien-2-yl triflates and chlorides was developed under metal-free conditions. Subtle electronic effects of the alkyl and aryl substituents on the carbon bearing the hydroxyl group has a profound impact on the reaction rate and efficiency such that vinyl triflates were obtained from electron-deficient substrates and trimethylsilyl triflate whereas vinyl chlorides were generated with an electron-rich substrate and trimethylsilyl chloride.

Cascade approach to stereoselective polycyclic ether formation: epoxides as trapping agents in the transposition of allylic alcohols

Complexity from simplicity: polycyclic ethers are synthesized by cascade reactions involving the use of epoxides as electrophilic traps in the transposition of allylic alcohols. Stereogenic centers are created by functionalizing prochiral sites under thermodynamic control, and remote stereoinduction can be achieved through the use of ketones as conduits.

Ring strain-promoted allylic transposition of cyclic silyl ethers

Relief of the ring strain of medium-sized rings promotes a regioselective allylic transposition of a C-O bond when catalyzed by rhenium oxide. Through the allylic transposition, eight-membered cyclic silyl ethers undergo ring contraction to the corresponding six-membered siloxacycles.

Regio- and stereocontrol in rhenium-catalyzed transposition of allylic alcohols

A hydroxyl group-directed, highly regio- and stereoselective transposition of allylic alcohols based on rhenium catalysis has been developed. The method is suitable for a direct isomerization of acetals into the thermodynamically preferred isomer as long as one of the hydroxyl groups is allylic. This method will expand the scope of rhenium-catalyzed alcohol transpositions for complex molecule synthesis.

Broadly applicable synthesis of 1,2,4,5-tetraoxanes

Re(2)O(7) is a mild and efficient catalyst for the high-yielding condensation of 1,1-dihydroperoxides with ketones or aldehydes to form 1,2,4,5-tetraoxanes, including targets not easily prepared via existing methodology. When applied in tandem with a recently reported Re(VII)-catalyzed synthesis of 1,1-dihydroperoxides, the reaction provides a high-yielding one-pot conversion of ketones or aldehydes to tetraoxanes.

Mild and efficient Re(VII)-catalyzed synthesis of 1,1-dihydroperoxides

Re2O7 in CH3CN is a remarkably efficient and mild catalyst for the peroxyacetalization of ketones, aldehydes, or acetals by H2O2 to generate 1,1-dihydroperoxides. Me3SiOReO3 and methyl rhenium trioxide (MTO) are also effective catalysts under these reaction conditions.