Photochemical and radical initiator-induced reaction of allylic indium compounds

Intramolecular cyclization of allylic radicals generated from allylindium compounds both by photolysis or by the reaction of radical initiators was examined. The photolysis of allylic indium compounds, prepared from 8-bromo- or 8-iodooct-1,6-dienes and powdered indium metal, led to the formation of the 5-exo-trig products. Benzoyl peroxide as a radical initiator was also effective for the cyclization. In contrast, the radical initiators with oxidizing nature, such as tert-butyl hypochlorite, induced iodocyclization producing iodomethylcyclopentane via an oxidation of the iodide on the indium atom.     

Stereodefined construction of trisubstituted alkenes by direct coupling reaction of allylating agents, alkynes, and organoboranes

Tandem reaction: The Pd-catalyzed three-component coupling reaction of an allylic alcohol, terminal alkyne, and organoborane to give (E)-1-substituted 2-alkyl-1,4-pentadienes, involving geminal allylation and alkylation at the acetylenic terminal carbon, is described. Bis-diene undergoes a similar multicomponent coupling reaction with acetylene and organoborane, involving cyclization of bis-π-allylpalladium, to form trans-allyl pentadienyl cyclic and heterocyclic compounds with excellent regio- and stereoselectivities (see scheme).     

DFT mechanistic study on nickel/IPr-catalyzed aldehyde–alkyne reductive couplings with trialkylsilane/dialkylsilane

Density functional theory calculations were carried out to reveal the mechanistic details of aldehyde–alkyne reductive couplings with trialkylsilane/dialkylsilane. The reaction with trialkylsilane is found to proceed through oxidative cyclization, Si-H/Ni-O σ-bond metathesis, and C(sp²)-H reductive elimination, leading to silylated allylic alcohols. The steric hindrance between the n-pent group of alkyne and iPr group of the NHC ligand determines the regioselectivity. While for the reaction with dialkylsilane, the present calculations propose a new mechanism, which consists of oxidative cyclization, Si−H/Ni−O σ-bond metathesis, Ni−C/Si−H σ-bond metathesis, and dehydrogenation, resulting in oxasilacyclopentenes. The calculated energy profiles rationalize the experimentally observed chemodivergence.     

Palladium(0)-catalyzed alkylative cyclization of alkynals and alkynones: remarkable trans-addition of organoboronic reagents

Palladium/monophosphine complexes catalyze trans-selective arylative, alkenylative, and alkylative cyclization reactions of alkynals and alkynones with organoboronic reagents. These reactions afford six-membered allylic alcohols with endo-tri- or tetrasubstituted olefin groups and/or five-membered counterparts with exo olefin groups. The ratios of these products are dramatically affected by alkyne substituents as well as the phosphine ligand. The remarkable trans-selectivity of the process results from the novel reaction mechanism involving oxidative addition without oxametallacycle formation.     

Copper‐Catalyzed Cyclization/aza‐Claisen Rearrangement Cascade Initiated by Ketenimine Formation: An Efficient Stereocontrolled Synthesis of α‐Allyl Cyclic Amidines

An efficient and convenient synthesis of α‐allyl cyclic amidines has been achieved by applying a novel cascade reaction. Copper(I)‐mediated in situ N‐sulfonyl ketenimine formation from the reaction of a terminal alkyne with sulfonyl azide is followed by an intramolecular nucleophilic attack on the central carbon atom by an allylic tertiary amine, and then an aza‐Claisen rearrangement takes place through a chair transition state to furnish the titled amidines with complete stereocontrol.     

Umpolung Reactivity of Ynamides: An Unconventional [1,3]‐Sulfonyl and [1,5]‐Sulfinyl Migration Cascade

A regioselective sulfonyl/sulfinyl migration cycloisomerization cascade of alkyne‐tethered ynamides is developed in the presence of XPhosgold catalyst. This reaction is the first example of a general [1,3]‐sulfonyl migration from the nitrogen center to the β‐carbon atom of ynamides, followed by umpolung 5‐endo‐dig cyclization of the ynamide α‐carbon atom to the gold‐activated alkyne, and final deaurative [1,5]‐sulfinylation. This process allows the synthesis of peripherally decorated unconventional 4‐sulfinylated pyrroles with broad scope from N‐propargyl‐tethered ynamides. In contrast, N‐homopropargyl‐tethered ynamides undergo intramolecular tetradehydro Diels–Alder reaction to provide 2,3‐dihydro‐benzo[f]indole derivatives. Control experiments and density‐functional theory studies were used to study the reaction pathways.     

Enantioselective Nickel‐Catalyzed Intramolecular Allylic Alkenylations Enabled by Reversible Alkenylnickel E/Z Isomerization

Enantioselective nickel‐catalyzed arylative cyclizations of substrates containing a Z ‐allylic phosphate tethered to an alkyne are described. These reactions give multisubstituted chiral aza‐ and carbocycles, and are initiated by the addition of an arylboronic acid to the alkyne, followed by cyclization of the resulting alkenylnickel species onto the allylic phosphate. The reversible E /Z isomerization of the alkenylnickel species is essential for the success of the reactions.     

Synthesis of secondary amines by titanium-mediated transfer of alkenyl groups from alcohols

Reaction of Ti(NMe2)4 with allyl alcohols and primary amines leads to the selective formation of secondary allylic amines. The allyl transfer from the alcohol to the amine occurs with selective allylic transposition. Due to substituent effects in the reactions, we postulate that the reaction occurs through a [2 + 2]/retro-[2 + 2]-cycloaddition mechanism. It was also found that a similar reaction could be accomplished with homoallylic alcohol. In this case, the more complex mechanism leads to the formation of 1-aza-spiro[5.5]undecane. Possible pathways for the homoallylic transfer and cyclization are discussed.     

Self‐Relay Gold(I)‐Catalyzed Pictet–Spengler/Cyclization Cascade Reaction for the Rapid Elaboration of Pentacyclic Indole Derivatives

Gold‐catalyzed cascade reactions allow the rapid elaboration of pentacyclic indolo[2,3‐a]quinolizidines from N‐allyl tryptamines and ortho‐alkynylarylaldehydes. The tandem process combines a gold‐catalyzed Pictet‐Spengler reaction and a cyclization occurring concomitantly with an allyl transfer from the nitrogen atom to the stilbene function. Various substituted allyls were successfully transferred, furnishing the products in yields typically ranging from 60–98 % in high diastereoselectivity. Tryptamines bearing a butenol chain undergo an additional cyclization to chiral hemiaminals in high diastereoselectivities.     

Palladium-catalyzed stereospecific synthesis of 2,6-disubstituted tetrahydropyrans: 1,3-chirality transfer by an intramolecular oxypalladation reaction

PdCl2(CH3CN)2 (10 mol %) catalyzed reactions of non-3-ene-2,8-diols 1 and 2 gave 2,6-disubstituted tetrahydropyrans 3 and 4 in excellent yields with high diastereoselectivities (>20:1). Intramolecular cyclizations of the hydroxy nucleophile to the chiral allylic alcohol take place efficiently under mild conditions. A new stereogenic center is generated on the tetrahydropyran ring by 1,3-chirality transfer from the chiral allylic alcohol via a syn-SN2' type process. Cis tetrahydropyran 3E was formed from syn-2,8-diols 1a and 2a, and trans tetrahydropyran 4E was formed from anti-2,8-diol 1b, stereospecifically. Cis tetrahydropyran bearing a cis alkene 3Z was obtained from 2b at -40 degrees C, while 4E was formed from 2b in the presence of catalytic amount of water at -40 degrees C. The face selectivity of these cyclizations can be rationalized by taking a favorable conformation of the intermediary Pd pi-complex with allylic alcohols, escaping the allylic strain and 1,3-diaxial interactions. A stereocontrolled synthesis of optically pure 2-alkenyl-6-methyltetrahydropyran 17 was achieved efficiently in four steps from 6-silyloxy-1-heptyne 13 with an aldehyde and included asymmetric alkynylation, partial reduction of alkyne, deprotection of the silyl group, and the stereospecific cyclization.     

Synthesis of Phenanthrene Derivatives by Intramolecular Cyclization Utilizing the [1,2]‐Phospha‐Brook Rearrangement Catalyzed by a Brønsted Base

The synthesis of functionalized phenanthrene derivatives was achieved by intramolecular cyclization utilizing the [1,2]‐phospha‐Brook rearrangement under Brønsted base catalysis. Treatment of biaryl compounds having an α‐ketoester moiety and an alkyne moiety at the 2 and 2′ positions, respectively, with diisopropyl phosphite in the presence of a catalytic amount of phosphazene base P2‐tBu provides 9,10‐disubstituted phenanthrene derivatives in high yields. This reaction involves the generation of an ester enolate through an umpolung process, that is, addition of diisopropyl phosphite to a keto moiety followed by the [1,2]‐phospha‐Brook rearrangement, the intramolecular addition to an alkyne, and the [3,3] rearrangement of the allylic phosphate moiety in a consecutive fashion.     

PdCl2催化炔酸烯丙酯环化反应的密度泛函研究

用密度泛函方法(DFT)研究了PdCl2催化炔酸烯丙酯环化反应的机理. 在B3LYP/6-311G**水平上优化了各反应中间体和过渡态的结构. 计算结果表明, 反应是放热的, 主要经历了炔键的卤钯化、烯烃对烯基钯的迁移插入以及β-杂原子消除等过程. 烯烃的迁移插入是反应的手性决定步骤, β-杂原子消除是反应的速率控制步骤. 理论预测的主要产物是与实验吻合的(Z,R)-α-亚烷基-γ-丁内酯.     

On the Mechanism of Au-Catalyzed Enynamide-yne Dehydro-Diels-Alder Reactions: An Experimental and Computational Study

Gold-catalyzed dehydro-Diels-Alder reactions of ynamide derivatives allow efficient access to a variety of N-containing aromatic heterocycles. A dual gold catalysis mechanism was postulated for transformations involving the formation of C−C bonds by reaction between a terminal alkyne and an enynamide fragment. In this article, complete experimental and computational investigations into the mechanism of such a transformation are reported. Support for a dual gold catalysis was found and it was shown that the concerted or stepwise nature of the cyclization event depends on the substitution of the ynamide moiety. The reaction was found to proceed in three stages: 1) formation of a σ,π-digold complex from the terminal alkyne, 2) cyclization to produce a gem-diaurated aryl complex, and 3) catalyst transfer to free the product and regenerate the σ,π-digold complex.     

Zirconocene-promoted synthesis of substituted tetrahydropyrans

Zirconocene reagents derived from zirconocene dichloride and two equivalents of butyllithium react with allylic, homoallylic diene ethers. Hydrolysis of the reaction products yields substituted tetrahydropyrans. The reaction is postulated to occur via cyclization of the diene to form a zirconacyclopentane. This cyclization occurs without allylic rearrangement.     

Synthesis of pyrrolo-pyrrolo-pyrazines via the Pd/C-catalyzed cyclization of N-propargyl pyrrolinyl-pyrrole derivatives

A novel and efficient synthesis of N-substituted dipyrrolo[1,2-a:2′,1′-c]pyrazine derivatives has been developed. The synthetic strategy relies on the synthesis of 4′,5′-dihydro-1H,3′H-2,2′-bipyrrole, followed by the reaction with propargyl bromide. Various substituents were introduced to the alkyne functionality using the Sonogashira coupling reaction. Aromatization of the dihydropyrrole ring followed by an intramolecular cyclization reaction between the alkyne functionality and the pyrrole nitrogen atom was catalyzed by Pd/C at high temperature to furnish the desired dipyrrolo-pyrazine skeleton.     

Enantioselective Nickel-Catalyzed Intramolecular Allylic Alkenylations Enabled by Reversible Alkenylnickel E/Z Isomerization

Enantioselective nickel-catalyzed arylative cyclizations of substrates containing a Z-allylic phosphate tethered to an alkyne are described. These reactions give multisubstituted chiral aza- and carbocycles, and are initiated by the addition of an arylboronic acid to the alkyne, followed by cyclization of the resulting alkenylnickel species onto the allylic phosphate. The reversible E/Z isomerization of the alkenylnickel species is essential for the success of the reactions.     

Nickeladihydrofuran. Key intermediate for nickel-catalyzed reaction of alkyne and aldehyde

The formation of a nickeladihydrofuran by oxidative cyclization of an alkyne and an aldehyde with nickel(0) has been demonstrated; the transformation of the nickeladihydrofuran into an enone by decomposition, a lactone by carbonylation and an allylic alcohol by treatment with ZnMe(2) suggests that nickeladihydrofuran is an important key intermediate in a variety of catalytic reactions.     

Synthesis of δ-bromo γ,δ-unsaturated carbonyl compounds via palladium-catalyzed bromoalkylation of alkynoates

Palladium-catalyzed regio- and stereoselective intermolecular tandem reaction of electron-deficient alkynes, CuBr(2), and allylic alcohol to synthesize δ-bromo-γ,δ-unsaturated carbonyls was developed. A mechanism involving bromopalladation of alkyne, followed by insertion of allylic alcohol and allylic hydrogen shift, is proposed. The shift of allylic hydrogen is the rate-limiting step in this reaction.     

Factors controlling the alkyne prins cyclization: the stability of dihydropyranyl cations

The relative stability of the intermediates involved in the alkyne Prins cyclization and the competitive 2-oxonia-[3,3]-sigmatropic rearrangement was studied. This rearrangement was shown to occur slowly under typical alkyne Prins cyclization conditions when the allenyl oxocarbenium ion that results from the rearrangement is similar to or higher in energy than the starting alkynyl oxocarbenium ion. The formal 2-oxonia-[3,3]-sigmatropic rearrangement may be disfavored by destabilizing the resultant allenyl oxocarbenium ion or by stabilizing an intermediate dihydropyranyl cation. The trimethylsilyl group as a substituent at the alkyne and electron-withdrawing groups (CH2Cl and CH2CN) located at the alpha-position to the carbinol center are shown to be effective. DFT calculations suggest that these substituents stabilize the dihydropyranyl cations, thus leading to a more uniform reorganization of the electronic density in the ring, and do not have a direct effect on the formally positively charged carbon atom.     

Alkyne haloallylation [with Pd(II)] as a core strategy for macrocycle synthesis: a total synthesis of (-)-haterumalide NA/(-)-oocydin A

The rarely used haloallylation reaction, first described by Kaneda and Teranishi in 1974, employs a Pd(II) catalyst to join an alkyne with an allylic halide to produce a 1-halo-1,4-diene subunit. It is shown here that functionalized and tertiary allylic chlorides perform well as substrates in this reaction under the action of PdCl2(PhCN)2 in THF solution. When the alkyne is added slowly to the reaction mixture, the two reactants can be used in a nearly equimolar ratio. This fact means (i) that reasonably complex pairs of alkyne and allylic halide substrates are tolerated and, therefore, (ii) that an intramolecular version of the reaction is suitable as a core strategy for complex molecule construction. The latter is demonstrated in the macrocyclization of 2b to 17b, which is the central step in the total synthesis of (-)-haterumalide NA/(-)-oocydin A (1) that is reported. The final key to the completion of the synthesis was the choice of the acid-labile PMB ester of 1 as the penultimate intermediate.