AuBr(3)-catalyzed [4 + 2] benzannulation between an enynal unit and enol

The reaction of enynals 1, including o-alkynylbenzaldehydes, and carbonyl compounds 2 in the presence of a catalytic amount of AuBr3 in 1,4-dioxane at 100 degrees C gave the functionalized aromatic compounds 3 in high yields. The AuBr3-catalyzed formal [4 + 2] benzannulation proceeds most probably through the coordination of the triple bond of 1 to AuBr3, the formation of a pyrylium auric ate complex via the nucleophilic addition of the carbonyl oxygen atom, the reverse electron demand-type Diels-Alder addition of the enols, derived from 2, to the auric ate complex, and subsequent dehydration and bond rearrangement. Similarly, the AuBr3-catalyzed reactions of 1 with acetal compounds afforded the corresponding aromatic compounds in good yields.     

Rh(I)-catalyzed intramolecular [3 + 2] cycloaddition of trans-vinylcyclopropane-enes

Vinylcyclopropane (VCP) has been well applied as a five-carbon component, rather than a three-carbon component, in transition-metal catalyzed cycloadditions. Here we demonstrate a Rh(I)-catalyzed [3 + 2] reaction of trans-VCP-enes, where VCP acts as a three-carbon synthon to furnish five-membered carbocycles. This novel cycloaddition is efficient in generating bicyclic cyclopentanes in good yields from simple and easily prepared substrates. When cis-VCP-ene is used as the substrate, VCP acts as a five-carbon unit to give a [5 + 2] cycloadduct. Rationalization of the [3 + 2] and [5 + 2] cycloadditions of VCP-enes has been proposed.     

Au(PPh3)OPOF2-catalyzed intramolecular [4+2] cycloaddition reaction of dienynes

Solvolysis of Au(PPh(3))PF(6) afforded Au(PPh(3))OPOF(2) which is an effective catalyst in the intramolecular [4+2] cycloaddition of unactivated dienynes bearing a terminal alkyne.     

Rh(I)-catalyzed intramolecular hetero-[4+2] cycloaddition of ω-alkynyl-vinyl oximes

Cationic rhodium(I) catalyst derived from [RhCl(cod)]₂ and AgSbF₆ in hexafluoroisopropanol (HFIP) efficiently catalyzed the formation of bicyclic pyridine derivatives by intramolecular hetero-[4+2] cycloadditions of ω-alkynyl-vinyl oximes.     

Rhodium(I)-catalyzed intramolecular pauson-khand-type [2 + 2 + 1] cycloaddition of allenenes

[reaction: see text] The novel [RhCl(CO)(2)](2)-catalyzed [2 + 2 + 1] cycloaddition of allenenes leading to the bicyclo[4.3.0]non-1(9)-en-8-one as well as the bicyclo[5.3.0]dec-1(10)-en-9-one skeletons has been developed. This method also provides a new procedure for the construction of the bicyclo[4.3.0]non-1(9)-en-8-one skeleton having an alkyl appendage at the ring juncture, which was hardly attained in a satisfactory yield by the Pauson-Khand reaction of the corresponding enynes.     

Rhodium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides

[reaction: see text] Cationic rhodium(I)-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were found to occur in good yields (70-87%). The halide moiety is compatible with the cycloaddition reactions, and no oxidative insertion to the alkynyl halide was observed. The halogen-containing cycloadducts could be transformed into a variety of products that are difficult or impossible to obtain via direct cycloaddition.     

Bioconjugation by copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition

The copper-catalyzed cycloaddition reaction between azides and alkynes functions efficiently in aqueous solution in the presence of a tris(triazolyl)amine ligand. The process has been employed to make rapid and reliable covalent connections to micromolar concentrations of protein decorated with either of the reactive moieties. The chelating ligand plays a crucial role in stabilizing the Cu(I) oxidation state and protecting the protein from Cu(triazole)-induced denaturation. Because the azide and alkyne groups themselves are unreactive with protein residues or other biomolecules, their ligation is of potential utility as a general bioconjugation method.     

Surface modification of magnetic nanoparticle via Cu(I)-catalyzed alkyne-azide [2 + 3] cycloaddition

The Cu(I)-catalyzed alkyne-azide [2 + 3] cycloaddition has been demonstrated to be an effective and orthogonal conjugation reaction to covalently immobilize biomolecules on magnetic nanoparticles (MNPs). The azido group on the MNP surface provides better conjugation efficiency with alkynated molecules. Moreover, the C-terminal alkynated protein was site-specifically immobilized on MNP. The protein binding activity presented by site-specific immobilization is higher than that by random amide bond formation.     

Rh(I)-catalyzed mild intramolecular [4+2] cycloaddition reactions of ester-tethered diene-yne compounds

The cationic rhodium(I) species derived from [Rh(COD)Cl]₂ and AgSbF₆ efficiently catalyze intramolecular [4+2] cycloadditions of ester-tethered 1,3-diene-8-yne derivatives such as 2-propynyl penta-2,4-dienoate and 2,4-pentadienyl propiolate derivatives in fluorinated alcohols.     

Asymmetric synergy between chiral dienes and diphosphines in cationic Rh(I)-catalyzed intramolecular [4 + 2] cycloaddition

High enantioselective [4 + 2] cycloaddition can be achieved by cationic Rh complexes bearing not only chiral phosphine but also chiral diene. This is the first example of asymmetric synergy between chiral dienes and diphosphines.     

Highly enantioselective Ag(i)-catalyzed [3 + 2] cycloaddition of azomethine ylides

A highly reactive Ag(I)-catalyzed [3 + 2] cycloaddition of azomethine ylides is founded using AgOAc as the catalytic precursor and phosphines as ligands. Using a new bis-ferrocenyl amide phosphine (FAP) as the ligand, we found that high enantioselectivities (up to 97% ee) have been achieved in the [3 + 2] cycloaddition of azomethine ylides. Up to four stereogenic centers can be established in this multicomponent coupling reaction from readily available materials such as aldehydes, aminoesters, and dipolarophiles.     

Rhodium-catalyzed [2 + 2 + 2] cycloaddition of alkenyl isocyanates and alkynes

A rhodium(I)-catalyzed [2 + 2 + 2] cycloaddition between alkenyl isocyanates and alkynes has been developed. Heating a mixture of an alkenyl isocyanate and a symmetrical internal alkyne in the presence of [Rh(ethylene)2Cl]2/P(4-OMe-C6H4)3 in toluene delivers substituted indolizinones and quinolizinones. Depending on the substrates, a rare fragmentation of the isocyanate unit can be involved within the cycloaddition process to furnish a vinylogous amide embedded in the indolizinone.     

Cu(I)-catalyzed highly exo-selective and enantioselective [3 + 2] cycloaddition of azomethine ylides with acrylates

[reaction: see text] A novel Cu(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides with acrylates has been developed. Up to 98/2 exo/endo selectivity and up to 98% enantiomeric excess have been achieved.     

Preparation of 3-azabicyclo [3.2.0] heptenones by intramolecular [2+2] cycloaddition

Pyrolysis of N-allyl enaminoesters is a general method for the stereoselective synthesis of 3-azabicyclo[3.2.0] heptenones ; the reaction involves a [2+2] intramolecular cycloaddition of an intermediate iminoketene.     

Stereocontrolled synthesis of bicyclic lactone derivatives via tungsten-mediated [3 + 2] cycloaddition of epoxides with a tethered alkynyl group

In the presence of BF3*Et2O, alkynyltungsten complexes underwent [3 + 2] cycloaddition with tethered epoxides to give bicyclic -lactones efficiently. Only one diastereomeric product was formed despite the presence of three stereogenic centers. A mechanism is proposed that involves formation of a tungsten-vinylidenium species via an SN2 attack of the epoxide carbon by an alkynyltungsten group to give a tungsten-enol ether species via counterattack at the central tungsten-vinylidenium carbon by the OBF3- terminus. Most of the tungsten enol ether species were too unstable for isolation and underwent hydrolysis to give only cis-fused -bicyclic lactones. This cyclization works for both cis- and trans-epoxides and tolerates various functional groups. In the case of trans-phenyl epoxide, the reaction led to an addition product via a 6-endo attack of epoxide by the tungsten fragment. This method provides a simple enantiospecific synthesis of complex bicyclic lactones if a chiral epoxide is used in the cyclization. It is also applicable to the one-pot synthesis of bicyclic unsaturated gamma-lactones if a suitable alkynyltungsten functionality is used.     

Palladium-catalyzed [3 + 2] intramolecular cycloaddition of alk-5-enylidenecyclopropanes

Readily accessible alk-5-enylidenecyclopropanes undergo [3 + 2] intramolecular cycloaddition reactions upon treatment with appropriate palladium complexes. The method allows the rapid and efficient assembly of a variety of bicyclo[3.3.0]octane systems with up to three stereocenters. Preliminary theoretical calculations uncovered previously unsuspected mechanistic possibilities based on either a concerted pallada-ene-like rearrangement or a stepwise process involving zwitterionic intermediates.     

Synthesis of a new conformationally defined serotonin homologue by intramolecular [4+2] cycloaddition

Treatment of 1-[(p-methoxyphenyl)sulfonyl]-2-methyl-3-[N-allyl-N-methyl-3-(2,6-dichlorobenzoyl)propylamino]indole 3 with triethylamine in chlorobenzene at 135°C results in the formation of 1,2,3,4,4a,5,6,11c-octahydro-7-[(p-methoxypyhenyl)-sulfonyl]-3-methyl-7H-pyrido[3,4-c]carbazole 1b in 58% yield. The reaction is selective in giving predominantly the trans isomer.