2-Activated 1,3-enynes in enantioselective synthesis

Recent Advancements in the Cyclization Strategies of 1,3-Enynes Towards the Synthesis of Heterocyclic/Carbocyclic Frameworks

1,3-Enynes have demonstrated their utility as valuable precursors to furnish a diverse range of heterocycles and carbocycles. Their unique structural characteristics enable a new toolbox to introduce requisite complexity in the molecular framework. Cyclization reaction is usually a simple and straightforward way to afford complex organic frameworks. Herein, we collated versatile cyclization strategies that have been developed by employing 1,3-enynes for the synthesis of heterocyclic and carbocyclic scaffolds. Divergent synthesis and mechanistic perspectives to define stereo-, regio- and chemo-selective outcomes of such reactions have also been highlighted.

Synthesis of tetrasubstituted furans through a Cu/base-mediated cascade reaction from terminal alkynes and 1,2-diketones

The expeditious construction of tetrasubstituted furans via Cu/base-promoted cascade reactions of terminal alkynes with 1,2-diketones is described. This reaction proceeds smoothly, involving the formation of multiple chemical bonds tolerating a wide range of functional groups. The mechanism underlying these transformations has been thoroughly investigated, suggesting that 1,3-diyne serves as a key intermediate generated in situ through oxidative coupling facilitated by the Cu catalyst.

Pd-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with enynes through non-rollover cyclometallation

Annulations through dual C-H activation represent a powerful tool to selectively assemble multi-cyclic scaffolds. We present herein a palladium-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with 1,6-enynes. This regioselective non-rollover cyclometallation was achieved through meticulous tuning of electronic factors of both the partners. This method is applicable to a wide range of protected o-arylanilines and enynes, and results in the regioselective preparation of benzo[f]isoindolyl derivatives in high yields with good diastereoselectivity (with respect to two types of stereogenic elements). Certain essential control experiments and kinetic isotope effect (KIE) studies were undertaken to elucidate the reaction mechanism, while subsequent transformations and a scale-up reaction were performed to substantiate the sturdiness of the transformation.

Enantioselective Au(I)-Catalyzed Cycloisomerization/Addition of Oxygen Nucleophiles to 2-Alkynylenones by the Tethered Counterion-Directed Catalysis Strategy

The tethered counterion-directed catalysis (TCDC) strategy enables the Au(I)-catalyzed highly enantioselective synthesis of bicyclic furan derivatives via a reaction sequence combining the cycloisomerization of 2-alkynyl enones and the addition of nucleophiles. A large range of oxygenated nucleophiles, such as water, alcohols, carboxylic acids, and peroxides, have successfully been used as nucleophiles, delivering the chiral furane derivatives in high enantioselectivities (mostly above 90% enantiomeric excess). The CPAphosAuCl complexes were used with catalytic loadings as low as 0.2 mol % in most cases, in combination with silver carbonate as the chloride abstractor.

Preparation of Benzo[a]fluorenes via Pd-Catalyzed Annulation of 5-(2-Bromophenyl)pent-3-en-1-ynes

A palladium-promoted cascade cyclization of 5-(2-bromophenyl)pent-3-en-1-ynes is developed for the synthesis of benzo[a]fluorene derivatives. The reaction proceeds with oxidative addition of C-Br, insertion, C-H activation, and reductive elimination in sequential steps.

Synthesis of Highly Substituted Furans via Intermolecular Enynone-Aldehyde Cross-Coupling/Cyclization Catalyzed by N-Heterocyclic Carbenes

A new strategy for facile access to multifunctionalized furans via N-heterocyclic carbene-catalyzed cross-coupling/cyclization of ynenones with aldehydes has been explored. This protocol features readily obtainable starting materials, mild and metal-free conditions, broad substrate scope, good functional group tolerance, excellent yields, and easy scale-up. Synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and postmodifications of the product.

Cooperative Gold(I)/DMAP Catalysis Enabled (2 + 3) Cycloadditions of Yne-Enones with Oxindole-Derived MBH Carbonates

A cooperative gold(I)/DMAP system catalyzes the (2 + 3) cycloadditions of yne-enones with oxindole-derived Morita-Baylis-Hillman (MBH) carbonates, yielding diverse bispiro-cyclopentene oxindole products. The mild, scalable protocol demonstrates broad substrate scope and excellent chemo- and diastereoselectivity. Mechanistic study reveals pivotal roles of both catalysts in the unique (2 + 3) cycloaddition. This strategy showcases superiority in achieving transformation with unique chemoselectivity and excellent diastereoselectivity, unattainable through traditional monocatalytic methodologies.

Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene ((CH3)2CCH2; X1A1)

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene (isobutylene; (CH3)2CCH2; X1A1) at a collision energy of 38 ± 3 kJ mol-1. The experimental results along with ab initio and statistical calculations revealed that the reaction has no entrance barrier and proceeds via indirect scattering dynamics involving C7H11 intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene. Further, the C7H11 intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH3 or CH2 group prior to atomic hydrogen loss preferentially leading to 1,2,4-trimethylvinylacetylene (2-methylhex-2-en-4-yne) as the dominant product. The molecular structures of the collisional complexes promote hydrogen atom loss channels. RRKM results show that hydrogen elimination channels dominate in this reaction, with a branching ratio exceeding 70%. Since the reaction of the 1-propynyl radical with 2-methylpropene has no entrance barrier, is exoergic, and all transition states involved are located below the energy of the separated reactants, bimolecular collisions are feasible to form trimethylsubstituted 1,3-enyne (p1) via a single collision event even at temperatures as low as 10 K prevailing in cold molecular clouds such as G+0.693. The formation of trimethylsubstituted vinylacetylene could serve as the starting point of fundamental molecular mass growth processes leading to di- and trimethylsubstituted naphthalenes via the HAVA mechanism.

Pd-Catalyzed Vicinal Intermolecular Annulations of Iodoarenes, Indoles, and Carbazoles with Enynes

Reaching the formidable C-H corners has been one of the top priorities of organic chemists in the recent past. This prompted us to disclose herein a vicinal annulation of 2-iodo benzoates, indoles, and carbazoles with N-embedded 1,6-enynes through 7-/8-membered palladacycles. The relay does not require the assistance of any directing group, leading to multicyclic scaffolds, which are readily diversified to an array of adducts (with new functional tethers and/or three contiguous stereocenters), in which we showcase a rare benzylic mono-oxygenation.

Diastereoselective Pd-catalyzed Decarboxylative (4+2) Cycloaddition Reactions of 4-Vinylbenzoxazinanones and 2-Nitro-1,3-enynes

A formal palladium-catalyzed decarboxylative (4+2) cycloaddition reaction between 4-vinylbenzoxazinanones and 2-nitro-1,3-enynes has been developed to produce highly valuable, densely functionalized tetrahydroquinolines in moderate to excellent yields with high diastereoselectivity under mild reaction conditions. The optimised protocol tolerates a range of substituted 2-nitro-1,3-enynes, which represent an under-utilized class of dipolarophile for transition-metal catalyzed cycloadditions. The employed reaction methodology facilitates efficient cycloaddition with both N-H- and N-Ts-4-vinylbenzoxazinanone dipole precursors. The stereochemistry of the major and minor diastereomeric (4+2) cycloadducts was determined by single crystal X-ray analyses. A mechanistic rationale for the high intrinsic diastereoselectivity and preliminary enantioselective experiments are also presented. The tetrahydroquinoline cycloadduct products feature numerous pendant functionalities, including a vinyl handle, an internal alkyne motif and a nitro functionality (which functions as a latent C-3 nitrogen substituent) for further synthetic manipulations.

trans-Hydroalkynylation of Internal 1,3-Enynes Enabled by Cooperative Catalysis

A cooperative catalyst system involving a Pd(0)/Senphos complex, tris(pentafluorophenyl)borane, copper bromide, and an amine base, is demonstrated to catalyze trans-hydroalkynylation of internal 1,3-enynes. For the first time, a Lewis acid catalyst is shown to promote the reaction involving the emerging outer-sphere oxidative reaction step. The resulting cross-conjugated dieneynes are versatile synthons for organic synthesis, and their characterization reveals distinct photophysical properties depending on the positioning of the donor/acceptor substituents along the conjugation path.

Palladium-Catalyzed Alkynylation of Enones with Alkynylsilanes via C-C Bond Activation

We report herein the synthesis of 1,3-enynes via palladium-catalyzed cross-coupling between enone derivatives and alkynylsilanes. The employment of an appropriate pyridine-oxazoline ligand is the key to the C-C cleavage and the high E/Z stereoselectivity. This protocol features broad substrate scope and wide functional-group tolerance, affording the desired products in moderate-to-good yields. Late-stage diversification of natural product β-ionone further demonstrated the synthetic utility of this protocol.

Gold(i)-catalyzed diastereo- and enantioselective [4 + 3] cycloadditions: construction of functionalized furano-benzoxepins

Highly diastereo- and enantioselective [4 + 3] cycloadditions of 2-(1-alkynyl)-2-alken-1-ones with o-QMs have been realized via a simple chiral gold catalysis, providing facile access to various functionalized furano-benzoxepins.

Synthesis of 2-acetal-1,3-enynes by Sonogashira reaction of bromovinyl acetals with alkynes: application to the formal synthesis of a glucagon antagonist

The preparation of functionalized 1,3-enynes bearing an acetal moiety at the 2-position has been studied through Sonogashira reaction of bromovinyl acetals with various alkyl- and aryl-substituted terminal alkynes.

Indium-Catalysed Transfer Hydrogenation for the Reductive Cyclisation of 2-Alkynyl Enones towards Trisubstituted Furans

Indium tribromide catalysed the transfer hydrogenation from dihydroaromatic compounds, such as the commercially available γ-terpinene, to enones, which resulted in the cyclisation to trisubstituted furan derivatives. The reaction was initiated by a Michael addition of a hydride nucleophile to the enone subunit followed by a Lewis-acid-assisted cyclisation and the formation of a furan-indium intermediate and a Wheland intermediate derived from the dihydroaromatic starting material. The product was formed by protonation from the Wheland complex and replaced the indium tribromide substituent. In addition, a site-specific deuterium labelling of the dihydroaromatic HD surrogates resulted in site specific labelling of the products and gave useful insights into the reaction mechanism by H-D scrambling.

Recent advances in the tandem annulation of 1,3-enynes to functionalized pyridine and pyrrole derivatives

Great progress has been made in the tandem annulation of enynes in the past few years. This review only presents the corresponding reactions of 1,3-enyne structural motifs to provide the functionalized pyridine and pyrrole derivatives. The functionalization reactions cover iodination, bromination, trifluoromethylation, azidation, carbonylation, arylation, alkylation, selenylation, sulfenylation, amidation, esterification, and hydroxylation. We also briefly introduce the applications of the products and the reaction mechanisms for the synthesis of corresponding N-heterocycles.

Access to Chiral Polycyclic 1,4-Dihydropyridines via Organocatalytic Formal [3 + 3] Annulation of 2-(1-Alkynyl)-2-alken-1-ones with 3-Aminobenzofurans

A rational designed tandem reaction of 2-(1-alkynyl)-2-alken-1-ones with 3-aminobenzofurans enabled by a chiral bifunctional catalyst is described, affording biologically significant polycyclic 1,4-dihydropyridines in moderate to good yields (43-82%) with good to excellent enantioselectivities (83-99%). This formal [3 + 3] annulation reaction reveals good practicality when conducted on a gram scale, and the cycloadduct has the capability for further elaborations.

Radical trans-Hydroboration of Substituted 1,3-Diynes with an N-Heterocyclic Carbene Borane

Monohydroboration of substituted 1,3-diynes with an N-heterocyclic carbene borane (NHC-borane) occurs under radical conditions using an azo initiator, such as ACCN and AIBN, and a thiol as a polarity-reversal catalyst. The reaction is highly regio- and stereoselective and provides stable NHC-(E)-alkynylalkenylboranes.

Highly regio- and stereo-selective heterogeneous 1,3-diyne hydrosilylation controlled by a nickel-metalated porous organic polymer

A porous organic polymer (POL-xantphos) was synthesized and employed as a heterogeneous ligand for nickel catalyzed highly regio- and stereo-selective 1,3-diyne hydrosilylation.

Ruthenium-Catalyzed trans-Hydroalkynylation and trans-Chloroalkynylation of Internal Alkynes

[Cp*RuCl]4 catalyzes the addition of iPr3SiC≡CX (X = H, Cl) across internal alkynes with formation of 1,3-enyne or 1-chloro-1,3-enyne derivatives, respectively; the reaction follows an unorthodox trans-addition mode. The well-balanced affinities of the different reaction partners to the ruthenium catalyst ensure that crossed addition prevails over homodimerization of the individual components, as can be deduced from spectroscopic and crystallographic data of various intermediates; this includes a dinuclear complex in which an internal alkyne bridges two [Cp*RuCl] fragments.