Organoseleno-Catalyzed Synthesis of α,ß-Unsaturated α'-Alkoxy Ketones from Allenes Enabled by Se···O Interactions.

α,ß-Unsaturated α'-alkoxy ketones have been prepared under mild conditions from allenes using water as the oxygen source and without the necessity of metals. The organocatalytic oxygenation-rearrangement sequence displays an exquisite chemo-, stereo-, and site-selectivity as well as good functional group compatibility. DFT calculations suggest that stabilizing noncovalent Se···O interactions may be responsible for the observed reactivity.

Allenols versus Allenones: Rhodium-Catalyzed Regiodivergent and Tunable Allene Reactivity with Triazoles.

2-Pyrrolines and 6-oxo-hexa-2,4-dienals have been prepared through the divergent reactions of 1-benzenesulfonyl-4-aryl-1,2,3-triazoles with functionalized allenes. The rhodium-catalyzed reactions between allenols and 1-benzenesulfonyl-4-aryl-1,2,3-triazoles yielded 2-pyrrolines. This transformation is compatible with the presence of aliphatic, aromatic, heterocyclic, amide, and halogen functional groups. Interestingly, a reactivity switch took place when the allene-tethered alcohol substrate was replaced by its ketone counterpart. When the rhodium-catalyzed reaction of 1-benzenesulfonyl-4-phenyl-1,2,3-triazole was performed with allenones, acyclic 6-oxo-hexa-2,4-dienals were stereoselectively formed as (2Z,4E) isomers.

Cationic AuIII versus AuI : Catalyst-Controlled Divergent Reactivity of Alkyne-Tethered Lactams.

Switchable reactivity through cationic gold-based catalyst control built on the oxidation state, namely cationic AuIII versus AuI , has been achieved in the direct functionalization of 2-azetidinone-tethered alkynyl indoles.

Metal-Catalyzed Cyclization Reactions of 2,3,4-Trien-1-ols: A Joint Experimental-Computational Study.

Controlled preparation of tri- and tetrasubstituted furans, as well as carbazoles has been achieved through chemo- and regioselective metal-catalyzed cyclization reactions of cumulenic alcohols. The gold- and palladium-catalyzed cycloisomerization reactions of cumulenols, including indole-tethered 2,3,4-trien-1-ols, to trisubstituted furans was effective, due to a 5-endo-dig oxycyclization by attack of the hydroxy group onto the central cumulene double bond. In contrast, palladium-catalyzed heterocyclization/coupling reactions with 3-bromoprop-1-enes furnished tetrasubstituted furans. Also studied was the palladium-catalyzed cyclization/coupling sequence involving protected indole-tethered 2,3,4-trien-1-ols and 3-bromoprop-1-enes that exclusively generated trisubstituted carbazole derivatives. These results could be explained through a selective 6-endo-dig cumulenic hydroarylation, followed by aromatization. DFT calculations were carried out to understand this difference in reactivity.

Stereoselective synthesis of strained cage compounds via gold-catalyzed allene functionalization.

The diastereoselective synthesis of strained adducts that show cage-like structures has been accomplished directly from allenyl-ß-lactams through gold catalysis.

Iron-catalyzed domino indole fluorination/allenic aza-Claisen rearrangement.

The synthesis of 2-allenyl-2-substituted-3,3-difluoroindolines has been accomplished, taking advantage of the reaction between N-allenyl-indoles and Selectfluor under iron catalysis.

Gold as Catalyst for the Hydroarylation and Domino Hydroarylation/N1-C4 Cleavage of ß-Lactam-Tethered Allenyl Indoles.

Gold-catalyzed hydroarylation reaction of ß-lactam-tethered allenyl indoles gives azeto-oxepino[4,5-b]indol-2-ones, tetrahydroazeto-azocino[3,4-b]indol-2-ones, and hexahydroazeto-azepino[3,4-b]indol-2-ones with very high levels of stereo- and regioselectivity, the 7-exo and 8-endo carbocyclization modes by attack of the indole group toward either the internal or the terminal allene carbon, respectively, being favored. Hydroarylation across the central carbon of the allene moiety has not been detected. The controlled gold-catalyzed annulations allowed the formation of fused ß-lactams without harming the sensitive four-membered heterocycle. Besides, a novel gold-catalyzed domino process, namely, the allenic hydroarylation/N1-C4 ß-lactam bond breakage to afford dihydro-oxepino[4,5-b]indole-4-carboxamides, has been discovered.

Divergent reactivity of homologue ortho-allenylbenzaldehydes controlled by the tether length: chromone versus chromene formation.

The divergent behavior of two homologue allenals, namely, 2-(buta-2,3-dienyloxy)- and 2-(propa-1,2-dienyloxy)benzaldehydes, as cyclization substrates is described. 2-(Buta-2,3-dienyloxy)benzaldehydes suffers a formal allenic carbocyclization reaction to afford chromenes, whereas 2-(propa-1,2-dienyloxy)benzaldehydes react to yield chromones. The formation of chromenes is strictly a formal hydroarylation process divided into two parts, namely, allenic Claisen-type rearrangement and oxycyclization. An unknown N-heterocyclic carbene (NHC)-catalyzed allenic hydroacylation reaction must be invoked to account for the preparation of chromones. ortho-Allenylbenzaldehydes bearing either electron-donating substituents or electron-withdrawing substituents worked well to afford both the hydroarylation and hydroacylation products. This unexpected difference in reactivity can be rationalized by means of density functional theory calculations.

Controlled heterocyclization/cross-coupling domino reaction of ß,γ-allendiols and α-allenic esters: method and mechanistic insight for the preparation of functionalized buta-1,3-dienyl dihydropyrans.

Starting from ß,γ-allendiols and α-allenic acetates, a chemo- and regiocontrolled palladium-catalyzed methodology has provided access to enantiopure 3,6-dihydropyrans that bear a buta-1,3-dienyl moiety. Thus, it has been demonstrated for the first time that the preparation of six-membered heterocycles through cross-coupling reactions of two different allenes can be accomplished. These heterocyclization/cross-coupling reactions have been developed experimentally and their mechanisms have additionally been investigated by a computational study.

Synthesis of fused-ß-lactams through selective gold-catalyzed oxycyclization of dioxolane-tethered enynes.

The gold-catalyzed preparation of 2-azetidinone-fused oxacycles was accomplished from ß-lactam-linked enynes through heterocyclization reaction taking advantage of the acetonide pendant group. While the synthesis of fused tetrahydrofuran-ß-lactams from 1,3-enynes could be considered as an unusual metal-catalyzed cyclization of enynols, α-alkoxy dioxolane-tethered 1,3-enynes exclusively undergo bis-oxycyclization to afford tricyclic bridged acetals.

Gold-catalysed tuning of reactivity in allenes: 9-endo hydroarylation versus formal 5-exo hydroalkylation.

The divergent gold-catalysed reactivity (C(sp(2))-H versus C(sp(3))-H) of aryloxy-tethered allenes has been uncovered.

Direct allenol-based stereocontrolled access to substituted (E)-1,3-enynes.

A stereoselective synthesis of 1-substituted (E)-2-aryl-but-1-en-3-ynes, including tetrasubstituted alkenes, has been developed from aryl-substituted α-allenols by treatment with the AcCl-NaOH (aqueous) system. This transformation might be explained through the elimination of acetic acid, made up of a δ-hydrogen and the acetate group in the initially formed α-allenic acetate.

Striking alkenol versus allenol reactivity: metal-catalyzed chemodifferentiating oxycyclization of enallenols.

An efficient chemodivergent metal-controlled methodology for the generation of different highly functionalized oxygen heterocycles from common enallenol substrates has been developed. Chemoselectivity control in the O-C functionalization of an enallenol can be achieved through the choice of catalyst: AuCl(3), PdCl(2), and [PtCl(2)(CH(2)=CH(2))](2) exclusively afford dihydrofurans through selective activation of the allenol moiety, whereas FeCl(3) solely gives tetrahydrofurans or tetrahydropyrans through selective activation of the alkenol moiety. We have also shown that a combination of metal-mediated hydroalkoxylation and allenic aminocyclization reactions can lead to a useful preparation of the tetrahydrofuro[3,2-b]piperidine core of the antimalarial alkaloid isofebrifugine. These divergent heterocyclization reactions have been developed experimentally and additionally, their mechanisms have been investigated by a theoretical study.

Fascinating reactivity in gold catalysis: synthesis of oxetenes through rare 4-exo-dig allene cyclization and infrequent ß-hydride elimination.

A novel reactivity in gold catalysis, namely the unusual preference for the 4-exo-dig cyclization in allene chemistry as well as the rare ß-hydride elimination reaction, was uncovered starting from readily available allenols.

Organocatalyzed enantioselective fluorocyclizations.

Enantioenriched fluorinated heterocycles can be prepared through fluorocyclizations of prochiral indoles (see scheme; Ts=tosyl, Bn=benzyl, Boc=tert-butoxycarbonyl). More than twenty examples for this cascade fluorination-cyclization, which is catalyzed by cinchona alkaloids and employs N-fluorobenzenesulfonimide as the electrophilic fluorine source have been explored, and an unprecedented catalytic asymmetric difluorocyclization has also been identified.

Metal-catalyzed cycloetherification reactions of ß,γ- and γ,δ-allendiols: chemo-, regio-, and stereocontrol in the synthesis of oxacycles.

Versatile routes that lead to a variety of functionalized enantiopure tetrahydrofurans, dihydropyrans, and tetrahydrooxepines are based on chemo-, regio-, and stereocontrolled metal-catalyzed oxycyclization reactions of ß,γ- and γ,δ-allendiols, which were readily prepared from (R)-2,3-O-isopropylideneglyceraldehyde. The application of Pd(II), Pt(II), Au(III), or La(III) salts as the catalysts gives controlled access to differently sized oxacycles in enantiopure form. Usually, chemoselective cyclization reactions occurred exclusively by attack of the secondary hydroxy group (except for the oxybromination of phenyl ß,γ-allenic diols 3b and 3d) to an allenic carbon atom. Regio- and stereocontrol issues are mainly influenced by the nature of the metal catalysts and substituents.

Cross-coupling/cyclization reactions of two different allenic moieties.

The allene moiety represents an excellent building block for allene cross-coupling cyclization reactions, affording heterocyclic skeletons in a single step. This strategy is of particular interest when two different allene derivatives are involved in a series of metal-catalyzed cross-coupling heterocyclization processes. This Concept article is focused on the Pd-catalyzed union of two different allenic moieties, with cyclization of at least one of them by intramolecular cyclometalation. These new, versatile, and highly effective transformations are complex multistep processes leading to potential privileged structures that could find wide applications in related medicinal chemistry.

Metal-catalyzed cyclization of beta- and gamma-allenols derived from D-glyceraldehyde--synthesis of enantiopure dihydropyrans and tetrahydrooxepines: an experimental and theoretical study.

Regiocontrolled metal-catalyzed preparations of enantiopure dihydropyrans and tetrahydrooxepines have been synthesized starting from beta- and gamma-allenols derived from D-glyceraldehyde. The Pd(II)-catalyzed cyclizative coupling reactions of beta-allenols 1 a and 1 b with allyl bromide effectively afforded enantiopure tetrafunctionalized dihydropyrans through a 6-endo oxycyclization protocol, whereas the gold-, platinum-, and palladium-mediated heterocyclization of gamma-allenol 2 furnished tetrahydrooxepines 13-16 through regioselective 7-endo-trig oxycyclization reactions. Moreover, density functional calculations were performed to predict the regioselectivity of the gamma-allenol cycloetherification to tetrahydrooxepines on the basis of both the tether nature and characteristics of the metals, and to gain an insight into the mechanism of the oxycyclization reactions.

Chemo- and regioselective palladium-catalyzed oxycyclization reactions of allendiols: preparation of five-, six-, and eight-membered cycles.

Positive discrimination: Chemo- and regioselective palladium-catalyzed cycloetherification of allendiols, namely beta,gamma- and gamma,delta-allendiols, may occur by judicious choice of palladium-catalyzed conditions owing to their potential ability to discriminate between both nucleophilic sites (see scheme).

Synthesis of spiroheterocycles by palladium-catalyzed domino cycloisomerization/cross-coupling of alpha-allenols and Baylis-Hillman acetates.

First insights into the reaction between a Baylis-Hillman adduct and an allene moiety have been obtained from the novel domino heterocyclization/cross-coupling reaction of alpha-allenols and Baylis-Hillman acetates, which furnishes [(2,5-dihydrofuran-3-yl)methyl]acrylate derivatives in moderate to good yields.