Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.

Regio- and Diastereoselective Construction of Functionalized Benzo[b]oxepines and Benzo[b]azepines via Recyclable Gold(I)-Catalyzed Cyclizations

The heterogeneous gold-catalyzed cyclization of (o-alkynyl)phenoxy- or N-(o-alkynylphenyl)tolylsulfonamidoacrylates with alcohols has been developed by using an MCM-41-anchored diphenylphosphine-Au(I) complex [MCM-41-Ph₂P-AuNTf₂] as the catalyst under mild reaction conditions, yielding diverse functionalized benzo[b]oxepines or benzo[b]azepines with good to high yields and excellent diastereoselectivity. This heterogenized gold(I) catalyst exhibits a comparable activity to homogeneous Ph₃PAuNTf₂ and can be facilely recovered by a simple filtration of the reaction solution and reused more than seven times with almost a consistent catalytic efficiency.

Annulative Morita-Baylis-Hillman reaction to synthesise chiral dibenzocycloheptanes

We describe the first metal-free and organocatalytic strategy to access highly functionalised dibenzocycloheptanes via a phosphine-promoted annulative Morita-Baylis-Hillman (MBH) reaction. The method is manipulated to access to chiral dibenzocycloheptanes as well. This work represents a rare entry for the construction of seven-membered carbocycles via the MBH route. The realisation of several bioactive molecules possessing the dibenzocycloheptane core makes this an attractive strategy.

Utilization of the Thorpe-Ingold Effect in the Synthesis of Cyclooctanoid Ring Systems via Anionic 6-exo-dig Cyclization/Claisen Rearrangement Sequence

We demonstrate a facile approach for the synthesis of gem-disubstituted cyclooctanoids, a motif found in several biologically active compounds. Appropriately substituted 1-alkenyl-5-pentyn-1-ols bearing gem-dialkyl substituents at either the C2, C3, or C4 position serve as useful precursors to a number of cyclooct-4-enone derivatives via a tandem, microwave-assisted oxyanionic 6-exo-dig cyclization/Claisen rearrangement reaction. gem-Dialkyl activation is necessary for these reactions to occur, as unactivated 1-alkenyl-5-pentyn-1-ols fail to undergo 6-exo-dig cyclization under the conditions employed. Further application of the methodology to the corresponding gem-dialkoxy system was also explored to facilitate access to more complex carbocycles.

Enantioselective Divergent Syntheses of (+)-Bulleyanaline and Related Isoquinoline Alkaloids from the Genus Corydalis

The isoquinoline alkaloids isolated from the genus Corydalis possess potent and diverse biological activities. Herein, a concise, divergent, and enantioselective route to access these natural products is disclosed. Key transformations of our approach include a challenging Zn-ProPhenol-catalyzed asymmetric Mannich reaction to build a quaternary stereogenic center and a rapid cationic Au-catalyzed cycloisomerization to the common structural skeleton of these natural products. Subsequent late-stage oxidations and modifications allow efficient access to the targeted alkaloids. Overall, seven natural products have been successfully synthesized in 6 to 10 steps from readily available starting materials, including (+)-corynoline, (+)-anhydrocorynoline, (+)-12-hydroxycorynoline, (+)-12-hydroxycorynoloxine, (+)-corynoloxine, (+)-6-acetonylcorynoline, and (+)-bulleyanaline.

Copper-Catalyzed Cascade Cyclization of Indolyl Homopropargyl Amides: Stereospecific Construction of Bridged Aza-[n.2.1] Skeletons

Catalytic cycloisomerization-initiated cascade cyclizations of terminal alkynes have received tremendous interest, and been widely used in the facile synthesis of a diverse array of valuable complex heterocycles. However, these tandem reactions have been mostly limited to noble-metal catalysis, and are initiated by an exo-cyclization pathway. Reported herein is an unprecedented copper-catalyzed endo-cyclization-initiated tandem reaction of indolyl homopropargyl amides, where copper catalyzes both the hydroamination and Friedel-Crafts alkylation process. This method allows the practical and atom-economical synthesis of valuable bridged aza-[n.2.1] skeletons (n=3-6) with wide substrate scope, and excellent diastereoselectivity and enantioselectivity by a chirality-transfer strategy. Moreover, the mechanistic rationale for this novel cascade cyclization is also strongly supported by control experiments, and is distinctively different from the related gold catalysis.

Stereoselective synthesis of 2,5-disubstituted pyrrolidines via gold-catalysed anti-Markovnikov hydroamination-initiated tandem reactions

A series of gold-catalysed intramolecular anti-Markovnikov hydroamination-initiated azidation, allylation and heteroarylation reactions of chiral homopropargyl sulfonamides have been developed. Various enantioenriched 2,5-disubstituted pyrrolidines are obtained in moderate to excellent yields with excellent enantioselectivities and generally high diastereoselectivities.

Domino C-F Bond Activation of the CF3 Group: Synthesis of Fluorinated Dibenzo[a,c][7]annulenes from 2-(Trifluoromethyl)-1-alkenes and 2,2'-Diceriobiaryls

The construction of ring-fluorinated seven-membered carbocycles was readily achieved via the domino S_N2'-type/S_NV reaction between 2-(trifluoromethyl)-1-alkenes and 1,4-carbodianions. The S_N2'-type reaction of 2-(trifluoromethyl)-1-alkenes with 2,2'-diceriobiaryls generated the intermediary 1,1-difluoro-1-alkenes bearing a monoceriobiaryl moiety, which in turn underwent intramolecular S_NV reaction to afford fluorinated 5H-dibenzo[a,c][7]annulenes.

Construction of dibenzo-fused seven- to nine-membered carbocycles via Brønsted acid-promoted intramolecular Friedel-Crafts-type alkenylation

Brønsted acid-promoted intramolecular hydroarylation of alkynylbenzenes carrying an arylalkyl group at the ortho-position leads to alkylidenedibenzo[a,d]cycloheptenes, -octenes and -nonenes in up to quantitative yield with complete regioselectivity. The scope and limitation of this reaction and application to the synthesis of tricyclic antidepressants are described.

Gold catalysis in total synthesis – recent achievements

The most recent achievements of gold catalysed transformations applied in total synthesis of natural products are reviewed and analysed.

A facile and highly chemoselective synthesis of 1-thia-3a,6-diaza-benzo[e]azulen-3-ones by 7-exo-dig/trig halocyclizations

The manuscript describes a study on relatively unexplored halogen mediated 7-exo-dig/trig cyclization reactions of 2-(2-amino-aryl)-3-prop-2-ynyl/allyl-thiazolidin-4-ones for the formation of thiazole condensed 1,4-benzodiazepines in good yields.

Anatomy of gold catalysts: facts and myths

This review article covers the main types of gold(i) complexes used as precatalysts under homogeneous conditions in organic synthesis and discusses the different ways of catalyst activation as well as ligand, silver, and anion effects.

Dual gold catalysis: synthesis of polycyclic compounds via C-H insertion of gold vinylidenes

New and interesting polycyclic compounds have been synthesized from non-conjugated diyne systems by dual gold catalysis. A quaternary carbon center in the backbone and the accompanying Thorpe-Ingold effect enabled the unprecedented insertion of sp(3) and sp(2) CH bonds that for the first time were incorporated within the backbone of the diyne system and allowed the construction of complex polycyclic carbon scaffolds inaccessible by previous approaches in which the CH bonds for the insertion were situated at the other end of the alkyne.