Enantioselective cyclization of enamide-ynes and application to the synthesis of the kopsifoline core

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Modular Synthesis of Polycyclic Alkaloid Scaffolds via an Enantioselective Dearomative Cascade

The polycyclic core of the akuammiline alkaloids can be synthesized from simple tryptamine and tryptophol derivatives via a Ag(I)-catalyzed enantioselective dearomative cyclization cascade sequence. The complex tetracyclic scaffolds are prepared via a rapid, versatile, three-step modular synthesis from simple commercially available indole derivatives in high yields and enantiomeric excess (up to 99% yield and >99% ee).

Modular synthesis and transition metal-free alkynylation/alkenylation of Castagnoli–Cushman-derived N,O- and N,S-heterocyclic vinyl chlorides

A modular and functional group-tolerant protocol for the transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides with terminal acetylenes and styrenes has been developed, leading to the epimerization-free synthesis of fully carbofunctionalized dihydro-1,4-oxazines/thiazines. Bicyclic morpholines have also been prepared through the interrogation of newly synthesized cross-conjugated dienes in Diels–Alder reactions. The use of environmentally benign reaction media endows the current strategy with a practical advantage.

A functional group-tolerant and transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides, which affords fully carbosubstituted dihydro-1,4-oxazines/thiazines as well as bicyclic morpholines, is described.

Iridium-catalyzed α-alkynylation of cyclic nonaromatic eneformamides: application to the synthesis of azapolycyclic architectures

A regioselective and chelation-assisted dehydrogenative alkynylation of cyclic nonaromatic eneformamides with terminal alkynes, under iridium catalysis, is described.

Chiral Phosphoric Acid Catalyzed Intramolecular Dearomative Michael Addition of Indoles to Enones

An enantioselective intramolecular dearomative Michael addition of indolyl enones is presented. In the presence of catalytic amount of chiral phosphoric acid, various enantioenriched spiro-indolenines bearing a quaternary stereogenic center were obtained with good yields and enantioselectivity (up to 97% ee) under mild reaction conditions.

Phosphine-catalyzed [3 + 2] annulation reaction: highly regio- and diastereoselective synthesis of 2-azaspiro[4.4]nonene-1,3-diones

A novel method for phosphine-catalyzed [3 + 2] annulation of γ-substituted allenoates with succinimides was developed, which was successfully applied to the synthesis of 2-azaspiro[4.4]nonene-1,3-diones derivatives in 50–96% yield, with high regio- and diastereoselectivities.

Direct metal-catalyzed regioselective functionalization of enamides

Enamides are stable enamine surrogates and provide key intermediates for the synthesis of small but complex nitrogen-containing compounds. Metal-catalyzed regioselective functionalization of enamides provides a rapid method to synthesize useful nitrogen containing heterocycles. This review discloses the recent progress made in the development of the C-H functionalization of enamides involving efficient and atom-economical routes. Syntheses of different heterocycles are classified based on the site reactivity of enamides and key mechanistic insights are given for each transformation.

Development of catalysts and ligands for enantioselective gold catalysis

During the past decade, the use of Au(I) complexes for the catalytic activation of C-C π-bonds has been investigated intensely. Over this time period, the development of homogeneous gold catalysis has been extraordinarily rapid and has yielded a host of mild and selective methods for the formation of carbon-carbon and carbon-heteroatom bonds. The facile formation of new bonds facilitated by gold naturally led to efforts toward rendering these transformations enantioselective. In this Account, we survey the development of catalysts and ligands for enantioselective gold catalysis by our research group as well as related work by others. We also discuss some of our strategies to address the challenges of enantioselective gold(I) catalysis. Early on, our work with enantioselective gold-catalyzed transformations focused on bis(phosphinegold) complexes derived from axially chiral scaffolds. Although these complexes were highly successful in some reactions like cyclopropanation, the careful choice of the weakly coordinating ligand (or counterion) was necessary to obtain high levels of enantioselectivity for the case of allene hydroamination. These counterion effects led us to use the anion itself as a source of chirality, which was successful in the case of allene hydroalkoxylation. In general, these tactics enhance the steric influence around the reactive gold center beyond the two-coordinate ligand environment. The use of binuclear complexes allowed us to use the second gold center and its associated ligand (or counterion) to exert a further steric influence. In a similar vein, we employed a chiral anion (in place of or in addition to a chiral ligand) to move the chiral information closer to the reactive center. In order to expand the scope of reactions amenable to enantioselective gold catalysis to cycloadditions and other carbocyclization processes, we also developed a new class of mononuclear phosphite and phosphoramidite ligands to supplement the previously widely utilized phosphines. However, we needed to judiciously design the steric environment to create "walls" that enclose the gold center. We also successfully applied these same considerations to the development of binuclear carbene ligands for gold. Finally, we describe the design of bifunctional urea-monophosphine ligands used in a gold-catalyzed three-component coupling.

Total syntheses of (-)-kopsifoline D and (-)-deoxoapodine: divergent total synthesis via late-stage key strategic bond formation

Divergent total syntheses of (-)-kopsifoline D and (-)-deoxoapodine are detailed from a common pentacyclic intermediate 15, enlisting the late-stage formation of two different key strategic bonds (C21-C3 and C21-O-C6) unique to their hexacyclic ring systems that are complementary to its prior use in the total syntheses of kopsinine (C21-C2 bond formation) and (+)-fendleridine (C21-O-C19 bond formation). The combined efforts represent the total syntheses of members of four classes of natural products from a common intermediate functionalized for late-stage formation of four different key strategic bonds uniquely embedded in each natural product core structure. Key to the first reported total synthesis of a kopsifoline that is detailed herein was the development of a transannular enamide alkylation for late-stage formation of the C21-C3 bond with direct introduction of the reactive indolenine C2 oxidation state from a penultimate C21 functionalized Aspidosperma-like pentacyclic intermediate. Central to the assemblage of the underlying Apidosperma skeleton is a powerful intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole that provided the functionalized pentacyclic ring system 15 in a single step in which the C3 methyl ester found in the natural products served as a key 1,3,4-oxadiazole substituent, activating it for participation in the initiating Diels-Alder reaction and stabilizing the intermediate 1,3-dipole.