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

Total Synthesis of Pyrrolidine and Piperidine Natural Products via TMSOTf-Mediated "5/6-endo-dig" Reductive Hydroamination of Enynyl Amines

Stereoselective syntheses of pyrrolidines and piperidines bearing hydrophobic chains have been achieved through a metal free, Lewis acid-mediated 5/6-endo-dig reductive hydroamination cascade of enynyl amines. The brevity of the developed strategy allowed for the collective stereoselective total synthesis of various alkaloids, including (±)-pyrrolidine cis-225H, (±)-epi-197B, (±)-epi-225C, the family of (+)-solenopsins and (+)-isosolenopsins, and the formal synthesis of (±)-bgugaine and (+)-azimic acid.

Brønsted acid-catalyzed asymmetric dearomatization for synthesis of chiral fused polycyclic enone and indoline scaffolds

In the past two decades, substantial advances have been made on the asymmetric alkyne functionalization by the activation of inert alkynes. However, these asymmetric transformations have so far been mostly limited to transition metal catalysis, and chiral Brønsted acid-catalyzed examples are rarely explored. Here, we report a chiral Brønsted acid-catalyzed dearomatization reaction of phenol- and indole-tethered homopropargyl amines, allowing the practical and atom-economical synthesis of a diverse array of valuable fused polycyclic enones and indolines bearing a chiral quaternary carbon stereocenter and two contiguous stereogenic centers in moderate to good yields with excellent diastereoselectivities and generally excellent enantioselectivities (up to >99% enantiomeric excess). This protocol demonstrates Brønsted acid-catalyzed asymmetric dearomatizations via vinylidene-quinone methides.

Gold-Catalyzed [4 + 1] Heterocyclization of Hydroxamic Acid and Nonactivated Alkyne: A Protocol to Construct 5-Methyl-1,4,2-dioxazole

A novel gold-catalyzed [4 + 1] heterocyclization of nonactivated alkyne and hydroxamic acid is developed for the regiospecific synthesis of 5-methyl-1,4,2-dioxazole, which is an important structural motif in various bioactive molecules. The current methodology is characterized by high efficiency, simple operation, mild reaction conditions, and good functional group compatibility. Moreover, gram-scale synthesis and synthetic application toward bioactive molecular skeletons have been realized.

Stereoselective Tandem Synthesis of Pyrrolidine Derivatives under Gold Catalysis: An Asymmetric Synthesis of (-)-Lepadiformine A

A Au-catalyzed tandem alkyne hydroamination/iminium ion formation/allylation reaction was developed for expedient access to pyrrolidine derivatives bearing a tetrasubstituted carbon stereocenter. The tandem reaction was successfully applied to a 12-step asymmetric synthesis of (-)-lepadiformine A, a marine cytotoxic tricyclic alkaloid.

Silver-Catalyzed Activation of Terminal Alkynes for Synthesizing Nitrogen-Containing Molecules

Alkynes are one of the most abundant chemicals in organic chemistry, and therefore the development of catalytic reactions to transform alkynes into other useful functionalities is of great value. In recent decades, extraordinary advances have been made in this area with transition-metal catalysis, and silver-based reagents are ideal for the activation of alkynes. This high reactivity is probably due to the superior π-Lewis acidic, carbophilic behavior of silver(I), allowing it to selectively activate carbon-carbon triple bonds (C≡C) through the formation of a silver-π complex. Within this field, we have been interested in the activation and subsequent reactions of readily accessible terminal alkynes for the synthesis of nitrogen-containing compounds, which has generally received less attention than methods involving internal alkynes. This is possibly due to the lack of suitable reactive reaction partners that are compatible under transition metals. Therefore, a thorough understanding of the factors that influence homogeneous silver catalysis and the identification of the appropriate reaction partners can provide a powerful platform for designing more efficient silver-catalyzed reactions of terminal alkynes. In this context, we envisioned that using readily available, environmentally benign, and inexpensive trimethylsilyl azide (TMSN₃) or an isocyanide as the nitrogen-donor would be the key to develop novel reactions of terminal alkynes.This Account describes our efforts since 2013 toward the development of novel silver-catalyzed tandem reactions of terminal alkynes with either TMSN₃ or isocyanides for the assembly of various nitrogen-containing compounds. The first section of this Account discusses the initial developments in the silver-catalyzed hydroazidation of terminal alkynes with TMSN₃ and the subsequent advances made in our laboratory. We first describe the discovery and experimental and computational mechanistic investigations of silver-catalyzed hydroazidation reactions, which is the most efficient strategy reported to date for accessing vinyl azides. Mechanistic study of this hydroazidation reaction provides an alternative activation mode for terminal alkyne conversion in transition metal catalysis. We then present the chemistry of in situ generated vinyl azides, including one-pot tandem radical addition/cyclization or migration reactions of terminal alkynes to access a variety of nitrogen-containing molecules. Finally, we discuss the one-pot, multistep tandem hydroazidation and 1,2-azide migratory gem-difluorination of terminal alkynes for the synthesis of β-difluorinated alkyl azides. The second section describes the silver-catalyzed coupling reactions between terminal alkynes and isocyanides, which offer a straightforward method for accessing synthetically useful building blocks, such as pyrroles, allenamides, benzofuran, vinyl sulfones, indazolines, propiolonitriles, and pyrazoles. The high efficiency, mild conditions, low cost, broad substrate scope, high chemo- and regioselectivity, step economy, and ecofriendliness of the developed approaches make them attractive and practical. The progress in this area provides guiding principles for designing new reactions of terminal alkynes that can be extended to various nitrogen-containing molecules of interest to medicinal and materials chemists.