Ynesulfonamide-Based Silica Gel and Alumina-Mediated Diastereoselective Cascade Cyclizations to Spiro[indoline-3,3′-pyrrolidin]-2-ones under Neat Conditions

Methodological Development and Applications of Tryptamine-Ynamide Cyclizations in Synthesizing Core Skeletons of Indole Alkaloids

Over the past two decades, synthetic strategies for synthesizing the skeletons of various indole alkaloids based on tryptamine-ynamide have been continuously developed and applied to the total syntheses or formal total syntheses of related molecules. In this synopsis, we summarized the cyclization pathways of tryptamine-ynamide under different catalytic conditions, emphasizing the reaction mechanism and applications in the syntheses of indole alkaloids.

Collective Syntheses of 8-Oxoprotoberberines via Sequential In(OTf)3-Catalyzed Cyclization and Pd(OAc)2-Catalyzed Heck Coupling

Six 8-oxoprotoberberines were synthesized collectively in four steps with acceptable yields (14-19%), of which the products 8-oxopalmatine, 8-oxopseudopalmatine, 8-oxoberberine, and 8-oxopseudoberberine come from nature. The synthetic route was featured with the In(OTf)₃-catalyzed cyclization and Heck coupling. Moreover, the syntheses of the natural products berberine, canadine, and iambertine were achieved via various reductions from 8-oxoberberine, which provided a concise approach to the syntheses of this kind of alkaloids.

Quantum Mechanical Prediction and Experimental Verification of Au(I)-Catalyzed Substitution-Controlled Syntheses of 1H-Pyrido[4,3-b]indole and Spiro[indoline-3,3'-pyridine] Derivatives

Density functional theory calculations were applied to predict the pathways of gold(I)-catalyzed cycloisomerization of the indole substrates with 1,6-enynes, which were consistent with the ensuing experimental results. The substitution-controlled synthesis led to the formation of 1H-pyrido[4,3-b]indole and spiro[indoline-3,3'-pyridine] derivatives in a tunable way. The reactions had good functional group tolerances, and a possible mechanism was proposed based on the computational and experimental results.

Computational insight into gold(I)-catalyzed intramolecular regioselectivity of tryptamine-ynamide cycloisomerizations

The regioselectivity for gold(I)-catalyzed intramolecular cycloisomerizations of tryptamine-ynamides has long been elusive despite various synthetic examples of similar substrates being available. Computational studies were carried out to provide insight into the mechanisms and the origin of the substrate-dependent regioselectivity of these transformations. Based on the analyses of non-covalent interactions, distortion/interaction, and energy decomposition on the interactions between the terminal substituent of alkynes and the gold(I) catalytic ligand, the electrostatic effect was determined to be the key factor for α-position selectivity while the dispersion effect was determined to be the key factor for β-position selectivity. Our computational results were consistent with the experimental observations. This study provides useful guidance for understanding other similar gold(I)-catalyzed asymmetric alkyne cyclization reactions.

I2 -Catalyzed Cycloisomerization of Ynamides: Chemoselective and Divergent Access to Indole Derivatives

Herein, an I₂ -catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom-economy. This protocol not only represents the first molecular-iodine-catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine-ynamides involving distinctively different C(sp³ )-C(sp³ ) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I₂ . Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5-exo-dig cyclization/rearrangement process.

Lewis Acid-Catalyzed Diastereoselective Domino Reaction of Ene-Ynamide with Trimethylsilyl Cyanide to Construct Spiroindolines

The Zn(OTf)₂-catalyzed domino reaction of enamide-ynamides in the presence of trimethylsilyl cyanide as an external nucleophile to construct spirocyclic indolines was developed. This domino reaction involved cyclization of enamide to ynamide to generate 4',5'-dihydrospiro[indoline-3,3'-pyrrol]-1'-ium followed by cyanide addition to produce spiroindolopyrrolidines with good diastereoselectivity.

Stereoselective Domino Reactions in the Synthesis of Spiro Compounds

This review summarizes the latest developments in asymmetric domino reactions, with the emphasis on the preparation of spiro compounds. Discussions on the stereoselectivity of the transformations, the reaction mechanisms, the rationalization of the stereochemical outcome, and the applications of domino reactions to the synthesis of biologically active molecules and natural products are included when appropriate.

1 Introduction

2 Asymmetric Domino Reactions

2.1 Domino Reactions Initiated by Michael Reactions

2.2 Domino Reactions Initiated by Mannich Reactions

2.3 Domino Reactions Initiated by Knoevenagel Reactions

2.4 Domino Reactions Initiated by Cycloaddition Reactions

2.5 Domino Reactions Initiated by Metal Insertion

2.6 Other Mechanisms

3 Conclusion

Ag(I)/PPh3-catalyzed diastereoselective syntheses of spiro[indole-3,4'-piperidine] derivatives via cycloisomerizations of tryptamine-ynamides

A Ag(I)/PPh₃-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4'-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π-π interactions.

Total Syntheses of Aspidospermidine, N-Methylaspidospermidine, N-Acetylaspidospermidine, and Aspidospermine via a Tandem Cyclization of Tryptamine-Ynamide

The total syntheses of aspidospermidine, N-methylaspidospermidine, N-acetylaspidospermidine, and aspidospermine were achieved from a common pentacyclic indoline intermediate. The common pentacyclic indoline intermediate was synthesized on a gram scale through a Stork-enamine alkylation of 1H-pyrrolo[2,3-d]carbazole derivatives, which were prepared through a Brønsted acid-catalyzed tandem cyclization of tryptamine-ynamide. The scalable synthesis of 1H-pyrrolo[2,3-d]carbazole afforded facile access and a practical approach to the Aspidosperma indole alkaloid family.

Competing H2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H₂ upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L^Ph]^3- with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L^Ph(Ni^II-H)₂]^- (3) are prone to intramolecular reductive H₂ elimination, and an equilibrium between 3 and orthometalated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal-metal cooperative phenyl group C(sp²)-H oxidative addition to the dinickel(I) intermediate [L^PhNi^I₂]^- (4). While Ni^I species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni^I complexes has rarely been described. The reversible H₂ and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [L^PhNi^I₂]^- intermediate 4 either via elimination of H₂ from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H₂ byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.

N-Fluorobenzenesulfonimide as a highly effective Ag(i)-catalyst attenuator for tryptamine-derived ynesulfonamide cycloisomerization

N-Fluorobenzenesulfonimide was identified for the first time as a unique Ag(i)-catalyst attenuator in the annulation of a tryptamine-derived ynesulfonamide to an azepino[4,5-b]indole.

Gold(i)-catalyzed pathway-switchable tandem cycloisomerizations to indolizino[8,7-b]indole and indolo[2,3-a]quinolizine derivatives

A common strategy was developed to access both indolizino[8,7-b]indole and indolo[2,3-a]quinolizine derivatives from tryptamine-N-ethynylpropiolamide substrates in a switchable fashion via tuning both the electronic effects and steric effects.