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.

Assembly of Complex 1,4-Cycloheptadienes by (4+3) Cycloaddition of Rhodium(II) and Gold(I) Non-Acceptor Carbenes

The formal (4+3) cycloaddition of 1,3-dienes with Rh(II) and Au(I) non-acceptor vinyl carbenes, generated from vinylcycloheptatrienes or alkoxyenynes, respectively, leads to 1,4-cycloheptadienes featuring complex and diverse substitution patterns, including natural dyctiopterene C' and a hydroxylated derivative of carota-1,4-diene. A complete mechanistic picture is presented, in which Au(I) and Rh(II) non-acceptor vinyl carbenes were shown to undergo a vinylcyclopropanation/Cope rearrangement or a direct (4+3) cycloaddition that takes place in a non-concerted manner.

Abnormal Nucleophilic Substitution on Methoxytropone Derivatives: Steric Strategy to Synthesize 5-Substituted Azulenes

Azulene is a non-alternant non-benzenoid aromatic system, and in turn, it possesses unusual photophysical properties. Azulene-based conjugated systems have received increasing interest in recent years as optoelectronic materials. Despite the routes available for the preparation of substituted azulene derivatives, there remain few methods that allow regioselective substitution on the seven-membered ring of azulenes due to the subtle reactivity difference among the various positions. This report explores the reactivity of substituted tropolones as the azulene precursors and also provides a new method to create 5-substituted azulenes. The reaction of cyanoacetate enolate with unsubstituted 2-methoxytropone affords azulene through the attack of the nucleophile on the C-2 center (normal pathway). We have observed that 3-substituted 2-methoxytropones undergo steric-guided nucleophilic addition at the C-7 center (abnormal pathway) to afford 5-substituted azulene derivatives. Based on this observation and DFT calculation, a new synthetic strategy is devised for the regioselective synthesis of 5-substituted multifunctional azulenes, which cannot be accessed by any other method.

Silver-Catalyzed Domino Reaction of ortho-Carbonylated Alkynyl-Substituted Arylaldehydes with Conjugated Dienes: Stereoselective Access to Indanone-Fused Cyclohexenes

A silver-catalyzed domino reaction of ortho-carbonylated alkynyl-substituted arylaldehydes with conjugated dienes is described here. Through this reaction, the synthesis of a variety of indanone-fused cyclohexene derivatives can be achieved efficiently. The formation of these tricyclic products could involve a key Diels-Alder reaction of in situ generated indanenone dienophiles with conjugated dienes. Particularly, the products can be accomplished in a high endo/exo selective way.

Gold-Catalyzed Reaction of ortho-Alkynylarylaldehydes with Conjugated Dienes: An Efficient Access to Highly Strained Tetracyclic Bridgehead Olefins

An unprecedented access to strained tetracyclic bridgehead alkenes by reaction of easily accessible ortho-alkynylarylaldehydes with conjugated dienes is described. The process involves a chemo- and stereo-selective, gold-catalyzed, tandem intermolecular [3+2] cycloaddition/Prins-type ring-closing reaction that allows generating structural complexity in a straightforward manner. This approach for the preparation of anti-Bredt compounds is synthetically superior to those previously reported: the procedure is easy to implement, operates under mild experimental conditions, is efficient, and exhibits a good substrate scope.

Gold-catalyzed π-directed regioselective cyclization of bis(o-alkynyl benzyl alcohols): rapid access to dihydroisobenzofuran derivatives

A tandem alkyne/alkene-directed 5-exo-dig cyclization and a Prins-type cyclization for the synthesis of dihydroisobenzofurans are described.

Gold-catalyzed cycloisomerization of 1,6,8-dienyne carbonates and esters to cis-cyclohepta-4,8-diene-fused pyrrolidines

A synthetic approach that provides access to cis-cyclohepta-4,8-diene-fused pyrrolidines efficiently through Au(I) -catalyzed cycloisomerization of 1,6,8-dienyne carbonates and esters at a low catalyst loading of 2 mol % is reported. Starting carbonates and esters with a pendant alkyl group on the terminal alkenyl carbon center were found to favor tandem 1,2-acyloxy migration/cyclopropanation followed by Cope rearrangement of the resulting cis-3-azabicyclo[3.1.0]hexane intermediate. On the other hand, substrates containing a terminal diene or starting materials in which the distal alkene moiety bears a phenyl substituent were observed to undergo competitive but reversible 1,3-acyloxy migration prior to the nitrogen-containing bicyclic ring formation. The delineated reaction mechanism also provides experimental evidence for the reversible interconversion between the oft-proposed organogold intermediates obtained in this step of the tandem process.

Gold-catalyzed synthesis of functionalized pyridines by using 2H-azirines as synthetic equivalents of alkenyl nitrenes

2H-Azirines are easily synthesized from the corresponding ketones and, despite possessing a CN bond embedded in a strained three-membered cycle, they are sufficiently stable to be isolated, stored, and manipulated. 2H-Azirines can be regarded as valuable synthetic equivalents of alkenyl nitrenes, however, reactions capitalizing on the cyclic strain of the heterocyclic motif and involving the cleavage of the CN single bond remain surprisingly limited. A gold-catalyzed reaction that allows the formation of polysubstituted functionalized pyridines from easily accessible 2-propargyl 2H-azirine derivatives was developed. This transformation, which corresponds to an unprecedented intramolecular transfer of an alkenyl nitrene to an alkyne, proceeds with low catalyst loading, is efficient, and exhibits a high functional-group tolerance and a wide substrate scope.