Construction of Fused Heterocyclic Architectures by Formal [4+1]/[3+2] Cycloaddition Cascade of Sulfur Ylides and Nitroolefins

Nitronate-aryne cycloaddition as a concise route to stereochemically complex fused benzisoxazolines and amino alcohols

The reaction of cyclic nitronates (isoxazoline N-oxides and 5,6-dihydro-4H-1,2-oxazine N-oxides) with Kobayashi's aryne precursors affords tricyclic benzene-fused nitroso acetals as a result of [3 + 2]-cycloaddition. The process is regio- and stereoselective in most cases and produces the target cycloadducts possessing up to four contiguous stereogenic centers. These nitroso acetals were shown to be convenient precursors of valuable polysubstituted aminodiols through catalytic hydrogenolysis of the N-O bonds. Also, the action of protic acids resulted in an unusual fragmentation of the cyclic nitroso acetal moiety through heterolytic N-O bond cleavage and Beckmann-type reaction. Using this acid-mediated reaction, the synthesis of a hitherto unknown hexahydrobenzo[4,5]isoxazolo[2,3-a]azepine scaffold was accomplished.

Organocatalytic Transformations from Sulfur Ylides

Sulfur ylides are an important class of organic compounds due to their ability to perform many different transformations that can give diverse and interesting products with a high degree of complexity. Although metal-catalyzed transformations are frequent in this class of compounds, organocatalyzed transformations remain scarce. From initial works, this review aims to show organocatalyzed transformations from sulfur ylides, involving cyclopropanation and formal N–H, S–H, and C–H insertion reactions, including enantioselective versions. The proposed mechanisms and the modes of activation of these organocatalysts will be covered. Furthermore, advances in this area and potential challenges to be circumvented in the near future will also be discussed.

Recent Developments in Stereoselective Reactions of Sulfonium Ylides

This review describes advances in the literature since the mid-1990s in the area of reactions of sulfonium ylide chemistry, with particular attention paid to stereoselective examples. Although the chemistry of sulfonium ylides was first popularized and applied in a substantial way in the 1960s, there has been sustained interest in the chemistry of sulfonium ylides since then. Many new ways of exploiting sulfonium ylides in productive stereoselective methodologies have emerged, often taking advantage of advances in organocatalysis and transition metal catalysis, to access stereodefined structurally complex motifs. The development of many different chiral sulfides over the last 20–30 years has also played a role in accelerating their study in a variety of reaction settings. In general, formal cycloaddition reactions ([2 + 1] and [4 + 1]) of sulfonium ylides follow a similar mechanistic pathway: initial addition of the nucleophilic ylide carbanion to an electrophile to form a zwitterionic betaine intermediate, followed by cyclization of the zwitterionic intermediate to afford the desired three-membered cyclic product (e.g., epoxide, cyclopropane, or aziridine), five-membered monocyclic (e.g., oxazolidinone), or fused bicyclic product (e.g., benzofuran, indoline).

[4+n] Annulation Reactions Using ortho-Chloromethyl Anilines as Aza-ortho-Quinone Methide Precursors

Aza-ortho-quinone methides are important reactive intermediates that have found broad applications in synthetic chemistry. Recently, 1,4-elimination of ortho-chloromethyl aniline derivatives has emerged as a novel, powerful and convenient method for aza-ortho-quinone methide generation. This review will highlight the recent applications of aza-ortho-quinone methide precursors in annulation reactions to access various biologically important nitrogen-containing heterocycles. The general mechanisms are briefly discussed as well.

1 Introduction

2 [4+n] Annulation Reactions Using ortho-Chloromethyl Anilines as Aza-ortho-Quinone Methide Precursors

2.1 [4+2] Annulation Reactions

2.2 [4+1] Annulation Reactions

2.3 [4+3] Annulation Reactions

3 Conclusion and Perspective

Construction of Saturated Oxazolo[3,2-b][1,2]oxazines via Tandem [3+2]-Cycloaddition/[1,3]-Rearrangement of Cyclic Nitronates and Ketenes

Reaction of six-membered cyclic nitronates with disubstituted ketenes affords hitherto unknown saturated oxazolo[3,2-b][1,2]oxazines possessing up to four contiguous stereogenic centers. The process involves a tandem of [3+2]-cycloaddition across the C═O bond of ketene, followed by a spontaneous [1,3]-rearrangement of transient vinylidene-substituted bicyclic nitrosoacetals. DFT calculations of the mechanism suggest that the [1,3]-O,C-shift proceeds through a recyclization of a biradical intermediate formed by an unusually mild homolytic cleavage of the N-O bond. The resulting products can be utilized as precursors of other fused 1,2-oxazines derivatives, in particular 1,2-oxazino-1,2,4-triazin-3-ones.

Efficient access to fluorescent benzofuro[3,2-b]carbazoles via TFA-promoted cascade annulations of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles

An efficient, TFA-promoted cascade annulation of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles was established.

Computational studies on [4 + 2] / [3 + 2] tandem sequential cycloaddition reactions of functionalized acetylenes with cyclopentadiene and diazoalkane for the formation of norbornene pyrazolines

The mechanistic pathways for the sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines, employed in the synthesis of pharmaceutically relevant compounds, have been studied computationally with DFT at the M06-2X/6-31G(d) and M06-2X/6-31G(d,p) levels of theory. We have established that, in the reaction of the parent (unsubstituted) acetylene with cyclopentadiene and dimethyl diazopropane, the order of the tandem addition has no substantial effects in product outcomes. The same product is obtained provided the reaction components remain the same for both [4 + 2]/[3 + 2] and [3 + 2]/[4 + 2] tandem addition sequences. The results indicate that the [4 + 2] Diels-Alder addition step is the rate-determining step irrespective of the addition order, while the 1,3-dipolar cycloaddition step has been found to generally proceed rapidly with very low activation energies. It has also been realized that the regio-, stereo-, and chemo-selectivities of the reaction are strictly dictated by the type of substituent on the parent acetylene. For substituted acetylenes, we conclude that the sequence of the tandem addition generally affects the type of isomeric product obtained. The [4 + 2]/[3 + 2] tandem addition sequence has been established to favor the exo stereo-selective isomer over the endo, whereas the [3 + 2]/[4 + 2] tandem addition sequence generally favors the endo product formation. Therefore, it is settled that the mechanistic route taken by any substrate in the [4 + 2]/[3 + 2] versus [3 + 2]/[4 + 2] sequential tandem cycloaddition is greatly affected by both electronic and steric factors. Electrophilicity indices calculations agree with the activation barriers obtained. Perturbation molecular orbital theory was employed to rationalize the results. Global reactivity indices calculations gave a good correlation with the activation energies. Graphical abstract The sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines.

Diastereoselective synthesis of 3,3-disubstituted 3-nitro-4-chromanone derivatives as potential antitumor agents

We report an efficient and highly diastereoselective protocol for the rapid construction of 3-nitro substituted 4-chromanones by an intramolecular Michael-type cyclization of α-nitro aryl ketones bearing unsaturated ester units.

Fluoronitroalkenes in tandem [4 + 1]/[3 + 2]-cycloaddition: one-pot three-component assembly of fluorinated bicyclic nitroso acetals

Novel fluorinated bicyclic nitroso acetals were obtained in a highly regio- and diastereo-selective manner via tandem [4 + 1]/[3 + 2]-cycloaddition of fluoronitroalkenes.

Beyond sulfide-centric catalysis: recent advances in the catalytic cyclization reactions of sulfur ylides

This Tutorial Review highlighted recent achievements on the catalytic cyclization reactions of sulfur ylide reagents, especially the asymmetric processes avoiding the use of structurally complex chiral sulfides.

Ylide formal [4 + 1] annulation

Ylide [4 + 1] annulation: beyond small-ring formation.

Double γ-alkylation of allylic phosphorus ylides: a unique access to oxa-bicyclic[3.3.0] diene skeletons

A new ylide-initiated tandem cyclization reaction based on γ-dialkylation of allylic ylides has been realized, delivering a series of [3.3.0] oxa-bicyclic dienes in high yields with perfect diastereoselectivity.

Development of cascade reactions for the concise construction of diverse heterocyclic architectures

Heterocyclic structural architectures occur in many bioactive natural products and synthetic drugs, and these structural units serve as important intermediates in organic synthesis. This Account documents our recent progress in the development of cascade reactions to construct complex carbocycles and heterocycles. We describe the rational design of cascade reactions and in-depth investigations of their mechanism as well as their applications in the synthesis of drugs, natural products, and related molecular analogs. Relying on knowledge about the dipole-type reactivity of sulfur ylides, we have developed three different types of cascade reactions: a [4 + 1] annulation/rearrangement cascade, a [4 + 1]/[3 + 2] cycloaddition cascade, and a Michael addition/N-alkylation cascade. Using these processes, we can generate oxazolidinones, fused heterocycles, and pyrrolines starting with simple and readily available substances such as nitroolefins and unsaturated imines. We have also developed corresponding enantioselective reactions, which are guided by axial chirality and asymmetric H-bonding control. In addition, by relying on the reactivity characteristics of newly designed acrylate-linked nitroolefins, we have disclosed an asymmetric Michael/Michael/retro-Michael addition cascade using the combination of a protected hydroxylamine and a bifunctional organocatalyst. Using this methodology, we prepared chiral chromenes in good yields and with high enantioselectivities. Moreover, a series of double Michael addition cascade reactions with anilines, thiophenols, and benzotriazoles generated highly functionalized chromanes. Via mechanistically distinct cascade processes that start with vinyl-linked indoles, we have synthesized polycyclic indoles. Intermolecular cross-metathesis/intramolecular Friedel-Crafts alkylation cascades, promoted by either a single ruthenium alkylidene catalyst or a sequence involving Grubbs' ruthenium catalyst and MacMillan's imidazolidinone catalyst, converted ω-indolyl alkenes into tetrahydrocarbazoles, tetrahydropyranoindoles, and tetrahydrocarbolines. In addition, we constructed tetrahydrocarbazoles and tetrahydroquinones using organocatalytic Friedel-Crafts alkylation/Michael addition cascades that used 2-vinyl indoles as common starting materials. Most green plants carry out photosynthesis to produce organic substances relying on readily available and renewable solar energy. In a related manner, we have also developed two cascade reactions that merge visible light-induced aerobic oxidation with either [3 + 2] cycloaddition/oxidative aromatization or intramolecular cyclization. These processes lead to the formation of pyrrolo[2,1-a]isoquinolines and enantiopure tetrahydroimidazoles, respectively.

Hydrogen-bond-mediated asymmetric cascade reaction of stable sulfur ylides with nitroolefins: scope, application and mechanism

A hydrogen-bond-mediated asymmetric [4+1] annulation/rearrangement cascade of stable sulfur ylides and nitroolefins was developed. This reaction provides a facile route to enantioenriched 4,5-substituted oxazolidinones in moderate to excellent isolated yields (65-96 %) with excellent stereocontrol (up to more than 95:5 d.r. and 97:3 e.r.). This methodology was successfully applied to the concise synthesis of two bioactive molecules. The stereocontrolled modes and mechanism have been proposed to explain the origin of this stereochemistry.

Amine-catalyzed formal (3 + 3) annulations of 2-(acetoxymethyl)buta-2,3-dienoate with sulfur ylides: synthesis of 4H-pyrans bearing a vinyl sulfide group

A DABCO-catalyzed (3 + 3) annulation between 2-(acetoxymethyl)buta-2,3-dienoate and sulfur ylides has been developed, which provides a facile method for the synthesis of 4H-pyrans bearing a vinyl sulfide group.