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.

A formal [4 + 1] cycloaddition reaction of Baylis–Hillman bromides with sulfur ylides: facile access to α-alkenyl lactones

A formal [4 + 1] cycloaddition reaction of Baylis–Hillman adducts with sulfur ylides has been developed for the first time.

Feeding sequence-regulated divergent (4 + 1 + 1’) annulations of α-bromo carbonyls and 1-azadienes via computational calculations-based mechanism elucidation

By investigating the mechanism of our previously reported (4 + 1 + 1) annulations of α-bromo carbonyls and 1-azadienes via density functional theory (DFT) calculations, the formation of a carbon...

Recent advances in the application of ylide-like species in [4+1]-annulation reactions: an update review

In this review, advances in [4+1]‐annulation reactions involving sulfonium, sulfoxonium and ammonium ylides, as well as diazo compounds and carbenes are summarized over the last 6 years. Newly emerged methods...

Mild synthesis of isoxazoline derivatives via an efficient [4 + 1] annulation reaction of transient nitrosoalkenes and sulfur ylides

An efficient [4 + 1] annulation between α-bromooximes and sulfur ylides via in situ generation of nitrosoalkenes under mild basic reaction conditions has been developed, providing an expeditious and scalable approach to synthesize biologically interesting isoxazoline derivatives with good to excellent yields.

The Cyclic Nitronate Route to Pharmaceutical Molecules: Synthesis of GSK's Potent PDE4 Inhibitor as a Case Study

An efficient asymmetric synthesis of GlaxoSmithKline’s potent PDE4 inhibitor was accomplished in eight steps from a catechol-derived nitroalkene. The key intermediate (3-acyloxymethyl-substituted 1,2-oxazine) was prepared in a straightforward manner by tandem acylation/(3,3)-sigmatropic rearrangement of the corresponding 1,2-oxazine-N-oxide. The latter was assembled by a (4 + 2)-cycloaddition between the suitably substituted nitroalkene and vinyl ether. Facile acetal epimerization at the C-6 position in 1,2-oxazine ring was observed in the course of reduction with NaBH₃CN in AcOH. Density functional theory (DFT) calculations suggest that the epimerization may proceed through an unusual tricyclic oxazolo(1,2)oxazinium cation formed via double anchimeric assistance from a distant acyloxy group and the nitrogen atom of the 1,2-oxazine ring.

Comparison of Substituting Ability of Nitronate versus Enolate for Direct Substitution of a Nitro Group

α-Nitrocinnamate underwent the conjugate addition of an active methylene compound such as nitroacetate, 1,3-dicarbonyl compound, or α-nitroketone, and the following ring closure afforded functionalized heterocyclic frameworks. The reaction of cinnamate with nitroacetate occurs via nucleophilic substitution of a nitro group by the O-attack of the nitronate, which results in isoxazoline N-oxide. This protocol was applicable to 1,3-dicarbonyl compounds to afford dihydrofuran derivatives, including those derived from direct substitution of a nitro group caused by O-attack of enolate. It was found the reactivity was lowered by an electron-withdrawing group on the carbonyl moiety. When α-nitroketone was employed as a substrate, three kinds of products were possibly formed; of these, only isoxazoline N-oxide was identified. This result indicates that the substituting ability of nitronate is higher than that of enolate for the direct S_N2 substitution of a nitro group.

Catalytic asymmetric (4 + 1) annulation of nitroalkenes with allylic acetates: stereoselective synthesis of isoxazoline N-oxides

An asymmetric (4 + 1) annulation of α-nitro cinnamates with Morita-Baylis-Hillman (MBH) acetates catalyzed by α-isocupreine is reported. It provides chiral isoxazoline N-oxides in moderate to good yields with 88-99% ee, and represents the first catalytic asymmetric (4 + 1) annulation of activated nitroalkenes with in situ generated ammonium ylides. It also affords a practical and efficient access to chiral isoxazoline N-oxides.

[4 + 1 + 1] Annulations of α-Bromo Carbonyls and 1-Azadienes toward Fused Benzoazaheterocycles

An unexpected [4 + 1 + 1] annulation between α-bromo carbonyls and 1-azadienes, derived from 2-methylenebenzofuran-3(2 H)-ones or 2-methylenebenzo[ b]thiophene-3(2 H)-ones, has been observed in the presence of DABCO and Cs₂CO₃. These reactions stand in contrast to the common [4 + 1] cyclization reactions of azadienes with the related sulfonium ylides. A range of fused benzofuro[3,2- b]pyridines and benzo[4,5]thieno[3,2- b]pyridines have been efficiently constructed in fair to excellent yields.

Formal (4+1)-Cyclization of Ammonium Ylides with Vinylogous para-Quinone Methides

The formal (4+1)-cyclization between in situ generated achiral carbonyl-stabilized ammonium ylides and vinylogous p-quinone methides can be carried out under operationally simple conditions, leading to two different double bond regioisomers depending on the used conditions. Besides these racemic approaches, also a first proof-of-concept for enantioselective versions of these reactions has been obtained.

Domino Reaction of Pyrrolidinium Ylides: Michael Addition/[1,2]-Stevens Rearrangement

A novel domino reaction featuring a Michael addition/[1,2]-Stevens rearrangement reaction of pyrrolidinium ylides with electrophilic alkenes is described. Ylides generated under mild conditions from 2-aryl- N-cyanomethyl- N-methylpyrrolidinium salts entered the Michael addition, followed by a [1,3]-hydrogen shift and finally the [1,2]-Stevens rearrangement to give 3-aryl-2-cyano-2-(2-EWG-ethyl)-1-methylpiperidines.

Ammonium Ylide Mediated Cyclization Reactions

The use of readily accessible ammonium ylides for (asymmetric) transformations, especially cyclization reactions, has received considerable attention over the past two decades. A variety of highly enantioselective protocols to facilitate annulation reactions have recently been introduced as an alternative to other common methods including S-ylide-mediated strategies. It is the intention of this short review to provide an introduction to this field by highlighting the potential of ammonium ylides for (asymmetric) cyclization reactions as well as to present the limitations and challenges of these methods.

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.

Review of the synthesis of acyclic and cyclic oxime ethers

Oxime ethers have attracted much attention due to their potential biological activities and wide variety of synthetic applications.

Ylide formal [4 + 1] annulation

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

Identification of the best-suited leaving group for the diastereoselective synthesis of glycidic amides from stabilised ammonium ylides and aldehydes

In comparison to the use of sulfur ylides, the use of ammonium ylides for the synthesis of epoxides is significantly less developed. As a part of our systematic investigations concerning the use of amide-stabilised ammonium ylides for the synthesis of glycidic amides we have focused on the identification of the best-suited amino leaving group for this purpose. Whereas tertiary amines like quinuclidine or cinchona alkaloids were found to be not suited for epoxide formation, trimethylamine was found to be the leaving group of choice, yielding trans-glycidic amides in excellent yields of > 90%.

Ammonium ylides for the diastereoselective synthesis of glycidic amides

A highly trans-selective protocol for the synthesis of glycidic amides was developed. This approach gave access to oxiranes by reacting stabilised ammonium ylides bearing an α-carbonyl group and aromatic aldehydes in moderate to good yields.