NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Synthesis of functionalized 3,2′-pyrrolidinyl spirooxindoles via domino 1,6-addition/annulation reactions of para-quinone methides and 3-chlorooxindoles

A highly efficient diastereoselective [4 + 1] cycloaddition of ortho-tosylaminophenyl-substituted p-QMs with 3-chlorooxindoles has been developed to afford 3,2′-pyrrolidinyl spirooxindoles.

Stereoselective synthesis of trifluoroethyl 3,2'-spirooxindole γ-lactam through the organocatalytic cascade reaction of 3-((2,2,2-trifluoroethyl)amino)indolin-2-one

Newly designed 3-((2,2,2-trifluoroethyl)amino)indolin-2-ones were used for the facile synthesis of chiral fluoroalkyl-containing 3,2'-spirooxindole γ-lactam products. The secondary amine-catalysed Michael/hemiaminalization cascade reaction of 3-((2,2,2-trifluoroethyl)amino)indolin-2-one with α,β-unsaturated aldehydes followed by oxidation can easily produce the desired products in high yields (up to 86%) with excellent enantioselectivities (up to 99% ee) and diastereoselectivities (up to >95 : 5 dr).

Strategies and methodologies for the construction of spiro-γ-lactams: an update

This review gives an insight into the most recent synthetic methodologies towards spiro-γ-lactams, a class of compounds that are present in a wide range of synthetic bioactive and naturally occurring molecules.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Asymmetric synthesis of γ-lactams under low-loading N-heterocyclic carbene catalysis

Low-loading N-heterocyclic carbene-catalyzed oxidative formal [3 + 2] annulation of enals with N-Ts diethyl aminomalonate has been successfully developed.

Catalytic asymmetric synthesis of 3,2'-pyrrolinyl spirooxindoles via conjugate addition/Schmidt-type rearrangement of vinyl azides and (E)-alkenyloxindoles

A catalytic asymmetric conjugate addition/Schmidt-type rearrangement of vinyl azides and (E)-alkenyloxindoles was realized. It afforded a variety of optically active 3,2'-pyrrolinyl spirooxindoles with high yields (up to 98%), and excellent diastereo- and enantioselectivities (up to 98% ee, >19 : 1 dr), even at the gram-scale in the presence of a chiral N,N'-dioxide-nickel(ii) complex. In addition, a possible catalytic cycle and transition state model were proposed to rationalize the stereoselectivity.

Enantioselective construction of dispirotriheterocycles featuring a 4-aminopyrazolone motif through a cascade Michael/cyclization process

A highly asymmetric approach to multicyclic dispiro [pyrazolone-pyrrolidinethione-oxindole] core structures bearing three contiguous stereogenic centers through a cascade Michael addition/cyclization reaction of 4-isothiocyanato pyrazolones with 3-ylideneoxindoles was developed.

NHC-Catalyzed [3 + 3] Annulation of Thioamides and Modified Enals for the Enantioselective Synthesis of Functionalized Thiazinones

N-Heterocyclic carbene (NHC)-catalyzed [3 + 3] annulation of thioamides with modified enals allowing the enantioselective synthesis of functionalized 1,3-thiazin-4-ones is reported. The NHC generated from the chiral triazolium salt was optimal and the reaction is initiated by the thia-Michael addition to catalytically generated α,β-unsaturated acylazolium intermediates derived from 2-bromoenals, followed by intramolecular cyclization. This operationally simple procedure offers a straightforward and rapid access to target compounds in moderate to good yields and enantiomeric ratio values.

A One-Pot Ring-Opening/Ring-Closure Sequence for the Synthesis of Polycyclic Spirooxindoles

One-pot ring-opening/ring-closure process of combining methyleneindolinone with 3-aminooxindole has been developed in this work. Novel polycyclic spirooxindoles were efficiently assembled under mild conditions in high yields (up to 95 %) with moderate to good diastereoselectivities (up to >95:5 d.r.) through simple filtration.

NHC-Catalyzed Generation of α,β-Unsaturated Acylazoliums for the Enantioselective Synthesis of Heterocycles and Carbocycles

This Account is aimed at highlighting the recent developments in the N-heterocyclic carbene (NHC)-catalyzed generation of α,β-unsaturated acylazolium intermediates and their subsequent reactivity with (bis)nucleophiles thereby shedding light on the power of this NHC-bound intermediate in organocatalysis. This key intermediate can be generated by the addition of NHCs to α,β-unsaturated aldehyde or acid derivatives. A wide variety of bisnucleophiles can add across the α,β-unsaturated acylazoliums to form various five and six-membered heterocycles and carbocycles. Moreover, suitably substituted nucleophiles can add to this intermediate and result in valuable products following cascade processes. Employing chiral NHCs in the process can result in the enantioselective synthesis of valuable compounds. In 2013, we developed a unified strategy for the enantioselective synthesis of dihydropyranones and dihydropyridinones by the NHC-catalyzed formal [3 + 3] annulation of 2-bromoenals with readily available 1,3-dicarbonyl compounds or primary vinylogous amides. This reaction takes place under mild conditions with low catalyst loading. Interestingly, employing enolizable aldehydes as the bisnucleophiles in this annulation afforded chiral 4,5-disubstituted dihydropyranones in spite of the competing benzoin/Stetter pathways. Moreover, the use of cyclic 1,3-dicarbonyl compounds such as 4-hydroxy coumarin/pyrazolone afforded the coumarin/pyrazole-fused dihydropyranones. In addition, a [3 + 2] annulation for the synthesis of spiro γ-butyrolactones was demonstrated using 3-hydroxy oxindoles as the bisnucleophile. The interception of α,β-unsaturated acylazolium intermediates with malonic ester derivatives having a γ-benzoyl group resulted in the enantioselective synthesis of functionalized cyclopentenes via a cascade process involving a Michael-intramolecular aldol-β-lactonization-decarboxylation sequence. The use of cyclic β-ketoamides as the coupling partner for catalytically generated α,β-unsaturated acylazoliums resulted in the enantioselective synthesis of spiro-glutarimide and the reaction proceeds in a Michael addition-intramolecular amidation pathway. We have recently demonstrated the enantioselective synthesis of tricyclic δ-lactones with three contiguous stereocenters by the reaction of enals with dinitrotoluene derivatives bearing electron-withdrawing groups, under oxidative conditions. This atom-economic cascade reaction proceeds in a Michael/Michael/lactonization sequence tolerating a range of functional groups. This technique was also used for the N-H functionalization of indoles for the enantioselective synthesis of pyrroloquinolines following the aza-Michael/Michael/lactonization sequence. The use of α-arylidene pyrazolinones as the bisnucleophiles for the tandem generation of dienolate/enolates combined with the NHC-catalyzed generation of α,β-unsaturated acylazoliums resulted in the enantioselective synthesis of pyrazolone-fused spirocyclohexadienones. This formal [3 + 3] annulation proceeds via the vinylogous Michael/spiroannulation/dehydrogenation sequence to afford spirocyclic compounds with an all-carbon quaternary stereocenter. It is reasonable to believe that the chemistry of α,β-unsaturated acylazoliums, catalytically generated through NHCs, will continue to flourish and will lead to amazing results. Future challenges in this area include the applications of this key intermediate in the synthesis of biologically active natural products and drugs.

New development in the enantioselective synthesis of spiro compounds

In this review we summarize the latest developments in the enantioselective synthesis of spirocompounds. The most important organometallic and organocatalytic methodologies are highlighted.

NHC-catalyzed regiodivergent syntheses of difunctionalized 3-pyrazolidinones from α-bromoenal and monosubstituted hydrazine

A formal [3 + 2] annulation of α-bromoenal with monosubstituted hydrazine could give 1,5 or 2,5-difunctionalized 3-pyrazolidinone regiodivergently by tuning the structure of the N-Heterocyclic Carbene (NHC) catalyst. Moderate to high yields, mild reaction conditions, good regioselectivity and potential biological significance of the final product have made this protocol attractive for the assembly of 3-pyrazolidinone.

Quinine-Based Trifunctional Organocatalyst for Tandem Aza-Henry Reaction-Cyclization: Asymmetric Synthesis of Spiroxindole-Pyrrolidine/Piperidines

A quinine-derived trifunctional sulphonamide catalyst has been developed for the effective asymmetric organocatalytic tandem aza-Henry reaction-cyclization of isatin-derived ketimines and nitroalkane-mesylates for the synthesis of spiro-pyrrolidine/piperidine-oxindoles. Demethylation of traditional bifunctional catalyst to incorporate an additional hydrogen bonding C6'-OH group plays the key role toward remarkable enantioselectivity.

Formal [3 + 4] Annulation of α,β-Unsaturated Acyl Azoliums: Access to Enantioenriched N-H-Free 1,5-Benzothiazepines

An unprecedented formal [3 + 4] annulation of α,β-unsaturated acyl azoliums with 2-aminobenzenethiols has been utilized to synthesize enantioenriched N-H-free 1,5-benzothiazepines, which are recognized as privileged structures in numerous biologically active scaffolds. This protocol offers a rapid and direct pathway to access the target compounds with high enantioselectivities and has been applied in the concise synthesis of chiral drug (R)-thiazesim.

N-heterocyclic carbene-catalyzed synthesis of spirocyclopentene-oxindoles from bromoenals

Spirocyclopentene-oxindoles were synthesized in good yields with good diastereo- and enantioselectivitiesviathe N-heterocyclic carbene-catalyzed reaction of bromoenals and oxindoles.

Transition-metal-free synthesis of indolizines via [3 + 2]-annulation from α-bromoenals and 2-substituted azaarenes

A transition-metal-free “one-pot” synthetic approach for the synthesis of indolizines from α-bromo-substituted enals and simple 2-substituted azaarenes has been developed.

Enantioselective synthesis of spiro γ-butyrolactones by N-heterocyclic carbene (NHC)-catalyzed formal [3 + 2] annulation of enals with 3-hydroxy oxindoles

The NHC-catalyzed enantioselective, oxidative formal [3 + 2] annulation of α,β-unsaturated aldehydes with 3-hydroxy oxindoles is presented.

Diastereoselective synthesis of cyclopentene spiro-rhodanines containing three contiguous stereocenters via phosphine-catalyzed [3 + 2] cycloaddition or one-pot sequential [3 + 2]/[3 + 2] cycloaddition

5-Spiro-cyclopentene-rhodanine derivatives have been synthesized through two different diastereoselective phosphine-catalyzed cascade reactions in excellent diastereoselectivities and good to excellent yields.