Diphenylprolinol Silyl Ether Catalysis in an Asymmetric Formal Carbo [3 + 3] Cycloaddition Reaction via a Domino Michael/Knoevenagel Condensation

l-Phenylalanine potassium catalyzed asymmetric formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone

A highly enantioselective formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone was developed.

Enantioselective organocatalytic Michael addition of isorhodanines to α,β-unsaturated aldehydes

The Michael reaction of substituted isorhodanines with α,β-unsaturated aldehydes in the presence of a catalytic amount of a chiral secondary amine is presented. This transformation proceeds in good to high yields furnishing the corresponding 4,5-disubstituted isorhodanine adducts in good to excellent enantioselectivities.

AlCl3-catalyzed [3 + 3] cycloaddition of chalcones and β-enamine ketones (esters): a highly efficient access to multisubstituted cyclohexa-1,3-dienamines

An efficient protocol was successfully achieved to give 1,3-cyclohexadiene derivatives in good to excellent yields from chalcones and β-enamine ketones (esters) promoted by AlCl₃.

Asymmetric α,γ-Regioselective [3 + 3] Formal Cycloadditions of α,β-Unsaturated Aldehydes via Cascade Dienamine-Dienamine Catalysis

Asymmetric α,γ-regioselective [3 + 3] formal cycloadditions of α,β-unsaturated aldehydes and 2-nitroallylic acetates have been developed for the first time. These reactions proceeded through a domino Michael addition-Michael addition sequence via an unusual cascade dienamine-dienamine catalysis of a chiral secondary amine, and multifunctional cyclohexene derivatives were generally constructed in moderate yields with excellent stereoselectivity after simple treatment with K2CO3.

Stereoselective synthesis of highly substituted cyclohexanes by a rhodium-carbene initiated domino sequence

A stereoselective synthesis of cyclohexanes bearing four stereocenters from vinyldiazoacetates and allyl alcohols by a rhodium-carbene initiated domino reaction is described. The reaction cascade features a tandem ylide formation/[2,3]-sigmatropic rearrangement, oxy-Cope rearrangement, and type II carbonyl ene reaction, all of which proceed with a high degree of stereocontrol. The products are routinely isolated with excellent stereocontrol (>97:3 dr, 99% ee).

Synergistic Cu–amine catalysis for the enantioselective synthesis of chiral cyclohexenones

An unprecedented utilization of 1,3-acetonedicarboxylic acid as a 1,3-bis-pro-nucleophile and a reactive acetone surrogate in enantioselective catalysis has been reported.

The organocatalyzed domino Michael–aldol reaction revisited. Synthesis of enantioenriched 3-hydroxycyclohexanone derivatives by reaction of enals with α,α′-diaryl-substituted acetone

The reaction of enals with α,α′-diaryl-substituted acetones (pK_a > 18) catalyzed by (S)-1-(2-pyrrolidinylmethyl) pyrrolidine provides direct access to enantioenriched 2,5,6-trisubstituted-3-hydroxy cyclohexanones.

A highly enantioselective, organocatalytic [3+2]-cycloannulation reaction towards the de novo-synthesis of 1-cyclopentenyl-α-keto esters

A novel [3+2]-cycloannulation process produces cyclopentenes with good yields and excellent enantioselectivity via a domino-vinylogous Michael–Knoevenagel reaction.

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

The combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels-Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention. In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing synthetic methodology today, involving organocatalysis or transition metal catalysis. With origins in organometallic or vinyliminium ion chemistry, this area has blossomed into a viable synthetic transformation for the construction of six-membered heterocyclic compounds containing one or more heteroatoms. The development of [3 + 3]-cycloaddition transformations has been achieved through identification of suitable and compatible reactive dipolar adducts and stable dipoles. The reactive dipolar species is an energetic dipolar intermediate that is optimally formed catalytically in the reaction. The stepwise process occurs with the reactive dipolar adduct reacting as an electrophile or as a nucleophile to form the first covalent bond, and this association provides entropic assistance for the construction of the second covalent bond and the overall formal [3 + 3]-cycloaddition. Organocatalysis is well developed for both inter- and intramolecular synthetic transformations, but the potential of transition metal catalysis for [3 + 3]-cycloaddition has only recently emerged. The key to the rapid development of the transition metal-based methodology has been recognition that certain catalytically generated vinylcarbenes are effective dipolar adducts for reactions with stable dipolar compounds, including aryl and vinyl ylides. In particular, metallo-enolcarbenes that are generated catalytically from conveniently prepared stable enoldiazoacetates or from donor-acceptor cyclopropenes are highly effective dipolar adducts for [3 + 3]-cycloaddition. The electron-donating oxygen of the silyl ether enhances electrophilic ring closure to the metal-bound carbon of the initial adduct from vinylogous addition, and this enhancement inhibits the alternative [3 + 2]-cycloaddition across the carbon-carbon double bond of the vinylcarbene. Catalytically generated metallo-enolcarbenes react under mild conditions with a broad spectrum of compatible stable dipoles, including nitrones, azomethine imines, ylides, and certain covalent precursors of stable dipoles, to form [3 + 3]-cycloaddition products having the β-ketoester functionality (in dihydrooxazines, tetrahydropyridazines, pyrazolidinone and pyraxole derivatives, dihydroquinolines, and quinolizidines, for example) in high yield. Two ways to access these metallo-enolcarbenes, either by dinitrogen extrusion from enoldiazoacetate esters or by rearrangement of donor-acceptor cyclopropenes, enhance the versatility of the process. The [3 + 3]-cycloaddition methodology is a complementary strategy to [4 + 2]-cycloaddition for the synthesis of heterocyclic compounds having six-membered rings. High levels of enantioselectivity are obtained with the use of chiral ligands on transition metal catalysts that include those on dirhodium(II) and silver(I).

Recent progress on asymmetric organocatalytic construction of chiral cyclohexenone skeletons

Chiral cyclohex-2-enones are important intermediates in synthetic chemistry as well as in the life science industries. In this focus review, recent advances in the organocatalytic asymmetric synthesis of chiral cyclohex-2-enone skeletons are summarized. The reaction mechanisms are also briefly discussed.

Bicyclic pyrazolidinone derivatives from diastereoselective catalytic [3 + 3]-cycloaddition reactions of enoldiazoacetates with azomethine imines

A highly regio- and diastereoselective synthesis of bicyclic pyrazolidinone derivatives by rhodium(II) acetate catalyzed [3 + 3]-annulation with enoldiazoacetates and azomethine imines has been achieved in high yield. A vinylogous reaction of the metal enol carbene with the azomethine imine initiates [3 + 3]-cycloaddition, whereas reaction at the carbene center effects N-N-cleavage of the azomethine imine.