Synthesis of Chiral 2‐Hydroxy‐1‐methylpropanoates by Rhodium‐Catalyzed Stereoselective Hydrogenation of α‐(Hydroxymethyl)‐acrylate Derivatives

The synthesis of chiral 3‐hydroxy‐2‐methylpropanoic acid esters (e.g., “Roche ester” 3a ) based on the rhodium‐catalyzed stereoselective hydrogenation of Baylis–Hillman reaction products was investigated. Full conversions and enantioselectivities of up to 99% at a substrate/catalyst ratio of up to 500/1 were achieved by application of bisphospholanes of the catASium M series as ancillary ligands. An interesting kinetic resolution was observed by the diastereoselective hydroxy‐directed hydrogenation of related racemic β‐branched precursors affording mainly anti‐isomers with up to 96%ee.     

Asymmetric Aza‐Morita–Baylis–Hillman‐Type Reactions: The Highly Enantioselective Reaction between Unmodified α,β‐ Unsaturated Aldehydes and N‐Acylimines by Organo‐co‐catalysis

The highly enantioselective organo‐co‐catalytic aza‐Morita–Baylis–Hillman (MBH)‐type reaction between N‐carbamate‐protected imines and α,β‐unsaturated aldehydes has been developed. The organic co‐catalytic system of proline and 1,4‐diazabicyclo[2.2.2]octane (DABCO) enables the asymmetric synthesis of the corresponding N‐Boc‐ and N‐Cbz‐protected β‐amino‐α‐alkylidene‐aldehydes in good to high yields and up to 99% ee. In the case of aza‐MBH‐type addition of enals to phenylprop‐2‐ene‐1‐imines, the co‐catalytic reaction exhibits excellent 1,2‐selectivity. The organo‐co‐catalytic aza‐MBH‐type reaction can also be performed by the direct highly enantioselective addition of α,β‐unsaturated aldehydes to bench‐stable N‐carbamate‐protected α‐amidosulfones to give the corresponding β‐amino‐α‐alkylidene‐aldehydes with up to 99% ee. The organo‐co‐catalytic aza‐MBH‐type reaction is also an expeditious entry to nearly enantiomerically pure β‐amino‐α‐alkylidene‐amino acids and β‐amino‐α‐alkylidene‐lactams (99% ee). The mechanism and stereochemistry of the chiral amine and DABCO co‐catalyzed aza‐MBH‐type reaction are also discussed.     

Access to Stereodefined Trisubstituted Alkenes via Rhodium‐Catalyzed 1,4‐Addition of Potassium Trifluoro(organo)‐ borates to Baylis–Hillman Adducts

In the presence of a rhodium catalyst, unactivated Baylis–Hillman adducts reacted regioselectively with potassium trifluoro(organo)borates to afford stereodefined trisubstituted alkenes with good yields. This highly efficient reaction (aerobic conditions, low temperature, absence of added phosphane ligand) is believed to proceed via a 1,4‐addition/β‐hydroxy elimination mechanism.     

Enantioselective Aza‐Morita–Baylis–Hillman Reaction Using Aliphatic α‐Amidosulfones as Imine Surrogates

The bifunctional catalyst 6′‐deoxy‐6′‐acylamino‐β‐isocupreidine ( 1 ) served both as a base to trigger the in situ generation of N‐sulfonylimine from readily available α‐amidosulfones and as a chiral nucleophile to initiate the enantioselective aza‐Morita–Baylis–Hillman (aza‐MBH) reaction. α‐Methylene‐β‐amino‐β‐alkyl carbonyl compounds, difficultly accessible previously, can now be synthesized in excellent yields and enantioselectivities.     

First Copper‐Catalyzed Intramolecular Amidation in Substituted 4‐Iodopyrazoles Leading to the Synthesis of Pyrazolo[4,3‐b]‐ pyridin‐5‐ones

An unprecedented copper‐catalyzed intramolecular amidation of substituted 4‐iodopyrazoles generated either via Baylis–Hillman or Horner–Wadsworth–Emmons chemistry for the synthesis of pyrazolo[4,3‐b]pyridine‐5‐ones is described. In addition, the effect of the stereochemistry of the acrylamide on the cross‐coupling reaction has been investigated and it is demonstrated that only the Z‐isomer is favoured to undergo the intramolecular cyclization.     

Chiral Thiourea‐Phosphine Organocatalysts in the Asymmetric Aza‐Morita–Baylis–Hillman Reaction

A new kind of bifunctional (thio)urea‐phosphine catalyst was synthesized and applied to the aza‐Morita–Baylis–Hillman reaction of N‐sulfonated imines with methyl vinyl ketone, phenyl vinyl ketone, ethyl vinyl ketone or acrolein. Moderate to excellent ee and yields of the products were obtained under mild reaction conditions.     

N‐Methylprolinol Catalysed Asymmetric Baylis−Hillman Reaction

N‐methylprolinol is used as a chiral base catalyst for the Baylis–Hillman reaction to obtain the adducts in good yields with moderate to good enantioselectivities in 1,4‐dioxane:water (1 : 1, v/v) under ambient conditions.     

A New One‐Pot Synthesis of Polysubstituted Indoles from Pyrroles and β‐Nitroacrylates

The reaction of β‐nitroacrylates with pyrroles, under solvent‐ and catalyst‐free conditions, allows the formation of Friedel–Crafts adducts which, after in situ treatment with Amberlyst 15 in isopropyl alcohol under reflux, provide polysubstituted indoles, via a benzannulation reaction, in a one‐pot process.     

A General Approach to the Synthesis of β2‐Amino Acid Derivatives via Highly Efficient Catalytic Asymmetric Hydrogenation of α‐Aminomethylacrylates

A new strategy was developed for the synthesis of a valuable class of α‐aminomethylacrylates via the Baylis–Hillman reaction of different aldehydes with methyl acrylate followed by acetylation of the resulting allylic alcohols and S_N2′‐type amination of the allylic acetates. Asymmetric hydrogenation of these diverse olefinic precursors using rhodium(Et‐Duphos) catalysts provided the corresponding β²‐amino acid derivatives with excellent enantioselectivities and exceedingly high reactivities (up to >99.5% ee and S/C=10,000). The first hydrogenation of (Z)‐configurated substrates was studied for the synthesis of β²‐amino acid derivatives. The high influence of the substrate geometry and steric hindrance on the reactivity and enantioselectivity was also disclosed for this reaction. This protocol provides a highly practical, facile and scalable method for the preparation of optically pure β²‐amino acids and their derivatives under mild reaction conditions.     

Diastereoselective Synthesis of Six‐Membered Carbocyclic Spirooxindoles via 6π‐Electrocyclization of 3‐Dienylidene‐2‐ oxindoles

The Wittig reaction of isatin derivatives with Morita–Baylis–Hillman bromides of cinnamaldehydes afforded 3‐dienylidene‐2‐oxindoles. These trienes were converted into the corresponding spirooxindoles in a stereoselective manner in refluxing toluene in good yields. The diastereomeric spirooxindoles could be obtained stereoselectively by adding a catalytic amount of palladium(II) acetate via the palladium‐catalyzed isomerization of EEE‐trienes to ZEE‐trienes followed by a more facile 6π‐electrocyclization process. The obtained spirooxindoles could be further functionalized by palladium‐catalyzed oxidative arylation, thionation with Lawesson’s reagent, catalytic hydrogenation and Friedel–Crafts‐type reaction.

     

Dendritic Chiral Phosphine Lewis Bases‐Catalyzed Asymmetric Aza‐Morita–Baylis–Hillman Reaction of N‐Sulfonated Imines with Activated Olefins

A series of polyether dendritic chiral phosphine Lewis bases was synthesized, and successfully applied to the asymmetric aza‐Morita–Baylis–Hillman reaction of N‐sulfonated imines (N‐arylmethylidene‐4‐methylbenzenesulfonamides) with methyl vinyl ketone (MVK), ethyl vinyl ketone (EVK), and acrolein to give the adducts in good to excellent yields along with up to 97 % ee, which are more effective than our previously reported original chiral phosphine Lewis bases. In addition, the dendrimer‐supported chiral phosphine Lewis bases can be easily recovered and reused.     

Heptakis(2,3‐di‐O‐methyl‐6‐O‐sulfopropyl)‐β‐cyclodextrin: A Genuine Supramolecular Carrier for Aqueous Organometallic Catalysis

The behavior of heptakis(2,3‐di‐O‐methyl‐6‐O‐sulfopropyl)‐β‐cyclodextrin as inverse phase transfer catalyst in biphasic Tsuji–Trost and hydroformylation reactions has been investigated. In terms of activity, this methylated sulfopropyl ether β‐cyclodextrin is much more efficient than the randomly methylated β‐cyclodextrin, which was the most active cyclodextrin known to date. From a selectivity point of view, the intrinsic properties of the catalytic system are fully preserved in the presence of this cyclodextrin as the chemo‐ or regioselectivity was found to be identical to that observed without a mass transfer promoter in the hydroformylation reaction. The efficiency of this cyclodextrin was attributed to its high surface activity and to the absence of interactions with the catalytically active species and the water‐soluble phosphane used to dissolve the organometallic catalyst in the aqueous phase.     

Chemoenzymatic Preparation of Enantiopure Homoadamantyl β‐Amino Acid and β‐Lactam Derivatives

Racemic cis‐10‐azatetracyclo[7.2.0.1^2,6.1^4,8]tridecan‐11‐one was prepared from homoadamant‐4‐ene by chlorosulfonyl isocyanate addition. The transformation of the β‐lactam to the corresponding β‐amino ester followed by Candida antarctica lipase A‐catalyzed enantioselective (E>>200) N‐acylation with 2,2,2‐trifluoroethyl butanoate afforded methyl (1R,4R,5S,8S)‐5‐aminotricyclo[4.3.1.1^3,8]undecane‐4‐carboxylate and the (1S,4S,5R,8R)‐butanamide with>99% ee at 50% conversion. Alternatively, transformation of the β‐lactam to the corresponding N‐hydroxymethyl‐β‐lactam and the following Pseudomonas cepacia (currently Burkholderia cepacia) lipase‐catalyzed enantioseletive O‐acylation provided the (1S,4S,6R,9R)‐alcohol (ee=87%) and the corresponding (1R,4R,6S,9S)‐butanoate (ee>99%). In the latter method, competition for the enzyme between the (1R,4R,6S,9S)‐butanoate, 2,2,2‐trifluoroethyl butanoate and the hydrolysis product, butanoic acid, tended to stop the reaction at about 45% conversion and finally gave racemization in the (1S,4S,6R,9R)‐alcohol with time.     

Enantioselective Copper(II)‐Catalyzed Conjugate Addition of Indoles to β‐Substituted Unsaturated Acyl Phosphonates

The first copper‐catalyzed enantioselective conjugate addition of indoles to β‐substituted unsaturated acyl phosphonates was successfully realized by using a heteroarylidene‐tethered bis(oxazoline) ligand. The reaction features high efficiency, cheap catalyst and broad generality. In the case of either β‐alkyl‐ or β‐aryl‐substituted unsaturated acyl phosphonates, the 3‐indolyl adducts were achieved in high yields with good to excellent enantioselectivities (up to 97% ee). The 3‐indolyl adducts can serve as important intermediates in the synthesis of indole alkaloids.

     

Enantioselective Synthesis of Optically Active Alkanephos‐ phonates via Rhodium‐Catalyzed Asymmetric Hydrogenation of β‐Substituted α,β‐Unsaturated Phosphonates with Ferrocene‐Based Monophosphoramidite Ligands

A series of chiral β‐substituted alkanephosphonates was synthesized in high enantioselectivities via the first rhodium‐catalyzed asymmetric hydrogenation of the corresponding β‐substituted‐α,β‐unsaturated phosphonates using a ferrocene‐derived monophosphoramidite ligand, with which up to 99.5% ee have been achieved for the hydrogenation of (E)‐substrates and 98.0% ee for (Z)‐substrates.     

Polymer‐Supported Lewis Bases for the Baylis–Hillman Reaction

The use of polymer‐supported Lewis bases such as PEG₄₆₀₀‐(PPh₂)₂ and poly(DMAP) in the Baylis–Hillman reactions of N‐tosylimines (ArCHNTs) 1 or the corresponding arenecarbaldehydes with α,β‐unsaturated ketones has been investigated. The corresponding Baylis–Hillman adducts are obtained in good yields. The polymer‐supported Lewis bases can be easily recovered by filtration and the Lewis base PEG₄₆₀₀‐(PPh₂)₂ can be reproduced by reduction with LiAlH₄ and CeCl₃.     

Palladium(II)‐Catalyzed 1,4‐Addition of Arylboronic Acids to β‐Arylenones for Enantioselective Synthesis of 4‐Aryl‐4H‐chromenes

The enantioselective 1,4‐addition of arylboronic acids to β‐arylenones to give β‐diaryl ketones was carried out at 0–25 °C in the presence of a dicationic palladium(II) catalyst, [Pd(S,S‐chiraphos)(PhCN)₂](SbF₆)₂. Addition of a silver salt such as silver tetrafluoroborate [AgBF₄] or silver hexafluoroantimonate [AgSbF₆] (5–10 mol %) was effective to achieve high enantioselectivities at low temperatures (92–99 % ee) and to reduce the catalyst loading to 0.05 mol %. The protocol provided a simple access to 4‐aryl‐4H‐chromenes. Optically active chromenes were synthesized with up to 99 % ee via dehydration of the 1,4‐adducts between arylboronic acids and β‐(2‐hydroxyaryl)‐α,β‐unsaturated ketones.     

Chiral Amino Diol Derivatives as New Modular Organocatalysts for the Enantioselective α‐Chlorination of Cyclic β‐Keto Esters

Highly modular chiral amino diol derivatives have been used as organocatalysts in the enantioselective α‐chlorination of cyclic β‐keto esters. Optimization of the catalyst structure and the reaction conditions has allowed the synthesis of optically active α‐chlorinated products with high enantioselectivities (up to 96% ee) using inexpensive commercially available N‐chlorosuccinimide (NCS) as the chlorine source under mild conditions.     

Intermolecular Gold(I)‐Catalyzed Alkyne Carboalkoxylation Reactions for the Multicomponent Assembly of β‐Alkoxy Ketones

A new gold(I)‐catalyzed multicomponent synthesis of β‐alkoxy ketones from aldehydes, alcohols, and alkynes is described. This atom economical synthesis was achieved through the use of the gold complex (SPhos)AuNTf₂ as a catalyst, and allows for the preparation of a diverse array of β‐alkoxy ketone products. Mechanistic studies illustrate that these reactions proceed via gold(I)‐catalyzed hydrolysis of the alkyne to an aryl ketone, which then undergoes an aldol reaction with an oxocarbenium ion generated in situ from the aldehyde and alcohol components.     

Diastereoselective and Enantioselective Epoxidation of Acyclic β‐Trifluoromethyl‐β,β‐Disubstituted Enones by Hydrogen Peroxide with a Pentafluorinated Quinidine‐Derived Phase‐Transfer Catalyst

An efficient catalytic asymmetric epoxidation of β‐trifluoromethyl‐β,β‐disubstituted unsaturated ketones has been achieved by a pentafluorine‐substituted phase‐transfer catalyst with hydrogen peroxide (30%). Thus, the β‐trifluoromethyl‐α,β‐epoxy ketones with a quaternary carbon centre were obtained in excellent diastereoselectivities (up to 100:1 dr) and excellent enantioselectivities (up to 99.7% ee). Low catalyst loading, recycle of catalyst, environmentally benign oxidant and easy transformation of the epoxides into medicinally important trifluoromethylated intermediate make our protocol much more practical.