Secondary-secondary diamine catalysts for the enantioselective Michael addition of cyclic ketones to nitroalkenes

Secondary-secondary diamines derived from S-proline are efficient catalysts for the ketone-nitroalkene Michael addition reaction. The stereoselectivity of the Michael addition is dependant on the pK_a of the N-substituted aminomethyl pendant in these diamines. N′-Aryl aminomethyl pyrrolidines provide γ-nitroketones with moderate to good enantiomeric excess (65-92%). Removal of the hydrogen-bond donor group by N-methylation results in a dramatic reduction of enantioselectivity (average ee 6%).     

Aromatic l-prolinamide-catalyzed asymmetric Michael addition of aldehydes to nitroalkenes

Two chiral aromatic l-prolinamides were synthesized in high overall yield (95%) from N-Boc-l-proline and served as organocatalysts in asymmetric Michael reactions of aldehydes to nitroalkenes. Under the optimized reaction conditions, (S)-N-tritylpyrrolidine-2-carboxamide 4 was found to be a highly efficient organocatalyst for the Michael addition, and the corresponding Michael adducts were obtained in good yields (up to 94%), with excellent enantioselectivities (up to 99% ee) and diastereoselectivities (up to 99:1 dr).     

Highly stereoselective Michael reduction/intramolecular Michael reaction cascade to synthesize trans-stereodiad comprising an all-carbon quaternary stereogenic center

A highly stereoselective Michael reduction/intramolecular Michael reaction cascade is described. The cascade is initiated by the regioselective Michael reduction of an α-methylidene ester with L-Selectride. This is followed by the highly stereoselective intramolecular Michael reaction which efficiently constructs a six-membered carbocyclic ring with formation of the trans-stereodiad, composed of an all-carbon quaternary center and a tertiary stereogenic center. The stereoselectivity is perfectly controlled by the choice of alkene geometry in the Michael acceptor.     

Bifunctional iminophosphorane superbases: Potent organocatalysts for enantio- and diastereoselective Michael addition reactions

Bifunctional iminophosphorane (BIMP) organocatalysts catalyze the enantio- and diastereoselective Michael additions of high pK_a cyclic malonamate ester pro-nucleophiles to nitroalkenes with reactivity profiles of up to 3 orders of magnitude greater than tertiary amine bifunctional Brønsted base/(thio)urea organocatalysts. The unrivalled performance of the BIMPs allows reaction times of challenging reactions to be slashed from weeks to minutes and has enabled new flow chemistry applications using polystyrene-supported versions contained within a flow reactor.     

Regioselective construction of polysubstituted pyridine ring from Baylis-Hillman adducts via sequential introduction of tosylamide, Michael reaction, aldol condensation, and elimination of TsH

Facile synthetic method of polysubstituted pyridine derivatives was developed starting from the Baylis-Hillman adducts. The reaction involved sequential introduction of tosylamide, Michael addition, aldol condensation, elimination of p-toluenesulfinic acid, and isomerization process.     

Asymmetric Michael reactions of α-substituted acetates with cyclic enones catalyzed by multifunctional chiral Ru amido complexes

Well-defined 16-electron chiral Ru amido complexes, Ru[(R,R)-diamine](η⁶-arene), efficiently catalyze asymmetric Michael additions of Michael donors to cyclic enones to give adducts in high yields and with excellent ee’s. β-Ketoesters or nitroacetate as Michael donors react with 2-cyclopentenone in toluene or t-butyl alcohol containing the Ru amido catalyst (S/C=50) to afford the Michael adduct in 99% yield and with up to 92% ee. The outcome of the reaction was delicately influenced by the structures of the diamine and arene ligands as well as reaction conditions.     

Solvent-free Brønsted acid-catalyzed Michael addition of nitrogen- and carbon-containing nucleophiles by ultrasound activation

A new method has been developed for the Michael addition of nitrogen- and carbon-containing nucleophiles to cyclic enones. Using this conjugate addition reaction, a variety of different nucleophiles can react with a range of cyclic enones in the presence of p-toluenesulfonic acid under solvent-free ultrasound irradiation conditions affording the corresponding C–N or C–C adducts in good to excellent yields. Comparatively, performing the reaction under ultrasound irradiation gives higher yields, is more efficient and environmentally benign than performing it at high pressure.     

Ferrocenophane-based bifunctional organocatalyst for highly enantioselective Michael reactions

Highly enantioselective Michael reactions between acetylacetone and trans-β-nitroolefins are achieved by a novel ferrocenophane-based tertiary amine-thiourea organocatalyst to provide the corresponding products in good to excellent yields (up to 95%) and enantioselectivities (up to 99% ee).     

Chiral primary amine thiourea promoted highly enantioselective Michael reactions of isobutylaldehyde with maleimides

Chiral primary amine thiourea catalysts were first successfully applied to promote Michael addition of isobutyraldehyde to maleimides. A variety of N-aryl and N-aliphatic maleimides provided Michael adducts in excellent yields (up to 98%) and enantioselectivities (up to 99% ee) with 5 mol % catalyst.     

Michael Initiated Ring Closure (MIRC) reaction on in situ generated benzylidenecyclohexane-1,3-diones for the construction of chromeno[3,4-b]quinoline derivatives

One-pot synthesis of chromeno[3,4-b]quinoline derivatives have been achieved in good yields through Michael Initiated Ring Closure (MIRC) by employing three-component condensation of aromatic aldehydes, 3-aminocoumarins, and cyclic 1,3-diketones in the presence of catalytic amount of p-toluenesulfonic (p-TSA) acid in ethanol under reflux condition. The salient features of this protocol are: simple reaction procedure, shorter reaction time, good yields, avoidance of aqueous work-up, and column-chromatographic separation. The merit of this process is highlighted by its high bond efficiency of producing three new bonds and one stereocenter in a single operation.     

Synthesis of isoxazolobenzoxepanes via Michael addition of indoles to nitroalkenes and sequential intramolecular nitrile oxide cycloaddition

Nitroalkenes derived from O-propargyl salicylaldehyde undergo facile Michael addition with indoles leading to indole-derived Michael adducts. Intramolecular nitrile oxide cycloaddition (INOC) of the Michael adducts results in isoxazolobenzoxepanes in good to excellent yields.     

Nitroalkenes as electrophiles in the asymmetric Michael reaction involving chiral imines/enamino esters

Addition of 2-nitropropene to the chiral imine derived from 2-methylcyclopentanone and (S)-1-phenylethylamine furnished the expected Michael adduct. The latter compound was then efficiently converted into (R)-pentalenone through a Nef reaction. Condensation of the enamino ester derived from 2-carbethoxycyclopentanone and (S)-1-phenylethylamine with 1-nitropropene gave with excellent diastereo- and enantioselectivity the corresponding Michael adduct.     

Phosphinoyl-aziridines as a new class of chiral catalysts for enantioselective Michael addition

A series of new optically pure phosphine oxides containing chiral aziridine subunit were synthesized in good yields and applied as organocatalysts in asymmetric Michael reaction of various aliphatic aldehydes with β-nitrostyrene. The corresponding organocatalysts were synthesized starting from optically pure aziridines in a few simple efficient steps. The appropriate Michael adducts were obtained in most cases in very high chemical yield (up to 97%), excellent enantioselectivity (up to 98% of ee) and diastereoselectivity (up to 95:5 of dr).     

Michael addition-based cyclization strategy in the total synthesis of Lycopodium alkaloids

Lycopodium alkaloids, a unique family of biologically important natural products isolated and characterized from various species of Lycopodium (sensu lato), have attracted extensive attention from chemists and pharmacists in the past three decades. Michael addition-based cyclization has been successfully employed as an elegant and efficient ring-construction protocol of constructing key cyclohexanone intermediates in the total synthesis of Lycopodium alkaloids. This mini-review chooses and summarizes several representative total syntheses of various Lycopodium alkaloids in which intramolecular Michael addition severed as the key methodology.     

Organocatalytic Michael addition of aldehydes to trisubstituted nitroolefins

The combination of O-TMS protected diphenyl-prolinol and benzoic acid was found to be effective to catalyze the Michael addition of aldehydes to 3-substituted 3-nitroacrylates. The reaction provided syn,anti-Michael adducts with good diastereoselectivity and excellent enantioselectivity. Some β-aryl and α-methyl substituted nitroolefins also worked under these conditions, although prolonging reaction time was required. These adducts could be used for assembling 2,3,4-trisubstituted pyrrolidines through simple hydrogenation.     

Michael addition reactions between various nucleophilic glycine equivalents and (S,E)-1-enoyl-5-oxo-N-phenylpyrrolidine-2-carboxamide,an optimal type of chiral Michael acceptor in the asymmetric synthesis of β-phenyl pyroglutamic acid and related compounds

(S)-5-Oxo-N-phenyl-1-[(E)-3-phenylacryloyl]pyrrolidine-2-carboxamide, easily prepared from inexpensive and readily available, in both enantiomeric forms, glutamic/pyroglutamic acid was designed as an optimal type of chiral Michael acceptor for reactions with a series of nucleophilic glycine equivalents. A study of the corresponding Michael addition reactions revealed that the new generation of modular glycine derivatives, as a counterpart to the Michael acceptor, provides for operationally convenient preparation of β-phenyl pyroglutamic acids and related compounds with virtually complete chemical and stereochemical outcomes.     

Highly enantioselective Michael addition of acetone to nitroolefins catalyzed by chiral bifunctional primary amine-thiourea catalysts with acetic acid

Highly enantioselective Michael reaction of acetone with a variety of nitroolefins catalyzed by N-[(1R,2R)-2-aminocyclohexyl]-N′-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-thiourea (1b) together with acetic acid is described. The Michael addition products were obtained in high yields (76-94%) and up to 96% ee.     

α-Thiosubstituted chiral imines/secondary enamines: their use in the asymmetric Michael reaction

The asymmetric Michael reaction between 2-thiosubstituted chiral imines/secondary enamines derived from (S)-1-phenylethylamine and electrophilic alkenes (methyl acrylate, MVK) was investigated. 2-Phenylthio derivatives furnished the expected Michael adducts with excellent ee. By contrast, an in situ elimination of p-toluenesulfenic acid took place when using the p-toluenesulfinyl analogs.     

Catalytic enantioselective Michael addition in the synthesis of α-aminophosphonates

Enantioenriched (S)-phosphoglutaminic acid derivatives with up to 72% ee were obtained in the catalytic enantioselective Michael reaction of the achiral phosphoglycine synthon. The scope and limitation of the process in terms of the catalyst (diverse (R,R)-TADDOL derivatives) and the base (solid alkali metal tert-butoxides) were examined. The nature of the transition complex was also investigated.     

N-Tosylimidates in highly enantioselective organocatalytic Michael reactions

N-Tosylimidates acted as nucleophiles in highly enantioselective organocatalytic Michael addition reactions to α,β-unsaturated aldehydes in the presence of a catalytic amount of trialkylsilyl-protected diarylprolinol. In particular, α-phenyl-substituted N-tosylimidates showed good reactivity. We also demonstrate that the kinetic acidity of the α-proton of α-phenyl N-tosylimidate as measured by proton/deuterium NMR exchange experiments is correlated with the potential of N-tosylimidates to act as nucleophiles in organocatalytic reactions.