Application of (S)- and (R)-methyl pyroglutamates as inexpensive, yet highly efficient chiral auxiliaries in the asymmetric Michael addition reactions

Methyl N-(E-enoyl)pyroglutamates, derived from inexpensive and readily available in both enantiomeric forms pyroglutamic acid were found to be an efficient Michael acceptors in the addition reactions with nucleophilic glycine equivalent allowing for an efficient practical asymmetric synthesis of β-substituted pyroglutamic acids and related compounds.     

Copper(II) tetrafluoroborate as a novel and highly efficient catalyst for Michael addition of mercaptans to α,β-unsaturated carbonyl compounds

Copper(II) tetrafluoroborate has been found to be a new and highly efficient catalyst for Michael addition of thiols to α,β-unsaturated carbonyl compounds under solvent-free conditions and in H₂O at room temperature. The reactions are very fast and are completed in 2 min to 1 h affording high yields. The rate of thiol addition was dependent on the steric hindrance at the β-carbon of the α,β-unsaturated carbonyl substrate. In the case of chalcones, the reactions are best carried out in MeOH as solvent.     

Chiral bidentate bis(n-heterocyclic carbene)-based palladium complexes bearing carboxylate ligands: highly effective catalysts for the enantioselective conjugate addition of arylboronic acids to cyclic enones

Axially chiral cis-chelated bidentate bis(N-heterocyclic carbene)-palladium(II) complexes with two weakly coordinating carboxylate ligands are effective catalysts for the asymmetric conjugate addition of arylboronic acids to cyclic enones, producing the corresponding adducts in moderate-to-high yields and with good-to-high enantioselectivities, in most cases under mild conditions.     

Evidence that protons can be the active catalysts in Lewis acid mediated hetero-Michael addition reactions

The mechanism of Lewis acid catalysed hetero‐Michael addition reactions of weakly basic nucleophiles to α,β‐unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone–catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of π‐olefin–metal complexes or for carbonyl–metal‐ion interactions was obtained. The reactions did not proceed in the presence of the non‐coordinating base 2,6‐di‐tert‐butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono‐ and dicarbonyl substrates have been rationalised. A ¹H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid‐catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.     

Syntheses of Polycyclic Natural Products Employing the Intramolecular Double Michael Reaction

The one-pot intramolecular double Michael reaction of compounds having two different α, β-unsaturated carbonyl groups to form polycyclic compounds can be carried out by three different methods: in the first the substrate is treated with lithium hexamethyldisilazide, in the second with chlorotrimethylsilane, triethylamine, and zinc chloride at an elevated temperature, and in the third with tert-butyldimethylsilyl trifluoromethanesulfonate and triethylamine. The reaction proceeds with complete regioselectivity and high stereoselectivity following a stepwise mechanism. Spiro-fused bicyclo[2.2.2]octane derivatives can be constructed with high stereoselectivity by the intromolecular double Michael reaction by using the first method. Enantiomerically pure atisine and the enantiomer of atisirene were synthesized stereoselectively by application of this methodology. The syntheses of steroids and angular triquinane-type sesquiterpenoids were possible with the second method. Heterocyclic compounds with a bridgehead nitrogen atom were obtained by the reaction of α, β-unsaturated amides following the second and third methods. The asymmetric synthesis of tylophorine with diastereofacial control was achieved by the intramolecular reaction according to the third method. The sulfur-mediated intramolecular double Michael reaction utilizing the third method produced trans-hydroindane derivatives. Furthermore, the intramolecular Michael-aldol reaction can be employed in synthesizing polycyclic systems with cyclobutane units by treatment with tert-butyldimethylsilyl trifluoromethanesulfonate in the presence of triethylamine. The intermolecular double Michael reaction and related reactions will also be described.     

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).     

Samarium diiodide mediated intramolecular cyclisation of mixed enone-enoate systems: a simple preparation of spirocyclic ethers

Samarium(II) diiodide mediated intramolecular cyclisation of mixed enone-enoate substrates in THF/MeOH is described. Spirocyclic ethers are obtained, and the stereodefined preparation of 1-oxaspiro[4.4]nonan-7-one products is included.     

Lithium tetrafluoroborate catalyzed highly efficient inter- and intramolecular aza-Michael addition with aromatic amines

Lithium tetrafluoroborate has been demonstrated for the first time to be an efficient catalyst in intermolecular aza-Michael addition aromatic amines to electron deficient alkenes. Suitability of the same catalyst in intramolecular aza-Michael addition leading 2-aryl-2,3-dihydroquinolin-4(1H) ones has also been described.     

A practical, green, and selective approach toward the synthesis of pharmacologically important quinone-containing heterocyclic systems using alumina-catalyzed Michael addition reaction

A convenient method for double Michael additions to quinone systems catalyzed by Al₂O₃, is reported. The advantages of this method include the use of a cheap and environment-friendly catalyst, a straightforward isolation of the pure product by filtration, high yields, and excellent selectivity, thus providing rapid access to useful building blocks for the preparation of biologically active quinones.     

A new type of organocatalyst for highly stereoselective Michael addition of ketones to nitroolefins on water

(S)-2-((Naphthalen-2-ylsulfonyl)methyl)pyrrolidine, prepared in three steps from (S)-N-Boc-2-[((4-toluenesulfonyl)oxy)methyl]pyrrolidine in 62% overall yield, was used as a new type of organocatalyst bearing a pyrrolidine and a sulfone moiety. It shows very high catalytic activity toward the direct asymmetric Michael reaction of cyclohexanone and nitroolefins. All the corresponding adducts can be furnished in 90-99% yields and with up to 98% ee and over 99:1 dr on water in the presence of this catalyst (15 mol %) without any additive.     

A highly efficient asymmetric Michael addition of anthrone to nitroalkenes with cinchona organocatalysts

A highly efficient asymmetric Michael addition of anthrone to nitroalkenes catalyzed by cinchona alkaloids was described. Up to 99% ee of the corresponding adduct was obtained.     

One-pot synthesis of highly substituted tetrahydrofurans from activated propargyl alcohols using Bu3P

General and robust synthesis of highly functionalized tetrahydrofuran ring was accomplished with activated propargyl alcohol and reactive Michael acceptors in the presence of catalytic Bu₃P.