Enantioselective Organocatalytic Michael Addition of Aldehydes to β-Nitrostyrenes

Highly efficient morpholine-based organocatalysts for the 1,4-addition reaction between aldehydes and nitroolefins: an unexploited class of catalysts

Many studies have demonstrated how the pyrrolidine nucleus is more efficient than the corresponding piperidine or morpholine as organocatalysts in the condensation of aldehydes with electrophiles via enamine. Focussing on morpholine-enamines, their low reactivity is ascribed to the presence of oxygen on the ring and to the pronounced pyramidalisation of nitrogen, decreasing the nucleophilicity of the enamine. Thus, the selection of efficient morpholine organocatalysts appears to be a difficult challenge. Herein, we reported on the synthesis of new organocatalysts belonging to the class of ß-morpholine amino acids that were tested in a model reaction, i.e., the 1,4-addition reaction of aldehydes to nitroolefins. Starting from commercially available amino acids and epichlorohydrin, we designed an efficient synthesis for the aforementioned catalysts, controlling the configuration and the substitution pattern. Computational studies indeed disclosed the transition state of the reaction, explaining why, despite all the limitations of the morpholine ring for enamine catalysis, our best catalyst works efficiently, affording condensation products with excellent yields, diastereoselection and good-to-exquisite enantioselectivity.

Stereoselective conjugate addition of ketones to alkylidene malonates using thiourea-sulfonamide organocatalyst

In this study, stereoselective conjugate addition of ketones to alkylidene malonates using organocatalyst has been developed. The reaction in the presence of 20 mol% of a novel thiourea-sulfonamide organocatalyst afforded conjugate adducts in moderate to high yields (up to 81%) under mild reaction conditions. Excellent diastereoselectivity (up to 98:2 dr) and enantioselectivity (up to 88% ee) were achieved.

Anti-Selective Organocatalytic Michael Addition between Phenylacetaldehyde and Nitrostyrene

Using the reaction between phenylacetaldehyde and nitrostyrene catalyzed by pyrrolidine as a simple model, we have studied the diastereochemical outcome of the organocatalytic Michael reactions between benzylic aldehydes and nitrostyrenes. We found that the anti adduct was obtained in high yield and diastereoselection as was demonstrated by the X-ray structure of the product. In situ NMR studies showed a different reaction pathway when compared to aliphatic aldehydes that yield the syn adduct as major isomer.

TiCl4-promoted condensation of methyl acetoacetate, isobutyraldehyde, and indole: a theoretical and experimental study

A combined DFT and experimental study of the title reaction points to a three-step mechanism: (1) titanium enolate formation, (2) Knoevenagel condensation of enolate and aldehyde, and (3) Michael addition of indole.

Diastereoselective multicomponent cascade reaction leading to [3.2.0]-heterobicyclic compounds

A general three-component triple cascade reaction through an iminium-enamine-iminium sequential activation initiated by a hetero-Michael addition to α,β-unsaturated aldehydes affords [3.2.0]heterobicycles in high diastereoselectivity. The rate and diastereoselectivity of the reaction depended on the (E)-4-heterocrotonate and size of the secondary amine. The enantiomers of the major diastereoisomer of oxa- and azabicyclo[3.2.0]heptane derivatives were separated by enzymatic kinetic resolution with immobilized Candida antarctica Lipase B (CALB), with E values up to 153. The absolute configuration of the nonacylated enantiomer of oxabicyclo[3.2.0]heptane was determined by single crystal X-ray analysis.

Organocatalytic asymmetric addition of malonates to unsaturated 1,4-diketones

The organocatalytic Michael addition of malonates to symmetric unsaturated 1,4-diketones catalyzed by thiourea and squaramide derivatives with Cinchona alkaloids afforded the formation of a new C–C bond in high yields (up to 98%) and enantiomeric purities (up to 93%). The absolute configuration of the product was suggested from comparison of the experimental and calculated VCD spectra of the reaction product 3a.

Electrophilicities of trans-β-nitrostyrenes

The kinetics of the reactions of the trans-β-nitrostyrenes 1a-f with the acceptor-substituted carbanions 2a-h have been determined in dimethyl sulfoxide solution at 20 °C. The resulting second-order rate constants were employed to determine the electrophile-specific reactivity parameters E of the trans-β-nitrostyrenes according to the correlation equation log k(2)(20 °C) = s(N)(N + E). The E parameters range from -12 to -15 on our empirical electrophilicity scale (www.cup.lmu.de/oc/mayr/DBintro.html). The second-order rate constants for the reactions of trans-β-nitrostyrenes with some enamines were measured and found to agree with those calculated from the electrophilicity parameters E determined in this work and the previously published N and s(N) parameters for enamines.

Catalytic asymmetric Michael addition with curcumin derivative

Catalytic asymmetric Michael additions with curcumin derivatives were achieved by a new series of tertiary amine-thiourea organocatalysts to afford the Michael adducts in high yields and excellent enantioselectivities.

Pyrrolidinyl-camphor derivatives as a new class of organocatalyst for direct asymmetric Michael addition of aldehydes and ketones to beta-nitroalkenes

Practical and convenient synthetic routes have been developed for the synthesis of a new class of pyrrolidinyl-camphor derivatives (7 a-h). These novel compounds were screened as catalysts for the direct Michael addition of symmetrical alpha,alpha-disubstituted aldehydes to beta-nitroalkenes. When this asymmetric transformation was catalyzed by organocatalyst 7 f, the desired Michael adducts were obtained in high chemical yields, with high to excellent stereoselectivities (up to 98:2 diastereomeric ratio (d.r.) and 99 % enantiomeric excess (ee)). The scope of the catalytic system was expanded to encompass various aldehydes and ketones as the donor sources. The synthetic application was demonstrated by the synthesis of a tetrasubstituted-cyclohexane derivative from (S)-citronellal, with high stereoselectivity.