Enantioselectivity in organocatalytic cascade double Michael addition reaction: a theoretical study

We have investigated important intermediates and key transition states of the organocatalyzed cascade double Michael addition using density functional theory. The calculated results suggest that the reaction contains intermolecular nucleophilic addition and intramolecular cyclization, both involving the formation of two stereocenters. The iminium-enamine catalysis of secondary amine unit enables the cascade addition to proceed consecutively. As an electron transport, the iminium attracts the electron stream to promote the nucleophilic addition. Then enamine causes the electron stream to catalyze the cyclization. As H bond donor, the catalyst forms three types of C-H···O H bond with substrates. The enantioselectivity and diastereoselectivity are dominated by the catalyst backbone. Two group links of pyrrole-phenyl and pyrrole-silyl ether orient the reaction in paths with smaller rotations of the linked single bonds. Our conclusion is supported by NBO analysis and the predicted ee, dr values according to the experiment.     

Enatioselective organocatalytic synthesis of highly functionalized tetrahydrothiophenes by a Michael-aldol cascade reaction

A catalytic asymmetric Michael-aldol cascade process for efficient synthesis of trisubstituted tetrahydrothiophenes is reported with high enantio- and diastereo-selectivities. Notably, three consecutive stereogenic centers including one chiral quaternary carbon center are efficiently created in the ‘one-pot’ operation.     

Asymmetric synthesis of trifluoromethyl substituted dihydropyrans via organocatalytic cascade Michael-hemiketalization reaction

Asymmetric conjugate addition of ethyl 4,4,4-trifluoroacetoacetate and other trifluoromethyl substituted nucleophiles to β,γ-unsaturated α-keto esters has been developed. The reaction efficiently provided dihydropyrans via cascade Michael-hemiketalization pathways. Quinine-derived thiourea was identified to be the best catalyst. A number of trifluoromethyl substituted dihydropyrans with three consecutive chiral centers were obtained in excellent yields, diastereoselectivities, and enantioselectivities. The product was readily transformed to the corresponding tetrahydropyridine without the loss of the optical purity.     

'Double asymmetric induction' as a novel tool for high stereocontrol in Baylis-Hillman reaction

The strategy of double asymmetric induction was utilized in Baylis-Hillman reaction for the first time by the coupling of chiral aldehydes with chiral acrylate (1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose-3-acrylate) to obtain corresponding adducts with high syn diastereoselectivities (de >90%) in moderate to good yields.     

Highly enantioselective organocatalytic cascade reaction for the synthesis of piperidines and oxazolidines

The synthesis of piperidines and piperidines derivatives in enantiopure fashion has been a challenging goal for organic chemists. In this report we developed a nice cascade reaction for piperidine derivatives based in an amidomalonate Michael addition to enals followed by an intramolecular hemiaminal formation with good yields and enantioselectivities. Moreover we studied the ‘in situ’ intramolecular cyclization of this hemiaminals with alcohols forming fused piperidine-oxazolidines.     

Stereoselective construction of fluorinated indanone derivatives via a triple cascade Lewis acid-catalyzed reaction

A one-pot three-component cascade reaction proceeds by way of a Lewis acid-catalyzed Knoevenagel condensation/Nazarov cyclization/electrophilic fluorination sequence to afford fluorinated 1-indanone derivatives in moderate to good yields with high diastereoselectivities.     

Electrospray mass spectrometry for detailed mechanistic studies of a complex organocatalyzed triple cascade reaction

The development of modular combinations of organocatalytic reactions into cascades has been shown to be an effective tool despite the fact that the mechanism of such a complex organocatalytic multistep cascade reaction still remains poorly understood. Here the detailed mechanistic studies of a complex organocatalytic triple cascade reaction for the synthesis of tetra-substituted cyclohexene carbaldehydes are reported. The investigation has been carried out using a triple quadrupole mass spectrometer with electrospray ionization. Important intermediates were detected and characterized through MS/MS studies. A detailed formation pathway is presented based on these characterized intermediates, and supporting the proposed mechanism of the formation of the substituted cyclohexene carbaldehydes.     

Enantioselective radical cyclizations: a new approach to stereocontrol of cascade reactions

Stereocontrol in a cascade radical addition-cyclization-trapping reaction was achieved by a new approach, which utilizes a hydroxamate ester moiety as a coordinating chiral Lewis acid tether between two radical acceptors. A remarkable feature of this reaction is the construction of three bonds and tertiary and quaternary stereogenic centers through both inter- and intramolecular carbon-carbon bond-forming processes. The chiral Lewis acid mediated reaction of oxime ethers also proceeded smoothly with good enantio- and diastereoselectivities, indicating the usefulness of the cascade approach for the asymmetric synthesis of various gamma-lactams.     

Lewis acid-mediated radical cyclization: stereocontrol in cascade radical addition-cyclization-trapping reactions

An efficient approach for achieving radical cyclizations by using hydroxamate ester as a coordination tether with Lewis acid was studied. The chiral Lewis acid-mediated cascade radical addition-cyclization-trapping reaction proceeded smoothly with good enantio- and diastereoselectivities, providing various chiral γ-lactams.     

Asymmetric epoxidation, Michael addition, and triple cascade reaction using polymer-supported prolinol-based auxiliaries

The applicability of polymer-supported diphenylprolinol derivatives in directing either asymmetric epoxidation or Michael addition of suitable α,β-unsaturated substrates has been assessed. Epoxidation of cinnamaldehyde in the solid state give poorer yields and stereoselectivities than in the solution-phase systems. In contrast Michael additions of several aldehyde enolates to 2-nitro-1-arylalkenes gave results that approached or surpassed those in solution, and could be extended successfully to a three-component Michael/Michael/aldol cascade process. Comparisons of the results of pendant- versus crosslinked functionalized resins in these applications were revealing of the benefits and limitations of each, as were attempts to reuse the polymer-bound auxiliaries.     

A highly efficient asymmetric organocatalytic aldol reaction in a ball mill

Anti-aldol products with up to >99 % enantiomeric excess (ee) have been obtained by proline catalysis in excellent yields under experimentally simple solvent-free conditions. Efficient mixing of all the components is accomplished by applying a mechanochemical technique (ball milling). The catalysis is air and moisture tolerant and can be performed with non-purified starting materials. Even mixtures of solely solid compounds react, giving (mostly solid) products through a partially homogeneous (honey-like) intermediate melt. Since the reactant ratio is almost 1:1 (avoiding the common excess of ketone), the product isolation is easy leading to high aldol product yields.     

Thermal effects in the organocatalytic asymmetric Mannich reaction

The proline-catalyzed direct asymmetric Mannich reaction between cyclohexanone, formaldehyde, and various anilines is thermally accelerated. With only 0.5 mol % of catalyst, Mannich products with up to 98% ee have been obtained after a short period of time in reactions performed under microwave irradiation. In situ reduction of the resulting ketones affords N-aryl amino alcohols in up to 86% yield.     

Tetrahydropyran rings from a Mukaiyama-Michael cascade reaction

A new annulation reaction leading to tetrahydropyrans has been discovered. The reaction of homoallylic enol ethers (e.g., 1) with alpha,beta-unsaturated ketones or esters begins with a Mukaiyama-Michael addition. The intermediate oxocarbenium ion undergoes a rapid 2-oxonia-Cope rearrangement, and the resulting zwitterion collapses to form a tetrahydropyran. The reaction is stereoselective with 3-butene-2-one, but leads to diastereomeric mixtures with ethyl acrylate. More complex enones, such as cyclohexenone, also undergo the reaction to produce fused ring products. The optical activity of the substrates is relayed in the tetrahydropyran products.     

Remarkable stereocontrol observed in the ring formation by intramolecular hosomi-sakurai reaction

The cyclization of 2-(3′,4′-dimethyl6?′-trimethylsilyl-4′-hexenyl)-2-cyclohexenone with TiCl₄, afforded stereospecifically 6α,7α-dimethyl-6β-vinyl-5βH-1-decalone derivative and the two-fold diastereoselection involved is reasoned in terms of orientation control and folding strain control.     

Multicomponent reaction to construct spirocyclic oxindoles with a Michael (triple Michael)/cyclization cascade sequence as the key step

Multicomponent cycloadditions with readily available isocyanides, allenoates, and isatylidene malononitriles are disclosed. This reaction, which does not require the aid of any catalyst, allows the efficient syntheses of spirocyclic oxindoles with excellent regioselectivity. Reactions with ethyl 2,3-butadienoate and various structurally diverse α- and γ-substituted allenoates are also fully explored. Remarkably, we have shown that the usual three-component process can be further developed into an unprecedented four-component cycloaddition in the presence of water, which provides a new strategy to access highly unusual tricyclic oxindoles. From a synthetic point of view, this protocol is very interesting considering the high level of complexity reached in one step. The mechanism is thought to proceed by a triple Michael/cyclization process by using allenoate as a three carbon atom component (3?C). Furthermore, multicomponent reaction with γ-substituted allenoate also results in a very interesting conversion. In such cases, the unusual cleavage of the "C=C" double bond of isatylidene malononitrile and one of the "C=C" double bonds of allenoate is always observed.     

Hexafluorobenzene: a powerful solvent for a noncovalent stereoselective organocatalytic Michael addition reaction

A dramatic enhancement of the diastereo- and enantioselectivity in the nitro-Michael addition reaction organocatalysed by a commercially available α,α-L-diaryl prolinol was disclosed when performing the reaction in unconventional hexafluorobenzene as a medium. DFT calculations were performed to clarify the origin of stereoselectivity and the role of C(6)F(6).     

Remote stereocontrol in the Nazarov reaction: a new approach to the core of roseophilin

Three different procedures are compared to obtain properly substituted divinyl ketones in which one of the double bonds is embedded in a five-membered heterocyclic structure and therefore suitable to produce cyclopenta-fused pyrrole derivatives by the acid-catalyzed Nazarov reaction. These, on treatment with TFA, afforded 2,4-cis-disubstituted 2,3,4,5-tetrahydro-1H-cyclopenta[b]pyrrol-6-ones with high stereocontrol. One of these Nazarov products was oxidized to the corresponding 4,5-dihydro-1H-cyclopenta[b]pyrrol-6-one derivative, thus obtaining an enantiopure key intermediate in the total synthesis of roseophilin.     

Highly enantioselective organocatalytic direct aldol reaction in an aqueous medium

We have demonstrated that small organic molecules 1 and 2 catalyzed the direct aldol reaction of both acyclic and cyclic ketones with different aldehydes in an excess of water/brine. Excellent enantioselectivities up to >99% and diastereoselectivities up to 99% with very good yields were obtained by using much lower catalyst loadings (0.5 mol %).