Enantioselective Total Synthesis of (+)-Eucophylline.

The total enantioselective synthesis of (+)-eucophylline 1 was achieved using as a key-structural motif a chiral piperidinone bearing the natural product all-carbon quaternary stereocenter. The elaboration of the latter is based on two strategies relying on the free-radical carbo-cyanation and sulfonyl-cyanation respectively of enantiopure substituted cyclopropenes and cyclobutenes. Co- or Ni-boride reduction of the nitrile functional group along with the cyclopropane and cyclobutane ring-opening then led to the formation of the chiral piperidinone ring. Further elaboration of the latter into the key 1-azabicyclo[3.3.1]nonane motif followed by its coupling with a 2-cyanoaniline allowed the formation of the tetrahydrobenzo[b][1,8]-naphthyridine skeleton of 1, which was finally accessible in 17 steps and 5.9 % overall yield from 1,1-dibromobutene.

Mechanistic Perspectives in the Regioselective Indole Addition to Unsymmetrical Silyloxyallyl Cations.

Our investigations on the reaction mechanism to account for regioselectivity on the addition of indoles to unsymmetrical silyloxyallyl cations are reported. Using both experimental and computational methods, we confirmed the significance of steric effects from the silyl ether group toward directing the inward approach of indoles, leading to nucleophilic attack at the less substituted electrophilic α'-carbon. The role of residual water toward accelerating the rate of reaction is established through stabilization of the participating silyloxyallyl cation.

Enantioselective Functionalization of Enamides at the ß-Carbon Center with Indoles.

We report an enantioconvergent approach for the functionalization of enamides at the ß-carbon atom, which involves a chiral Brønsted acid induced tautomerization of 2-amidoallyl into 1-amidoallyl cations. These putative reactive intermediates were produced by ionization of racemic α-hydroxy enamides with a chiral Brønsted acid and captured with substituted indoles in a highly regio- and enantioselective manner.

Visible-Light-Mediated Addition of Phenacyl Bromides onto Cyclopropenes.

Visible-light-promoted addition of α-bromoacetophenones onto the cyclopropene π-system in the presence of the fac-Ir(ppy)3 catalyst was shown to afford the corresponding 1(4H)-naphthalenones. The syn-carboarylation of the cyclopropene is followed by a cyclopropane ring opening under the basic conditions, allowing the formation of two C-C bonds and the generation of 1(4H)-naphthalenones bearing an all-carbon benzylic quaternary stereocenter.

Regioselective Functionalization of Enamides at the α-Carbon via Unsymmetrical 2-Amidoallyl Cations.

A new method to functionalize enamides via an intermediacy of unsymmetrical 2-amidoallyl cations is reported. Generated under mild Brønsted acid catalysis, these reactive species were found to undergo addition with various nucleophiles at the less substituted α-carbon to produce highly substituted enamides in high yields with complete control of regioselectivity.

Free-Radical Carbocyanation of Cyclopropenes: Stereocontrolled Access to All-Carbon Quaternary Stereocenters in Acyclic Systems.

Free-radical carbocyanation of cyclopropenes offers straightforward access to tetrasubstituted cyclopropanes in satisfying yields with moderate diastereoselectivity. The incorporation of various functional groups on the cyclopropane ring allows a subsequent base-mediated ring-opening reaction leading to functionalized acyclic systems having an all-carbon quaternary stereocenter.

Functionalization of Silyldienol Ethers at the γ-Position via 2-Silyloxypentadienyl Cations.

This report describes Brønsted acid catalyzed de novo synthesis of silyldienol ethers bearing tetrasubstituted double bonds via an intermediacy of 2-silyloxypentadienyl cations. The reactivity of these novel cationic intermediates could be modulated and harnessed toward direct nucleophilic additions regioselectively at the γ-position to produce highly functionalized silyldienol ethers with tunable control of the resulting double bond geometry.

Brønsted acid catalyzed α'-functionalization of silylenol ethers with indoles.

A new method which enables carbon-carbon bond formation at the α'-position of silylenol ethers by using catalytic amounts of pyridinium triflate is reported. This chemistry successfully produces, structurally challenging, highly substituted indole-containing silylenol ethers in excellent yields with complete regiocontrol, presumably through silyloxyallyl cation intermediates. Despite the use of Brønsted acid, the silylenol ether moiety does not undergo protodesilylation, thus underscoring the very mild reaction conditions.

Cooperative benzylic-oxyallylic stabilized cations: regioselective construction of α-quaternary centers in ketone-derived compounds.

We describe a novel reactivity of benzylic-stabilized oxyallyl cations towards regioselective construction of carbon quaternary centers. These synthetically useful intermediates were readily generated upon ionization of aryl substituted α-hydroxy methylenol ethers with catalytic, mild Brønsted acid. The emerging unsymmetrical oxyallyl cations were then directly captured by indoles and other nucleophiles with exquisite control of regioselectivity, predictably at the electrophilic carbon bearing the alkyl substituent to produce highly functionalized, value-added enol ethers.

One-pot asymmetric synthesis of seven-membered carbocycles cyclohepta[b]indoles via a sequential organocatalytic Michael/double Friedel-Crafts alkylation reaction.

A new method has been developed for the enantioselective synthesis of highly functionalized cyclohepta[b]indoles with high enantioselectivity (up to 96% ee). The process combines an enantioselective organocatalytic Michael addition and a highly efficient double Friedel-Crafts reaction sequence in one pot with good yields and stereoselectivity. The structures and absolute configurations of the products were confirmed by X-ray analysis.

One-pot organocatalytic enantioselective domino double-Michael reaction and Pictet-Spengler-lactamization reaction. A facile entry to the "inside yohimbane" system with five contiguous stereogenic centers.

An expedited method was developed for the enantioselective synthesis of dodecahydrobenz[a]indolo[3,2-h]quinolizine containing five contiguous stereogenic centers with high enantioselectivities (>99% ee). The methodology comprises a domino organocatalytic double Michael reaction and Pictet-Spengler-lactamization reaction. The structures and absolute configurations of the appropriate products were confirmed by X-ray analysis.

Organocatalytic asymmetric anti-selective Michael reactions of aldehydes and the sequential reduction/lactonization/Pauson-Khand reaction for the enantioselective synthesis of highly functionalized hydropentalenes.

A new method has been developed for the enantioselective synthesis of highly functionalized hydropentalenes bearing up to four stereogenic centers with high stereoselectivity (up to 99% ee). This process combines an enantioselective organocatalytic anti-selective Michael addition with a highly efficient one-pot reduction/lactonization/Pauson-Khand reaction sequence. The structures and absolute configurations of the products were confirmed by X-ray analysis.

Organocatalytic Michael-Knoevenagel-hetero-Diels-Alder reactions: an efficient asymmetric one-pot strategy to isochromene pyrimidinedione derivatives.

Synthesis of isochromene pyrimidinedione derivatives having five stereocenters has been achieved by a one-pot Michael-Knoevenagel condensation-inverse-electron-demand hetero-Diels-Alder reaction of α, ß-unsaturated aldehydes, olefinic nitroalkanes, and 1,3-dimethylbarbituric acid via a one-pot strategy with excellent diastereo- and enantioselectivities (up to 99% ee). The structures and absolute configurations of the products were confirmed by X-ray analysis.

Dynamic kinetic asymmetric synthesis of five contiguous stereogenic centers by sequential organocatalytic stetter and Michael-aldol reaction: enantioselective synthesis of fully substituted cyclopentanols bearing a quaternary stereocenter.

A synthesis of fully substituted cyclopentanes bearing a quaternary carbon center and five contiguous stereogenic centers has been achieved by sequential organocatalyzed Stetter and Michael-Aldol reactions of heteroaromatic aldehydes, nitroalkenes, and α,ß-unsaturated aldehydes via the [1 + 2 + 2] annulation strategy with dynamic kinetic asymmetric transformation and excellent enantioselectivities (up to >99% ee).

Sequential organocatalytic Stetter and Michael-Aldol condensation reaction: asymmetric synthesis of fully substituted cyclopentenes via a [1 + 2 + 2] annulation strategy.

A stereoselective synthesis of fully substituted cyclopentenes has been achieved by a sequential organocatalyzed Stetter and Michael-aldol condensation of aromatic aldehydes, nitroalkenes, and α,ß-unsaturated aldehydes via the [1 + 2 + 2] annulation strategy with excellent diastereoselectivities and enantioselectivities (up to >99% ee).