Organocatalytic [6+4] Cycloadditions via Zwitterionic Intermediates: Chemo-, Regio-, and Stereoselectivities.

The mechanisms and origins of chemo- and stereoselectivities of the organocatalytic [6+4] cycloaddition between 2-cyclopentenone and tropone have been investigated by a combined computational and experimental study. In the presence of a cinchona alkaloid primary amine catalyst and an acid additive, 2-cyclopentenone forms a cross-dienamine intermediate that subsequently undergoes a stepwise [6+4] cycloaddition reaction via a zwitterionic intermediate. The rate-determining transition state features a strong hydrogen-bonding interaction between the tropone oxygen atom and the protonated quinuclidine directing the reaction course leading to a highly periselective [6+4] cycloaddition. The importance of the strong hydrogen-bonding interaction is also demonstrated by the influence of the concentration of the acid additive on the yields and enantioselectivities of the reaction. The corresponding [4+2] cycloaddition reaction has a much higher energy barrier. The enantioselectivity of the [6+4] cycloaddition originates from different repulsive hydrogen-hydrogen interactions that distinguish the diastereomeric transition states.

Cycloaddition Reactions: Why Is It So Challenging To Move from Six to Ten Electrons?

High on cycloaddi(c)tions: The impact of the Woodward-Hoffmann [4n+2] rule for thermally allowed cycloaddition reactions and the challenges associated with moving from six to ten electrons are discussed in this Essay.

Organocatalytic stereoselective [8+2] and [6+4] cycloadditions.

Cycloadditions that involve more than six π electrons are termed higher-order cycloadditions and are an excellent tool for solving complex synthetic challenges, as they provide direct access to polycyclic scaffolds that contain medium-sized rings. They have interesting synthetic potential for the discovery of new bioactive molecules and in natural product synthesis. It is peculiar that stereocontrolled [8+2] and [6+4] cycloadditions have been largely neglected for the past 50 years. Here we demonstrate a cross-dienamine activation of 2-cyclopentenone and the unprecedented endocyclic linear-dienamine activation of 2-cyclohexenones and 2-cycloheptenones. These dienamine intermediates undergo aminocatalytic stereoselective [8+2], [6+4] and formal [4+2] cycloadditions with various heptafulvenes. The periselectivities of the cycloadditions are controlled based on the ring size of the 2-cycloalkenones and the substitution patterns of the heptafulvenes. The chiral products obtained undergo various chemical and photochemical single-step transformations that give access to other classes of all-carbon polycyclic scaffolds.

Direct Access to Multifunctionalized Norcamphor Scaffolds by Asymmetric Organocatalytic Diels-Alder Reactions.

A general organocatalytic cross-dienamine activation strategy to produce chiral multifunctionalized norcamphor compounds having a large diversity in substitution pattern is presented. The strategy is based on a Diels-Alder reaction of an amino-activated cyclopentenone reacting with most common classes of electron-deficient olefins, such as nitro-, ester-, amide-, and cyano-substituted olefins, chalcones, conjugated malononitriles, CF3-substituted enones, and fumarates. The corresponding norcamphor derivatives are formed in good yield, excellent enantioselectivities, and with complete diastereoselectivity. Furthermore, it is demonstrated that quaternary stereocenters and spiro norcamphor compounds can be formed with high stereoselectivity. The present development provides a simple, direct, and efficient approach for the preparation of important norcamphor scaffolds.

Stereoselective Retentive Domino Transmetalations of Secondary Alkyllithium Compounds to Functionalized Secondary Alkylcopper Reagents.

Functionalized secondary alkyllithium reagents obtained by I/Li exchange from the corresponding secondary alkyl iodides undergo two successive transmetalations with Me3 SiCH2 ZnBr⋅LiBr and CuBr⋅2 LiCl⋅Me2 S to provide functionalized secondary alkylcopper compounds with high retention of configuration. These alkylcopper derivatives react further with electrophiles such as alkynyl esters, acid chlorides, allylic chlorides, ketals, ethylene oxide, and 3-iodocyclopentanone with high retention of configuration. A related sequence of transmetalations with MeMgI and LaCl3 ⋅2 LiCl allows a retentive addition of secondary alkyllithium reagents to acetone. The influence of the solvent on the configurational stability of secondary alkylzinc reagents is described.

Organocatalytic cascade reactions: towards the diversification of hydroisochromenes and chromenes through two different activation modes.

The organocatalytic enantioselective syntheses of functionalized hydroisochromenes and chromenes by trienamine-mediated [4+2]-cycloaddition/nucleophilic ring-closing and iminium-ion/aminal-mediated oxa-Michael/Michael/nucleophilic ring-closing with 2-nitroallylic alcohols are presented. The corresponding cycloadducts, with up to five stereocenters, are formed in good yield and excellent enantioselectivities. The synthetic applications of the obtained products have been demonstrated.

Organocatalytic cascade reactions: diversity-oriented synthesis for the construction of hydroisoquinoline scaffolds.

The organocatalytic enantioselective synthesis of highly functionalized hydroisoquinolines by trienamine-mediated [4+2]-cycloaddition/nucleophilic ring-closing reaction cascade sequence of cyanoacrylamides with 2,4-dienals is presented. The corresponding cycloadducts are formed in high yields and excellent stereoselectivities. Moreover, a series of transformations demonstrate the synthetic application of the obtained hydroisoquinolines.

Diastereoselective synthesis of open-chain secondary alkyllithium compounds and trapping reactions with electrophiles.

A practical stereoselective iodide-lithium exchange was used in the first general preparation of functionalized stereodefined acyclic secondary nonstabilized lithium reagents from the corresponding secondary alkyl iodides. These lithium reagents react with various electrophiles including carbon electrophiles with high retention of configuration. Kinetic data on the configurational stability of these acyclic alkyllithium reagents are given. This methodology offers a new entry to chiral synthons for the stereoselective synthesis of open-chain molecules.

Reductive lithiation of methyl substituted diarylmethylsilanes: application to silanediol peptide precursors.

Reductive lithiation of methyl-substituted diarylmethylsilanes using lithium naphthalenide represents a practical method for the preparation of the corresponding silyl lithium reagents. Their addition to chiral sulfinimines affords versatile precursors to silanols and silanediols. The replacement of the currently used diphenylsilane motif by a more labile diarylsilane moiety allows the selective hydrolysis of one or two aryl groups by treatment with TFA.

Further studies toward the stereocontrolled synthesis of silicon-containing peptide mimics.

Further studies are reported on the utilization of the versatile reaction between chiral sulfinimines and alkyldiphenylsilyl lithium reagents with the goal of preparing a wide range of silanediol-based protease inhibitors. In particular, focus has been placed to demonstrate how a number of genetically encoded amino acid side chains such as serine, threonine, tyrosine, lysine, proline, arginine, aspartate and asparagine might be incorporated into the overall approach. Efforts to apply this synthetic methodology for accessing biologically relevant silanediol dipeptide mimics are also described. This includes the synthesis of a potential inhibitor of the human neutrophil elastase, as well as a diphenylsilane mimic of a hexapeptide fragment of the human islet amyloid polypeptide.