Asymmetric Conjugate Additions of Carbonyl Compounds to Nitroalkenes under Solvent-Free Conditions Using Fluorous Diaminomethylenemalononitrile Organocatalyst

Dipeptide-catalysed Michael reaction under physiological conditions: Examination of potential bioorthogonality

Reactions for drug synthesis under cell-like conditions or even inside living cells can potentially be used e.g., to minimize toxic side effects, to maximize bioactive compound efficacy and/or to address drug delivery problems. Those reactions should be bioorthogonal to enable the generation of drug-like compounds with sufficiently good yields. In the known bioorthogonal Michael reactions, using thiols and phosphines as nucleophiles (e.g., in CS and CP bond formation reactions) is very common. No bioorthogonal Michael addition with a carbon nucleophile is known yet. Therefore, the development of such a reaction might be interesting for future drug discovery research. In this work, the metal-free Michael addition between cyclohexanone and various trans-β-nitrostyrenes (CC bond formation reaction), catalysed by a dipeptide salt H-Pro-Phe-O-Na+, was investigated for the first time in the presence of glutathione (GSH) and in phosphate-buffered saline (PBS). We demonstrated that with electron-withdrawing substituents on the aromatic ring and in β-position of the trans-β-nitrostyrene yields up to 64% can be obtained under physiological conditions, indicating a potential bioorthogonality of the studied Michael reaction. In addition, the selected Michael products demonstrated activity against human ovarian cancer cells A2780. This study opens up a new vista for forming bioactive compounds via CC bond formation Michael reactions under physiological (cell-like) conditions.

Recent Advances in Asymmetric Synthesis of Pyrrolidine-Based Organocatalysts and Their Application: A 15-Year Update

In 1971, chemists from Hoffmann-La Roche and Schering AG independently discovered a new asymmetric intramolecular aldol reaction catalyzed by the natural amino acid proline, a transformation now known as the Hajos-Parrish-Eder-Sauer-Wiechert reaction. These remarkable results remained forgotten until List and Barbas reported in 2000 that L-proline was also able to catalyze intermolecular aldol reactions with non-negligible enantioselectivities. In the same year, MacMillan reported on asymmetric Diels-Alder cycloadditions which were efficiently catalyzed by imidazolidinones deriving from natural amino acids. These two seminal reports marked the birth of modern asymmetric organocatalysis. A further important breakthrough in this field happened in 2005, when Jørgensen and Hayashi independently proposed the use of diarylprolinol silyl ethers for the asymmetric functionalization of aldehydes. During the last 20 years, asymmetric organocatalysis has emerged as a very powerful tool for the facile construction of complex molecular architectures. Along the way, a deeper knowledge of organocatalytic reaction mechanisms has been acquired, allowing for the fine-tuning of the structures of privileged catalysts or proposing completely new molecular entities that are able to efficiently catalyze these transformations. This review highlights the most recent advances in the asymmetric synthesis of organocatalysts deriving from or related to proline, starting from 2008.

Metal-Ion Specific Recognition with Amplified Transcription from Subnanometer to Submillimeter or Real-Time Domain by Self-Assembled Vanadyl Quartets

Vanadyl(V) complexes 1 and 2 bearing a nematic liquid crystal (LC) like a p-heptoxyphenyl group or a fluorous-tag p-nonafluoroheptoxyphenyl (NFH) group at the C5 position of the N-salicylidene template were designed and synthesized. Each complex was subjected to MVO₃-induced self-assembly to form metal-ion, encapsulated quartet clusters 3-M and 3'-M. The Na⁺ in cluster complex 3-Na or 3'-Na can be readily replaced by Rb⁺, Ag⁺, or Hg²⁺ in an aqueous layer to form cluster complexes by ion swapping at the H₂O/CDCl₃ bilayer interface. Selectivity profiles were examined with alkali-metal ions, Ag⁺, and Hg²⁺ through metal-ion competition experiments. The 3'-Na has an exclusive selectivity for Hg²⁺ in the presence of Zn²⁺ and Cd²⁺. Cluster complexes 3-M were utilized as chiral dopants to nematic LC materials. The effects of the encapsulated metal ions within the alkali family and Ag⁺ on Cano's line widths and helical pitch changes were viewed in wedge cells under a polarized microscope. Their correlations with the ionic radius were identified. The subnano information of the metal ions can thus be asymmetrically amplified to Cano's line spacings of the submilimeter domain. Conversely, the effects of the encapsulated alkali metal ions and Hg²⁺ in 3'-M on the interactions of their NFH tails toward fluorous silica gel (FSG) were performed via HPLC analyses. Their retention times became longer as the sizes of encapsulated, alkali metal ions increased. The increasing ion size from Na⁺ to Cs⁺ caused the four lower rim NFH tags of the cluster to be closer due to reduced cone angles. Their interactions among NFH tail groups on FSG became larger, thus leading to distinctive separations with t_R from 7.36 to 10.27 min. The retention time difference between 3'-Na and 3'-Hg on HPLC was ∼3.6 min, resulting in discernible separation. The individual ion size differences on the subnano scale can thus be amplified and unambiguously established in the real time domain.

A New Organocatalytic Desymmetrization Reaction Enables the Enantioselective Total Synthesis of Madangamine E

The enantioselective total synthesis of madangamine E has been completed in 30 steps, enabled by a new catalytic and highly enantioselective desymmetrizing intramolecular Michael addition reaction of a prochiral ketone to a tethered β,β′-disubstituted nitroolefin. This key carbon–carbon bond forming reaction efficiently constructed a chiral bicyclic core in near-perfect enantio- and diastereo-selectivity, concurrently established three stereogenic centers, including a quaternary carbon, and proved highly scalable. Furthermore, the pathway and origins of enantioselectivity in this catalytic cyclization were probed using density functional theory (DFT) calculations, which revealed the crucial substrate/catalyst interactions in the enantio-determining step. Following construction of the bicyclic core, the total synthesis of madangamine E could be completed, with key steps including a mild one-pot oxidative lactamization of an amino alcohol, a two-step Z-selective olefination of a sterically hindered ketone, and ring-closing metatheses to install the two macrocyclic rings.

Density Functional Theory Study on the Mechanism of Iridium-Catalyzed Benzylamine ortho C-H Alkenylation with Ethyl Acrylate

Iridium-catalyzed oxidative o-alkenylation of benzylamines with acrylates was enabled by the directing group pentafluorobenzoyl (PFB). Density functional theory calculations were performed to explore the detailed reaction mechanism. The calculated results reveal that N-deprotonation prior to C-H activation is favored over direct C-H activation. Moreover, C-H activation is reversible and not the rate-determining step, which has been supported by the experimental observation. The regio- and stereoselectivity of ethyl acrylate insertion are controlled by the steric effect and the carbon atom with a larger orbital coefficient of the π* antibonding orbital in the nucleophilic attack, respectively. The migratory insertion of ethyl acrylate is computationally found to be rate-determining for the whole catalytic cycle. Finally, the seven-membered ring intermediate IM11 undergoes a sequential N-protonation and β-H elimination with the assistance of AcOH, rather than β-H elimination and reductive elimination proposed experimentally, to afford the o-alkenylated product. IM11 is unable to directly cyclize through C-N reductive elimination because both sp³-hybridized N and C atoms are unfavorable for N-C reductive elimination. The origin of the directing group PFB preventing the product and intermediates undergoing aza-Michael addition has been explained.

Iridium-Catalyzed Benzylamine C-H Alkenylation Enabled by Pentafluorobenzoyl as the Directing Group

The first iridium-catalyzed oxidative alkeynylation of benzylamines with acrylates enabled by a new directing group pentafluorobenzoyl has been developed. The reaction proceeded efficiently in the presence of silver acetate as oxidant and chlorobenzene as solvent. A good range of benzylamines could be selectively monoalkenylated without interfering with further aza-Michael addition. The kinetic isotope effect experiments showed that C-H activation is not the rate-limiting step. In addition, a five-membered iridacycle species was isolated and established as the possible key intermediate.

Cinchona-Diaminomethylenemalononitrile Organocatalyst for the Highly Enantioselective Hydrophosphonylation of Ketones and Enones

The use of diaminomethylenemalononitrile (DMM) organocatalyst to promote the challenging 1,2-hydrophosphonylation of simple ketones and enones, which are also called α,β-unsaturated ketones, is proposed and validated. This reaction provided the corresponding chiral α-hydroxy phosphonates in high to excellent yields and with enantioselectivity up to 96% ee.