Catalytic Access to Chiral δ-Lactams via Nucleophilic Dearomatization of Pyridine Derivatives

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Enantioselective Dearomatization of Pyridinium Salts by Copper-Catalyzed C4-Selective Addition of Silicon Nucleophiles

A copper-catalyzed C4-selective addition of silicon nucleophiles released from an Si-B reagent to prochiral pyridinium triflates is reported. The dearomatization proceeds with excellent enantioselectivity using Cu(CH3CN)4PF6 as the precatalyst and (R,R)-Ph-BPE (1,2-bis[(2R,5R)-2,5-diphenylphospholan-1-yl]ethane) as the chiral ligand. A carbonyl group at C3 is required for this, likely acting a weak donor group to preorganize and direct the nucleophilic attack towards C4. The resulting 4-silylated 1,4-dihydropyridines can be further converted into functionalized piperidine derivatives.

Stereodivergent Synthesis of All Stereoisomers of 2,3-Disubstituted δ-Lactam Derivatives via Organocatalytic Cascade Reactions and Base-Induced Epimerization

A protocol was developed to achieve stereodivergent synthesis of stereoisomers of δ-lactam bearing vicinal chiral centers. Organocatalytic cascade reactions were employed to produce the target products as the kinetic products, which exhibited remarkable enantioselectivities. In the presence of DBU, the kinetic product underwent epimerization to form a thermodynamically more stable diastereomer without loss in enantioselectivity. By simply switching the chiral organocatalyst and its enantiomer, we can efficiently obtain four stereoisomers with high enantioselectivities.

Brook Rearrangement as Trigger for Dearomatization Reaction: Synthesis of Non-Aromatic N-Heterocycles

The [1,2]-Brook rearrangement stands as a potent technique for constructing complex molecules. In this study, we showcase its power in the dearomatization of aromatic N-heterocycles. Through a concise four-step process that integrates lithiation, nucleophilic addition, Brook rearrangement and dearomatization reaction, we demonstrate a versatile strategy for generating diverse non-aromatic N-heterocycles which exhibit ambident reactivities. Various acyl silanes, halo-pyridines, and quinolines have been explored within this context. The synthetic utility of this methodology is demonstrated through the construction of complex architectures.

Dearomatization of Pyridines: Stereoselective Synthesis of Functionalized [1,2,4]Triazolo[4,3-a]pyridines

The stereoselective preparation of functionalized [1,2,4]triazolo[4,3-a]pyridines from N-tosylhydrazones and pyridines was developed through the dearomatization of pyridines. The current transformation features good step- and atom-economy, high diastereoselectivity, and the efficient formation of four new carbon-heteroatom bonds in the corresponding product tetrahydro pyridines.

The Construction of Highly Substituted Piperidines via Dearomative Functionalization Reaction

Nitrogen heterocycles play a vital role in pharmaceuticals and natural products, with the six-membered aromatic and aliphatic architectures being commonly used. While synthetic methods for aromatic N-heterocycles are well-established, the synthesis of their aliphatic functionalized analogues, particularly piperidine derivatives, poses a significant challenge. In that regard, we propose a stepwise dearomative functionalization reaction for the construction of highly decorated piperidine derivatives with diverse functional handles. We also discuss challenges related to site-selectivity, regio- and diastereoselectivity, and provide insights into the reaction mechanism through mechanistic studies and density functional theory computations.

Recent Progresses in the Catalytic Stereoselective Dearomatization of Pyridines

1,2- and 1,4-dihydropyridines and N-substituted 2-pyridones are very important structural motifs due to their synthetic versatility and vast presence in a variety of alkaloids and bioactive molecules. In this article, we gather and summarize the catalytic and stereoselective synthesis of partially hydrogenated pyridines and pyridones via the dearomative reactions of pyridine derivatives up to mid-2023. The material is fundamentally organized according to the type of reactivity (electrophilic/nucleophilic) of the pyridine nucleus. The material is further sub-divided taking into account the nucleophilic species when dealing with electrophilic pyridines and considering the reactivity manifold of pyridine derivatives behaving as nucleophiles at the nitrogen site. The latter more recent approach allows for an unconventional entry to chiral N-substituted 2- and 4-pyridones in non-racemic form.