Chemodivergent Preparation of Various Heterocycles via Phase‐Transfer Catalysis: Enantioselective Synthesis of Functionalized Piperidines

Catalytic role of the enol ether intermediate in the intramolecular Stetter reaction: a computational perspective

The intramolecular Stetter reaction catalyzed by a carbene is investigated by density functional theory (DFT) calculations and kinetic simulations. Catalyst 1 first reacts with aldehyde 2 to give the primary adduct (PA). The PA undergoes the intramolecular oxa-Michael reaction to irreversibly generate enol ether intermediate 9. The conversion of the enol ether to the Breslow intermediate (BI) requires the assistance of a base such as the PA. The next step involves formation of a carbon-carbon bond through the Michael addition, and expulsion of the catalyst generates the Stetter product 7. Calculations show that the catalytic cycle is composed of two irreversible processes: the first one involves the exergonic formation of the enol ether intermediate, while the second one is the conversion of the enol ether to the final product. Kinetic simulations using initial concentrations of [1]0 = 0.005 M and [2]0 = 0.025 M demonstrate that under a steady-state condition, 35% of the catalyst rests on the state of the enol ether (0.0018 M). The catalyst resting state therefore consists of the unbound form (the free catalyst) and its bound form (the enol ether species). According to variable time normalization analysis, the reaction exhibits a second-order dependence (first order in catalyst and first order in substrate), which agrees with experiments. The oxa-Michael reaction to form the enol ether is identified to be turnover limiting in the intramolecular Stetter reaction, which rationalizes the observed electronic effect of the Michael acceptor on the reactivity, as well as the measured isotope effect with respect to the aldehydic proton/deuteron. The base that participates in the BI formation has a significant effect on the build-up of the resting state 9 and the active catalyst concentration. In addition, the thermodynamic stability of the enol ether is found to depend on the tether length between the aromatic aldehyde and the Michael acceptor, as well as the chemical nature of the carbene catalyst. The favorability for the oxa-Michael reaction is therefore suggested to govern the reactivity of the intramolecular Stetter transformation.

Recent Advances in Organocatalyzed Asymmetric sulfa-Michael Addition Triggered Cascade Reactions

The sulfa-Michael addition reaction is a crucial subset of the Michael addition reaction, and aroused the interest of numerous synthetic biologists and chemists. In particular, sulfa-Michael addition triggered cascade reaction has developed quickly in recent years because it offers an efficient method to construct C-S bonds and other bonds in one approach, which is widely applicable for building chiral pharmaceuticals, their intermediates, and natural compounds. This review emphasizes the recent advancements in sulfa-Michael addition-triggered cascade reactions for the stereoselective synthesis of sulfur-containing compounds, including sulfa-Michael/aldol, sulfa-Michael/Henry, sulfa-Michael/Michael, sulfa-Michael/Mannich and some sulfa-Michael triggered multi-step processes. Moreover, some reaction mechanisms and derivatization experiments are introduced appropriately.

A Diastereoselective Route to Benzoannelated Bridged Sultams

A practical diastereoselective method for the synthesis of benzoannelated tri- and tetracyclic bridged sultams has been developed. The synthetic route employs widely available, substituted o-iodoanilines and is based on intramolecular Michael addition followed by cycloalkylation with α,ω-dihaloalkanes, or vice versa. The target compounds were isolated in good yields and the diastereoselectivity of the reaction could be easily controlled by the order of Michael addition and cycloalkylation steps.

Cobalt-Catalyzed Chemoselective Transfer Hydrogenative Cyclization Cascade of Enone-Tethered Aldehydes

The ligand-free Co-catalyzed chemoselective reductive cyclization cascade of enone-tethered aldehydes with i-PrOH as the environmentally benign hydrogen surrogate is developed by this study. Mechanistic studies disclosed that such a protocol is initiated by an ortho-enone-assisted Co(I)-catalyzed reduction of the aldehyde functionality with i-PrOH. Meanwhile, the selectivity from the Michael-Aldol cycloreduction cascade to the oxa-Michael cascade is feasible and readily adjusted by the addition of steric Lewis bases, such as TEMPO and DABCO, delivering substituted 1H-indenes and dihydroisobenzofurans, respectively.

The oxa-Michael reaction in the synthesis of 5- and 6-membered oxygen-containing heterocycles

In this review, we provide an updated account on the recent advances and applications of oxa-Michael reaction in the synthesis 5- and 6-membered monocyclic oxygen-containing heterocyclic compounds published in the literature since 2013 to date.