Carbenium ion trapping using sulfonamides: an acid-catalysed synthesis of pyrrolidines by intramolecular hydroamination

Reusable Selenenyl Iodide-Initiated Cascade Cyclization of Polyenes with N-terminating Groups

Although cascade cyclization of polyenes has advanced remarkably, most reported examples are limited to carbon- or oxygen-terminating groups, with nitrogen-terminating cyclizations remaining rare. In this work, we developed a cascade cyclization of polyenes with N-terminating groups, initiated by an isolable selenenyl iodide (RSeI) bearing a cavity-shaped substituent. Acid-labile substrates were successfully employed in the cascade cyclization. Compared with commonly used organoselenium reagents, a selenenyl iodide, characterized by its unique soft electrophilic nature, proved to be the most effective in promoting the cascade reaction. Furthermore, the stabilizing effect of the cavity-shaped substituent enabled the isolation of a selenenic acid (RSeOH), which was generated via oxidative β-selenoxide elimination from the cyclized product during its derivatization to an olefin, as a stable compound. We also developed a method for regenerating the starting selenenyl iodide from the selenenic acid. The reusability of the selenenyl iodide renders the series of molecular transformations environmentally benign by reducing unwanted organoselenium waste.

Concise practical avenues toward 5,5-spiro-α-prolines

This article describes concise and practical synthetic approaches toward the multigram-scale preparation of value-added conformationally constrained 5-spirocyclic α-prolines from readily accessible starting materials. Direct carboxylation of 2-spiropyrrolidines is a novel and promising approach to achieve this goal, particularly for substrates with no acidic centers within the spiro-linked portion of the pyrrolidine core. The method allows for quick, one-step preparation of the target chemicals in good yields. An alternative synthetic avenue was designed for substrates unsuitable for direct carboxylation. This 4-step protocol involves common laboratory practice transformations and is comparable in its efficiency to the carboxylation method. This study concludes a series of our investigations on the elaboration of efficient methods for the construction of spiro-proline frameworks.

Iridium Acylnitrenoid-Initiated Biomimetic Cascade Cyclizations: Stereodefined Access to Polycyclic δ-Lactams

Ring-fused azacyclic compounds are important building units in the synthesis of biorelevant natural products, pharmaceutical agents, and molecular materials. Herein, we present a new approach to these condensed azacycles by a biomimetic cascade cyclization of arylalkenyl dioxazolones. This cascade reaction was found to proceed with excellent stereoselectivity and a high functional group tolerance. The substrate scope of arylalkenyl dioxazolones turned out to be highly flexible and extendable to additional terminating subunits, such as heteroaryl and alkynyl moieties. This biomimetic cyclization was elucidated to be initiated by an intramolecular transfer of the in situ generated electrophilic Ir-acylnitrenoid to the tethered olefinic double bond, leading to a key N-acylaziridine intermediate, which is in turn reacted with pendant (hetero)arenes or alkynes in a highly regio- and stereoselective manner to produce ring-fused azacyclic compounds.

Hydroamination versus Allylic Amination in Iridium-Catalyzed Reactions of Allylic Acetates with Amines: 1,3-Aminoalcohols via Ester-Directed Regioselectivity

In the presence of a neutral dppf-modified iridium catalyst and Cs₂CO₃, linear allylic acetates react with primary amines to form products of hydroamination with complete 1,3-regioselectivity. The collective data, including deuterium labeling studies, corroborate a catalytic mechanism involving rapid, reversible acetate-directed aminoiridation with inner-sphere/outer-sphere crossover followed by turnover-limiting proto-demetalation mediated by amine.

Brønsted acid-catalysed intramolecular hydroamination of unactivated alkenes: metal triflates as an in situ source of triflic acid

Hydroamination of alkenes or alkynes is one of the most straightforward methods to form C-N bonds and nitrogen-containing heterocycles. A simple Lewis acid Al(OTf)₃ was found to be an effective precatalyst for the hydroamination of unactivated primary and secondary alkenylamines between 110 and 150 °C. Subsequent studies show that other metal triflates are also effective precatalysts for the hydroamination reactions. For metal triflate salts, mechanistic studies, including kinetics, are consistent with the in situ generation of triflic acid, which likely serves as the active catalyst.

A general protocol to afford enantioenriched linear homoprenylic amines

The reaction of a readily obtained chiral branched homoprenylamonium salt with a range of aldehydes, including aliphatic substrates, affords the corresponding linear isomers in good yields and enantioselectivities.

Zinc-mediated highly α-regioselective prenylation of imines with prenyl bromide

A highly α-regioselective prenylation of imines has been successfully developed. The efficiency of this approach is confirmed by a wide range of imines including N- and C-aryl aldimines, N-alkyl aldimines, C-alkyl aldimines, and N- and C-aryl ketimines. The approach uses prenyl bromide as the prenyl source and inexpensive and convenient zinc as the mediator as well as environmentally benign 1,3-dimethyl-2-imidazolidinone (DMI) as the solvent.

Stereochemistry and mechanism of the Brønsted acid catalyzed intramolecular hydrofunctionalization of an unactivated cyclic alkene

Through employment of deuterium-labeled substrates, the triflic acid catalyzed intramolecular exo addition of the X-H(D) (X=N, O) bond of a sulfonamide, alcohol, or carboxylic acid across the C=C bond of a pendant cyclohexene moiety was found to occur, in each case, with exclusive formation (≥90%) of the anti-addition product without loss or scrambling of deuterium as determined by (1)H and (2)H NMR spectroscopy and mass spectrometry analysis. Kinetic analysis of the triflic-acid-catalyzed intramolecular hydroamination of N-(2-cyclohex-2'-enyl-2,2-diphenylethyl)-p-toluenesulfonamide (1a) established the second-order rate law: rate=k(2)[HOTf][1a] and the activation parameters ΔH(++)=(9.7±0.5) kcal mol(-1) and ΔS(++)=(-35±5) cal K(-1) mol(-1). An inverse α-secondary kinetic isotope effect of k(D)/k(H) =(1.15±0.03) was observed upon deuteration of the C2' position of 1a, consistent with partial C-N bond formation in the highest energy transition state of catalytic hydroamination. The results of these studies were consistent with a mechanism for the intramolecular hydroamination of 1a involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C3' position of 1a coupled with intramolecular anti addition of the pendant sulfonamide nitrogen atom to the alkenyl C2' position.