Organocatalyzed enantioselective fluorocyclizations

Catalytic Aminohalogenation of Alkenes and Alkynes

Catalytic aminohalogenation methods enable the regio- and stereoselective vicinal difunctionalization of alkynes, allenes and alkenes with amine and halogen moieties. A range of protocols and reaction mechanisms including organometallic, Lewis base, Lewis acid and Brønsted acid catalysis have been disclosed, enabling the regio- and stereoselective synthesis of halogen-functionalized acyclic amines and nitrogen heterocycles. Recent advances including aminofluorination and catalytic enantioselective aminohalogenation reactions are summarized in this review.

Enantioselective bromoaminocyclization of allyl N-tosylcarbamates catalyzed by a chiral phosphine-Sc(OTf)3 complex

An effective enantioselective bromoaminocyclization of allyl N-tosylcarbamates catalyzed by a chiral phosphine-Sc(OTf)3 complex is described. A wide variety of optically active oxazolidinone derivatives containing various functional groups can be obtained with high enantioselectivities.

Solvent-dependent enantiodivergence in the chlorocyclization of unsaturated carbamates

A remarkable solvent-controlled enantiodivergence is seen in the hydroquinidine 1,4-phthalazinediyl diether ((DHQD)2PHAL)-catalyzed chlorocyclization of unsaturated carbamates. Eyring plot analyses of this previously unreported reaction are used to probe and compare the R- and S-selective pathways. In the CHCl3/hexanes solvent system, the pro-R process shows a surprising increase in selectivity with increasing temperature. These studies point to a strongly solvent-dependent entropy-enthalpy balance between the pro-R and pro-S pathways.

N-indolyltriethylborate: a useful reagent for synthesis of C3-quaternary indolenines

N-Indolyltriethylborate was found to be a useful reagent for dearomatizing C3-alkylation of 3-substituted indoles with both activated and nonactivated alkyl halides to give C3-quaternary indolenines, pyrroloindolines, furoindoline, and hexahydropyridoindoline under mild reaction conditions. The utility of these reagents was demonstrated in the syntheses of a pyrroloindoline-4-cholestene hybrid and debromoflustramine B.

Silver-catalyzed radical aminofluorination of unactivated alkenes in aqueous media

We report herein a mild and catalytic intramolecular aminofluorination of unactivated alkenes. Thus, with the catalysis of AgNO3, the reactions of various N-arylpent-4-enamides with Selectfluor reagent in CH2Cl2/H2O led to the efficient synthesis of 5-fluoromethyl-substituted γ-lactams. A mechanism involving silver-catalyzed oxidative generation of amidyl radicals and silver-assisted fluorine atom transfer was proposed.

Regio- and enantioselective aminofluorination of alkenes

Enantio- and regioselective: The intramolecular enantioselective aminofluorination of unactivated olefins was achieved by using a chiral iodo(III) difluoride salt. A highly regioselective aminofluorination of styrenes to access 2-fluoro-2-phenylethanamines was also developed.

C2-symmetric cyclic selenium-catalyzed enantioselective bromoaminocyclization

A catalytic asymmetric bromocyclization of trisubstituted olefinic amides that uses a C(2)-symmetric mannitol-derived cyclic selenium catalyst and a stoichiometric amount of N-bromophthalimide is reported. The resulting enantioenriched pyrrolidine products, which contain two stereogenic centers, can undergo rearrangement to yield 2,3-disubstituted piperidines with excellent diastereoselectivity and enantiospecificity.

Asymmetric ion-pairing catalysis

Charged intermediates and reagents are ubiquitous in organic transformations. The interaction of these ionic species with chiral neutral, anionic, or cationic small molecules has emerged as a powerful strategy for catalytic, enantioselective synthesis. This review describes developments in the burgeoning field of asymmetric ion-pairing catalysis with an emphasis on the insights that have been gleaned into the structural and mechanistic features that contribute to high asymmetric induction.

Organocatalytic asymmetric one-pot sequential conjugate addition/dearomative fluorination: synthesis of chiral fluorinated isoxazol-5(4H)-ones

A facile one-pot sequential conjugate addition/dearomative fluorination transformation of isoxazol-5(4H)-ones with nitroolefins and N-fluorobenzenesulfonimide (NFSI) has been developed. By using a bifunctional chiral tertiary amino-thiourea catalyst, a series of chiral fluorinated isoxazol-5(4H)-ones containing one fluorine-substituted quarternary stereocenter were obtained in high yields with high enantio- and diastereoselectivities. Further transformation of adducts could afford isoxazolidin-5-one derivatives with three contiguous stereocenters.

Catalytic, asymmetric halofunctionalization of alkenes--a critical perspective

Despite the fact that halogenation of alkenes has been known for centuries, enantioselective variants of this reaction have only recently been developed. In the past three years, catalytic enantioselective versions of halofunctionalizations with the four common halogens have appeared and although important breakthroughs, they represent just the very beginnings of a nascent field. This Minireview provides a critical analysis of the challenges that accompany the development of general and highly enantioselective halofunctionalization reactions. Moreover, the focus herein, diverges from previous reviews of the field by identifying the various modes of catalysis and the different strategies implemented for asymmetric induction.

Direct conversion of indoles to 3,3-difluoro-2-oxindoles via electrophilic fluorination

3,3-Difluoro-2-oxindoles can be obtained directly from indoles in moderate yields via electrophilic fluorination using N-fluorobenzenesulfonimide as a mild fluorinating reagent. The presence of tert-butyl hydroperoxide during the reaction, together with additional heating after quenching the reaction with triethylamine, is beneficial to the formation of the desired product.

Total synthesis and biological evaluation of phidianidines A and B uncovers unique pharmacological profiles at CNS targets

The synthesis of phidianidines A and B, the first 1,2,4-oxadiazole-containing alkaloid, from the marine opisthobranch mollusk Phidiana militaris is reported. The synthesis proceeds in six steps from known indole acetic acids in 39.9% (phidianidine A) and 21% (phidianidine B) overall yields from commercially available materials. Biological characterization found that phidianidines A and B are selective inhibitors of DAT (versus SERT and NET) and a selective, potent ligand and partial agonist of the μ opioid receptor (versus δ- and κ-opioid receptors). Moreover, neither phidianidines A and B are cytotoxic, and thus represent an attractive starting point for chemical optimization; therefore, we piloted a number of chemistries and prepared a diverse series of unnatural analogs.