Enantioselective bromolactonization of cis-1,2-disubstituted olefinic acids using an amino-thiocarbamate catalyst

Enantioselective halogenative semi-pinacol rearrangement: extension of substrate scope and mechanistic investigations

The present Full Paper article discloses a survey of our recent results obtained in the context of the enantioselective halogenation-initiated semi-pinacol rearrangement. Commencing with the fluorination/semi-pinacol reaction first and moving to the heavier halogens (bromine and iodine) second, the scope and limitations of the halogenative phase-transfer methodology will be discussed and compared. An extension of the fluorination/semi-pinacol reaction to the ring-expansion of five-membered allylic cyclopentanols will be also described, as well as some preliminary results on substrates prone to desymmetrization will be given. Finally, the present manuscript will culminate with a detailed mechanistic investigation of the canonical fluorination/semi-pinacol reaction. Our mechanistic discussion will be based on in situ reaction progress monitoring, complemented with substituent effect, kinetic isotopic effect and non-linear behaviour studies.

A facile synthesis of isoxazolo[3,4-a]pyrrolizine and isoxazolo[4,3-c]pyridine derivatives via intramolecular nitrone cycloaddition reaction

Isoxazolo [3,4-a]pyrrolizine and isoxazolo[4,3-c]pyridine are formed by a facile and efficient intramolecular [3 + 2] nitrone cycloaddition, forming two new rings and three contiguous stereocentres with high diastereoselectivity and good yields.

Recent advances in asymmetric intra- and intermolecular halofunctionalizations of alkenes

This review seeks to provide coverage on recent advances in catalytic enantioselective halofunctionalization of alkenes. The aim is to give an overview of various reports, highlighting the new reaction types and strategies developed during the past two years. The scope and challenges of intra- and intermolecular reaction variants are discussed as well.

N-Bromoacetamide-mediated domino cyclization and elimination of homoallylic trichloroacetimidates: a novel approach toward the synthesis of 1-bromo-2-amino-3-butene derivatives

A practical synthesis of 1-bromo-2-amino-3-butene derivatives from homoallylic trichloroacetimidates was reported. Simply heating the mixture of substrates and N-bromoacetamide in DMF at 90 °C would give the desired products in moderate to excellent yields. The reaction may proceed through a domino bromo-cyclization and elimination pathway. The synthesis of 4,5-dihydrooxazoles from 1-bromo-2-amino-3-butene derivatives was also investigated.

Diastereoconvergent synthesis of trans-5-hydroxy-6-substituted-2-piperidinones by addition-cyclization-deprotection process

A diastereoselective one-pot approach to access trans-5-hydroxy-6-substituted-2-piperidinones by an addition-cyclization-deprotection process has been developed, in which the stereogenic center at the C-6 position was solely controlled by α-OTBS group. The utility of this transformation is demonstrated by the asymmetric synthesis of the enantiomer of (-)-CP-99,994.

Enantioselective haloetherifications catalyzed by 1,1'-bi-2-naphthol (BINOL) phosphates: from symmetrical alkenediols to simple alkenols

Asymmetric haloetherifications can be conducted using 1,1'-bi-2-naphthol (BINOL) phosphates as catalyst. In combination with simple N-haloamides such as N-iodopyrrolidinone or N-bromosuccinimide, good enantioselectivities can be achieved. However, depending on the substrate, the choice of BINOL phosphate is important, and different catalysts show remarkably different selectivities.

Cooperative activation with chiral nucleophilic catalysts and N-haloimides: enantioselective iodolactonization of 4-arylmethyl-4-pentenoic acids

Chiral triaryl phosphates promote the enantioselective iodolactonization of 4-substituted 4-pentenoic acids to give the corresponding iodolactones in high yields with high enantioselectivity. N-Chlorophthalimide (NCP) is employed as a Lewis acidic activator and oxidant of I2 for the present iodolactonization. In combination with 1.5 equivalents of NCP, only 0.5 equivalents of I2 are sufficient to generate the iodinating reagent.

Desymmetrization of diolefinic diols by enantioselective amino-thiocarbamate-catalyzed bromoetherification: synthesis of chiral spirocycles

A facile, efficient, and highly diastereo- and enantioselective bromoetherification of diolefinic diols has been developed using an amino-thiocarbamate catalyst. Further manipulations of the bromoether products enabled entry into a new class of spirocycles which are distinctively lacking in the literature.

Intermolecular bromoesterification of conjugated enynes: an efficient synthesis of bromoallenes

Highly functionalized bromoallenes were prepared by intermolecular 1,4-bromoesterification of conjugated enynes.

A trinuclear Zn3(OAc)4-3,3′-bis(aminoimino)binaphthoxide complex for highly efficient catalytic asymmetric iodolactonization

Using a harmony of the tri-zinc atoms, 1 mol% Zn₃(OAc)₄-3,3′-bis(aminoimino)binaphthoxide catalyzed asymmetric iodolactonization in up to 99.9% ee.

Dissecting the stereocontrol elements of a catalytic asymmetric chlorolactonization: syn addition obviates bridging chloronium

We report absolute and relative stereochemistry of addition in enantioselective chlorolactonizations of 4-phenyl-4-pentenoic acid and its related t-butyl ester, catalyzed by (DHQD)2PHAL. Predominant syn addition of the chlorenium and the nucleophile across the olefin is observed. As shown by isotopic labeling, NMR spectroscopy, and derivative studies, the two new stereocenters formed by addition across the double bond are set independently and influenced by different factors. These findings suggest a stepwise process via an intermediate capable of lactone closure with either stereochemistry, in contradistinction to the more familiar scenario in which anti addition is dictated by a bridging chloronium ion intermediate.

Kinetic resolution of unsaturated amides in a chlorocyclization reaction: concomitant enantiomer differentiation and face selective alkene chlorination by a single catalyst

The first example of a kinetic resolution via chlorofunctionalization of olefins is reported. The enantiomers of racemic unsaturated amides were found to have different hydrogen-bonding affinities for chiral Lewis bases in numerous solvents. This interaction was exploited in developing a kinetic resolution of racemic unsaturated amides via halocyclization. The same catalyst serves to both "sense chirality" in the substrate as well as mediate a highly face-selective chlorine delivery onto the olefin functionality, resulting in stereotriad products in up to 99:1 dr and up to 98.5:1.5 er. The selectivity factors were typically greater than 50 to allow for the simultaneous synthesis of both the products and unreacted substrates in highly enantioenriched form at yields approaching 50%. The reaction employs catalytic amounts (≤0.50 mol %) of a commercially available and recyclable organocatalyst.

Asymmetric 5-endo chloroetherification of homoallylic alcohols toward the synthesis of chiral β-chlorotetrahydrofurans

An asymmetric 5-endo chloroetherification of homoallylic alcohols is successfully developed that employs an easily available quaternary ammonium salt derived from cinchonine as a conventional organocatalyst. This approach provides ready access to β-chlorotetrahydrofurans in high enantioselectivities.

Stereoselective halocyclization of alkenes with N-acyl hemiaminal nucleophiles

Halocyclization of alkenes was realized using N-acylhemiaminal nucleophiles. High diastereoselectivity could be achieved for the formation of three stereogenic centers in this halogen-mediated cyclization reaction. We also demonstrated that enantioselective bromocyclization of alkenes using N-acylhemiaminal nucleophiles was possible.

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.

Lewis basic selenium catalyzed chloroamidation of olefins using nitriles as the nucleophiles

A Lewis base catalyzed chloroamidation of olefinic substrates was achieved using diphenyl selenide as the catalyst. The reaction conditions are mild and suitable for a wide range of substrates including those which are acid labile.

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.

Enantioselective iodolactonization of disubstituted olefinic acids using a bifunctional catalyst

The enantioselective iodolactonizations of a series of diversely substituted olefinic carboxylic acids are promoted by a BINOL-derived, bifunctional catalyst. Reactions involving 5-alkyl- and 5-aryl-4(Z)-pentenoic acids and 6-alkyl- and 6-aryl-5(Z)-hexenoic acids provide the corresponding γ- and δ-lactones having stereogenic C-I bonds in excellent yields and >97:3 er. Significantly, this represents the first organocatalyst that promotes both bromo- and iodolactonization with high enantioselectivities. The potential of this catalyst to induce kinetic resolutions of racemic unsaturated acids is also demonstrated.

Catalytic desymmetrization of cyclohexadienes by asymmetric bromolactonization

Asymmetric bromolactonization of prochiral cyclohexadiene derivatives with N-bromosuccimide proceeded in the presence of (DHQD)(2)PHAL as a chiral catalyst to afford the corresponding bromolactones with up to 93% ee. This reaction was also applicable to the kinetic resolution of a racemic cyclic ene-carboxylic acid, where the starting material was recovered with high enantioselectivity.

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.

Enantioselective synthesis of 2-substituted and 3-substituted piperidines through a bromoaminocyclization process

A catalytic enantioselective bromocyclization of olefinic amides using amino-thiocarbamates as the catalysts has been developed. The resulting enantioenriched 2-substituted 3-bromopiperidines can readily be transformed to 3-substituted piperidines through a silver salt-mediated rearrangement. This process has been applied to the synthesis of a dopaminergic drug, Preclamol.