Cinchona Alkaloids as Organocatalysts in Enantioselective Halofunctionalization of Alkenes and Alkynes

The Knoevenagel condensation using quinine as an organocatalyst under solvent-free conditions

An organocatalytic Knoevenagel condensation has been developed for the synthesis of electrophilic alkenes using natural quinine under green reaction conditions.

BINOL-derived bifunctional sulfide catalysts for asymmetric synthesis of 3,3-disubstituted phthalides via bromolactonization

An efficient enantioselective synthesis of 3,3-disubstituted phthalides possessing a chiral quaternary carbon center was achieved via catalytic asymmetric bromolactonization using BINOL-derived bifunctional sulfide catalysts.

Catalytic, Enantioselective Synthesis of Cyclic Carbamates from Dialkyl Amines by CO2-Capture: Discovery, Development, and Mechanism

Cyclic carbamates are a common feature of small-molecule therapeutics, offering a constrained hydrogen bond acceptor that is both polar and sterically small. Methods for their preparation most often focus first on amino alcohol synthesis and then reaction with phosgene or its equivalent. This report describes an enantioselective synthesis of cyclic carbamates in which carbon dioxide engages an unsaturated basic amine, facilitated by a bifunctional organocatalyst designed to stabilize a carbamic acid intermediate while activating it toward subsequent enantioselective carbon-oxygen bond formation. Six-membered cyclic carbamates are prepared in good yield with high levels of enantioselection, as constrained 1,3-amino alcohols featuring a chiral tertiary alcohol carbon. Spectroscopic analysis (NMR, DOSY) of various substrate-reagent combinations provides insight into the dominant species under the reaction conditions. Two peculiar requirements were identified to achieve highest consistency: a "Goldilocks" amount of water and the use of a noncrystalline form of the ligand. These atypical features of the final protocol notwithstanding, a diverse range of products could be prepared. Their functionalizations illustrate the versatility of the carbamates as precursors to enantioenriched small molecules.

Catalytic Regio- and Enantioselective Haloazidation of Allylic Alcohols

Herein we report a highly regio- and stereoselective haloazidation of allylic alcohols. This enantioselective reaction uses readily available materials and can be performed on a variety of alkyl-substituted alkenes and can incorporate either bromine or chlorine as the electrophilic halogen component. Both halide and azido groups of the resulting products can be transformed into valuable building blocks with complete stereospecificity. The first example of an enantioselective 1,4-haloazidation of a conjugated diene is reported as well as its application to a concise synthesis of an aza-sugar.

Design of Chiral Bifunctional Dialkyl Sulfide Catalysts for Regio-, Diastereo-, and Enantioselective Bromolactonization

Although a wide variety of chiral organocatalysts have been developed for asymmetric transformations, effective chiral dialkyl sulfide organocatalysts remain relatively rare and under-developed, despite the potential utility of dialkyl sulfide catalysts. Herein, we report the development of chiral bifunctional dialkyl sulfide catalysts possessing a urea moiety for regio-, diastereo-, and enantioselective bromolactonization. The importance of the bifunctional design of chiral sulfide catalysts was clearly demonstrated in the present work. The roles of both the sulfide and urea moieties of the catalyst were clarified based on the results of experimental and theoretical investigation.

Atroposelective Catalytic Asymmetric Allylic Alkylation Reaction for Axially Chiral Anilides with Achiral Morita-Baylis-Hillman Carbonates

A highly efficient method to access axially chiral anilides through asymmetric allylic alkylation reaction with achiral Morita-Baylis-Hillman carbonates by using a biscinchona alkaloid catalyst was reported. Through the atroposelective approach, a broad range of axially chiral anilide products with different acyl groups, such as substituted phenyl, naphthyl, alkyl, enyl, styryl, and benzyl, were generated with very good yields, moderate to excellent cis: trans ratios, and good to excellent enantioselectivities. The reaction can be scaled up, and the synthetic utility of axially chiral anilides was proved by transformations. Moreover, the linear free energy relationship analysis was introduced to investigate the reaction.

Synthesis of Enantioenriched Bromohydrins via Divergent Reactions of Racemic Intermediates from Anchimeric Oxygen Borrowing

We report a chiral phosphoric acid catalyzed bromocyclization/regiodivergent reaction of racemic intermediates sequence, which is enabled by anchimeric oxygen borrowing. Different types of alkenes are applicable, and both enantiomers of the bromohydrin products were obtained in generally excellent yields and enantioselectivities. In addition, an example of enantioconvergent synthesis from the two isomeric products is presented.

Absolute and relative facial selectivities in organocatalytic asymmetric chlorocyclization reactions

For four related 1,1-disubstituted olefins, (DHQD)₂PHAL-catalyzed asymmetric chlorocyclization delivers Cl⁺ uniformly to one π face, but cyclizes with strong but differing net syn vs. anti addition.

Though (DHQD)₂PHAL-catalyzed chlorocyclizations of 1,1-disubstituted olefins show useful (and in some cases, reversible) asymmetric induction, stereochemically complete descriptions of these alkene additions have remained largely unknown. Herein, based on a combination of NMR, derivative, isotope labeling, and computational studies, we present detailed stereochemical analyses of chlorocyclizations of nucleophile-tethered 1,1-disubstituted styryl systems. The selectivities of the two asymmetric bond-forming processes, namely electrophilic chlorine attack and nucleophilic ring closure, are thus mapped out independently. Under the established optimal conditions, four related chlorocyclizations were subjected to this analysis. All showed a strong preference for Cl⁺ delivery from the same face of the alkene. However, depending on reaction conditions and substrate identity (carboxylic acid, amide or carbamate), the internal nucleophiles may close with a strong net preference for either syn or anti addition relative to the Cl atom. Studies of both uncatalyzed and (DHQD)₂PHAL-catalyzed processes place new boundary conditions on the role of the catalyst in these reactions.

Synthesis of pyridine trans-tetrafluoro-λ6-sulfane derivatives via radical addition

Pyridine-trans-SF₄-alkenes or -alkanes have been synthesized for the first time via the radical addition reactions of pyridine-trans-SF₄Cl to alkynes or alkenes in good to high yields.

Natural products as inspiration for the development of new synthetic methods

Natural products have played an important role in shaping modern synthetic organic chemistry. In particular, their complex molecular skeletons have stimulated the development of many new synthetic methods. We highlight in this article some recent examples of synthetic design inspired by the biosynthesis of natural products.

Highly Regio- and Enantioselective Vicinal Dihalogenation of Allyl Amides

We report a highly regio-, diastereo- and enantioselective vicinal dihalogenation of allyl amides. E- and Z-alkenes with both aryl and alkyl substituents were compatible with this chemistry. This is the result of exquisite catalyst controlled regioselectivity enabling use of electronically unbiased substrates. The reaction employs commercially available catalysts and halenium sources along with cheap inorganic halide salts to affect this transformation. A preliminary effort to extend this chemistry to heterodihalogenation is also presented.

Catalytic enantioselective bromohydroxylation of aryl olefins with flexible functionalities

A highly enantioselective bromohydroxylation of aryl olefins with flexible functionalities has been achieved with (DHQD)₂PHAL as a catalyst and H₂O as a nucleophile, giving a variety of optically active bromohydrins with up to 98% ee.

Synthesis of phostones via DABCO-catalyzed bromocyclization of alkenylphosphonic acid monoesters

A series of phostones were synthesized via DABCO-catalyzed bromocyclization reaction under mild conditions with high endo regioselectivity. The obtained diastereomers were separated and the relative configurations were confirmed.

Chiral N,N′-dioxide-Sc(NTf2)3 complex-catalyzed asymmetric bromoamination of chalones with N-bromosuccinimide as both bromine and amide source

A chiral N,N′-dioxide-Sc(NTf₂)₃ complex catalytic system has been developed to catalyze the asymmetric bromoamination reaction of chalones with N-bromosuccinimide.

Toward Catalytic, Enantioselective Chlorolactonization of 1,2-Disubstituted Styrenyl Carboxylic Acids

An investigation into the use of Lewis base catalysis for the enantioselective chlorolactonization of 1,2-disubstituted alkenoic acids is described. Two mechanistically distinct reaction pathways for catalytic chlorolactonization have been identified. Mechanistic investigation revealed that tertiary amines predominately operate as Brønsted rather than Lewis bases. Two potential modes of activation have been identified that involve donation of electron density of the carboxylate to the C═C bond as well hydrogen bonding to the chlorinating agent. Sulfur- and selenium-based additives operate under Lewis base catalysis; however, due to the instability of the intermediate benzylic chloriranium ion, chlorolactonization suffers from low chemo-, diastereo-, and enantioselectivities. Independent generation of the benzylic chloriranium ion shows that it is in equilibrium with an open cation, which leads to low specificities in the nucleophilic capture of the intermediate.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Phosphine Oxide-Sc(OTf)3 Catalyzed Highly Regio- and Enantioselective Bromoaminocyclization of (E)-Cinnamyl Tosylcarbamates. An Approach to a Class of Synthetically Versatile Functionalized Molecules

A highly regio- and enantioselective bromoaminocyclization of (E)-cinnamyl tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)3 complex is described. A wide variety of optically active aryl 5-bromo-1,3-oxazinan-2-ones can be obtained with high yield and enantioselectivity.

Organoselenium and DMAP co-catalysis: regioselective synthesis of medium-sized halolactones and bromooxepanes from unactivated alkenes

A co-catalytic system has been developed for the synthesis of medium-sized halolactones and bromooxepanes possessing high transannular strain. Mechanistic studies reveal that quaternary selenium intermediate is involved in catalysis.

Asymmetric chlorination of 4-substituted pyrazolones catalyzed by natural cinchona alkaloid

A natural quinidine catalyzed asymmetric chlorination of 4-substituted pyrazolones was revealed in high yield and excellent enantioselectivity.

Chiral Phosphine Oxide-Sc(OTf)3 Complex Catalyzed Enantioselective Bromoaminocyclization of 2-Benzofuranylmethyl N-Tosylcarbamates. Approach to a Novel Class of Optically Active Spiro Compounds

An efficient enantioselective bromoaminocyclization of 2-benzofuranylmethyl N-tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)(3) complex is described. A wide variety of optically active spiro benzofuran oxazolidinones can be obtained with high enantioselectivities.

Catalytic Enantioselective 1,4-Iodofunctionalizations of Conjugated Dienes

The first catalytic enantioselective 1,4-iodofunctionalizations of conjugated dienes have been developed. Starting from β,γ,δ,ε-unsaturated oximes and 4-Ns hydrazones, these N-iodosuccinimide-mediated reactions are catalyzed by newly modified tertiary aminothiourea derivatives and furnish Δ(2)-isoxazoline and Δ(2)-pyrazoline derivatives, respectively, containing an (E)-allyl iodide group at the quaternary stereogenic center generally in high yield and with excellent enantioselectivity (up to 98.5:1.5 er).

Enantioselective 6-exo-Bromoaminocyclization of Homoallylic N-Tosylcarbamates Catalyzed by a Novel Monophosphine-Sc(OTf)3 Complex

A highly enantioselective 6-exo-bromoaminocyclization of (E)-homoallylic N-tosylcarbamates catalyzed by a novel monophosphine-Sc(OTf)3 complex is described, giving a wide variety of optically active oxazinanones with high enantioselectivities.

Organocatalytic asymmetric chlorinative dearomatization of naphthols

An organocatalytic asymmetric chlorinative dearomatization of naphthols was realized for the first time, providing chiral naphthalenones with a Cl-containing all-substituted stereocenter in excellent yields and enantioselectivity (up to 97% yield and 96% ee). The reaction features mild reaction conditions, good tolerance of diverse functional groups and simple reaction operation.