Asymmetric, Organocatalytic Bromolactonization of Allenoic Acids

Structure-Enantioselectivity Relationship (SER) Study of Cinchona Alkaloid Chlorocyclization Catalysts

Various structural elements of the Cinchona alkaloid dimers are interrogated to establish a structure-enantioselectivity relationship (SER) in three different halocyclization reactions. SER for chlorocyclizations of a 1,1-disubstituted alkenoic acid, a 1,1-disubstituted alkeneamide, and a trans-1,2-disubstituted alkeneamide showed variable sensitivities to linker rigidity and polarity, aspects of the alkaloid structure, and the presence of two or only one alkaloid side group defining the catalyst pocket. The conformational rigidity of the linker-ether connections was probed via DFT calculations on the methoxylated models, uncovering especially high barriers to ether rotation out of plane in the arene systems that include the pyridazine ring. These linkers are also found in the catalysts with the highest enantioinduction. The diversity of the SER results suggested that the three apparently analogous test reactions may proceed by significantly different mechanisms. Based on these findings, a stripped-down analogue of (DHQD)2PYDZ, termed "(trunc)2PYDZ", was designed, synthesized, and evaluated, showing modest but considerable asymmetric induction in the three test reactions, with the best performance on the 1,1-disubstituted alkeneamide cyclization. This first effort to map out the factors essential to effective stereocontrol and reaction promotion offers guidance for the simplified design and systematic refinement of new, selective organocatalysts.

Enantioselective Lewis base catalysed allylation of picoline- and quinaldine-based latent pronucleophiles

Picolines and quinaldines are valuable building blocks and intermediates in the synthesis of natural products and pharmaceuticals. Functionalization of the methyl group in picolines and quinaldines under mild conditions is challenging. We report that the concept of latent pronucleophiles enables Lewis base catalysed allylation of picolines and quinaldines with allylic fluorides starting from silylated picolines and quinaldines. Reactions afford enantioenriched allylation products when chiral Lewis base catalysts are used. The allylation products can be rapidly transformed to quinolizine-4-ones.

Catalytic Enantioselective Halocyclizations to Access Benzoxazepinones and Benzoxazecinones

We report a catalytic asymmetric halocyclization protocol to furnish benzoxazepinones and benzoxazecinones using (DHQ)₂PHAL as the catalyst. Various halogenated benzoxazepinones and benzoxazecinones were achieved in excellent yields and enantioselectivities under mild conditions. A cocrystal structure of the substrate and the catalyst was studied.

Catalytic enantioselective iodolactonization reactions

The halolactonization reaction is a useful chemical transformation for the construction of lactones from γ- or δ-substituted alkenoic carboxylic acids or carboxylic esters. Traditionally, the stereoselectivity of these reactions has been controlled by the substrates or the reagents. The substrate-controlled method has been extensively studied and applied in the synthesis of many natural products. However, catalytic, enantioselective iodolactonizations of γ- or δ-substituted alkenoic carboxylic acids have only recently been developed. This review article highlights the advances that have emerged over the last decade.

Enantioselective Halolactonizations Using Amino-Acid-Derived Phthalazine Catalysts

Amino-acid-derived phthalazine catalysts have been designed and synthesized for enantioselective halolactonization of prochiral dienoic acids. The scope of the reaction is evidenced by 17 examples of spiro α-exo-methylene-halolactones with up to 99.8% enantiomeric excess. The resulting enantio-enriched spiro halolactone products are found to exhibit potent antitumor effects. In addition, both antipodes of products with equally excellent enantioselevity could be obtained since a pair of enantiomeric catalysts is guaranteed.

Enantioselective Bromolactonization of Trisubstituted Olefinic Acids Catalyzed by Chiral Pyridyl Phosphoramides

Enantioselective bromolactonization of trisubstituted olefinic acids producing synthetically useful chiral lactones with two contiguous asymmetric centers has remained mainly unexplored except for the 6-exo cyclization mode. In this work, the 5-exo- and 6-endo modes of bromocyclization of trisubstituted olefinic acids were enabled for the first time using N-bromosuccinimide and a pyridyl phosphoramide catalyst. The utility of the resulting bromolactones was demonstrated by transformations harnessing reactive alkyl bromide moieties without losing stereochemical information. Optimization studies and control experiments revealed that the basicity of pyridine moieties and presence of N-H protons in the phosphoramide species strongly affected both the reactivity and enantioselectivity parameters.

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.

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.

Homohalocyclization: Electrophilic Bromine-Induced Cyclizations of Cyclopropanes

An efficient method for the halocyclization of cyclopropanes has been developed. The cyclopropanes undergo a 1,3-addition reaction to form homohalocyclization products compared to conventional alkene halocyclizations. The reaction can be induced by various electrophilic halogenating agents including 1,3-dibromo-5,5-dimethylhydantoin and N-iodosuccinimide. In cyclopropane derivatives with a preexisting stereocenter, excellent induced diastereoselectivities can be observed.

Enantioselective iodolactonization of allenoic acids

An enantioselective iodolactonization reaction of allenoic acids has been developed using a trisimidazoline catalyst and I₂.