No abstract

Substituted and annulated pyrroles containing chiral centers are potentially accessible by means of an organocatalytic enantioselective Friedel-Crafts alkylation (FCA) reaction. They are interesting compounds because of their abundant natural occurrence and their different biological activities. In comparison to indoles, for which a myriad of asymmetric procedures involving the FCA have been developed, pyrroles have been relatively less used as nucleophilic counterparts in this reaction. However, in the last two decades, huge synthetic efforts have been performed around this interesting asymmetric transformation and several methodologies have been devised. In this context, this review article provides a general overview of the organocatalytic enantioselective FCA chemistry of pyrroles, which has not been revised to date. Reactions 2.4. Brønsted Acid Catalyzed Intramolecular FCA Reactions 3.Organocatalytic FCA Reactions with Dihydroindoles 4.Conclusions and Outlook

The asymmetric intramolecular aza-Michael reaction (IMAMR) is a very convenient strategy for the generation of heterocycles bearing nitrogen-substituted stereocenters. Due to the ubiquitous presence of these skeletons in natural products, the IMAMR has found widespread applications in the total synthesis of alkaloids and biologically relevant compounds. The development of asymmetric versions of the IMAMR are quite recent, most of them reported in this century. The fundamental advances in this field involve the use of organocatalysts. Chiral imidazolidinones, diaryl prolinol derivatives, Cinchone-derived primary amines and quaternary ammonium salts, and BINOL-derived phosphoric acids account for the success of those methodologies. Moreover, the use of N-sulfinyl imines with a dual role, as nitrogen nucleophiles and as chiral auxiliaries, appeared as a versatile mode of performing the asymmetric IMAMR.

The intramolecular 1,3-dipolar cycloaddition of ortho-substituted 1,1,1-trifluoromethylstyrene-derived nitrones is described. Tricyclic fused isoxazolidines were obtained as major or exclusive products, in contrast to the case for nonfluorinated substrates, which rendered the bridged derivatives. This change in the regioselectivity was attributed to the electronic and, particularly, steric requirements of the trifluoromethyl group in comparison to the methyl group. It is worth mentioning that trifluoromethylstyrenes have been employed for the first time as dipolarophiles in a 1,3-dipolar intramolecular cycloaddition reaction, leading to the corresponding isoxazolidines bearing a quaternary trifluoromethyl moiety. Finally, the synthetic utility of the developed methodology has been illustrated with the synthesis of a family of bicyclic fluorinated 1,3-amino alcohols.

The preparation of nonanomeric C-acyl-saccharides has been developed from two different carboxylic acid feedstocks. This transformation is driven by the synergistic interaction of an electron donor−acceptor complex and Ni catalysis. Primary-, secondary-, and tertiary redox-active esters are incorporated as coupling partners onto preactivated pyranosyl-and furanosyl acids, preserving their stereochemical integrity. The reaction occurs under mild conditions, without stoichiometric metal reductants or exogenous catalysts, using commercially available Hantzsch ester as the organic photoreductant.