Rosaria Schettini

Enantioselective Alkylation of Amino Acid Derivatives Promoted by Cyclic Peptoids under Phase-Transfer Conditions

The effects of substituents and cavity size on catalytic efficiency of proline-rich cyclopeptoids under phase-transfer conditions were studied. High affinity constants (Ka) for the sodium and potassium cations, comparable to those reported for crown ethers, were observed for an alternated N-benzylglycine/L-proline hexameric cyclopeptoid. This compound was found to catalyze the alkylation of N-(diphenylmethylene)glycine cumyl ester in values of enantioselectivities comparable with those reported for the Cinchona alkaloid ammonium salts derivatives (83-96% ee), and with lower catalyst loading (1-2.5% mol), in the presence of a broad range of benzyl, allyl and alkyl halides.

Switchable Diastereoselectivity in the Fluoride-Promoted Vinylogous Mukaiyama–Michael Reaction of 2-[(Trimethylsilyl)oxy]furan Catalyzed by Crown Ethers

The fluoride-promoted vinylogous Mukaiyama-Michael reaction of 2-[(trimethylsilyl)oxy]furan with diverse α,β-unsaturated ketones is described. The TBAF-catalyzed VMMR afforded high anti-diastereoselectivity irrespective of the solvents used. The KF/crown ethers catalytic systems proved to be highly efficient in terms of yields and resulted in a highly diastereoselective unprecedented solvent/catalyst switchable reaction. Anti-adducts were obtained as single diastereomers or with excellent diastereoselectivities when benzo-15-crown-5 in CH2Cl2 was employed. On the other hand, high syn-diastereoselectivities (from 73:27 to 96:4) were achieved by employing dicyclohexane-18-crown-6 in toluene. On the basis of DFT calculations, the catalysts/solvent-dependent switchable diastereoselectivities are proposed to be the result of loose or tight cation-dienolate ion pairs.

Highly Diastereoselective Crown Ether Catalyzed Arylogous Michael Reaction of 3-Aryl Phthalides

The first arylogous Michael reaction of 3-aryl phthalides has been developed. The reaction, promoted by catalytic amounts of KOH or K3PO4 and dibenzo-18-crown-6, affords the corresponding 3,3-disubstituted phthalides in good to high yields and as single diastereomers in nearly all studied cases.

Phakellistatins: An Underwater Unsolved Puzzle

A critical summary on the discovery of the nineteen members of the phakellistatin family (phakellistatin 1–19), cytotoxic proline-rich cyclopeptides of marine origin, is reported. Isolation, structural elucidation, and biological properties of the various-sized natural macrocycles are described, along with the total syntheses and the enigmatic issues of the cytotoxic activity reproducibility.

Cation-Induced Molecular Switching Based on Reversible Modulation of Peptoids Conformational States

Peptoids are oligomers of N-substituted glycines with predictable folding and strong potentials as guest-binding receptor molecules. In this contribution, we investigate the structural features of a series of designed symmetric cyclic octamer peptoids (with methoxyethyl/propargyl side chains) as free hosts and reveal their morphologic changes in the presence of sodium and alkylammonium guests as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts, reporting the first case of reversible adaptive switching between defined conformational states induced by cationic guests (Na+ and benzylammonium ion) in the peptoid field. The reported results are based on 1H NMR data, theoretical models, and single-crystal X-ray diffraction analysis. They represent initial steps toward deciphering the unique conformational states of cyclic octamer peptoids as supramolecular hosts with the aim to fully disclose their functional and dynamic properties.