Organocatalysis is becoming an important tool in polymer science because of its versatility and specificity. To date a limited number of organic catalysts have demonstrated the ability to promote stereocontrolled polymerizations. In this work we report one of the first examples of chirality transfer from a catalyst to a polymer in the organocatalyzed ring-opening polymerization (ROP) of rac-lactide (rac-LA). We have polymerized rac-LA using the diastereomeric densely substituted amino acids (2S,3R,4S,5S)-1-methyl-4-nitro-3,5-diphenylpyrrolidine-2-carboxylic acid (endo-6) and (2S,3S,4R,5S)-1-methyl-4-nitro-3,5-diphenylpyrrolidine-2-carboxylic acid (exo-6), combined with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a cocatalyst. Both diastereoisomers not only showed the ability to synthesize enriched isotactic polylactide with a P higher than 0.90 at room temperature but also were able to preferentially promote the polymerization of one of the isomers (l or d) with respect to the other. Thus, exo-6 preferentially polymerized l-lactide, whereas endo-6 preferred d-lactide as the substrate. Density functional theory calculations were conducted to investigate the origins of this unique stereocontrol in the polymerization, providing mechanistic insight and explaining why the chirality of the catalyst is able to define the stereochemistry of the monomer insertion.

An unexpected 1,3-dioxa-[3,3]-sigmatropic rearrangement during the treatment of aryl-and alkenyl-substituted allylic alcohols with activated isocyanates is reported. The reorganization of bonds is highly dependent on the electron density of the aromatic ring and the nature of isocyanate used. This metal-free tandem reaction from branched allyl alcohols initiated by a carbamoylation reaction and followed by a sigmatropic rearrangement thus offers a new access to (E)-cinnamyl and conjugated (E,E)-diene carbamates, such as N-acyl and N-sulfonyl derivatives. A computational study was conducted in order to rationalize this phenomenon, and a rearrangement progress kinetic analysis was performed.

The organocatalytic properties of unnatural aamino acids are reviewed. Post-translational derivatives of natural a-aminoa cids include 4-hydroxy-l-proline and 4amino-l-proline scaffolds, and also prolineh omologues. The activity of synthetic unnatural a-amino acid-based organocatalysts, such as b-alkyl alanines, alanine-based phosphines, and tert-leucine derivatives, are reviewed herein. The orga-nocatalytic properties of unnatural monocyclic,b icyclic, and tricyclicproline derivatives are also reviewed. Severalfamilies of these organocatalystsp ermitthe efficient and stereoselective synthesis of complex natural products. Most of the reviewedo rganocatalysts accelerate the reported reactions through covalent interactions that raise the HOMO (enamine intermediates) or lower the LUMO (iminiumintermediates).Scheme1.Ageneral (simplified) mechanism of addition reactionsbye namine organocatalysis promoted by a-amino acid derivatives.[a] M.

The asymmetric synthesis of bicyclic highly substituted tetrahydropyrans is described. The reaction is catalyzed by unnatural γ-dipeptides based on densely substituted L-and Dproline derivatives. This organocatalytic one-pot reaction takes place among a ketone, a nitroalkene, and an aldehyde to yield an octahydro-2H-chromene scaffold. Monomeric species, from which the corresponding γ-dipeptides are synthesized, cannot catalyze the reaction, thus confirming the emergent nature of the catalytic behavior of these dimeric species. Note pubs.acs.org/joc