Trasformazioni asimmetriche indotte da cristallizzazione di miscele diastereoisomeriche di aldeidi alpha-epimerizzabili

In the last few years organic chemistry has focused attention on enantiomeric resolution. Among the several techiniques, crystallization-induced diastereoisomeric transformation (CIDT) aroused the interest because of high yields, as well as to meet the criteria of green chemistry. The process is applied in the specific way for a racemic mixtures of α- epimerizable aldehydes, in order to obtain enatiomerically enrichment mixtures. This technique involves the transformation of a racemic mixture of enantiomers into a diasteroisomeric one by a reaction with a enantiopure auxiliary (Betti’s base). Then, to mixture of diastereoisomers is applied the acid-catalyzed enrichment process: in solution, the epimerization of more soluble diastereoisomer occurs, accompanied by precipitation and hence the removal of the less soluble one from the equilibrium. Finally, through the hydrolysis reaction, it was possible to recover the enantiomerically enriched aldehydes.

Regio- e stereocontrollo nell'addizione viniloga di alchilidenossindoli a nitrostireni via organocatalisi.

The topic of this work is the simultaneous activation, promoted by 9-epi-NH2-DHQA-TU, of alkylideneoxindole and nistirene derivatives, respectively via base catalysis and hydrogen-bond catalysis. The chosen substrates, of high biological interest, are used as starting materials for a vinylogous Michael addition where we wish to control the stereochemistry of the two asymmetric carbons far away from the active site, respectively in γ and δ position. Due to the particular structure of the starting oxindoles, it is hereby presented the first variant of this reaction performed at its highest level of stereochemical complexity. It is possible as a matter of fact, to generate 24 isomers of the product. Specifically, given that the nucleophilic attack can occur from various, non equivalent regions of the starting molecule, our main goal was to achieve a complete regio- and stereocontrol of the reaction. We have verified that the reported organocatalyzed vinylogous reaction represents a valid integration of the metal-catalyzed one, since it affords highly stereochemically complex products in good to high yields and excellent optical purity.