Short double-stranded RNAs, which are known as short interfering RNA (siRNA), can be used to specifically down-regulate the expression of the targeted gene in a process known as RNA interference (RNAi). However, the success of gene silencing applications based on the use of synthetic siRNA critically depends on efficient intracellular delivery. Polycationic branched macromolecules such as poly(amidoamine) (PAMAM) dendrimers show a strong binding affinity for RNA molecules and, hence, can provide an effective, reproducible, and relatively nontoxic method for transferring siRNAs into animal cells. Notwithstanding these perspectives, relatively few attempts have been made so far along these lines to study in detail the molecular mechanisms underlying the complexation process between PAMAMs and siRNAs. In this work we combine molecular simulation and experimental approaches to study the molecular requirements of the interaction of RNA-based therapeutics and PAMAM dendrimers of different generations. The dendrimers and their siRNA complexes were structurally characterized, and the free energy of binding between each dendrimer and a model siRNA was quantified by using the well-known MM/PBSA approach. DOSY NMR experiments confirmed the structural in silico prediction and yielded further information on both the complex structure and stoichiometry at low N/P ratio values. siRNA/PAMAM complex formation was monitored at different N/P ratios using gel retardation assays, and a simple model was proposed, which related the amount of siRNA complexed to the entropy variation upon complex formation obtained from the computer simulations.

In the past few decades natural and non-natural prolines-and among the latter, a-quaternary derivatives are of great interest-have found wide application in the design of new organocatalysts [1] and chiral auxiliaries [2] for asymmetric synthesis. Furthermore, non-natural proline units have been incorporated in new peptide chains, affording peptidomimetics with different conformational flexibility and correlated drastic changes of their secondary structures. [3] In the best cases, these alterations enhance the resistance of the modified proteins to metabolic and chemical degradation and hence their overall biological activity.A number of strategies [4] -and, among them, those that adopt the well-known Kawabatas principles of "memory of chirality" (MOC) [5] and of "chiral non-racemic enolate" [6] emerge as the most appealing-have been proposed for the stereoselective synthesis of quaternary a-amino acids. In particular, the synthetic methods for quaternary prolines [7] involve the enantioselective functionalization of l-proline derivatives, for example, through self-reproduction of chirality, diastereoselective alkylation, or transfer of stereochemical information via cyclic ammonium ylides.Recently, we reported a straightforward protocol for the enantioselective synthesis of quaternary N-alkyl-a-4-nitrophenyl-a-amino tert-butyl esters, through N-alkylation of the corresponding a-N-(4-nitrophenyl)sulfonamido esters, followed by degradative rearrangement, with loss of SO 2 .[8]The one-pot overall process is highly stereoselective, ees up to 98 %; in contrast, the reaction conducted with the preformed N-alkyl-a-N-(4-nitrophenyl)sulfonamido ester gave the corresponding quaternary amino ester with very low ees, for example, 28 % ee in the case of N-allyl phenylalanine derivative. In the present paper we describe the rearrangement under basic conditions of N-(arylsulfonyl)proline tert-butyl esters 1, which showed a different behavior of that found for N-alkylated open-chain sulfonamido esters. Preliminary runs (Table 1), conducted on N-(4-nitrobenzenesul-[a] F.

The 'one-pot' stereoselective conversion of N-(4-nitrobenzene)sulfonyl-alpha-amino acid tert-butyl esters into the corresponding N-alkyl-alpha-(4-nitrophenyl)-alpha-amino esters has been realized through N-alkylation of the starting amido esters, followed by N-C(alpha) migration of the p-nitrophenyl group and the loss of sulfur dioxide; the asymmetric induction is determined by an intermediate non-racemic enolate, without the need of an external source of chirality.

The "memory of chirality" stereodivergent synthesis of polyfluorobenzo[d]sultams has been developed. The interest of this protocol resides in the possibility of using the chirality of a starting sulfonamide single enantiomer to synthesize the target sultams in both absolute configurations, by using different base systems, under homogeneous conditions.