The "azido gauche effect" was examined both experimentally and theoretically and was found to determine the conformation of, for example, (4R)- and (4S)-azidoproline (Azp) derivatives. For (4R)Azp derivatives, the azido gauche effect induces a preferred C(4)-exo conformation of the pyrrolidine ring, which leads to stabilization of the s-trans amide conformer of, e.g., Ac-(4R)Azp-OCH(3) (5R) via an n-->pi interaction between the nonbonding electrons of the oxygen of the acetyl group and the carbonyl group of the ester. For (4S)Azp derivatives, the azido gauche effect results in a C(4)-endo conformation of the pyrrolidine ring that does not allow for this stabilizing n-->pi interaction of the s-trans conformer. Consequently, a significantly higher s-trans:s-cis amide conformer ratio is observed for (4R)Azp compared to (4S)Azp derivatives (e.g., 6.1:1 versus 2.6:1 in D(2)O for Ac-(4R)Azp-OCH(3) (5R) compared to Ac-(4S)Azp-OCH(3) (5S)). These conformational preferences are reflected in the higher tendency of (4S)Azp-containing peptides to form cyclic peptides with all-cis amide bonds compared to (4R)Azp derivatives. Ab initio calculations demonstrate that the strength of the azido gauche effect is comparable to that of the well-known "fluorine gauche effect". For azidoethane derivatives N(3)-CH(2)CH(2)-X (X = N(3), NHCOH, NHAc, or N(CH(3))Ac), the ab initio calculations revealed energy differences of 5-13 kJ mol(-)(1) between the anti and gauche conformations in favor of the gauche conformer. Calculations were also performed for the (4R)Azp and (4S)Azp derivatives 5R and 5S, supporting the experimentally observed data.

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Dengue fever is a viral disease that affects 50-100 million people annually and is one of the most important emerging infectious diseases in many areas of the world. Currently, neither specific drugs nor vaccines are available. Here, we report on the discovery of new inhibitors of the viral NS5 RNA methyltransferase, a promising flavivirus drug target. We have used a multistage molecular docking approach to screen a library of more than 5 million commercially available compounds against the two binding sites of this enzyme. In 263 compounds chosen for experimental verification, we found 10 inhibitors with IC(50) values of <100 microM, of which four exhibited IC(50) values of <10 microM in in vitro assays. The initial hit list also contained 25 nonspecific aggregators. We discuss why this likely occurred for this particular target. We also describe our attempts to use aggregation prediction to further guide the study, following this finding.

During the synthesis of the new antimalarial drug candidate NITD609, a high degree of diastereoselectivity was observed in the Pictet-Spengler reaction. By isolating both the 4E and 4Z imine intermediates, a systematic mechanistic study of the reaction under both kinetic and thermodynamic conditions was conducted. This study provides insight into the source of the diastereoselectivity for this important class of compounds.