Exceedingly high enantioselectivity in a catalytic reaction can be realized even when the chirality resides only in the counteranion of the catalyst. A salt (1) composed of an achiral ammonium cation and a chiral phosphate counteranion catalyzes asymmetric transfer hydrogenations of aromatic and aliphatic α,β‐unsaturated aldehydes with a Hantzsch ester in excellent enantioselectivities (see scheme).

A set of vectors for improved tetracycline-dependent gene regulation in Staphylococcus aureus is presented. Plasmid pRAB11 was generated from pRMC2 by adding a second tet operator within the TetR-regulated promoter P xyl/tet . Pronounced repression was observed in the absence of anhydrotetracycline (ATc) combined with high induction in the presence of the drug, as demonstrated for pRAB11 bearing staphylococcal nuclease nuc1, lacZ or gfp. Also, in plasmid pCG261, the pRAB11 tetR-P xyl/tet regulatory architecture permitted tight repression and a stepwise increase in transcript amounts of the target gene rny (putative RNase) correlated with rising ATc concentrations. Additionally, pRAB11-derived vectors harbouring semi-rationally designed P xyl/tet -like fragments, mutated at up to six defined positions, were constructed. Sixteen mutant sequences with single to quadruple exchanges were analysed for transcriptional strength and ATc-dependent inducibility. A set of promoters with gradually decreased activities and improved repression is presented. Finally, the implementation of reverse TetR revtetR-r2, which exhibits three amino acid exchanges and binds to tetO in the presence of ATc, yielded an efficiently co-repressible vector within the pRAB11 system. Intriguingly, revtetR was found to contain a fourth mutation only after propagation in S. aureus. We predict that the described vectors constitute valuable tools for staphylococcal genetics.

Außerordentlich hohe Enantioselektivitäten können auch dann erzielt werden, wenn nur das Anion eines Katalysatorsalzes chiral ist. Ein aus einem achiralen Ammonium‐Kation und einem chiralen Phosphat‐Anion zusammengesetztes Salz (1) katalysiert die asymmetrische Transferhydrierung von aromatischen und aliphatischen α,β‐ungesättigten Aldehyden in Gegenwart eines Hantzsch‐Esters mit ausgezeichneten Enantioselektivitäten (siehe Schema).