Determination of enantiomeric excess of nipecotic acid as 1-(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl) derivatives.

For the enantiopure synthesis of novel chiral GABA uptake inhibitors, nipecotic acid (1) is an important key precursor. To characterize accurately the pharmacological activity of these interesting target compounds, the determination of the correct enantiomeric purity of nipecotic acid as the starting material is indispensable. In this report, a sensitive high-performance liquid chromatography (HPLC) based method for the separation and quantitation of both enantiomers of nipecotic acid as 1-(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl) derivatives (5) on a Chiralpak ID-3 column (Daicel, Illkirch, France) was established. UV/Vis-detection at 490 nm was chosen to ensure a selective determination of even highly enantioenriched samples. Reliability was demonstrated by validation of specificity, linearity, lower limit of quantification (LLOQ), accuracy, and precision. By spiking highly enantiopure samples with small amounts of racemic rac-5, it was proven that the established HPLC method is able to detect even slight changes in enantiomeric excess (ee) values. Thus, accurate determination of ee values up to 99.87% ee for (R)-5 and 99.86% ee for (S)-5 over a linear concentration range of 1-1500 µM for (R)-5 and of 1-1455 µM for (S)-5 could be demonstrated.

Robust visible light photoswitching with ortho-thiol substituted azobenzenes.

Introduction of S-ethyl groups in all four ortho positions of azobenzene prevents reduction of the azo group by intracellular glutathione, while enhancing the absorptivity to ~10,000 M(-1) cm(-1) in the blue and green regions of the visible spectrum. cis-to-trans isomerization occurs thermally on the minutes timescale. Further, this substitution pattern permits switching with red light, a color that is more penetrating through biological tissues than other parts of the visible spectrum.