Modification and Optimization of the Bis-picolylamide-Based Relay Protection for Carboxylic Acids to be Cleaved by Unusual Complexation with Cu²⁺ Salts

A simple modification of our recently published protection scheme for carboxylic acids as amides resulted in a new protecting group with significantly improved properties. It requires shorter reaction times for deprotection and allows us to replace Cu­(OTf)₂ by CuCl₂, indicating at the same time the importance of the nature of the anion of the Cu²⁺ source. Since the new scheme fulfills all criteria required for an ideal protection group it should find widespread application in synthetic organic chemistry

Stable isotope phosphate labelling of diverse metabolites is enabled by a family of 18O-phosphoramidites

Stable isotope labelling is state-of-the-art in quantitative mass spectrometry, yet often accessing the required standards is cumbersome and very expensive. As 18O can be derived from heavy water (H218O), it is comparably cheap and particularly suited for labelling of phosphorylated compounds, provided the introduction is straight-forward and phosphate neutral loss in the ion source can be avoided. Here, a unifying synthetic concept for 18O-labelled phosphates is presented, based on a family of modified 18O2‑phosphoramidite reagents. This flexible toolbox offers access to major classes of biologically highly relevant phosphorylated metabolites as their isotopologues including - but not limited to - nucleotides, inositol phosphates, -pyrophosphates, and inorganic polyphosphates. 18O-enrichment ratios >95% and good yields are obtained consistently in gram-scale reactions, while enabling late-stage labelling. We demonstrate the utility of the 18O labelled inositol phosphates and pyrophosphates by assignment of these metabolites from different biological matrices, such as mammalian cell lysates, slime mold and plant samples. We demonstrate that phosphate neutral loss is negligible in an analytical setup employing capillary electrophoresis electrospray ionization triple quadrupole mass spectrometry