# Identification of Natural Isonitriles Through Ligation to an Azomethine Imine Probe

## TL;DR

A new method using an azomethine imine probe allows for the selective detection of natural isonitriles in biological samples, including the discovery of a new fungal isonitrile.

## Contribution

A reactivity-based screening method using an AMI probe for chemoselective detection of natural isonitriles, including chiral discrimination and detection in complex matrices.

## Key findings

- The AMI probe reacts with various isonitriles to form stable conjugates, enabling their detection.
- The ligation creates a stereogenic center, distinguishing chiral and achiral isonitriles.
- A unique reactivity was observed in isonitriles with an α-COOH group.

## Abstract

Natural isonitriles are promising lead compounds for medicinal chemistry. Their identification is, however, challenging due to the hydrolytic lability of the isonitrile group. Here, we use the azomethine imine (AMI)–isonitrile ligation in a reactivity‐based screening protocol for the chemoselective derivatization of natural isonitriles. The herein developed AMI probe rapidly reacts with isonitriles—primary, secondary, tertiary, as well as aromatic—to stable conjugates. A dibromide mass tag enables the detection of low‐abundance isonitriles, even in complex biological matrices. The ligation establishes a new stereogenic center, thereby allowing facile distinction between achiral and chiral isonitriles by the formation of racemates or diastereoisomers, respectively. In addition, a unique reactivity of isonitriles bearing an α‐COOH group was unraveled. This AMI probe enabled the detection of known bacterially produced isonitriles and the identification of a novel fungal isonitrile.

The azomethine imine (AMI)–isonitrile (NC) ligation allowed for the chemoselective detection of natural isonitriles in bacteria and fungi. The reactivity‐based screening establishes a new stereogenic center, thereby allowing for facile distinction between chiral and achiral isonitriles. The work also unraveled a unique reactivity of isonitriles bearing an α‐COOH group.