Pyrazolines and their pyrazole congeners are important heterocyclic building blocks
with numerous applications in the fine chemical industries. However, traditional routes
towards these entities are based on multistep syntheses generating substantial amounts
of chemical waste. Here we report an alternative approach using UV-light to convert
tetrazoles into pyrazolines via a reagent-free photo-click strategy. This route generates
nitrile imine dipoles in situ that are trapped with different dipolarophiles rendering
a selection of these heterocyclic targets in high chemical yields. A continuous flow
method is ultimately realized that generates multigram quantities of product in a
safe and readily scalable manner thus demonstrating the value of this photochemical
approach for future exploitations in industry.
Key words
flow chemistry - photochemistry - pyrazoline - tetrazole - click reaction - drug-like
heterocycles
