# Continuous-Flow Photochemical Isomerization of Humulones to Isohumulones

## TL;DR

This paper describes a continuous-flow method to convert humulones into isohumulones using UV light, improving production efficiency.

## Contribution

The study introduces a novel continuous-flow photoreactor for efficient isomerization of humulones into trans-isohumulones.

## Key findings

- A continuous-flow reactor using UV LEDs achieved a throughput of 0.43 g/h for trans-n-isohumulone.
- UV light at 365 nm was most effective for isomerization compared to white and blue light.
- The method enables stereoselective conversion of humulones into trans-isohumulones.

## Abstract

Humulones are a family of homolog natural products obtained from the strobiles of humulus lupulus, or hops plants. Structurally, they consist of substituted phloroglucinols with two isoprenyl side chains, a carbonyl group and a quaternary ring carbon substituted with a hydroxyl group. The three most prominent homologs are n-, co- and ad-humulone, containing isobutyl, isopropyl and secbutyl ketone groups, respectively. When solutions of humulones are exposed to UV light, they undergo stereoselective isomerization to the five-membered ring trans-isohumulones. A photoreactor was assembled from strip LEDs in close contact with UV-transparent tubing. This reactor allowed continuous-flow chemical synthesis of the isohumulones. The yield, conversion and product throughput are compared for the humulones, using LEDs emitting white, blue and ultraviolet light (visible, 400 nm, and 365 nm, respectively). Using an optimized continuous-flow reactor, a throughput of 0.43 g/h was obtained for trans-n-isohumulone.