# Glucuronidated Hydroxyphenylacetic and Hydroxyphenylpropanoic Acids as Standards for Bioavailability Studies with Flavonoids

## TL;DR

This study synthesizes glucuronides of phenolic acids to serve as standards for studying flavonoid bioavailability in biological systems.

## Contribution

The successful synthesis and in vivo validation of six glucuronides as potential polyphenolic metabolites.

## Key findings

- Six glucuronides were synthesized using Schmidt imidate with moderate to excellent yields.
- Two glucuronides were identified in rat plasma after hawthorn extract administration, confirming in vivo formation.
- The synthesized compounds were fully characterized and used in a pilot metabolic study.

## Abstract

Glucuronidation is a major phase II biotransformation
of (poly)­phenols
leading to potentially bioactive metabolites. Due to the limited availability
of authentic standards, in this work, we have focused on the glucuronidation
of a series of mono- and dihydroxyphenolic acids. Their reactivity
with two glucuronidation reagents, 2,3,4-triaceto-1-bromo-α-d-glucuronic acid methyl ester and Schmidt imidate, was investigated.
The use of Schmidt imidate led to the successful synthesis of six
target glucuronides in moderate to excellent yields. Subsequent deprotection
of these compounds afforded the final glucuronides of 2-hydroxyphenylacetic,
3-hydroxyphenylacetic, 4-hydroxyphenylacetic, 3,4-dihydroxyphenylacetic,
3-(4-hydroxyphenyl)­propionic, and 3-(3,4-dihydroxyphenyl)­propionic
acid. These compounds, which are plausible polyphenolic metabolites,
were fully characterized and used in a pilot metabolic study in rats
after administration of a hawthorn berry extract (475 mg/kg). UHPLC-HRMS
analysis identified two of the glucuronides, namely 4-hydroxyphenylacetic
acid glucuronide and 3-hydroxyphenylacetic acid glucuronide in rat
plasma, confirming their in vivo formation.