Synthesis of 3,3‘,5-Trihydroxybiphenyl-2-carboxylic Acid, a Component of the Bitterest Natural Product Amarogentin and Its Coenzyme A and N-Acetyl Cysteamine Thiol Esters

3,3',5-Trihydroxybiphenyl-2-carboxylic acid (6), an ester component of the bitter-tasting natural products amarogentin and amaroswerin, was synthesized in six steps in 13.6% overall yield. Its N-acetyl cysteamine thiol ester (9) and its coenzyme A thiol ester (8), a likely biosynthetic precursor of the amarums, were also prepared.