Substituted 4-hydroxy-2-furoic acids are obtained by the fermentation of an asterriquinone, which is a natural product having a 2,5-dioxy-3,6-bis(indolyl)quinone structure. The compounds modulate insulin receptor tyrosine kinase activity and may be useful in the treatment of diabetes and other diseases or conditions characterized by impaired endogenous insulin production or an impaired response to endogenous insulin.