A new antifungal substance, azoxybacilin (an unusual amino acid with an azoxy moiety) and its derivatives have been synthesized from Boc-L-Asp-O(t)Bu utilizing the Moss procedure for the preparation of the azoxy moiety. The ester derivative, Ro 09-1824, showed more potent antifungal activity and a broader antifungal spectrum than azoxybacilin did.