Two new mycolactones, mycolactones S1 and S2, were isolated from culture agar of Mycobacterium ulceranssubsp. shinshuense. Their structures were established in a three-step procedure: (1) probable structures were speculated from MS analysis; (2) candidates were synthesized; (3) HPLC profiles were established for identification of the natural products. Newly isolated mycolactones correspond to the âoxidized formsâ of mycolactone A/B, the causative toxin of Buruli ulcer, isolated from Mycobacterium ulcerans. [ABSTRACT FROM AUTHOR]