Papyracillicacid (1), an antibiotic isolated from the ascomyceteLachnumpapyraceum, owes its bioactivities to its reactivity towards nucleophiles. When reacted with cysteine and cysteine methyl ester, it exclusively added the thiol groups to the exomethylene double bond. Both papyracillicacid (1) and its analoguepenicillicacid (2) react with pyridine to indolizine derivatives during acetylation