A novel type of tetrahydroisoquinoline alkaloid, antidesmone (1), was isolated from Antidesma membranaceum Mull. Arg.. The structure of 1 was determined to be (5S)-1-hydroxy-4-methoxy-3-methyl-5-octyl-5,6,7,8-tetrahydroisoquidolin-8-one by spectroscopic methods (MS, H-1, C-13 2D NMR, CD) and chemical derivatisation. The absolute (S)-configuration was determined by quantumchemical calculation of CD spectra. (C) 1999 Elsevier Science Ltd. All rights reserved.