The 6alpha,7beta-dihydroxyvouacapan-17beta-oic acid (1) and methyl 6alpha,7beta-dihydroxyvouacapan-17beta-oate (2) are natural products isolated from the hexane extract of Pterodon polygalaeflorus Benth fruits. Here we describe the preparation and characterization by infrared and nuclear magnetic resonance spectroscopy, and by elemental analysis of the novel derivative methyl 6alpha,7beta-thiocarbonyldioxyvouacapan-17beta-oate (5), from the ester 2. The structure of compound 5 was determined by X-ray diffraction. At the concentration of 100 ppm, this compound inhibited the radicle growth of Sorgum bicolor L. (-28%) and showed no significant effect on Cucumis sativus L..