Display omitted •Total syntheses of 4-methylthio-3-butenylisothiocyanate and raphanusanin.•MTBI and raphanusanin are phototropism-regulating substances of radish hypocotyls.•Mechanisms of phototropism: Cholodny–Went theory and Bruinsma–Hasegawa theory. Total syntheses of 4-methylthio-3-butenylisothiocyanate (MTBI, 1) and raphanusanin (2), which are growth inhibitors involved in the phototropism of radish (Raphanus sativus) hypocotyls, were achieved from commercially available thiolane in four and five steps, respectively. MTBI (1) was prepared via the ring opening of thiolane with sodium azide, double bond introduction by oxidation, and isothiocyanation. Cyclization of MTBI to raphanusanin (2) was achieved under mild conditions using sauer Al2O3. Synthetic MTBI (1) and raphanusanin (2) showed potent bioactivity in a unilateral administration test and a segment elongation test using radish hypocotyls.