Three castasterone (CS) derivatives with varied side-chain moieties, 26,27-bisnorcastasterone (20S-bisilorCS), 20-epi-26,27-bisnorcastastcrone (20R-bisnorCS), and 21,26,27-trisnorcastasterone (trisnorCS), were synthesized stereoselectively from either stigmasterol or dehydroisoandrosterone. The 50% effective doses (ED50, nmol/plant) in the concentration-response Curve for brassinolide-like activity in the rice lamina inclination assay were determined to be 0.020 nmol (pED(50) = 10.7) for 20S-bisnorCS, 3.2 nmol (pED50 = 8.5) for 20R-bisnorCS, and 2.0 nmol (pED50 = 8.7) for trisnorCS. An analog containing an ester linkage between the steroid and the side-chain moiety of 20S-bisnorCS was also synthesized and its activity was evaluated to be 3.2 nmol (pED50 = 8.5), being equipotent to 20R-bisnorCS and trisnorCS. The activity of 20S-bisnorCS was 1/40 that of CS. The conformation analysis was conducted using a systematic search, showing that the activity decreases with an increase in the degree of freedom of the side chain of the steroidal skeleton. (c) 2005 Elsevier Ltd. All rights reserved.