Prostephanaberrine (1), a new hasubanan alkaloid, was isolated from the fresh fruits of Stephania japonica. The new alkaloid and its acetate 2 were converted into the metaphanine-type base stephanaberrine (3). The reaction sequence and spectral data showed that the structures of 1 and 3 are related to prometaphanine (7) and metaphanine (10). © 1986, American Chemical Society. All rights reserved.