Roemeria hybrida (L.) DC. (Papaveraceae) of Turkish origin has yielded the new prosporphine alkaloids (-)-roemerialinone (3), (-)-isoorientalinone (4), (-)-isoroemerialinone (5), (-)- 11,12-dihydroorientalinone (6), (+)-8, 9-dihydroisoroemerialinone (9) and (-)-α-roemehybrine (11). Catalytic reduction of (-)-isoorientalinone (4) led to (+)-8, 9-dihydroisoorientalinone (8) which corresponds to the partly characterized "(+)-dihydroorientalinone" originally obtained from Papaver orientale. The previously known (-)-roehybrine (10) of undetermined structure was also reisolated, and its structure was elucidated. Known proaporphines present are (-)-mecambrine (1) and (-)-orientalinone (2). The isolation of such pairs of diastereomeric proaporphines as 2 and 4, and 3 and 5, points to the fact that enzyme catalyzed intramolecular oxidative coupling of a specific tetraoxygenated tetrahydrobenzylisoquinoline may occur in either of two modes, depending upon the folding of the pendant benzylic ring. © 1987.