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The stereochemistry of the enzymic conversion of rot-2'-enonic acid into deguelin, mediated by deguelin cyclase, has been studied. Using both an enzyme preparation and seedlings of Tephrosia voge//ii, it is shown that (6aS,12aS,5'R/S)-5'hydroxy-4',5'-dihydro[6',6'-C3H3]deguelin is not an acceptable intermediate: no evidence for other oxygenated intermediates was found.The (pro-R)and (pro-S) -6'-methyl groups of deguelin were identified by synthesis from [ (€)-4'-13C] rot-2'-enonic acid. Addition of benzeneselenenyl chloride gives two diastereoisomeric 5'-(phenyl selenides) of 4',5'-dihydrodeguelin which are separated and their stereochemistry established by Xray crystallography. Elimination of selenoxide from the (5's) -stereoisomer then gives (6'R) -deguelin (6,28.20): (6'S)-deguelin has S, 28.52.Although a chemical conversion of [4'-13C]rot-2'-enonic acid into labelled deguelin produces a 1 : 1 distribution of label between the (pro-R)and (pro-S) -6'-methyls, the enzyme-mediated conversion results unexpectedly in a 76% incorporation into the (pro-R)-and 24% into the (pro-S)-form. The stereochemistry of the removal of the key 1 'hydrogens in rot-2'-enonic acid was therefore examined.