A minor alkaloid was isolated from Crotalaria assamica, and its structure was determined by spectroscopic methods as the 9-O-(y-lactone) of(2',3'dimethyl-4'-hydroxypentane-2',3'dicarboxyl)-retronecine [1]. The new alkaloid was named assamicadine. As we reported previously, monocrotaline has been isolated from Crotalaria assamica Benth. (Leguminosae), which occurs in the southern parts of China (1,2). The seeds of C. assamica yield about 3.5% of monocrotaline. During the further investigation of the seeds, another new pyrrolizidine alkaloid has been isolated which has been named assamicadine. This alkaloid occurs only in traces. Its structure 1 as the 9-O-(y-lactone) of (2', 3'-dimethyl-4' hydroxy-pentane-2 ' , 3 '-dicarboxy1) retronecine was determined by its physical and spectroscopic properties.The ir spectrum of 1 showed characteristic signals for a free hydroxyl group at 3373 cm-', a saturated ester at 1736 cm-l, and a y-lactone structure at 1778 cm- '. The eims spectrum had a molecular ion peak {MI+ of m/z 309 (4%), which can be correlated to the formula C16H23NO5. Characteristic peaks appear at m/z 266, 265, 256, 211, 155, 139, 138, 137, 136, 111,94,93,80.This fragmentation indicated the presence of the necine, retronecine, or one of its isomers. Alkaline saponification yielded a necine, whose spectroscopic data verify the structure to be that of retronecine.The characteristic intensities of the ms fragments at m/z 136, 137, and 138 support the presence of a C-9-monoester structure (3). The 'H-nmr chemical shift of H-7 at 4.46 ppm is an indication for the presence of a free hydroxyl group in position 7. The DEPT spectrum of 1 gave the signals of five CH, four CH, and three Me groups. The exact classification of the 'H-nmr signals was made by the 'H- 'H-correlation spectrum. The 2D NOESY of assamicadine was used to determine the relative configuration of the molecule. The cross-signals in this spectrum showed an nOe effect indicating that H-2' is sterically near to the methylprotons of Me-1". The same is true for H-4' and Me-4". In addition, it was indicated that H-2' and Me-2" are on the same side of the ring plane of the lactone {la and lb}. In the cd spectrum we observed a negative Cotton effect at 203 nm. The rule of Klyne and Beecham (5,6) concerning y-lactones suggested that the p carbon lies under the ring plane of the y lactone. The ester carbonyl group in position 3' is either pseudo-axial or pseudo-equatorial. H-2', H-4', and Me-2" are on the same side of the lactone ring system. The relative configuration of the lactone is either 2'S, 3'R, 4'R [la] or 2'R, 3'S, 4'S [lb].