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Reaction of nopinone (1) with 3-chloroperbenzoic acid in CHCl3 leads, in a very slow reaction, to the two lactones 2 and 3 in the ratio 9:1. Methylnopinone (4) under the same conditions gives the homologous lactones 6 and 7, but in the reverse ratio, together with minor amounts of the stereoisomers 8 and 9 arising from the presence of a small amount of the trans-isomer in the starting material. For the successful isolation of some of these lactones, it is essential to remove all traces of EtOH from the solvent CHCl3.