Relative Stabilities of the Desmotroposantonins

Article abstract of DOI:10.1021/jo00389a045

Equilibration of (-)-α-desmotroposantonin methyl ether (6), (+)-β-desmotroposantonin methyl ether (7), and isohyposantonin (8) with K2CO3 in xylene gives the same 56:44 mixture of isomers at C-11.Although acid-catalyzed isomerization of (-)-α-desmotroposantonin (2) affords (+)-β-desmotroposantonin (3) in good yield, the deoxy analogue of 2, isohyposantonin (8), gives an approximately 1 to 1 mixture of 8 and β-desmotroposantonin analogue (11) with acid 10 as the major product.These results indicate that the published data which indicate that the β-isomers are significantly more stable than their α-epimers are based on reactions in which equilibrium was not reached.NMR studies at 200 MHz show that the conformation of the lactone ring in the α- and β-isomers is not the same.A conformation is suggested for α-desmotroposantonin on the basis of the NMR data, and an explanation is offered for the stability relationships in the desmotroposantonin series.