N-methylated products of the Solanum steroidal alkaloids tomatidine and solasodine

Abstract

The Solanum steroidal alkaloids tomatidine and solasodine contain a spiro-ring junction with azaketal functionality. The conversion of the natural products to their N-methylated derivatives involves intermediates in which the formerly spiro-carbon (C-22) is sp² hybridized, and therefore the stereochemical information at this centre is lost. ¹³C N.m.r. analysis is used to show that methylation of tomatidine (22S,25S) results in one (22S,25S) product, while the same treatment on solasodine (22R,25R) affords two isomers that can equilibrate in solution, with 22R,25R(major) and 22S,25R(minor) stereochemistry. The ¹³C n.m.r.-derived conformations of the products suggest an explanation for these results.