Evidence for the structure, and for a non-chair conformation in ring F of two isomeric N-formylsolasodines

Abstract

Formylation of solasodine results in the formation of two different 3,N-diformylsolasodines whose isomerism is due to a difference in stereochemistry at C22 rather than restricted rotation about the C=N partial double bond of the amide or nitrogen inversion as previously proposed. Mass spectra, 1H and I3C n.m.r. spectra have shown the isomers to possess 22R,25R (major) and 22S,25R (minor) stereochemistry. The 22S,25R isomer is thermodynamically preferred at elevated temperatures while the 22R,25R isomer is kinetically favoured upon recyclization of the ring-F opened intermediate which forms upon heating of either isomer. A non-chair ring-F conformation is proposed for each isomer on the basis of 13C and 1H n.m.r. spectra.