Assignment of H-1 and C-13 spectra and investigation of hindered side-chain rotation in lupeol derivatives

Complete H-1 and C-13 spectral assignments are reported for lupeol (1a) and two derivatives where the C-30 methyl group is replaced by CH2OH (1b) and HC=O (1c). Compound 1c shows conformationally dependent substituent effects on H-1 chemical shifts. It also shows line broadening of some C-13 signals at 25 degrees C, suggesting hindered rotation of the side-chain group. Thi s is confirmed by low-temperature spectra which show splitting of broadened peaks into pairs in a ca 2:1 area ratio. The free energy of activation of hindered rotation is estimated as 13.5 kcal mol(-1). By contrast, 1a shows no evidence of hindered rotation down to -40 degrees C although NOE data su ggest the presence of two conformers. Spartan molecular mechanics calculati ons confirm the presence of two stable conformers for 1a and 1c but overest imate the rotational barrier in 1a. The additional barrier in 1c probably r eflects loss of conjugative stabilization during rotation. Copyright (C) 20 00 John Wiley & Sons, Ltd.