Burns, Darcy; Reynolds, William F.; Buchanan, Greg O.; Reese, Paul B.; Enriquez, Raul G.
Author Affiliation, Ana.
Department of Chemistry
Assignment of 1H and 13C spectra and investigation of hindered side-chain rotation in lupeol derivatives
Magnetic Resonance in Chemistry
Date of Publication
Complete 1H and 13C 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 1H chemical shifts. It also shows line broadening of some 13C signals at 25°C, suggesting hindered rotation of the side-chain group. This is confirmed by two low-temperature spectra which shows splitting of broadened peaks into pairs in a ca 2 : 1 area ratio. The free energy of activation of hindered rotation is estimated as 13.5kcal mol-1. By contrast, 1a shows no evidence of hindered rotation down to –40°C although NOE data suggest the presence of two conformers. Spartan molecular mechanics calculations confirm the presence of two stable conformers for 1a and 1c but overestimate the rotational barrier in 1a . The additional barrier in 1c probably reflects loss of conjugative stabilization during rotation.....