Structural and chiroptical analysis of naturally occurring (–)-strychnine

Abstract

Structural aspects such as chemical exchange, dimerization, solvent association, nitrogen inversion and protonation status of strychnine were investigated using experimental and calculated data. The information was mainly interpreted in view of a successful determination of the absolute configuration (AC) with strychnine (base and salt) as test molecule due to its importance in chemistry. By geometry optimization a stable isomer of protonated strychnine was found with an inverted nitrogen, however, 25 kcal/mol higher in energy. It is shown that solvent association can be assumed in protic solvents such as methanol and dimerization to a small extent in polar/protic solvents. However, the monomeric structural model neglecting explicit solvent molecules still allows the correct prediction of the AC of base and hydrochloride using optical rotation and ECD data.