Conformational Rigidification via Derivatization Facilitates the Determination of Absolute Configuration Using Chiroptical Spectroscopy:  A Case Study of the Chiral Alcohol endo-Borneol

We demonstrate that derivatization of the OH group of endo-borneol, 1, leads to conformational rigidification. Conformational analysis (CA) of 1 and its methyl, acetate, tert-butyl, and trimethylsilyl derivatives, 2−5, is carried out using ab initio density functional theory (DFT). The number of thermally accessible stable conformations is reduced from 3 in 1, to 2 in 2, and to 1 in 3−5. Comparison of IR and vibrational circular dichroism (VCD) spectra of 1 and 3−5, calculated using DFT, to experimental spectra unambiguously confirms the DFT CA. The determination of absolute configurations (ACs) of chiral molecules via analysis of chiroptical spectra using DFT methods increases in complexity and decreases in reliability as the number of populated conformations increases. Our results for endo-borneol support the conclusion that, in the case of chiral alcohols, derivatization can lead to substantial rigidification and, as a result, significantly facilitate the determination of ACs.