Complete Assignments of 1H and 13C NMR Chemical Shift Changes Observed upon Protection of Hydroxy Group in Borneol and Isoborneol and Their DFT Verification.

Complete assignments of the 1H and 13C NMR chemical shifts for the monoterpenes, borneol 1a and isoborneol 2a, as well as their derivatives (1b-1g and 2b-2g), in which the secondary hydroxy group is protected with various protecting groups, have been made in various solvents. Upon protection of the hydroxy groups in 1a and 2a, many protons and carbons within the bicyclic ring exhibited downfield or upfield shifts in their chemical shift values, facilitating the unambiguous assignments of these protons and carbons. These chemical shift values also showed excellent correlations with those obtained from density functional theory (DFT) calculations. Furthermore, the anisotropic effect of the benzene ring was estimated by the analysis of the iso-chemical shielding surface (ICSS) resulting from substituents introduced to the hydroxyl groups of 1a and 2a.