Abstract
The chiroptical properties of an iridoid glycoside ajugol were fully investigated by quantum chemical calculations of specific optical rotation at the sodium D line ([α]D value), optical rotatory dispersion (ORD), and electronic circular dichroism (ECD) using time-dependent density functional theory (TDDFT). TDDFT calculations of the [α]D value and ORD of ajugol over the range of 365–633 nm were in good agreement with the experimental data. The predicted ECD spectrum of ajugol was also consistent with the experiment, showing a strong negative Cotton effect (CE) at around 190 nm and a weak positive CE at around 220 nm. Our results unambiguously determined the absolute configuration (AC) of the aglycone part of ajugol as (1S, 5R, 6R, 8S, 9S) and supported the generally accepted AC assignments of iridoid glycosides. Semi-empirical rule for the enol ether chromophore, basis set selection, and effect of glucose group on ECD spectra were also discussed in the case of ajugol.
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