Study on absolute configurations of α/α′ chiral carbons of thiodiketopiperazines by experimental and calculated circular dichroism spectra

We report here new investigations of the CD of TDKPs (including new structural types). Some unprecedented rules between ACs and CD spectra are deduced from the studies of experimental and calculated CD of them as follows: (1) the CD band around 270 nm could be applied to determine the ACs of the chiral carbons at α/α′ positions in TDKPs. (2) The intensities of CE around 270 nm followed the decreasing trend: disulfide bridges >S-methyl groups> single sulfur bridges. (3) While the sulfur atom attached to the α position of the saturated cyclohexanone in the axial bond form, the CE in the 300 nm can be used to determine the AC of the chiral carbon at the α position of the six-membered ring. (4) The apparent violation of the skewed diene rule in 18 may be due to a strong interaction of the diene, and the S-methyl group and sulfur bridge.