Chiroptical spectra of bicyclic selenolactams. Optical activity of the singlet–triplet transition
摘要:
Two selenolactams with rigid 2-azabicyclo[2.2.1]heptane skeletons were prepared by reaction of the corresponding lactams with P4Se10. A comparison of their UV-vis and CD spectra with those of the carbonyl and thiocarbonyl analogues showed a similar character of the lowest-energy electronic transitions and the same signs of the corresponding Cotton effects. The MCD spectra of selenolactams revealed that their n-pi* absorption band is dominated by the singlet-triplet component. A very weak CD corresponding to this component has an opposite sign to its much stronger singlet-singlet counterpart observed at the blue edge of the n-pi* band. (C) 1999 Elsevier Science Ltd. All rights reserved.
Conformational properties and chiroptical spectra of lactams and thiolactams with 2-azabicyclo[2.2.1]heptane, 2- and 3-azabicyclo[3.2.1]octane skeletons
摘要:
The CD spectra of several bicyclic lactams and thiolactams were measured in different solvents. The concentration dependence of the spectra observed in hydrocarbon solvents was attributed to shifts in the equilibrium between monomer and hydrogen-bonded dimer forms. The CD of some compounds is characterized by unusually strong Cotton effects resulting from non-planarity of the amide bonds due to internal strain of the bicyclic skeletons. The X-ray crystallographic structures of 2a,c, 3b,d and 4a,b showed different degrees of distortion of the amide or thioamide moieties from planarity, which causes inherent chirality of the chromophores and profoundly affects the Cotton effect sign and magnitude. This distortion also restricts application of the sector rules for prediction of the n-pi* CD sign, since they can be used only for compounds with planar chromophores. (C) 1999 Elsevier Science Ltd. All rights reserved.