CIRCULAR-DICHROISM AND ABSOLUTE-CONFIGURATION OF AZAFLAVANONES AND THIAFLAVANONES

The correlation between the absolute configuration and CD of aza- and thiaflavanone (1, 2) is discussed: the 2R configuration for the laevor otatory enantiomers has been established by a comparison of their CD d ata with those of (S)-(-)-3. For the Cotton effect within the n --> pi band of 1 and 2 the same helicity rule is valid as for the homochira lly analogous flavanones (3). Starting from (S)-(-)-flavanone (3) and cholesterol (S), we have synthesized the chromane derivatives 4 and 20 , respectively. The 1L(b) band of the chromane chromophore has been un equivocally identified in the range between 255 and 290 nm in the CD s pectra of 4 and 20. For this Cotton effect the opposite helicity rule is valid as compared with the homochirally analogous tetralins: the P/ M helicity of the hetero ring leads to a negative/positive CD within t he 1L(b) band. The modification of the helicity rule can be explained with the help of Petruska's ''q values''.