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The correlation between the absolute configuration and CD of aza-and thiaflavanone (1, 2) is discussed: the 2R configuration for the laevorotatory enantiomers has been established by a comparison of their CD data with those of ( S ) -( -) -3 . For the Cotton effect within the n + I[* band of 1 and 2 the same helicity rule is valid as for the homochirally analogous flavanones (3). Starting from (S)-( -)-flavanone (3) and cholesterol Recently, the optical resolution of azaflavanone[l3I (1) and thiaflavanone[I41 (2) has been described but the absolute configuration of their enantiomers has not been elucidated. Since the laevorotatory flavanones possess, without exception, S configuration it seemed to be also interesting to examine the validity of this empirical rule for their aza and thia analogs. In order to obtain chiroptical and stereochemical information we have carried out CD measurements in the UV absorption region of 1-3 in solvents of different polarity.(8-( -)-Flavanone ["] has been selected as a suitable reference compound, whose CD spectra have been investigated respectively. The lLb band of the chromane chromophore has been unequivocally identified in the range between 255 and 290 nm in the CD spectra 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 the 'Lb band. The modification of the helicity rule can be explained with the help of Petruska's "q values". in the range from 400 to 200 nm and compared with those of (-)-1 and (-)-2. In order to assign the origin of the Cotton effects of n + R* and 'Lb in the above-mentioned spectra unequivocally, we have synthesized the chromane derivatives 4 and 20 with known absolute configuration. In this paper we describe these syntheses and discuss the respective CD spectra. SynthesesThe synthesis of (S)-(-)-flavan (4) has been achieved by Clemmensen reduction [16] of the carbonyl group of (a-(-)-

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