Labile atropisomers

Atropisomers are a particular class of conformers that are enantiomers (with the same energy and all the characteristics of enantiomers such as not being superimposable mirror images, opposite ORD, ECD, and VCD) and that can be interconverted through a suitable process (see below). We are of the opinion that the occurrence of a stereogenic element is more important than the interconversion barrier, the limit of which is very variable (see cryo-chiral chromatography) and completely arbitrary. We think the suitable term should be isolable atropisomers in opposition to labile atropisomers when the interconversion is too fast for isolation. 

In each mesophase mixture, the dominant atropisomer after photo or thermal resolution experiments was S(+). The sole exception occurred during irradation of a 3% BN solution in mixture C. Since the sample solidified partially during the experiment, the mechanism by which the R(-) atropisomer arose is unclear. The thermal lability of the atropisomers toward interconversion and the possible contribution of cholesteric contaminants in recovered BN samples make an accurate assessment of atropisomeric excess a formidable task. Extreme care was taken to handle all solutions containing BN during work-up at temperatures which preclude significant thermal racemization at 25°C, the half-life for racemization is ca. 10 h in normal isotropic solvents all manipulations were conducted at 4°C or below. 

The main difficulties associated with constructing the biaryl moiety as a single atropisomer early in the synthesis lies within the arene substitution pattern. The thermal isomerization barrier for a steganone precursor that carries only three ortho substituents is extremely sensitive to the exact identity of those substituents. Specifically, when one of the three groups (e.g., a formyl moiety) adds only a small amount to the inversion barrier and a second substituent (e.g., a methoxy group) is also relatively small, the barrier to inversion renders the biaryl stereochemically labile even at 0 For example, at least one of... 

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