Conformational Cycloenantiomerism in l,2-Bis(l-bromoethyl)-3,4,5,6-tetraisopropylbenzene

The concept of cycloenantiomerism is critically reexamined. It is shown that suitably substituted derivatives of hexaisopropylbenzene (1) illustrate a novel type of stereoisomerism, in which enantiomers are distinguished by the sense of conformational orientation of side chains for a given configurational distribution pattern of stereocenters. Such isomers, which may be described as conformational cycloenantiomers, have now been synthesized for the first time. Photobromination of l,2-diethyl-3,4,5,6-tetraisopropylbenzene yields a mixture of the two diastereomeric l,2-bis(l-bromoethyl)-3,4,5,6-tetraiso-propylbenzenes (2), from which the (1'RS,2'SR) isomer (2a) is isolated by HPLC. This isomer is a racemic mixture of conformational cycloenantiomers. VT-NMR measurements yield a lower limit of AG* = 24 kcal mol^-1for the enantiomerization of 2a.