Restricted Rotation Involving the Tetrahedral Carbon. XVII. Isolation of Three “Diastereomeric” Rotamers of a Triptycene Type Compound and Stereoselectivity in the Synthesis

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Abstract
Three rotameric forms of 2,3-dichloro-9-(1,1-dimethyl-2-phenylethyl)-11,12-bis(methoxycarbonyl)-9,10-dihydro-9,10-ethenoanthracene (3) were isolated as stable entities at room temperature. The structural assignment of the rotamers based on the PMR characteristics. Since the synthesis of 3 produced only the ap form, another reaction involving benzyne was carried out to test the generality of the stereoselectivity. Thus the ap form and the sc form of 2,3-dichloro-9-(1,1-dimethyl-2-phenylethyl)triptycene (4) were prepared separately. Equilibration of isomers of 4 was carried out and the barrier to isomerization was obtained as 36.6 kcal/mol.