The Synthesis and the Stereochemistry of 4-Chromanones and 4-Chromanols with Bulky Substituents

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Abstract
The synthesis and the stereochemistry of 4-chromanones and 4-chromanols with bulky substituents have been investigated. In 3-substituted chromanones, the t-butyl group exists mainly in the quasi-axial position, while the phenyl group is still in the quasi-equatorial environment. The preferred conformations of the isopropyl, and of the t-butyl group in cis- and trans-2-substituted chromanols, are equatorial, whereas trans-3-t-butylchromanol and its benzoate exist mainly as the conformations in which the t-butyl group is in the axial position. The conformational preference of these compounds can reasonably be explained by considering the steric interactions (A(1,3) strain, gauche, and diaxial repulsion) of these groups. The magnitude of the A(1,3) strain due to the hydroxyl group was estimated to be 1.3 kcal/mol.