Application of the exciton chirality method for the stereochemical assignment of remote groups. Determination of the α- or β-configuration of a 2-hydroxy-substituent in 2,17- and 2,16-dihydroxy-A-nor-5α-androstane and -estrane derivatives

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Abstract
The c.d. of dibenzoate and bis-p-methoxybenzoate derivatives of 2,17- and 2,16-dihydroxy-A-nor-5α-androstanes and -estranes has been shown to arisa principally from the coulombic coupling of the long-axis polarized excitation moments of the two benzoate groups. Analysis of the c.d. spectra in thelight of the excitation chirality method allows an immediate determination of the α- or β-configuration of the 2-OH group, providing that the configuration of the 17- or 16-OH group is known. The c.d. spectra are qualitatively unaffected by the presence of additional ethynyl or methyl substituents in the molecule. The sign of the lower-energy component of the excitation c.d. couplet is given by the sign of the torsion angle θ= O(2)–C(2)–C(17)–O(17), or O(2)–C(2)–C(16)–C(16), the values of which can be deduced from X-ray data or Dreiding models. In spite of the weak coupling, due to the remoteness of the two interacting chromophores, observed and calculated rotational strengths are in fairly satisfactory agreement.

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