7 ベンゾエートセクター則

抄録

Brewster's benzoate rule is widely employed to determine the absolute configuration of cyclic sec-alcohols, but is not applicable to certain types of compounds, e. g., those in which the carbinyl carbon is flanked by two methylene groups (No. 4 in Table 1), and those in which one of the carbons adjacent to the carbinyl carbon is the bulkier and also the more polar (Fig. 3). I. The Benzoate Sector Rule We have found that the strong Cotton effect of benzoates due to π-π^* intramolecular charge transfer transition at ca 225mμ permits one to predict the absolute configurations of various cyclic sec-hydroxyl groups including those mentioned above. The sectors are set up as shown in Fig. 1. The preferred conformation of the benzoyloxy group is assumed to be the one in which it lies staggered between the carbinyl hydrogen and the smaller substituent. The benzoate is viewed from the para-position and the rotatory contribution of α,β- and β,γ-bonds are considered. The bonds falling in the shaded sectors in Fig. 1 make a positive contribution to the sign of the 230mμ Cotton effect. The contribution of a double bond would be larger than that of a single bond because of the larger polarizability. Pertinent examples of the application of the benzoate sector rule, mainly from the steroid field, are listed in Table 1. This sector rule is corroborated by simple LCAO MO calculations. II. Optical Rotatory Power of the Benzoate Group The fact that the benzoate Cotton effect was assigned to the π-π^* intramolecular charge transfer transition was ascertained by observing the red shift caused by para-substituents. (Table 2). This is of practical value since the benzoate absorption can be shifted to longer wavelengths when results are ambiguous because of overlap of the benzoate and substrate absorptions (No. 15, Table 1). III. Optical Rotation of α-glycol dibenzoates Dibenzoates show two Cotton effects of opposite sign centered at 225mμ, in which the sign located at the longer wavelength depends on the chilarity of the glycol. (Table 3).