Biphenyl active configurations

In both the biphenyl case and the benzyl case the aromatic ring is free to rotate and if sufficient energy is available at the active site the molecule could simply twist to an active configuration. However, this would not be true if the 2- and 2 -positions of the biphenyl alcohol were bridged with an alkyl chain where the conformation is fixed within narrow limits that depend on the length of the chain (17). ... 

The molecules most profitably studied in connection with purely steric isotope effects have been isotopically substituted biphenyl derivatives. Mislow et al. (1964) reported the first more or less clearcut example of this kind in the isotope effect in the configurational inversion of optically active 9,10-dihydro-4,5-dimethylphenanthrene (7), for which an isotopic rate ratio ( d/ h) of 1-17 at 295-2°K in benzene solution was determined. The detailed conformation of the transition state is not certain in this case, as it involves the mutual passage of two methyl groups, and thus it is difficult to compare the experimental results with... 

Considerable interest has been shown in dibenz[c,e]azepines as examples of optically active 2,2-bridged biphenyls (68MI51600). The absolute configurations of some dibenz[c,e]azepines have been determined (b-77SH(2)305), and the CD spectra of the hydro-2-benzazepin-l-one (22) recorded (78CHE538). In contrast, to analogous five- and six-membered lactams, azepinone (22) has a (+) Cotton effect. 

Berson, J. A., and M. A. Greenbaum Asymmetric induction studies with optically active biphenyls. V. On the unreliability of absolute configurational assignments based on hydride reductions ofphenylglyoxylatcs. J. Amer. chem. Soc. 81, 6456 (1959). 

Optically active biphenyl derivatives (Section 13-5 A) are racemized if the two aromatic rings at any time pass through a coplanar configuration by rotation about the central bond. This can be brought about more or less easily by heat, unless the 2,2 -ortho substituents are very large. 

This first synthesis of triphyophylline (23) confirms its relative trans configuration of the two secondary methyl groups. A synthesis of this main Triphyophyl-lum alkaloid in optically active form, which is supposed to establish the chirality at the biphenyl linkage as well as the entire absolute stereochemistry, is under investigation in the author s laboratory. 

The optical activity of the tri- and tetra-orr/zo substituted biaryls (atropisomers) is a consequence of the chiral axis, present in this structure. The configuration R- or S-) of these compounds is matched according to the Cahn-Ingold-Prelog (CIP) system and an additional sequence rule Near groups precede far groups. An example with 2-chloro-6-methoxy-2 -nitro-6 -carboxy-l,T-biphenyl (9) is illustrative. Figure 2 ... 

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