Abstract
Mixtures of the diastereomers of 2,2,3,5,6‐pentamethylheptane were prepared in two ways, either starting with compounds of (3__R__)‐configuration, or from compounds of (5__R__)‐configuration. Comparison of the GC. and optical rotatory power of the fractions of these two mixtures permitted the unambiguous assignment of the absolute configuration and molar rotatory power to the various diastereomers ([M] = + 119.1° for the (3__R__, 5__R__)‐ and [M] = + 79.8° for the (3__R__, 5__S__)‐diastereomer). The very high molar rotatory power which was expected on the basis of the conformational analysis carried out with a rotational‐isomeric‐3‐states model is interpreted as arising from the molecular ‘conformational rigidity’, i.e. from the presence of only few conformers. Conformational properties of these compounds were computed using a new approach, which scans the whole space of each bond (2 π) in 5° steps and calculates the conformational energy based upon semiempirical potential functions. The conformational flexibility of each bond of the two diastereomers is evaluated in terms of the a priori probability density function of that bond. This allows us to analyze in detail how configurational differences affect conformational properties. The molar rotatory power of the two diastereomers as calculated with a new method recently developed in our group is in excellent agreement with experimental data. The molar rotatory power is analyzed in terms of the contribution of the single bonds.