Optical Rotatory Dispersion and the Twist Conformation of Cyclohexanones.

The Octant Rule1 provides a theoretical basis for the study of optical rotatory dispersion curves of ketones. The application of this rule to extensive collections of data for cyclohexanone types from our two laboratories has been described elsewhere.2 3 Among the hundreds of curves recorded, a few stand out by reason of their unusually large amplitudes (positive or negative), which cannot be rationlized even in a roughly quantitative fashion by the usual octant treatment as applied to an all-chair conformation. Recently,4-8 there has been much interest in the existence of boat and modified boat conformations of cylohexane rings. Several authors9-11 have discussed forms intermediate between chairs and boats but the discussion has often been in rather general terms. However, Johnson et al.11 have recently provided thermodynamic evidence for the existence of an intermediate "twist" form in a lactone derived from trans-decalin. It is now suggested that this twist conformation provides a possible explanation of some of the grossly abnormal amplitudes found in decalone derivatives; the concept has also been used in dealing with some monocylic compounds.12 The twist conformation consists of two sets of contiguous coplanar C-atoms, 2, 1, 2' and 3, 4, 3'; C-1 and C4 are on the line of intersection of the two planes and may be called the "points" of the twist form. The special properties of the twist for our present purposes are those of compounds in which the carbonyl carbon is one of the "points" (as in perspective drawing I). Twist conformations in which the cart bonyl carbon is in another position are less dramatic in their chirality and are dealwith briefly later (cf. VIIIA).