Optical Activity in Adamantane Derivatives

The four bridgehead positions of adamantane are formally analogous to the four tetrahedral valences of carbon. Adamantanes with four different bridgehead substituents are, therefore, chiral. 

In general, small specific rotations should be expected for such adamantane derivatives since a large reduction in optical rotatory power should occur when pairwise interactions are greatly reduced by distance. The resolution of several tetrasubstituted adamantanes (62-65) has been attempted 178 179 247f Mes-ured rotations, as expected 4 are quite low. Only 65, the formal analogue of lactic acid, has been proven to be optically active 178 179f by a confirmatory approach. 

While a small specific rotation ( 1 °C) may be observed for this compound (optical purity unknown) l79 conversion of 65 to l-methyl-3-methylene-7- 

Quantitative conclusions regarding the reduction of optical rotatory power of these derivatives by the increased distance of the pairwise interactions must, of course, await optical purity determinations. Qualitatively, however, expectations are obviously confirmed 179  

Since the adamantane ring system is composed of three interlocking cyclohexane rings, all in the perfect chair conformation, /3-sub stituted adamanta-nones are also ideal models for establishing quantitative substituent contributions to the optical rotatory dispersion of cyclohexanones in the undisturbed chair form. A variety of optically active /3-equatorial and /3-axial substituted adamantanones have been synthesized 18°-184) and their circular dichroism determined 184-185) jji generai) axial polar substituents (C02R, Cl, 

---