Why (1S)-camphanates are excellent resolving agents for helicen-1-ols and why they can be used to analyze absolute configurations

The questions considered in this paper are why, as agents for resolving hel icenols, camphanate esters are particularly effective, and why, in all 19 e xamples studied, when the (1S)-camphanates of(P)- and (M)-helicen-1-ols are chromatographed on silica gel, the former has the lower Rf. Models are pro posed for the favored conformations of the esters, and to support the model s, evidence is provided from five X-ray diffraction analyses and four ROESY analyses supplemented by molecular mechanics calculations. The essential d iscovery is that, presumably to avoid a steric interaction between a methyl on the camphanate's bridge and the helicene skeleton, the O=CCO conformati on is anti-periplanar in (M)-helicenol camphanates and syn-periplanar in (P )-helicenol camphanates. In the former, the lactone carbonyl points toward the helicene ring system, and in the latter, it points away.