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Why (1S)-Camphanates Are Excellent Resolving Agents for Helicen-1-ols and Why They Can Be Used to Analyze Absolute
1Department of Chemistry, Columbia University, New York, New York 10027, and Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716.
Abstract:
The questions considered in this paper are why, as agents for resolving helicenols, camphanate esters are particularly effective, and why, in all 19 examples studied, when the (1S)-camphanates of (P)- and (M)-helicen-1-ols are chromatographed on silica gel, the former has the lower R(f)(). Models are proposed for the favored conformations of the esters, and to support the models, evidence is provided from five X-ray diffraction analyses and four ROESY analyses supplemented by molecular mechanics calculations. The essential discovery is that, presumably to avoid a steric interaction between a methyl on the camphanate's bridge and the helicene skeleton, the O=CCO conformation 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.