Correlation of molecular mechanics and NMR data: Conformation and configuration of the Meerwein lactones

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Abstract
A method for relating averaged NMR parameters with point‐by‐point geometries for MM2 calculations is given for mobile molecules, e.g. δ‐lactones. This method permits the identification of cis/trans geometry in cases where a complete set of isomers is not available. 2‐Oxa‐3r,4c,5t‐triphenylcyclohexanone is prepared by condensation of 1,2‐diphenylethanone with 3‐phenylpropenal. The initially formed formed biscarbonyl adduct undergoes an internal Cannizzaro reaction forming a hydroxy ester, which lactonizes during work‐up. The stereochemistry of the hydride transfer in the Cannizzaro reaction is consistent with the Felkin rule for asymmetric induction. The configurations of this and related lactones were studied through observation of NMR coupling constants, reductions to hemiacetals and molecular mechanics calculations. The most stable conformations are found to be a boat form for the parent lactone and a half‐chair form for two isomers. The half‐chair and boat forms lie near in energy for all isomers. The calculation techniques outlined illustrate that it is incorrect to consider only chair and boat forms; many intermediate conformations exist. Difficulties with previous IR methods of analysis are discussed.

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