Detection of boat conformations in the triterpene friedelin by methyl-to-methyl nuclear overhauser enhancements
All the methyl-proton resonances of friedelin (1) have been assigned by n.O.e.-difference spectroscopy. The observation of substantial enhancements between Me-26 and Me-28, and between Me-28 and Me-30 can be explained by invoking boat conformations for rings D and E, in agreement with the crystal structure of a related compound. Methyl assignments were independently confirmed by lanthanide-shift experiments. Methyl spin–lattice relaxation times reflect steric crowding by other methyl groups and may provide a useful assignment tool.