Conformational study of oxacyclononan-2-one by dynamic NMR spectroscopy and molecular mechanics calculations

The nine-membered lactone, oxacyclononan-2-one (1), was found by low-temper ature H-1 and C-13 NMR spectroscopy to have two conformations with populati ons of 26.(9) and 73.(1)% at -161.0 degrees C. Free-energy barriers of 6.41 and 6.19 kcal/mol for interconversion of conformations at - 140.1 degrees C were calculated from the C-13 spectrum of the carbonyl carbon. The H-1 NM R spectrum of the CH2O protons splits into two bands of equal intensity at low temperatures, due to slowing of the exchange of geminal hydrogens, and the free-energy barrier for this process was 6.68 kcal/mol at coalescence ( - 122.6 degrees C). The NMR results for the compound are discussed in terms of the conformations predicted by Allinger's MM3 program. (C) 1999 Elsevie r Science B.V. All rights reserved.