Fig. 5. Conformers of urea 6 in CDCl3 and acetone-d6. In CDCl3, the major H-bonded urea 6a shows downfield shift of an NH proton, whereas the minor urea conformation 6b was observed in EXSY spectrum after NH irradiation. The higher population of 6b in acetone was the result of increased polarity and H-bond accepting ability of the solvent.4.
