18 proton nmr are significantly different. That of 7b appears at 0.6 ppm, considerably shielded with respect to the absorption observed in 7a (1.1 ppm). Compound 7b undergoes a deuterium for proton exchange at C3 when a sample in CDCla is shaken with DzO (requiring 48 hr), a reaction similarly reported for 5.2 The remaining methine proton, on C,, then appears as a doublet of quartets at 6 3.4 ppm with a vicinal PH coupling constant of ...
