Chapter 22, Problem 45P

The ${}^{1}H$ NMR data for the two anomers included very comparable peaks in the $\delta$ 2.0–5.6 region but differed in that, as their highest $\delta$ peaks, anomer V had a doublet at $\delta$ 5.8 (1H, $\text{J}=12$ Hz) while anomer VI had a doublet at $\delta 6.3\left(1H,\text{ J}=4Hz\right)$.

(a) Which proton in these anomers would be expected to have these highest $\delta$ values?

(b) Why do the signals for these protons appear as doublets?

(c) The relationship between the magnitude of the observed coupling constant and the dihedral angle (when measured using a Newman projection) between C−-H bonds on the adjacent carbons of a C−-C bond is given by the Karplus equation. It indicates that an axial–axial relationship results in a coupling constant of about 9 Hz (observed range is 8–14 Hz) and an equatorial–axial relationship results in a coupling constant of about 2 Hz (observed range is 1–7 Hz). Which of V and VI is the αa anomer and which is the $\beta$ anomer?

(d) Draw the most stable conformer for each of V and VI.