The 1H NMR spectrum of 1a exhibits signals at 3.74 and 3.39 ppm, representing the positions C1 and C3, respectively, while the protons at positions C2 and C4 produce signals at 5.63 and 5.52 ppm, respectively. Explain why the protons are grouped this way, and why there is such a large difference in chemical shifts between the two groups of protons