The hydrogens of the OCH2 group are enantiotopic in A, but
diastereotopic in B. Thus in A, these hydrogens are not distinguished by
NMR and appear as a quartet due to coupling to the adjacent CH3
group. In B however, the OCH2 hydrogens will have different
chemical shifts and will couple to one another. Thus each of the OCH2
hydrogens will give a doublet of quartets (eight lines), sixteen lines
in total.
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