Resonance stabilization methoxyl group

Resonance forms show that the methoxyl group effectively stabilizes the sigma complex if it is ortho or para to the site of substitution, but not if it is meta. Resonance stabilization is provided by a pi bond between the —OCH3 substituent and the ring. 

Why is it that only the hydroxyl group at the anomeric carbon is substituted by a methoxyl group in this reaction The first step is protonation of one of the many OH groups (Rg- 22.39). All the OH groups are reversibly protonated, but only one protonation leads to a resonance-stabilized cation when a molecule of... 

The difference (280 mV) in E1/2 for the two compounds can be attributed to intramolecular H-bonding of the hydroxyl group in la. This interaction stabilizes the resultant radical anion. In lb the resonance effect of the methoxyl group hinders reduction, and H-bonding is eliminated. Data for several related naphthoquinones assist in elucidating important chemical effects. Diospyrin reduces at a value close to that (-0.15 V) reported for the anticancer 5-hydroxy 1,4-naphthoquinone... 

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