Lead(IV) ester and electrochemical oxidations of phenolic compounds: a comparative study
Oxidation of 3-acetyl-1 -propyl-5,6,7,8-tetrahydro-2-naphthol (6) with lead(IV) acetate or lead(IV) benzoate is shown to produce almost entirely the o-quinol esters, (8) and (20), respectively. By contrast, electrochemical oxidation of (6) in acetic acid at a platinum anode leads largely to the corresponding p-quinol acetate (7) in addition to (8), and also to other products [(10) and (11)]. Structure (8) was assigned unambiguously from a two-dimensional homonuclear shift-correlated 13C spectrum obtained by using a symmetrised INADEQUATE program, and an X-ray crystal structure determination was carried out on the benzoate (20). Treatment of the o-quinol acetate (8) with acetic anhydride containing sulphuric acid leads to the diacetate (9a), whereas dissolution of the p-quinol acetate (7) in dichloromethane containing ethanolic sulphuric acid produces the phenol (12). Saponification of either (12) or (9a) gives the 2′,6′-dihydroxyacetophenone (9b), a useful precursor to proxicromil (2).