The Electrochemistry of Biologically Active 2-Hydroxy Naphthoquinones: The Role of Hydrogen Bonding. Reductive Silylation and Acetylation, Useful Electrochemical Transformations
2-Hydroxy-3-alkenylnaphthoquinones showed typical voltammograms in aprotic medium (DMF/TBAP 0.1 M). They are represented by two pair of peaks with reversible or quasi-reversible nature, depending on the position of the double bond, and one or more intermediate shoulders. The first pair of peaks, in lapachol, corresponds to a quasi-reversible monoelectronic transfer, forming a semiquinone strongly stabilized by intramolecular hydrogen bonding, contrary to the reported proton reduction of one of the possible tautomeric forms of the hydroxyquinones. High yield reductive acetylation was obtained by co-eletrolysis of lapachol with acetic anhydride, at the potential of the first wave, on mercury or platinum electrodes. The reduction process depends on the nature and concentration of proton sources. Silyl ethers, key intermediates for modification of heterocyclic quinones, were prepared through electrolysis at Epc2, specially in THF. Triacetyldihydrolapachol showed a significant trypanocidal activity.
KeywordsIntramolecular Hydrogen Bonding Acetic Anhydride Silyl Ether Proton Source Aprotic Medium
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