Two new compounds from the electroreduction of hypostictic acid

Abstract

The electrochemical reduction of hypostictic acid ( 1) (1,4-dihydroxy-10-methoxy-5,8,11-trimethyl-1H-benzo[ e]furo[3 ′,4 ′:3,4]benzo[ b][1,4]dioxepine-3,7-dione) on glassy carbon electrodes was studied for the first time in DMF by cyclic voltammetry, rotating disc and ring electrodes, and long-term controlled-potential electrolysis. Parameters involving data from cyclic voltammetry and the rotating disc electrode, such as E p1 vs. log ν, E p/2,1− E p1 vs. log ν, α app vs. log ν, and E 1/2 vs. log ω, have shown behavior that can be correlated to a stepwise dissociative electron transfer. The reduction mechanism of 1 proceeds through a disproportionation step followed by the absorption of a proton from 1 (self-protonation step), producing hypostictinolide ( 4) (4-hydroxy-10-methoxy-5,8,11-trimethyl-1H-benzo[ e]furo[3 ′,4 ′:3,4]benzo[ b][1,4]dioxepine-3,7-dione). The overall reaction is 2 RHX + 2e − → RH 2 + RX − + X −. The two other new compounds formed by long-term controlled-potential electrolysis-compound 5 (2-hydroxy-4-methoxy-3,6-dimethyl-1H-benzoic acid-7-hydroxy-6-methyl-1-oxo-1,3-dihydro-isobenzofuran-5-yl-ester) and compound 9 (8-hydroxy-3-methoxy-1,4,9-trimethyl-11-oxo-11H-dibenzo[ b, e][1,4]dioxepine-7-carbaldehyde) suggest that 4 is easily modified by chemical and/or electrochemical reactions. Compound 4 suffers electrochemical reduction in the same potential range as 1 and follows a mechanism involving the transfer of two heterogeneous electrons in order to produce 5. The overall reaction is RH 2 + 2 RHX + 2e − → RH 4 + 2 RX −. Compound 4 appears to undergo also a chemical reaction whose product can follow an electrochemical mechanism similar to that from 1 to 4, thus producing 9. Compounds 4, 5, and 9 were elucidated by HRMS and 1D- and 2D-NMR methods. D 0=4.6 × 10 −6 cm 2 s −1 and k 0=9.4 × 10 −3 cm s −1 were found for the electrochemical reduction of 1.