We also examined electrochemical reactions of monocyclized by-products 3a and 4b (Scheme 3B). From 4b, only a trace amount of 2a was obtained. In contrast, 2a was obtained in 52% yield from 3a. These results indicate that brominated byproduct 4b could not be converted to 2a, and only nonbrominated mono-cyclized product 3a, which would be the intermediate of the double-cyclization, could be transformed to 2a. To confirm the intermediate by a simple NMR analysis, we chose dimethyl-substituted 1b as a substrate and the electrolysis of 1b with 0.5 F mol -1 of electricity was carried out. Electrolysis with less electricity afforded 2b in only 5% yield and mono-cyclized 3b was obtained as a major product (Scheme 3C), suggesting that this reaction would be a step-wise reaction through the mono-cyclized intermediate. To confirm the proton source for 3b, we next carried out the electrochemical reaction of 1b in CD3CN and found that partially D-labelled 3b was obtained (27%D) (Scheme 3D). In contrast, 82% D-labelled 3b was obtained when the reaction was performed with D2O (30 equiv) (Scheme 3E). These results suggest that acetonitrile would act as a hydrogen source, but H2O should serve as the major proton source. H2O would also act as a trapping agent of the eliminated MOM group. When the electrochemical reaction of 1a was carried out with 1-dodecanol (5 equiv) instead of H2O, 2a was obtained in a yield similar to that with H2O (81% yield), and 1-(methoxymethoxy)dodecane was obtained in 19% yield (Scheme 3F). This result means that oxocarbenium ion generated in situ was partially trapped by 1-dodecanol. H2O should play a similar role in the electrochemical reaction.