TETRAHEDRON LETTERS Tetrahedron Letters 43 (2002) 3301–3304 Pergamon Anodic oxidation of 4-methoxy-1-naphthol Hesham R. El-Seedi, Shosuke Yamamura and Shigeru Nishiyama* Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3 -14 -1, Kohoku -ku, Yokohama 223 -8522, Japan Received 28 January 2002; accepted 15 March 2002 Abstract—Anodic oxidation of 4-methoxy-1-naphthol 1 in the presence of nucleophiles provided the corresponding products 8–12 and the dimers 13 and 17 were also produced. The reaction mechanism of the oxidation reaction including the [3+2] cycloaddition was investigated. © 2002 Elsevier Science Ltd. All rights reserved. Anodic oxidation of phenol derivatives is well known to provide a variety of products possessing an aromatic and/or non-aromatic structure. 1 In particular, the latter conversion into cyclohexane derivatives might be com- parable to those of the Birch reduction. While a num- ber of investigations have indicated a versatile availability of the phenolic oxidation, only a few have been reported in relation to naphthol derivatives. In particular, reaction of the electrolytically generated active species, which are stabilized with the attached phenyl residue, with olefinic nucleophiles would provide an access to multi-cyclic substances. 2 As part of our extensive electrochemical investigation, study of 4- methoxy-1-naphthol 1 would be included to understand their scope and limitation as substrates in phenolic oxidations. 3 General procedure for anodic oxidation. Compound 1 (0.33 mmol) was electrolyzed with appropriate alkenic nucleophiles such as isosafrole 2, 1-methoxy-4- propenylbenzene 3, dihydrofuran 4 and dihydropyran 5 (5–15 equiv. mol to 1) [anode: a glassy carbon beaker, cathode: a platinum wire, a divided cell through glass- filters, n Bu 4 NBF 4 as supporting salts (240 mg), solvent (25–35 ml)]. The results are summarized in Table 1 and Fig. 1. Among solvents assessed for the electrolysis employing 2 and 3 as nucleophiles, CH 3 CN effected the expected couplings leading to 8 4 and 9 2a (entries 1 and 4), whereas Ac 2 O or CH 3 CN–Ac 2 O provided considerable amounts of undesired quinone 6 or acetate 7. Accord- ingly, the following entries employed CH 3 CN as a solvent, while such acidic solvents as CH 3 CN–AcOH, 2a Ac 2 O, 2b or CH 3 NO 2 –AcOH 2c were used in the case of phenol derivatives. The anodic oxidation commenced with the one-electron oxidation of 1 to give the radical A, followed by the second abstraction of an electron to the cation B (Scheme 1). Upon capture of the nucleo- philes presented, the cation B produced such [3+2] cycloaddition products as 8, 9, 10 4 and 11. 4 This reac- tion feature was comparable to that of phenol deriva- tives. 2 Compounds 10 and 11 were obtained in better yields than those derived from 4-methoxyphenol, 2b owing to the stabilizing effect of the attached phenyl residue of the naphthol molecule. Without such interac- tion of the nucleophiles, B was transformed into the corresponding naphthoquinone 6. In entries 7 and 8 using 4 as nucleophile, the C -glycosyl product 12 4 was preferentially produced rather than benzofuran 10, whereas 10 was obtained in moderate yield under the current-increased CPE conditions (entry 9). The assembly of 12 should adopt an entirely differ- ent reaction pathway from that described in Scheme 1, as the sp 2 carbons adjacent to the ring oxygen in dihydrofuran or pyran generally behave as elec- trophiles, which could not react at the C-2 position. A plausible mechanism is that acidic properties under the reaction conditions produced O -glycoside 14, followed by the OC glycosyl migration, leading to 15, which on two-electron oxidation gave 12 (Scheme 2). To ascertain this possibility, 1 was reacted with 4 in the presence of a catalytic amount of TsOH. The reaction required 14 h to give a mixture of 14 (70%) and 15 (30%), while 14 was quantitatively obtained after 1 h, which was a similar reaction time to the anodic oxida- Keywords : anodic oxidation; 4-methoxy-1-naphthol; electroorganic chemistry; cycloaddition. * Corresponding author. Tel./fax: +81-45-566-1717; e-mail: nisiyama@chem.keio.ac.jp 0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(02)00544-0