Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) Electroinduced oxidative transformation of 2,5-dioxabicyclo[4.4.0]decanes into 5-(1,3-dioxolan-2-yl)- and 5-(dimethoxymethyl)pentanoates Yuri N. Ogibin,* Alexander O. Terent’ev, Alexey I. Ilovaisky and Gennady I. Nikishin N. D. Zelinsky Institute of Organic Chemistry, Russsian Academy of Sciences, 117913 Moscow, Russian Federation.Fax: + 7 095 135 5328 Anodic oxidation of 2,5-dioxabicyclo[4.4.0]decane 1a, 1-methoxy-2,5-dioxabicyclo[4.4.0]decane 1b and 1-hydroxy-2,5-dioxabicyclo[ 4.4.0]decane 1c in methanol in the presence of tetrabutylammonium fluoroborate as a supporting electrolyte induces the electrooxidative transformation of substrates 1a and 1b into methyl 5-(dimethoxymethyl)pentanoate and of substrate 1c into methyl 5-(1,3-dioxolan-2-yl)pentanoate.Recently, we found the electroinduced oxidative rearrangement of 1,6-dimethoxy-2-oxabicyclo[n.4.0]alkanes into w-(2-methoxytetrahydrofur- 2-yl)alkanoates:1 This finding provoked us to investigate the behaviour of 2,5- dioxabicyclo[4.4.0]decane 1a, 1-methoxy-2,5-dioxabicyclo- [4.4.0]decane 1b and 1-hydroxy-2,5-dioxabicyclo[4.4.0]decane 1c under similar electrolysis conditions.† In this communication, we report the results obtained by the electrolysis.Methyl 5-(dimethoxymethyl)pentanoate 2 is formed as the main product from bicyclodecanes 1a and 1b in 75% yield, and methyl 5-(1,3- dioxolan-2-yl)pentanoate 3 is formed from bicyclodecane 1c in 90% yield (Scheme 1). The transformation of 1a–c into esters 2 and 3 resulted from the electrolysis of 1a–c at room temperature in methanol in the presence of tetrabutylammonium fluoroborate as a supporting electrolyte in an undivided cell equipped with a platinum or † Starting materials.trans-2,5-Dioxabicyclo[4.4.0]decane 1a2 was prepared from epoxycyclohexane by the acid-catalysed reaction with 2-chloroethanol followed by the treatment of the resulting 2-(b-chloroethoxy)- cyclohexanol with an alcoholic potassium hydroxide solution (60% overall yield). 1-Methoxy-2,5-dioxabicyclo[4.4.0]decane 1b3 was the product of the acid-catalysed addition of methanol to 2,5-dioxabicyclo- [4.4.0]dec-1(6)-ene.4 1-Hydroxy-2,5-dioxabicyclo[4.4.0]decane 1c was synthesised by a known procedure4 from cyclohexanone in 40% yield; ethylene ketal of 2-hydroxycyclohexanone was formed along with 1c in the same yield.graphite anode and a stainless steel cathode under passages of 2–4 F mol–1 of electricity (Table 1).‡ The structures of esters 2 and 3 were established on the basis of 1H and 13C NMR spectra,§ which contained signals due to dimethoxymethyl (dH 3.28, 4.31; dC 52.5, 64.7 and 104.1), methoxycarbonyl (dH 3.63; dC 51.3, 173.8) and 1,3-dioxolanyl (dH 3.87, 4.81) groups, and by comparison of their hydrolysis product with the authentic methyl 6-oxohexanoate.6 The formation of two types of products from structurally similar starting substrates indicates a difference in the mechanisms of their electrochemical transformations.A rearrangement related to that observed for 1,6-dimethoxy-2-oxabicyclo[n.4.0]alkanes,1 occurs only in the case of bicyclodecane 1c.The electrolysis of bicyclodecanes 1a and 1b gives ester 2 and is not accompanied by the rearrangement. Ester 2 is formed from substrate 1a through the intermediate formation of bicyclodecane 1b. Scheme 2 illustrates the proposed mechanism for the transformation of substrates 1a–c into esters 2 and 3. The electrochemical process begins with electron transfer from electrophorus ethylenedioxy fragments of bicyclodecanes 1a–c.It is possible by two routes of further transformation of the resulting radical cations; one route starts with the deprotonation of radical cations and the formation of radicals A (route i), and the other route starts with the cleavage of the bridgehead C–C bond and the formation of distonic radical cation8 B (route ii). Similar radical cations also arise from subsequent electrochemical transformations of radicals A.The transformations of radical cations derived from bicyclodecanes 1a and 1b,c follow routes i and ii, respectively. The conversion of distonic ions B (X = OH) electrogenerated from substrate 1c into the final product (ester 3) is accompanied by the deprotonation, rearrangement and decyclization via oxonium ions F.¶ ‡ Electrolysis of dioxabicycloalkanes 1a–c (typical procedure).A solution of an electrolyte (9 mmol), compound 1 (5 mmol) and n-decane (internal standard, 3 mmol) in MeOH (15–25 ml) was placed in an undivided cell5 and then electrolysed at a constant current (0.5 A) and room temperature under intense stirring until more than 90% of 1 was converted.The solvent was removed, the residue was extracted with hexane (2×20 ml), and the combined extracts were concentrated. The products were isolated by vacuum distillation or flash chromathography with hexane–ethyl acetate (1%) as an eluent and then analysed. § 1-Methoxy-2,5-dioxabicyclo[4.4.0]decane 1b.3 1H NMR (200MHz, CDCl3) d: 1.55–1.80 (m, 8H, CH2), 3.23 (s, 3H, MeO), 3.30 (m, 1H, CH), 3.46 and 3.82 (m, 4H, OCH2CH2O). 13C NMR (50 MHz, CDCl3) d: 96.4 (O–C–O), 80.8 (CH–O), 64.8, 60.2 (O–C–C–O), 46.9 (MeO), 29.82, 27.96, 24.18, 21.82 (CH2). Methyl 5-(dimethoxymethyl)pentanoate 2.6 1H NMR (200 MHz, CDCl3) d: 1.35 (m, 2H, CH2), 1.60 (m, 4H, CH2), 2.30 (t, 2H, CH2COO), 3.28 (s, 6H, OMe), 3.63 (s, 3H, MeOCO), 4.31 (t, 1H, CHOMe). 13C NMR (50 MHz, CDCl3) d: 178.8 (O=C–O), 104.1 (O–CH–O), 64.7, 52.5, 51.3 (OMe), 33.8, 32.8, 24.6, 24.0 (CH2). Methyl 5-(1,3-dioxolan-2-yl)pentanoate 3.7 1H NMR (200 MHz, CDCl3) d: 1.42–1.63 (m, 6H, CH2), 2.30 (t, 2H, CH2COO, J 7.5 Hz), 3.63 (s, 3H, MeO), 3.87 (m, 4H, OCH2CH2O), 4.81 (t, 1H, OCHO, J 4.9 Hz). ¶ The participation of cyclic oxonium ions in the isomerization of linear aliphatic methoxy-substituted carbonium ions was found in ref. 9. O (CH2)n OMe OMe 1 2 3 4 5 6 – e, MeOH O COOMe OMe n O O (CH2)4 X O O COOMe 4 OMe MeO COOMe – e, MeOH X = H, OMe X = OH 4 1a–c a X = H b X = OMe c X = OH 2 3 O H COOMe 4 4 Scheme 1 H3O+ H3O+Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) A gain in energy as a result of decyclization of the strained 10-membered ring system seems to be a driving force for this process.Distonic ions B (X = OMe) derived from substrates 1a and 1b are likely to be turned to the final product (ester 2) as a result of simultaneously occurring electrooxidation and alcoholysis of radical and cationic centres and by the interaction of cationic intermediates D and cyclic ortho ether 5 with methanol.The protons generated during the electrooxidation of methanol and the alcoholysis of intermediates B and D are a possible catalyst for the reaction of this ortho ether with methanol. The reduction of the protons at a cathode to produce molecular hydrogen does not permit them to be accumulated in the electrolysis products in a concentration sufficient for catalysing the complete conversion of the ortho ether into ester 2.This was supported by the presence of signals typical of protons of the methoxy group (dH 3.13) and 13C nuclei (dC 115.5) of the ortho ether group10 in the NMR spectra of the electrolysis products of 1a. Thus, the electroinduced oxidative rearrangement of 2-oxaand 2,5-dioxabicycloalkanes is not a general case, and it is typical of only a limited range of compounds of this kind, such as 1,6-dimethoxy-2-oxabicyclo[n.4.0]alkanes and 1-hydroxy-2,5- dioxabicyclo[4.4.0]decane.This work was supported by the Russian Foundation for Basic Reseach (grant no. 97-03-33159). References 1 Yu. N. Ogibin, A. O. Terent’ev, A. I. Ilovaisky and G. I. Nikishin, Mendeleev Commun., 1998, 239. 2 F. R. Larsen and A. Neese, J.Am. Chem. Soc., 1975, 97, 4345. 3 D. Lalandais, C. Baequet and J. Einhorn, Tetrahedron, 1981, 37, 3131. 4 I. R. Fjeldskaar, P. Rongved and L. Skattebol, Acta Chem. Scand., 1987, B41, 477. 5 Yu. N. Ogibin, A. I. Ilovaisky and G. I. Nikishin, Izv. Akad. Nauk, Ser. Khim., 1994, 1624 (Russ. Chem. Bull., 1994, 43, 1536). 6 G. A. Tolstikov, M. S. Miftakhov, F. A. Valeev, R. R.Akhmetvaleev, L. M. Khalilov and A. A. Panasenko, Zh. Org. Khim., 1985, 21, 72 [J. Org. Chem. USSR (Engl. Transl.), 1985, 21, 65]. 7 T. Nakamura, H. Sawada and M. Nakayma, Jpn. Kokai Tokyo Koho, Japanese Patent 02 48.585 (90 48.585), 1991 (Chem. Abstr., 1991, 113, P 41219). 8 K. M. Stirk, L. K. Kiminkinen and H. I. Kenttamaa, Chem. Rev., 1992, 92, 1649. 9 (a) E. L. Allred and S. Winstein, J.Am. Chem. Soc., 1967, 89, 3991; (b) E. L. Allred and S. Winstein, J. Am. Chem. Soc., 1967, 89, 4012. 10 (a) P. Deslongchamps, J. Lessard and Y. Nadeau, Can. J. Chem., 1985, 63, 2485; (b) P. Deslongchamps, J. Lessard and Y. Nadeau, Can. J. Chem., 1985, 63, 2493. O O (CH2)4 i (X = H) – H+ 1a–c – e 1a–c O O (CH2)4 MeO A – e, MeOH 1b O O (CH2)4 B X MeOH – e O O (CH2)4 MeO 5 O O (CH2)4 D MeO OMe OMe X = OMe – H+ – e, MeOH OMe O O (CH2)4 C O – H+ X = OH 2 MeOH OMe (CH2)4 G O (CH2)4 F O O (CH2)4 E O 3 – e O O OH O O Scheme 2 – H+ ii X = OMe, OH – H+ aOn a converted bicyclodecane basis. Table 1 Electroinduced transformation of 2,5-dioxabicyclo[4.4.0]decane 1a, 1-methoxy-2,5-dioxabicyclo[4.4.0]decane 1b, and 1-hydroxy-2,5- dioxabicyclo[4.4.0]decane 1c to methyl 5-(dimethoxymethyl)pentanoate 2 and methyl 5-(1,3-dioxalan-2-yl)pentanoate 3. Entry Bicycloalkane Anode Q/F mol–1 Conversion (%) Product Yield (%)a 1 1a Pt 2.0 85 1b + 2 50 + 28 2 1a Pt 3.0 95 1b + 2a 34 + 44 3 1a Pt 4.0 100 1b + 2a 23 + 57 4 1b Pt 2.0 90 2 80 5 1c C 4.0 100 3 90 Received: 12th March 1999; Com. 99/1460