IntroductionAlthough the 2,3,7-trioxabicyclo[2.2.1]hept-5-enes 2, usually known as furan endo-peroxides, were postulated by Schenck 1 in 1944 as unstable primary products formed when furans 1 were subject to sensitized or auto-sensitized photo-oxygenation, these peroxides were not isolated until 1980. 2 At that time our group carried out the dye-sensitized photooxygenation of furans substituted at C-3 with an electron-withdrawing group on the basis that delocalization of the π-electrons in the furan endo-peroxide structure by an electron-withdrawing substituent would ensure that the reactivity of this system lay between that of the monoozonides of cyclobutadiene derivatives and of the more stable ozonides of cyclobutene derivatives. 3 Thus, endo-peroxides 2 which were sufficiently stable to be isolated and characterized by analytical and spectral data were obtained (Scheme 1). 2 Successively, a systematic study on the reaction of singlet oxygen with furans substituted at C-2 and C-5 with aryls, 4 alkyls 5 or hydrogens 6 showed that the furan endoperoxides are formed in any case. However, the latter undergo thermal rearrangement by various pathways depending chiefly on the nature of the substituents at C-1 and C-4, 4-6 although the reaction conditions are equally important. 7 These endo-peroxides generally react with primary and secondary alcohols to give 2-alkoxy-5-hydroperoxy-2,5-dihydrofurans 3 (Scheme 1) or products which are believed to derive from these compounds. 2,5c Alternatively, they fail to add alcohols at all, as is the case with electron-deficient endo-peroxides 8,6a,6c whose behaviour confirms the addition pathway suggested. 5c
Scheme 1In the present account, the authors wish to describe their recent progress concerning the reaction of singlet oxygen with 2-alkoxyfurans which leads to 1-alkoxy-2,3,7-trioxabicyclo[2.2.1]hept-5-enes 5 having a bicyclic monoperoxy ortho ester structure (Scheme 2). The dyesensitized photo-oxygenation of the parent 2-methoxyfuran (4a) was carried out several years ago by Gollnick et al. 5c in an extensive study on the addition of singlet oxygen to furans. However, this part of the subject was only touched on and the results were examined only superficially. Nevertheless, endo-peroxide 5a was identified by 1 H NMR spectrum recorded at -70 °C and at higher temperatures it gave only resinous material.
Scheme 2
Synthetic EntryIn order to obtain more manageable 1-alkoxy-2,3,7-trioxabicyclo[2.2.1]hept-5-enes 5 than rather 5a, we performed dyesensitized photo-oxygenation at a low temperature (-20 or -60 °C) in CDCl 3 /CFCl 3 on 2-alkoxyfurans 4 substituted at C-3 and/or C-4 with electron-withdrawing groups and at C-5 with a phenyl substituent. [8][9][10] Then, in order to confirm the behaviour of the endo-peroxides 5 obtained, we extended the dye-sensitized photo-oxygenation to 2-alkoxyfurans variously substituted while working at a temperature range of -80 to -70 °C. [11][12][13][14] In any case the endo-peroxides 5, which are thermally less stable than those bearing aryls, ...
