Various alkenes and dienes, such as (R)-or (S)-limonene 2, 2-carene 6, 3-carene 8, (R)-a-pinene 10, (S)-a-pinene 12, and endo-dicyclopentadiene 14 were transformed into the corresponding mono-and bis-epoxides by epoxidation with dimethyldioxirane (as an acetone solution). The selectivity observed in these epoxidations is explained by the assumption of hydrogen bonding between bridge protons and the dioxirane.Dimethyldioxirane (DMD, in situ or isolated as solution) has been established 1 as a widely-used oxidant, due to the convenience of its isolation 2 from cheap, commercially available starting materials, its efficiency, and its performance under strictly neutral conditions. Its reaction mode includes epoxidation 3 of alkenes, oxygen insertion 4 into s-bonds, and heteroatom oxidation. 5 Alkene epoxidation has found numerous applications 6 in the total syntheses of natural products. However, there are only few impressive results of regio-and diastereoselective epoxidations with DMD, i.e. the regioselective epoxidation 7 of a disubstituted double bond over a trisubstituted enol ether moiety, and the exclusive epoxidation 8 at the endocyclic double bonds of pentafulvenes.We thought that this powerful oxidant might be more regio-and diastereoselective than initially assumed. A more systematic investigation of simple strained alkenes could lead to interesting observations concerning transition states of the epoxidation mechanism. Herein, we report the DMD epoxidations of simple bicyclic alkenes and dienes yielding often regio-and diastereoselectively the corresponding mono-and bis-epoxides.Epoxidation of (R)-(+)-limonene R-2 with dimethyldioxirane at -5°C yields a 65:35 mixture of mono-epoxides R-3, R-4 (66% yield at 90% conversion) and bis-epoxide R-5 (34% yield at 90% conversion) (Scheme 1). The mixture of epoxides R-3, R-4 was separated from R-5 by column chromatography; addition of another equivalent of dimethyldioxirane led to the isolation of bis-epoxide R-5. The corresponding 8,9-epoxide was not detected in the crude reaction mixture, a fact that indicates the electrophilic character of dimethyldioxirane and its preference for the more electron-rich double bond of the molecule. Although the diastereoselectivity was poor [ratio R-3/R-4 = 1:8], it is better than many other oxidation protocols involving oxidants such as mCPBA 9 or PhIO, 10 H 2 O 2 11 and various Fe-porphyrins. The diastereoselectivity was influenced by the co-solvent used in the epoxidation (Table 1). When acetonitrile or methanol, were used as co-solvents (entries 4, 6) the conversion was quantitative but the diastereoselectivity was low, while in the case of methanol the formation of bis-epoxide was negligible (1 %). In contrast, when dichloromethane was used (entries 1, 3), the conversion was smaller, the diastereoselectivity better, and bis-epoxide R-5 was formed in larger amounts. The best diastereoselectivity was observed in the case of petroleum ether (entry 5), where also the lowest conversion was obtained.
Scheme 1It seems that the dias...