A Radical Alternative to the Anionic Oxy-Cope Rearrangement
β Scribed by Rachel Chuard; Anne Giraud; Philippe Renaud
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 77 KB
- Volume
- 41
- Category
- Article
- ISSN
- 0044-8249
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β¦ Synopsis
The anionic oxy-Cope rearrangement has emerged as a highly useful tool for organic synthesis. [1, 2] Various 1,5-dien-3ols readily undergo this electronic reorganization, and the usually easy preparation of the precursors contributes to the popularity of this reaction. However, a limitation of this sigmatropic rearrangement is the requirement of proximal double bonds. Indeed, the distance and the angular relationship between the two double bonds should be close to optimal values in order to achieve the rearrangement. [2, 3] For example, with 1,5-dien-3-ols that result from the addition of alkenyl anions to 2-norbornenones, the double bonds are not proximal and therefore anionic sigmatropy is not possible. Two protocols that allow the inversion of the configuration of the quaternary center have been developed to remedy this problem. However, both are lengthy (four steps) and not high-yielding (Scheme 1). [4, 5] We report herein a mechanistically different approach to solve this problem: a radical alternative to the oxy-Cope rearrangement. A cascade reaction that involves the fragmentation of an alkoxyl radical followed by a 6-endocyclization process leads to the oxy-Cope product (Scheme 2). According to this mechanism, the relative configuration of the alcohol has no influence on the efficacy of the rearrangement. Related anionic stepwise mechanisms have been observed for anionic amino-Cope rearrangements [6] as well as for oxy-Cope rearrangements of 4-phenylthio-1,5-dien-3-alkoxides. [7, 8] We investigated several methods to generate the desired alkoxyl radical A, starting from 2-norbornenone. Treatment COMMUNICATIONS
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