The Eschenmoser fragmentation
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
965
2025-07-27
609
The Eschenmoser fragmentation
In the late 1960s, the Swiss chemist Albert Eschenmoser discovered an important reaction that can be used to achieve similar ring expansions and that now bears his name, the Eschenmoser fragmentation. The starting material for an Eschenmoser fragmentation is the epoxide of an α,β-unsaturated ketone. The fragmentation happens when this epoxyketone is treated with tosylhydrazine, and one of the remarkable things about the product is that it is an alkyne. The fragmentation happens across the epoxide (shown in black), and the product contains both a ketone (in a different place from the ketone in the starting material) and an alkyne. You can see how in this case hydrogenation of the triple bond can give muscone (R=Me) or exaltone (R=H).
The Eschenmoser fragmentation does not have to be a ring expansion, and it is a useful synthetic method for making keto-alkynes. The following reaction, which we will use to dis cuss the fragmentation’s mechanism, was used to make an intermediate in the synthesis of an insect pheromone, exo-brevicomin.
The reaction starts with formation of the tosylhydrazone from the epoxyketone. The tosyl hydrazone is unstable with respect to opening of the epoxide in an elimination reaction, and it is this elimination that sets up the familiar 1, 2, 3, 4 system ready for fragmentation. The ‘push’ comes from the newly created hydroxyl group, and the ‘pull’ from the irresistible concerted loss of a good leaving group (Ts−) and an even better one (N2). Notice how all the (green) bonds that break are parallel to one another, held anti-periplanar by two double bonds. Perfect!
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