Fragmentation of a three-membered ring
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
967
2025-07-28
469
Fragmentation of a three-membered ring
This synthesis does not look as though it will lead to nootkatone because the fragmentation product still requires a great deal of modification. It has the advantage that the stereochemistry is correct at one centre at least. The sequence starts from natural (–)-carone: conjugate addition of the enolate to butenone without control leads to a bicyclic diketone with one extra stereogenic centre. The enone adds to the bottom face of the enolate opposite the dimethyl cyclopropane ring so the methyl group is forced upwards.
Now the diketone is cyclized by a Robinson-style aldol condensation in HCl to give a bi cyclic enone. But during the reaction, a new six-membered ring has been formed while the old three-membered ring has disappeared, evidently by fragmentation.
The fragmentation is pulled by the enone (with some help from the acid) and pushed by the stability of a tertiary carbocation as well as the release of strain as the single bond that is frag mented is in a three-membered ring.
Addition of a proton to the end of the enol and a chloride ion to the cation gives the product. The further development of this compound into nootkatone is beyond the scope of this book.
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