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Regiospecific singlet oxygen oxidation of (-)R-tetrahydrohumulone and hexahydrocolupulone

✍ Scribed by D. De Keukeleire; E. Siaens; M. Verzele

Publisher
Wiley (John Wiley & Sons)
Year
2010
Weight
453 KB
Volume
85
Category
Article
ISSN
0037-9646

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✦ Synopsis

Abstract

Hexahydrocolupulone (4) and (‐)R‐tetrahydrohumulone (5) are regiospecifically oxidized by singlet oxygen at ring carbon atom 4. The reaction proceeds by addition of singlet oxygen to the δ4‐5 enolic function with formation of a perepoxide intermediate. Rearrangement affords the 4‐hydroperoxy‐derivative, which is relatively stable only in the case of 4. Peroxide bond splitting and consequent radical reactions produce stable secundary products: 4‐hydroxy‐hexahydrocolupulone (7) in the oxidation of 4; tetrahydrohumulinone (9) in the oxidation of 5. The reactivity at the C~4~ enolic centre is comparable to that of trisubstituted alkenes. In contradiction with what has been found in the usual oxidation conditions, the hop bitter α‐acids are more sensitive towards singlet oxygen than the corresponding β‐acids.

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