F ft U

germacrene A

Figure 5.30

parthenolide

parthenolide transannular cyclization initiated via protonated epoxide; compare formation of guaiyl cation, Figure 5.27

Guaiyl Cation

attack of sulphydryl in protein on to resultant H carbocation

nucleophilic attack of sulphydryl on to a,P-unsaturated lactone: Michael reaction

nucleophilic attack of sulphydryl on to a,P-unsaturated lactone: Michael reaction

HO H

sesquiterpene lactone irreversibly bound to protein

HO H

sesquiterpene lactone irreversibly bound to protein

Figure 5.31

second alkylation site. Cytotoxic agents may irreversibly alkylate critical enzymes that control cell division, whilst allergenic compounds may conjugate with proteins to form antigens which trigger the allergic response. The beneficial effects of parthenolide and structurally related compounds in feverfew have been demonstrated to relate to alky-lation of thiol groups.

a-Santonin (Figure 5.29) has been identified as the principal anthelmintic component of

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