Xp

from bisabolyl cation (see Figure 5.27)

air oxidation to hydroperoxide

Xp amorpha-4,11-diene

Xp photoxidation to hydroperoxide

HO2T artemisinic acid photoxidation to hydroperoxide

HO2C' dihydroartemisinic acid from bisabolyl cation (see Figure 5.27)

HO2T'

formation of hemiacetal, then lactone

HO2T'

HO2T

HO2T

formation of hemiacetal, then lactone hydrolysis of hemiketal

an enzyme preparation has also catalysed this transformation

artemisinin (qinghaosu)

O^ HO2T

an enzyme preparation has also catalysed this transformation cleavage of hydroperoxide, ring expansion, and quenching of carbocation

i oV

dihydroarteannuin B

artemisinin (qinghaosu)

HO2T

dihydroartemisinic acid hydroperoxide

HO2T

dihydroartemisinic acid hydroperoxide

dihydroarteannuin B

Figure 5.34

antimalarial component in Artemisia annua *, a Chinese herbal drug. There is currently strong research effort to produce artemisinin or analogues as new antimalarial drugs, since many of the current drugs have become less satisfactory due to resistance (see quinine, page 362).

The 11-carbon ring of the humulyl carboca-tion may be retained, as in the formation of humulene (Figure 5.36), or modified to give the caryophyllyl cation containing a nine-membered ring fused to a four-membered ring, as in 0-caryophyllene (Figure 5.36). Humulene is found

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