RiR2 CH3 R30

by 1,5 hydrogen atom abstraction and which are postulated to be important in the antimalarial action of artemisinin-like compounds (Cumming et al., 1998). The in vitro antiplasmodial activities of these derivatives supported the above hypothesis and the most active compounds (e.g. 116) were of comparable potency to artemisinin. For a full discussion of the mechanisms involved, see chapter 13.

Singh, (1990), and Singh et al. (1995), have reported the synthesis of a series of a new class of active monocyclic 1,2,4-trioxanes 117-120. The antiplasmodial IC50 of trioxanes 117-119 ranges from 2.86 to 222 mg/ml with respect to an IC50 of 0.65 mg/ml for 1 against chloroquine resistant strains of P. falciparum. However, several of the derivatives 120 possessed promising blood schizontocidal activity against P. berghei in mice.

Two synthetically racemic bicyclic czs-fused cyclopentene 1,2,4-trioxanes, 121 and 122 displayed high antimalarial activities which are commensurate with those of 1 and arteether, 38 (Peters et al., 1993). Two major exo- 1,2-diols for each racemate were prepared by Jefford et al. (1994). These diols are very useful as new chiral ligands for catalysis. Later, the above authors prepared the enantiomerically pure isomers of both 121 and 122 (Jefford et al., 1995). Jefford, (1996), has also reported a series of 1,2,4-trioxanes, 123, for the potential treatment of tropical diseases including malaria.

A series of more than 20 cyclic peroxy ketals have also been prepared and assessed for in vitro antiplasmodial activities (Posner et al., 1998b). Seven com-

113, R= pCH3OCH2Ph

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