Exo gem-difluoromethylene-artemisinins (8) has been designed to mimic artemisinin. The classical Wittig olefination reaction applied to artemisinin failed. An alternative reaction involving the generation of an alpha-CF3 carbanion, from the corresponding bromide 6, allowed the access to the target compound 8, and could also be exemplified in sugar series. The replacement of the carbonyl function by a difluoroethylene moiety resulted in a better antimalarial activity. (C) 2006 Elsevier B.V. All rights reserved.
S<sub>N</sub>/S<sub>N</sub>‘ Competition: Selective Access to New 10-Fluoro Artemisinins
In this paper, we report a simple route to accede to a new family of C-10 fluorinated derivatives of artemisinin 7. We demonstrated that nucleophilic substitution of the allylic bromide 6 with alcohols can occur at carbon 10 (compounds 7) under solvolytic conditions (S-N'/S-N ratio, 87:13). Furthermore, using the particular properties of hexafluoroisopropanol (HFIP), we are able to increase the selectivity of the substitution. Primary alcohols are completely selective for allylic substitution. With amines as nucleophiles, selectivity of substitution is dependent on their nucleophilicity, but attack at carbon 16 was always favored. However, the S-N'/S-N ratio could be slightly increased by adding HFIP, which is able to modulate their nucleophilicity through hydrogen bonding. In preliminary in vitro assessments, these new compounds, 7, exhibited a satisfying activity against malaria.