Enantioselective synthesis of encecaline-derived potent antimalarial agents

Abstract

The high antiplasmodial activity of racemic benzylamines rac-1 and rac-2 stimulated the synthesis of pure enantiomers. Ellman's chiral sulfinamides (S)-6 and (R)-6 were used as chiral auxiliaries. Condensation of prochiral ketone 5 with enantiomerically pure sulfinamides (S)-6 and (R)-6 and subsequent NaBH₄ reduction provided predominantly unlike configured diastereomers (S,R)-8 and (R,S)-8 (ratio unlike-8 : like-8 = 90 : 10). The same transformation of phenol 4 led to the diastereomeric sulfinamides (S,R)-12 and (S,S)-12 in the ratio 60 : 40. Acid hydrolysis of the diastereomerically pure sulfinamides followed by monobenzylation yielded the enantiomerically pure benzylamines (R)-1, (S)-1, (R)-2 and (S)-2. The enantiomeric purity of the products was proven by chiral HPLC and the absolute configuration by CD-spectroscopy. Generally, benzylamines with (R)-configuration show higher antiplasmodial activity than their corresponding (S)-configured enantiomers. Phenol (R)-2 represents a very potent lead against P. falciparum, with an IC₅₀ value of only 0.026 μM against the NF54 strain. The very high eudismic ratio of 34 indicates the enantioselective interaction of phenol (R)-2 with a particular target protein of P. falciparum.