Enantioselective Synthesis of Iclaprim Enantiomers―A Versatile Approach to 2-Substituted Chiral Chromenes

Both enantiomers of the DHFR inhibitor iclaprim (R)-1 and (S)-1 were synthesized from the cyclopropyl homoallyl alcohols (R)-6 and (S)-6, respectively. As key steps these transformations include a Mitsunobu reaction and the formation of the diaminopyrimidine unit prior to a novel cyclization procedure to obtain the desired chromene heterocycle. The moderate enantioselectivity of the products (R)-1 and (S)-1 is related to the Mitsunobu reaction, which unfortunately did not proceed with complete inversion of configuration.