Synthesis of Novel Chiral (Thio)ureas and Their Application as Organocatalysts and Ligands in Asymmetric Synthesis

Abstract

The synthesis of novel chiral (thio)ureas 1–10 and 14–26 is described. These (thio)ureas incorporate chiral auxiliaries derived from (R)- or (S)-α-phenylethylamine, (R)-phenylglycine, or (1R,2S)-ephedrine. The phenylethyl group in compounds 1–10 and 21–24 adopts a particular orientation in the molecular structure as a consequence of 1,3-allylic strain with the (thio)carbonyl group. Ureas 1–10 were tested as Lewis basic organocatalysts in epoxide ring opening and in aldolic condensation, and it was found that the tetrasubstituted urea (R,R)-2 afforded the best results in terms of reaction yields. (Thio)ureas 20–26 were examined as ligands in the enantioselective diethylzinc addition to benzaldehyde, observing that C2-symmetric chiral urea (R,S,R,S)-20 provides the expected carbinol in nearly quantitative yield and in up to 62% enantiomeric excess.