Stereochemical relay through oligoureas - Is it possible to control the orientation of the helix with a chiral group A? - Is it possible to control a stereoselective reaction on B ? P M 11/11/2004

Synthesis

nABStepsYields 1 s-Bu H457% 2 H647% 3 H848% 3 i-Pr 835% 3 s-Bu Br8??% 3 i-Pr Br817%

Synthesis Symmetrical oligoureas

Synthesis Symmetrical oligoureas

Synthesis

Conformation of the ureas (Et) 5.81 ppm (i-Pr) 5.88 ppm 5.65 ppm ≈ 10 Å Conformation in solution (CDCl 3 ) and in the single cristal Two conformations at low temperature R = Et 2 conformers 50/50 à -60°C R = i-Pr 2 conformers 55/45 à -30°C

Conformation of the ureas Conformation in solution (CDCl 3 ) and in the single cristal 5.96 ppm With an asymmetric centre (s-Bu, racemic) MP Two enantiomer helices Each imposes a different conformation of a same enantiomer of the s-Bu

Conformation of the ureas Conformation in solution (CDCl 3 ) of the oligoureas 6.02 ppm 5.91 ppm 6.03 ppm 5.91 ppm Symmetrical tetraureaunsymmetrical tetraurea 5.91 ppm 6.00 ppm symmetrical octaurea

Conformation of the ureas Conformation in solution (CDCl 3 ) of the oligoureas 6.03 ppm 5.92 ppm6.05 ppm Chiral sulfoxide 5.93 ppm6.01 ppm 6.04 ppm

A chiral i-propyl group as a sensor ?? How to know if the sulfoxide controls the sense of the helix in solution ?

Chiral i-propyl as a sensor Thayumanavan, S.; Basu, A.; Beak P. J. Am. Chem. Soc. 1997, 119, Dynamic Thermodynamic Resolution R = CD 3 de = 65% R = 13 CH 3 de = 81% R = CD 3 87% R = 13 CH 3 77% R = CD 3 52% R = 13 CH 3 68%