Multigram solution-phase synthesis of three diastereomeric tripeptidic second-generation dendrons based on (2S,4S)-,...

Three diastereomeric second-generation (G2) dendrons were prepared by using (2S,4S)-, (2S,4R)-, and (2R,4S)-4-aminoprolines on the multigram scale with highly optimized and fully reproducible solution-phase methods. The peripheral 4-aminoproline branching units of all the dendrons have the 2S,4S configuration throughout, whereas those units at the focal point have the 2S,4S, 2S,4R, and 2R,4S configurations. These latter configurations led to the dendrons being named (2S,4S)-1, (2S,4R)-1, and (2R,4S)-1, respectively. The 4-aminoproline derivatives used in this study are new, although many closely related compounds exist. Their syntheses were optimized. The dendron assembly involved amide coupling, the efficiency of which was also optimized by employing the following well-known reagents: EDC/HOBt, DCC/HOSu, TBTA/HOBt, TBTU/HOBt, BOP/HOBt, pentafluorophenol, and PyBOP/HOBt. It was found that the use of PyBOP is by far the best for dendrons (2S,4S)-1 and (2R,4S)-1, and pentafluorophenol active ester is best for (2S,4R)-1. Because of their multigram scale, all couplings were done in solution instead of by solid-phase procedures. Purifications were, nevertheless, easy. The optical purities of the key intermediates as well as the three G2 dendrons were analyzed by chiral HPLC analysis. These novel, diastereomeric second-generation dendrons have a rather compact and conformationally highly rigid structure that makes them interesting candidates for applications, for example, in the field of dendronized polymers and in organocatalysis.