Influence of remote carbamate protective groups on the β-selectivity in rhamnosylations

In this work, we present the synthesis of a series of l-thiorhamnosyl donors containing O-carbamate protective groups and the study of their influence on the selectivity in rhamnosylations. It is found that a carbamate on the C-4 position increased the β selectivity compared with carbamates on the C2 or C3 positions, respectively, and when no carbamate group was installed. In addition it is found that the observed β selectivity was greater when the 4-O carbamate had less electron withdrawing groups on the nitrogen. The influence of using triflic acid catalysis was studied as well and it was found to lower the β-selectivity. In addition a new efficient one step synthesis of selectively 2,4-O-benzylated rhamnosides was established using phase transfer catalysis.