Steroidal guanidines as enantioselective receptors for N-acyl α-amino acids.: Part 1.: 3α-Guanylated carbamates derived from cholic acid

Receptors 5-11, bearing guanidinium, carbamate and (in some cases) other functional groups, were synthesized from cholic acid 1. These cations were shown to extract chiral carboxylate anions from aqueous buffer into chloroform with significant enantioselectivities. The most successful receptors bore two carbamate substituents and achieved, in the best cases, ratios (L:D) of 7-10:1 for a series of five N- acetyl alpha -amino acids.