First Steroidal Carbamoyloxa-Bridged Cyclophanes: Regioselective Macrocyclization of Methyl Cholate by Double Transesterification

Facile regioselective macrocyclization of methyl cholate (1) using dimethyl α,α,α’,α’-tetramethyl-m-xylylenedicarbamate (2) by transesterification was achieved, in refluxing toluene (0.004 M), using 3% of SnCl₂ and 1% of glycoluril as an additive. After a reaction time of 2 hours, the kinetically favorable 3 was obtained selectively in excellent yield (94%). Conversion of the 16-membered 3 to the 18-membered cyclophane 4 was achieved by establishing an equilibrium (7 days), to give the thermodynamically stable 4 in practical yield (68%). This mild conversion of the regiochemistry of the steroidal macrocycles is remarkable as a characteristic of the carbamoyloxa-bridged cyclophanes. The new steroidal carbamoyloxa-bridged cyclophanes is the first example of a pair of the regioisomeric steroidal cyclophanes.

cholic acid - cyclophane - macrocyclization - carbamate - ring transformation