Synthesis and muscarinic M2 subtype antagonistic activity of unnatural ent-himbacine and an enantiomeric pair of (2′S,6′R)-Diepihimbacine

The title three compounds ent-1, 6, and ent-6 were synthesized by coupling the chiral sulfone 4 or ent-4 with the chiral piperidinaldehyde 5 or ent-5, which were readily prepared following the synthetic routes previously established by the novel total synthesis of natural himbacine 1. Their muscarinic M2 subtype binding affinity was evaluated in comparison to that of 1, disclosing that the stereochemistry of both the tricyclic moiety and the piperidine part of 1 plays crucial roles in its potent activity.