An NMR and theoretical study of the conformation and internal flexibility of butaclamol hydrochloride

A theoretical (MM2) and experimental (1H and 13C NMR) study of butaclamol hydrochloride in CDCl3 has been done in order to determine preferred conformations and internal molecular flexibility of this molecule. The theoretical calculations suggest the presence of four low-energy conformations, two of which involve a trans junction of the D and E rings, with the other two involving a cis I ring junction. An alternative cis junction (cis II) was excluded on energetic grounds. The 1H NMR data strongly suggest the presence of a trans D-E ring junction and are consistent with a chair conformation of the E ring. 13C spin-lattice relaxation time measurements show that most of the molecule is rigid, although there is some degree of mobility in the seven-membered B ring, associated with rapid flipping of the bridging C8 and C9 carbons between two skewed conformations, which have previously been referred to as conformer A and conformer B (Laus et al. Heterocycles 1984, 22, 311).