Rotational Spectra of Tetracyclic Quinolizidine Alkaloids: Does a Water Molecule Flip Sparteine?

Abstract

Sparteine is a quinolizidine alkaloid used as chiral auxiliary in asymmetric synthesis. We examine whether hydration by a single molecule can flip sparteine from the most stable trans conformation to the bidentate cis arrangement observed in catalytic complexation to a metal center. Sparteine and the dimer sparteine-water were generated in a supersonic jet expansion with H216O and H218O, and characterized by broadband chirped-pulse microwave spectroscopy. Despite the bidentate water dimer was predicted with larger binding energy, a single isomer was observed for the monohydrated cluster, with sparteine retaining the trans conformation observed for the free molecule. The absence of the bidentate dimer is attributed to kinetic control of the cluster formation, favoring the pre-expansion most abundant monomer. The structural properties of the O-H···N hydrogen bond in the dimer are compared with complexes of other secondary and tertiary amines.