X-Ray Crystal Structure of Woodinine and Conformational Analysis by Means of Semiempirical and 1H-NMR Methods

Abstract

The mol. structure of (-)-woodinine (I), a carboline-based alkaloid with antibacterial and antimycobacterial activities, was investigated by X-ray crystallog., NMR, and semiempirical quantum chem. methods. The X-ray crystal structure of I showed the indole ring in the expected planar conformation, the pyrrolidine ring in an envelope conformation, and a weak intramol. hydrogen bond between the pyrrolidine nitrogen and the proton of the indole nitrogen. NMR expts. indicated that this hydrogen bond is not present in soln. and that further differences exist between the crystal and the soln. structures of I. By semiempirical quantum chem. methods, different local energy min. conformations of I, resulting from inversions within the piperidine moiety and from rotation of the pyrrolidine ring, were calcd. The totality of all NOE signals can only be explained to originate from a population of some of these conformers, and addnl. from different envelope and twisted conformations of the pyrrolidine ring.