Abstract
The title compound [systematic name: (1R*, 8S)-2-acetamidoocta-hydro-pyrrol-izin-4-ium chloride-N-[(1R, 8S)-hexa-hydro-1H-pyrrolizin-2-yl)acetamide (1/1)], 2(C9H16N2O)·HCl or C9H17N2O+·Cl-·C9H16N2O, arose as an unexpected product when 1-exo-acetamido-pyrrolizidine (AcAP; C9H16N2O) was dissolved in CHCl3. Within the AcAP pyrrolizidine group, the unsubstituted five-membered ring is disordered over two orientations in a 0.897 (5):0.103 (5) ratio. Two AcAP mol-ecules related by a crystallographic twofold axis link to H+ and Cl- ions lying on the rotation axis, thereby forming N-H⋯N and N-H⋯Cl⋯H-N hydrogen bonds. The first of these has an unusually short N⋯N separation of 2.616 (2) Å: refinement of different models against the present data set could not distinguish between a symmetrical hydrogen bond (H atom lying on the twofold axis and equidistant from the N atoms) or static or dynamic disorder models (i.e. N-H⋯N + N⋯H-N). Computational studies suggest that the disorder model is slightly more stable, but the energy difference is very small.
Highlights
In the course of our ongoing studies on the biosynthesis of loline alkaloids (Schardl et al, 2007; Pan et al, 2018), we had occasion to prepare 1-exo-acetamidopyrrolizidine (C9H16N2O; AcAP) by reduction of the corresponding oxime (Fig. 1) (Pan et al, 2014)
This paper describes the structure of crystalline 2AcAPÁHCl, which features an unusually short +N—HÁ Á ÁN interaction
Aside from an unusually short N—HÁ Á ÁN hydrogen bond, which will be discussed in subsequent sections, all geometrical parameters are within their expected ranges (e.g., Allen et al, 1987)
Summary
In the course of our ongoing studies on the biosynthesis of loline alkaloids (Schardl et al, 2007; Pan et al, 2018), we had occasion to prepare 1-exo-acetamidopyrrolizidine (C9H16N2O; AcAP) by reduction of the corresponding oxime (Fig. 1) (Pan et al, 2014). As part of our effort to prove unambiguously that the major diastereomer obtained in this reaction was the exo diastereomer, we attempted to obtain crystals of AcAP that were suitable for X-ray analysis. We obtained sufficiently high-quality crystals by recrystallization from CHCl3, but to our surprise, the analysis showed that the crystals were not AcAP, but 2AcAPÁHCl, with the HCl presumably originating from amine-promoted decomposition of CHCl3. This paper describes the structure of crystalline 2AcAPÁHCl, which features an unusually short +N—HÁ Á ÁN interaction.
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