(‒)-Sparteine (1) and (–)-(α)-isosparteine (2) are members of the lupine alkaloid family.[1-2] Sparteine has found extensive use in asymmetric organic transformations, including lithiations and Pd-catalyzed oxidations.[4-7] (α)-Isosparteine, which can be made from sparteine, has been utilized as a chiral ligand for a limited number of stereoselective reactions.[8-9] The two compounds differ in that 1 displays an exo-endo arrangement of the bridgehead hydrogens at C-11 and C-6, respectively, while 2 retains an exo-exo arrangement of these atoms (Figure 1). This study is focused on assigning 1H chemical shifts and coupling constants and 13C chemical shifts for N-methyl derivatives of sparteine and isosparteine, both of which have been fully characterized by X-ray crystallography. X-ray analysis of (N-methyl)-(–)-sparteinium iodide (3) revealed a chair-chair-boat-chair conformation (Figure 1),[10-11] and its 1H and 13C NMR chemical shift assignments were reported by Duddeck and co-workers in 1995. An X-ray analysis of (N-methyl)-(α)-isosparteinium iodide (4) showed an all-chair conformation in which the N-CH3 group is positioned in close proximity to the transannular nitrogen lone pair, resulting in a +NCH•••N hydrogen bond. Our group has harnessed the bridging geometry in 4 with an equilibrium isotope effect to investigate 1H and 3H chemical shift differences in (N-CH2D) and (N-CHDT) isotopologs of 4.[14-15] Simeonov, Duddeck, and co-workers have previously reported 1H and 13C NMR chemical shift assignments for 4 dissolved in DMSO-d6. We noticed discrepancies between our 1H and 13C assignments for 3 and 4 and values reported in the earlier studies. This was especially true for the 1H data for 4, where 16 out of 27 assignments differ from the previously reported values. Spectral assignments for 3 and 4 are also compared with quantum-mechanically computed 13C and 1H NMR chemical shifts[17-21] to further validate the assignments reported here.
K. Kolahdouzan, O. M. Ogba, D. J. O’Leary, Magn. Reson. Chem. 2019, 57, 55, https://doi.org/10.1002/mrc.4792.
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