NMR Spectra of Sparteine N1-oxide and α-Isosparteine N-oxide

Sparteine N1-oxide and α-isosparteine N-oxide were prepared and their structures determined for the first time by 1H- and 13C-NMR spectroscopy using two-dimensional techniques. The N-oxide effects were also calculated.


Introduction
The wide use of quinolizidine alkaloids in chemistry is related first of all with a possibility of configurational-conformational changes that could take place in the bis-quinolizidine skeleton. Naturally occurring (-)-sparteine is an equilibrium mixture in which the conformer possessing a boat ring C and trans junction of rings C/D predominates [1][2][3][4]. On the other hand, the less stable all-chair conformer participates in complex formation [5][6][7]. The compound α-isosparteine, consisting of two trans-quinolizidine systems, exists solely in an all-chair conformation, and similarly in the free base form [8] and in metal complexes [5][6][7]9,10]. As a continuation of our study on the complex forming ability of bis-quinolizidine alkaloids [6,7,9,10], the choice of N-oxides as ligands was made. We have already obtained the complexes of sparteine N16-oxides with lithium [11] and zinc [12] salts. This time the subject of our study was the synthesis of the complexes of sparteine N1-oxide, sparteine epi-N-oxide and α-isosparteine N-oxide.
NMR spectroscopy is known to permit observation of conformational changes taking place in the structure of the ligands during complexation reactions. Moreover, a comparison of the chemical shifts of carbon atoms and protons of the initial alkaloid and the complex formed, enables determination of the effects of complexation. The present work is a continuation of our studies on the structural investigation of bis-quinolizidine alkaloids [13][14][15][16][17]. The NMR data of sparteine N16-oxide and sparteine epi-N-oxide have been presented before [11,18]. In this paper we present the NMR spectra of sparteine N1-oxide and α-isosparteine N-oxide. For each of them the N-oxide effects were determined. The two conformers of sparteine form three isomeric mono N-oxides: sparteine N1-oxide (1), sparteine N16-oxide (2) and sparteine epi-N-oxide (3). In the reactions of sparteine with H 2 O 2 a 1:3 mixture of the two sparteine N-oxides: sparteine N 1 -oxide and sparteine N16-oxide is obtained [19,20]. Sparteine N1-oxide (1) occurs in a chair-boat type equilibrium involving inversion of the lone pair on the N16 atom [20]; the N16-oxide of sparteine (2), previously thought to adopt the all chair conformation, has been found recently to have ring C in a boat conformation and a cis C/D ring junction [21]. Sparteine epi-N-oxide (3) has the C ring boat conformation and must be obtained by NaBH 4 reduction of lupanine N-oxide [20]. α-Isosparteine N-oxide (4) has a conformation identical to that of the free base ( Figure 1) [19].

Results and Discussion
In order to obtain the NMR spectra of the two conformers of the N1-oxide of sparteine (1a and 1b), the NMR spectra were measured in two solvents: CDCl 3 and DMSO-d 6 . The NMR spectrum of sparteine N1-oxide recorded in DMSO-d 6 solution is typical of that expected for pure 1a conformer, while the spectrum recorded in CDCl 3 seems to be that of sparteine N1-oxide hydrochloride (all-chair conformation) since in its 1 H-NMR spectrum the signal of the "acid" proton appears at 17.5 ppm.
The 1 H-and 13 C-NMR data for sparteine N1-oxide (conformer 1a), sparteine N1-oxide hydrochloride (1-HCl) and free base of sparteine are collected in Table 1. The N-oxide effect can be derived as a difference in the chemical shifts of the appropriate carbon atoms in N-oxide and its basic amine. This effect is superimposed by the solvent effect as the solvent is changed from CDCl 3 (free base of sparteine) into DMSO-d 6 (sparteine N1-oxide). The 13 C chemical shifts for C7, C8, C9, C12, C13, C14 and C15 carbon atoms of conformer 1a approximate the analogous δ C values of sparteine [22]. This result corroborates the presence of chemically unchanged C and D rings preserving the transquinolizidine form in the N-oxide. The deshielding influence of the N-oxide function, generated on the N1 nitrogen of the alkaloid considered, causes a down-field shift of the α carbons, i.e. C2 (Δδ C = 13.4 ppm), C6 (Δδ C = 4.3 ppm) and C10 (Δδ C = 8.7 ppm), as compared with respective sparteine δ C values. Table 1. 13 C-and 1 H-NMR chemical shifts of sparteine, sparteine N1-oxide (conformer 1a) and sparteine N1-oxide hydrochloride (1-HCl) (δ in ppm).
Analysis of the chemical shifts given in Table 1 shows that the conformational changes are observed for the bis-quinolizidine skeleton in sparteine N1-oxide hydrochloride on passing from the free base to the N-oxide salt. The γ-gauche effects which usually accompany a conformational change from boat-chair to all-chair [23] are observed at carbon atoms C12 (Δδ C = 10.0 ppm), C14 (Δδ C = 7.4 ppm) and C17 (Δδ C = 9.5 ppm) . The conformational changes are superimposed by the N-oxide effect  assuming the greatest values for the carbon atoms in the α position with respect to the N-oxide group: C2 (+10.9 ppm), C6 (+6.6 ppm) and C10 (+9.1 ppm).
The β-effect influencing the secondary carbon atoms in the outer ring (A) is negative, amounting to -5.7 and -3.1 ppm for 1a and -5.3 and -4.8 ppm for 1-HCl. In the inner ring (B), the oxidation effects on on the tertiary carbon atoms differ only slightly from 0. The γ-effect in the outer ring amounts to ca. -1.4 ppm (1a) and -2.7 ppm (1-HCl), in the inner rings (B and C), it amounts ca. -1.3, -5.0, -6.4 ppm for the boat conformer 1a and -3.4, -5.8 and -9.5 ppm for the hydrochloride salt. In the proton spectra, the greatest N-oxidation effect is observed for C2, C6 and C10 ( Table 1). The protons connected with these carbons appear within the range 5.00 > δ H > 3. 10. The NMR spectra of α-isosparteine N-oxide and the free base of α-isosparteine are shown in Table 2. The spectra of both compounds display the 13 C chemical shift of C8 characteristic of the α-isosparteine skeleton structure (32.8 ppm for α-isosparteine and 31.7 ppm for the N-oxide). The relatively rigid skeleton of α-isosparteine allows us to determine the N-oxide effects more precisely than in the flexible sparteine. The α N-oxide effect for the metine carbon atom (C6) is +7.2 ppm, and for methylene carbon atoms (C2, C10) +11.0 and + 9.8 ppm, respectively, while the value of the N-oxide β effect on carbon atoms C3, C5, C7 and C9 range from -4.2 to +0.9 ppm. The γ-effect in the A ring amounts to ca. -1.1 ppm. In the rings B and C, it is generally greater and amounts to ca. -5.1 ppm. Chemical shift changes (from -0.6 to -3.1 ppm) are noted on the carbon atoms of ring D. These changes follow mainly from the presence of the N-O group. It seems probable that the effect of a slight change in the geometry of the molecule involving greater deformation of rings B and C than that of ring A (following an elongation of the N-C bonds as a result of introducing the N + −Ofunction) is imposed on the direct N-oxidation effect in the inner rings. In the proton spectra, the greatest Noxidation effect ( Table 2) is for H6 (Δδ H = 1.88 ppm). Other large effects are those on the α-axial protons (above 1.70 ppm). Also significant are the effects on α-equatorial protons (0.80 ppm).
The conformational assignment for compounds 1a, 1-HCl and 2 was also carried out by comparison of the experimental 13 C-NMR chemical shifts with those predicted by DFT/CSGT shielding calculations. The results of these calculations for the optimized structures, together with the experimental values are listed in Table 3. The correlation coefficient (R 2 ) for carbon chemical shifts is 0.97 for 1a, 0.98 for 1-HCl and 0.98 for 2. The results suggest that the structures of the N-oxides investigated in solution and under vacuum are the same. Table 3. Comparison of CSGT chemical shifts (δ, in ppm) calculated at the DFT level of theory for sparteine N1-oxide (conformer 1a), sparteine N1-oxide hydrochloride (1-HCl) and α-isosparteine N-oxide (2). (2) δ theor.

Conclusions
The NMR data of sparteine N1-oxide and α-isosparteine N-oxide have been presented. The spectrum of sparteine N1-oxide recorded in DMSO-d 6 solution is typical of that expected for the boat conformer 1a, while the spectrum recorded in CDCl 3 solution turned out to be a spectrum of sparteine N1-oxide hydrochloride (in the all chair conformation 1b). The similarity of the conformation of 1a and 4 to that of their free bases allowed us to determine the N-oxidation effect better than previously possible.

General
The 1 H-and 13 C-NMR spectra (including 1 H-1 H COSY, 13 C-1 H COSY) were measured on a Varian 300 Mercury spectrometer operating at 300.13 and 75.462 MHz, respectively, and at ambient temperature, using ~0.5 M solutions in CDCl 3 and DMSO-d 6 with TMS as internal reference. The conditions of the spectra recording were: 13
α-Isosparteine N-oxide (4). α-Isosparteine (234 mg) was dissolved in methanol (6 mL) and 30% aqueous hydrogen peroxide (4 mL) was added. The reaction was complete after 1 day (TLC). A small amount of palladium on asbestos was added to decompose excess H 2 O 2 and filtered off after 12 h. The solvents were evaporated under reduced pressure giving a white crystalline product. Yield: 64%.

DFT calculations
The 13 C-NMR absolute shielding constants (σ values) were calculated at the B3LYP/DFT level with the continuous set of gauge transformations (CSGT) method using the (6)6-311+G basis set. The calculated magnetic shieldings were converted into the δ chemical shifts using the calculated 13 C absolute shieldings in TMS (177.3) at the same level of theory [3].