1590
J. Org. Chem. 2001, 66, 1590-1599
NAD/NADH Models with Axial/Central Chiralities: Superiority of the Quinoline Ring System Yuji Mikata,* Kiyoko Mizukami, Kayo Hayashi, Sawako Matsumoto, Shigenobu Yano, Norimasa Yamazaki,† and Atsuyoshi Ohno† Department of Chemistry, Faculty of Science, Nara Women’s University, Nara 630-8506, Japan
[email protected] Received May 31, 2000
Precursors of NAD model compounds 1c and 3a,b were successfully resolved into their atropisomers with respect to carbamoyl rotation. Atropisomers of quinoline derivatives are much more stable than pyridine derivatives as determined by cyclic voltammetry and X-ray crystallography. The 1,4-reduction of NAD model compound 4 was successfully achieved, affording novel NADH model compound 5. The rotational properties of the side chain of 5 were investigated by means of dynamic NMR. The rotational rate and syn/anti ratio, which indicate the orientation between carbonyl oxygen and hydrogen at the 4-position, are significantly affected by addition of magnesium ion. In the rotational transition state, the double-bond character of the Ccarbonyl-Namide bond is disrupted judging from the activation parameters. The oxidation of chiral 5 with p-benzoquinone in the presence of magnesium ion catalyst gave predominantly one enantiomer of 4. On the other hand, oxidation of 5 with p-chloranil (tetrachloro-p-benzoquinone) in the absence of magnesium ions affords the opposite enantiomer of 4 as the major product. The product enantiomer ratio is parallel to the syn/anti ratio in the starting material, indicating the importance of ground state conformation to stereochemistry of the reaction. Introduction There is controversy in the literature over how enzymes discriminate between the two hydrogen atoms in NADH coenzyme.1 Some enzymes use the HA hydrogen in the C4 position of the nicotinamide ring of the coenzyme, while others use the HB hydrogen, even when the same reaction is catalyzed (Scheme 1).2,3 Among several possible explanations,4-8 there are interesting reports that the carbonyl orientation of NAD coenzyme plays an important role in determining the reaction face of NAD in model systems.9-13 It has been suggested that the carboxamide moiety of natural NAD binds to amino acid residues in the active center of the enzyme and the carbonyl oxygen orients the reacting hydrogen in the * To whom correspondence should be addressed. Tel/Fax: +81-74220-3392. † Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan. (1) You, K. CRC Crit. Rev. Biochem. 1985, 17, 313. (2) Benner, S. A.; Nambiar, K. P.; Chambers, G. K. J. Am. Chem. Soc. 1985, 107, 5513. (3) Allemann, R. K.; Hung, R.; Benner, S. A. J. Am. Chem. Soc. 1988, 110, 5555. (4) Nambiar, K. P.; Stauffer, D. M.; Kolodzı`ej, P. A.; Benner, S. A. J. Am. Chem. Soc. 1983, 105, 5886. (5) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon Press: Oxford, 1983; p 340. (6) Oppenheimer, N. J. J. Am. Chem. Soc. 1984, 106, 3032. (7) Schneider-Bernlo¨hr, H.; Adolph, H.-W.; Zeppezauer, M. J. Am. Chem. Soc. 1986, 108, 5573. (8) Glasfeld, A.; Benner, S. A. Eur. J. Biochem. 1989, 180, 373. (9) Ohno, A.; Ohara, M.; Oka, S. J. Am. Chem. Soc. 1986, 108, 6438. (10) Ohno, A.; Kashiwagi, M.; Ishihara, Y.; Ushida, S.; Oka, S. Tetrahedron 1986, 42, 961. (11) Ohno, A.; Ogawa, M.; Oka, S. Tetrahedron Lett. 1988, 29, 1951. (12) Ohno, A.; Ogawa, M.; Mikata, Y.; Goto, M. Bull. Chem. Soc. Jpn. 1990, 63, 813. (13) Ohno, A.; Mikata, Y.; Goto, M.; Kashiwagi, T.; Tanaka, T.; Sawada, M. Bull. Chem. Soc. Jpn. 1991, 64, 81.
Scheme 1
nicotinamide ring.14 Quantum-mechanical calculations in simple model systems support the hypothesis that the stabilizing effect of the transition state orients the hydride transfer syn to the carbonyl group.15 In light of these proposals, construction of NADH model systems where the carbonyl oxygen sticks out of the nicotinamide plane is of significant interest. As part of our program to elucidate the nature of coenzyme specificity, we reported that NAD model compounds possessing two methyl groups in the 2- and 4-position of pyridine or quinoline ring systems and two alkyl groups at the carbamoyl nitrogen have enantiomers with respect to the carbonyl rotation.9-13 To maintain the stability of the atropisomers, two alkyl substituents on the amide nitrogen are prerequisites for the 2,4-dimethyl nicotinamide ring system. However, in the NAD model compounds having a 2,4-dimethylpyridine ring, the carbamoyl nitrogen substitution results in difficulty in obtaining the NADH models by 1,4-reduction.16 (14) Eklund, H.; Samama, J.-P.; Jones, T. A. Biochemistry 1984, 23, 5982. (15) Donkersloot, M. C. A.; Buck, H. M. J. Am. Chem. Soc. 1981, 103, 6554. (16) de Kok, P. M. T.; Donkersloot, M. C. A.; van Lier, P. M.; Meulendijks, G. H. W. M.; Bastiaansen, L. A. M.; van Hooff, H. J. G.; Kanters, J. A.; Buck, H. M. Tetrahedron 1986, 42, 941.
10.1021/jo000829w CCC: $20.00 © 2001 American Chemical Society Published on Web 02/08/2001
NAD/NADH Models with Axial/Central Chirality Chart 1
J. Org. Chem., Vol. 66, No. 5, 2001 1591
of the ground state conformation in the NADH model reaction using this compound.28 Here, the differences in stability of atropisomers and in reactivity of 1,4-reduction between 2,4-dimethylpyridine- and 2,4-dimethylquinoline-type NAD model compounds are discussed in terms of enantiomer separation, hydrosulfite reduction, cyclic voltammetry, and X-ray crystallographic studies. We also report the rotational properties of side chains of quinoline-type NADH model compounds as well as stereospecific reactions of chiral NADH model compounds. A portion of these results have been reported as a preliminary communication.29 Results
Vekemans and co-workers synthesized a corresponding NADH model compound, 3-(N,N-dimethylcarbamoyl)1,2,4-trimethyl-1,4-dihydronicotinamide (I, Chart 1), by a synthetic route other than the 1,4-reduction of the corresponding pyridinium ion.17 However, from the viewpoint of coenzyme modeling, redox reversibility is an indispensable part of NAD/NADH model systems. The C3-C6 strapped NAD model compound (II) with a fixed carbonyl dipole reported by the same group18 undergoes 1,4-reduction mediated by sodium hydrosulfite. This reduction occurred with high face selectivity (>95%), but the results must be considered as the sum of electronic and steric19 effects. We have developed C2,C3-annulated NADH model compounds with fixed carbonyl groups arising from the steric hindrance between an o-methyl group of the aryl substituent at C2 and a methyl group at N1 (III), derived from the Na2S2O4 reduction of the corresponding NAD model compound.20-23 Bourguignon and co-workers have also reported the effect of a chiral auxiliary having an alcohol group in the amide substituent of the C2,C3-annulated 1,4-dihydropyridine systems (IV).24-27 In the present paper, we demonstrate the superiority of quinoline-type NADH model compounds having unannulated amide moieties with simple structures. We previously reported the formation of a quinoline-type NADH model compound (V) by the reduction of corresponding quinolinium ion10 and proposed the importance (17) de Kok, P. M. T.; Bastiaansen, L. A. M.; Van Lier, P. M.; Vekemans, J. A. J. M.; Buck, H. M. J. Org. Chem. 1989, 54, 1313. (18) de Kok, P. M. T.; Buck, H. M. J. Chem. Soc., Chem. Commun. 1985, 1009. (19) Kanomata, N.; Nakata, T. J. Am. Chem. Soc. 2000, 122, 4563. (20) Ohno, A.; Tsutsumi, A.; Kawai, Y.; Yamazaki, N.; Mikata, Y.; Okamura, M. J. Am. Chem. Soc. 1994, 116, 8133. (21) Ohno, A.; Tsutsumi, A.; Yamazaki, N.; Okamura, M.; Mikata, Y.; Fujii, M. Bull. Chem. Soc. Jpn. 1996, 69, 1679. (22) Ohno, A.; Ishikawa, Y.; Yamazaki, N.; Okamura, M.; Kawai, Y. J. Am. Chem. Soc. 1998, 120, 1186. (23) Ohno, A.; Oda, S.; Ishikawa, Y.; Yamazaki, N. J. Org. Chem. 2000, 65, 6381. (24) Combret, Y.; Duflos, J.; Dupas, G.; Bourguignon, J.; Que´guiner, G. Tetrahedron: Asymmetry 1993, 4, 1635. (25) Be´dat, J.; Ple´, N.; Dupas, G.; Bourguignon, J.; Que´guiner, G. Tetrahedron: Asymmetry 1995, 6, 923. (26) Be´dat, J.; Levacher, V.; Dupas, G.; Que´guiner, G.; Bourguignon, J. Chem. Lett. 1995, 327. (27) Be´dat, J.; Levacher, V.; Dupas, G.; Que´guiner, G.; Bourguignon, J. Chem. Lett. 1996, 359.
Synthesis and Optical Resolution of NAD Model Precursors. The reaction of secondary amines with activated 2,4-dimethylpyridine-3-carboxylic acid30 or 2,4dimethylquinoline-3-carboxylic acid10 gives the corresponding precursors of NAD model compounds 1 and 3 (Schemes 2 and 3). Synthetic attempts to obtain N,Ndiisopropylcarbamoyl-2,4-dimethylquinoline via similar methods were unsuccessful, resulting instead in N-isopropylcarbamoyl-2,4-dimethylquinoline in good yield (Scheme 4). The existence of enantiomer pairs in these compounds with respect to carbamoyl rotation was confirmed by HPLC analysis (Daicel CHIRALCEL OD using i-PrOH/hexane ) 9/1 as an eluent). Resolution of these compounds into their enantiomers by preparative HPLC was successful for the compounds 1c and 3a,b. Enantiomers of 1c and 3a,b showed satisfactory values and signs in optical rotation and CD spectra (Table 1 and Figure 1). The absolute configuration of axial chirality in these compounds was assigned from the sign of their CD spectra by comparison to those reported by Vekemans and co-workers.17,31-33 Reduction of NAD Model Compounds by Sodium Hydrosulfite. The Menschutkin reactions of compounds 1 and 3 by alkyl halides to obtain the corresponding NAD model compounds 2 and 4 were performed in acetonitrile (Schemes 2 and 3). These reactions require heat, which results in carbamoyl moiety rotation during the reaction. The racemic NAD compounds showed two peaks in analytical HPLC corresponding to their enantiomers; however, they could not be separated by preparative conditions. Assignment of the absolute configuration with respect to the axial chirality for these peaks was determined from their retention time in HPLC.29 The reduction of these NAD models to NADH model compounds for the pyridine ring series with the N1methyl group (2a-c) resulted in recovery of unreacted starting material. In contrast, reduction of the quinolinetype compounds (4a,b) by sodium hydrosulfite gave the corresponding NADH model compounds (5a,b) in 8090% yield (Scheme 3). Other pyridine derivatives (2d,e) (28) Okamura, M.; Mikata, Y.; Yamazaki, N.; Tsutsumi, A.; Ohno, A. Bull. Chem. Soc. Jpn. 1993, 66, 1197. (29) Mikata, Y.; Hayashi, K.; Mizukami, K.; Matsumoto, S.; Yano, S.; Yamazaki, N.; Ohno, A. Tetrahedron Lett. 2000, 41, 1035. (30) Ohno, A.; Ikeguchi, M.; Kimura, T.; Oka, S. J. Am. Chem. Soc. 1979, 101, 7036. (31) Bastiaansen, L. A. M.; Kanters, J. A.; van der Steen, F. H.; de Graaf, J. A. C.; Buck, H. M. J. Chem. Soc., Chem. Commun. 1986, 536. (32) Bastiaansen, L. A. M.; Vermeulen, T. J. M.; Buck, H. M.; Smeets, W. J. J.; Kanters, J. A.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1988, 230. (33) Vekemans, J. A. J. M.; Boogers, J. A. F.; Buck, H. M. J. Org. Chem. 1991, 56, 10.
1592
J. Org. Chem., Vol. 66, No. 5, 2001
Mikata et al. Scheme 2
Scheme 3
Scheme 4
Table 1. Summary of Optical Resolution of 1c and 3a,b compd 1c 3a 3b
HPLC fraction
Cotton effect at ∆ (265 nm)
[R]D
absolute confign
first second first second first second
+ + +
+39 (c ) 0.75) -34 (c ) 0.75) -27 (c ) 0.79) +30 (c ) 0.70) -30 (c ) 0.89) +32 (c ) 0.82)
R S R S R S
with large N1 substituents, such as propyl or benzyl groups, afforded a dealkylated product (1c) after treatment with sodium hydrosulfite (Scheme 5). Compound 1c was obtained quantitatively via Hoffmann-type elimination by stirring 2d in aqueous sodium carbonatedichloromethane without sodium hydrosulfite, indicating the difficulty of reduction of pyridine-type NAD model compounds under similar conditions. Cyclic Voltammetry of NAD Model Compounds. The cyclic voltammetry (CV) studies of 2a-c and 4a,b as well as pyridine derivatives 6 (elimination of 2,4dimethyl substituent from 2b, Chart 2) and 7 (substitution of 2-H of 6 to chlorine) in DMF were performed in order to clarify the factors stabilizing the NAD• radical. An irreversible voltammogram with a reduction peak potential Epc ) -1.94 V (vs Fc/Fc+) was obtained in the CV measurement of compound 2a at a scan rate of 100 mV/s (Figure 2a). This irreversibility was maintained at higher sweep rates (up to 2 V/s), indicating that the pyridinium radical produced via one-electron reduction is unstable in DMF and undergoes further reaction immediately. Compounds 2b,c, 6, and 7 showed similar CV behavior. In contrast, the quinolinium ion 4a afforded a reversible voltammogram where E1/2 ) -1.10 V (Epc ) -1.43 V) at a scan rate of 100 mV/s, indicating higher stability for the quinolinium radical and higher electron-accepting
character of the quinolinium ring (Figure 2b). The reduction peak potentials and CV behaviors of pyridineand quinoline-type NAD model compounds are summarized in Table 2. X-ray Crystallography. Recrystallization of racemic 2a and 4a from CH2Cl2/EtOH afforded yellow crystals suitable for X-ray crystallography. The crystal data, experimental conditions, atomic coordinates, bond lengths, and angles are presented in the Supporting Information. Rather long Cring(sp2)-Ccarbonyl(sp2) bond distances are observed (1.51(1) Å for 2a and 1.518(8) Å for 4a). The crystal structures of these compounds (Figures 3 and 4) indicate the CdO bond is perpendicular to the aromatic plane and the amide moieties including the two methyl carbons at the amide nitrogen (CON(Cmethyl)2) are in a planar configuration. Conformational Analysis of 5. As anticipated, the axial chirality was lost upon reduction of quinolinium ion 4 to 5. At -40 °C in CD3CN solution, the two diastereomers of 5 arising from the central chirality at the 4-position and axial chirality at the C3 position were distinguished by 1H NMR spectroscopy. The magnetically different species become observable on the NMR experimental time scale at low temperature. The isomers with a 4-H resonance at lower field in the 1H NMR were assigned to the syn isomer, where the carbonyl oxygen is pointing in the same orientation as the hydrogen atom at the C4 position, because of the anisotropic effects of the carbonyl oxygen.28,35 The syn/anti isomer ratio was 45/55 for 5a and 40/60 for 5b. The rates for carbonyl rotation at each temperature were estimated by line shape analysis. Figure 5a shows partial 1H NMR spectra for the N1-Me signal of 5b at various temperatures and their calculated spectra. As shown in Figure 5b, the carbonyl rotation was significantly suppressed in the presence of magnesium ion, which is generally used as a catalyst for NADH model reactions. A similar investigation was performed for compound 5a using the 4-H signal with decoupling at the (34) Schmidt, R. R.; Berger, G. Chem. Ber. 1976, 109, 2936. (35) Bean, J. W.; Nelson, D. J.; Wright, G. E. Biochem. Pharmacol. 1986, 35, 1011.
NAD/NADH Models with Axial/Central Chirality
J. Org. Chem., Vol. 66, No. 5, 2001 1593
Figure 1. CD spectra of enantiomers of (a) 1c, (b) 3a, and (c) 3b in EtOH.
Scheme 5
Table 2. Reduction Peak Potential (Epc), CV Behavior, and Activity of 1,4-Reduction of NAD Models compd
Epc/V vs Fc/Fc+
CV behavior
1,4-reduction with Na2S2O4
2a 2b 2c 4a 4b 6 7
-1.94 -1.84 -1.87 -1.43 -1.45 -1.62 -1.30
irreversible irreversible irreversible reversible reversible irreversible irreversible
no no no yes yes yesa nob
Chart 2 a
Figure 2. Cyclic voltammogram of (a) 2a and (b) 4a in DMF. Scan rate ) 100 mV/s. Potentials were referred to aqueous Ag/AgCl.
4-Me resonance (Figure S1, Supporting Information). The activation parameters (∆G*, ∆H*, and ∆S*) for carbonyl rotation in the presence/absence of magnesium ion, calculated from the Arrehenius plot (R > 0.999), are listed in Table 3 together with those for compound V from our previous work.28 The syn/anti ratio changed significantly by addition of magnesium ion affording the syn conformer predominantly (syn/anti ) 79/21 for 5a and 75/25 for 5b).
Reference 34. b Gave compound 8.
Figure 3. ORTEP drawing of cation part of 2a.
Optical Resolution of 5. Chiral HPLC using the same column as in the resolution of the axially chiral quinoline precursors (vide supra) separated the racemic materials 5 into their enantiomers. The enantiomers were well resolved (eluent: i-PrOH/hexane ) 20:1 for 5a and 9/1 for 5b); however, there was an extra peak in the HPLC trace (Figure S2). A similar observation was reported by Vekemans and co-workers in the case of compound I.17 The 1H NMR spectra of the racemic compounds of 5 showed that there was a minor (
