Mechanisms of the oxidations of NAD(P)H model Hantzsch 1,4-dihydropyridines by nitric oxide and its donor N-methyl-N-nitrosotoluene-p-sulfonamide

Article abstract of DOI:10.1021/jo000484h

4-Substituted derivatives of Hantzsch 1,4-dihydropyridine were treated by nitric oxide (NO) or its donor N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS) to give the corresponding pyridine derivatives. When the 4-substituted group was methyl, ethyl, n-propy

Full text of DOI:10.1021/jo000484h

8158
J . Org. Chem. 2000, 65, 8158-8163
Mech a n ism s of th e Oxid a tion s of NAD(P )H Mod el Ha n tzsch
1,4-Dih yd r op yr id in es by Nitr ic Oxid e a n d Its Don or
N-Meth yl-N-n itr osotolu en e-p-su lfon a m id e
Xiao-Qing Zhu,* Bing-J un Zhao, and J in-Pei Cheng*
Department of Chemistry, Nankai University, Tianjin 300071, China
xqzhu@nankai.edu.cn
Received March 29, 2000
4-Substituted derivatives of Hantzsch 1,4-dihydropyridine were treated by nitric oxide (NO) or its
donor N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS) to give the corresponding pyridine deriva-
tives. When the 4-substituted group was methyl, ethyl, n-propyl, and aryl groups, it was preserved,
but when the group was isopropyl or benzyl one, it was lost. 2,3-Dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) was used in place of NO and MNTS to react with the 4-substituted Hantzsch
1,4-dihydropyridines, no the corresponding 4-dealkyl Hantzsch pyridines were obtained from all
the reactions. 1-Benzyl-1,4-dihydronicotinamide (BNAH), a close analogue of Hantzsch 1,4-
dihydropyridine (HEH), was used instead of HEH to react with either of NO and MNTS, no reactions
were observed for 3 days. Replacement of HEH by N-d-HEH and HEH-4,4-d2 to react with NO,
MNTS and DDQ gave the observed kinetic isotope effects of 3.1 and 1.4 for NO, 1.1 and 1.3 for
MNTS, and 1.1 and 2.1 for DDQ, respectively. When p-dinitrobenzene, an electron-transfer inhibitor,
was added into the title reaction systems, no remarkable inhibitory effect was observed. These
results indicated that the oxidation of HEH by NO was initiated by hydrogen transfer from the
N1-position to give the corresponding aminyl radical, which then underwent homolytic cleavage to
become the final aromatized product (A). But the reaction of HEH with MNTS was initiated by
nitrosation to give the corresponding N-nitroso compound, which was subsequently subjected to
two steps of homolytic cleavage to afford the aromatized Hantzsch pyridine A.
In tr od u ction
an oxidation of NAD(P)H coenzyme by NO (eq 1),
whereas some other people6 promote an alternate opinion
that NO from diverse sources in aqueous solution at
physiological pH does not appreciable alter either the
molecular properties or redox function of NAD(P)H. Thus,
the following questions need to be further clarified: (i)
Can NO oxidize NAD(P)H in vivo? (ii) If the oxidation
could take place, how does it take place?
Great interest has been accumulated in recent years
in the chemistry of nitric oxide (NO) upon many striking
discoveries of its active roles in a wide range of human
physiological processes,1 and the papers concerning the
chemistry of NO have been gradually increasing. The
published reports seem to be classified into three types:
(i) the reactivities of NO with some biologically important
molecules;2 (ii) nitric oxide group transfer (including
release) in some biological systems or the models;3 (iii)
development of the detection method of NO using organic
reactions.4 However, to the best of our knowledge, very
little research has hitherto been reported in the literature
on the reaction mechanisms of NO or its donor with some
biologically important molecules.
NAD(P)H + 2NO + H+ f NAD(P)+ + N2O + H2O
(1)
To further probe the possibility of the reaction (eq 1)
and elucidate the details of the reaction mechanism
(provided that the reaction could take place), the chemical
mimic method will be used in this work as a powerful
tool. As is well known, Hantzsch 1,4-dihydropyridine
derivatives which contain the 1,4-dihydropyridine struc-
ture of natural reduced nicotinamide adenine dinucle-
otide [NAD(P)H] coenzyme and possess a high biological
activity as a class of useful drugs, particularly as anti-
oxidants, can be chosen as models of NAD(P)H to react
with NO or its donor N-methyl-N-nitrosotoluene-p-sul-
fonamide (MNTS) to mimic the oxidation of NAD(P)H by
NO in vivo. Ohsawa and co-workers, in their previous
The reduced form of nicotinamide adenine dinucleotide
[NAD(P)H] is a typical coenzyme, which plays a vital role
in many biological redox reactions. Much research5
suggested that the fungal denitrification process contains
* To whom correspondence should be addressed. Tel: 86-22-
23508548. Fax: 86-22-23502458.
(1) (a) Moncada, S.; Dalmer, R. M. J .; Higgs, E. A. Pharmacol. Rev.
1991, 43, 109. (b) Stamler, J . S.; Singel, D. J .; Loscalzo, J . Science,
1992, 258, 1898. (c) Feldman, P. L.; Griffith, O. W.; Stuehr, D. J . Chem.
Eng. News, 1993, Dec 20, 26. (d) Butler, A. R.; Williams, D. L. H. Chem.
Soc. Rev. 1993, 22, 233.
(2) (a) Itoh, T.; Nagata, K.; Okada, M.; Ohsawa, A. Tetrahedron Lett.
1995, 36 2269. (b) J anzen, E. G.; Wilcox, A. L.; Manoharan, V. J . Org.
Chem. 1993, 58, 3597. (c) Wilcox, A. L.; J anzen, E. J . J . Chem. Soc.
Chem. Commun. 1993, 1377.
(3) Hrabie, J . A.; Klose, J . R. J . Org. Chem. 1993, 58, 1472.
(4) (a) Korth, H.-G.; Ingold, K. U.; Sustmann, R.; de Groot, H.; Sies,
H. Angew. Chem., Int. Ed. Engl. 1992, 31, 891. (b) Afzal, M.; Walton,
J . C. BioMed. Chem. Lett. 1996, 6, 2329.
(5) Shoun, H.; Sudo, Y.; Beppu, T. J . Biochem. 1983, 94, 1219. (b)
Shoun, H.; Suyama, W.; Yasui, T. FEBS Lett. 1989, 244, 11. (c) Kizawa,
H.; Tomura, D.; Oda, M.; Fukamizu, A.; Hoshino, T.; Gotoh, O.; Yasui,
T.; Shoun. H. FEBS Lett. 1991, 266, 10632. (d) Shoun, H.; Tanimoto,
T. FEBS Lett. 1991, 266, 11078. (e) Nakakara, K.; Tanimoto, T.;
Hatano, H.; Usuda, K.; Shoun, H. J . Biol. Chem. 1993, 268, 8350.
(6) Ewing, J . F.; J anero, D. R.; Grinnell, T. A.; Schroeder, J . D.;
Garvey, D. S. Archives Biochem. Biophys. 1997, 131-139.
10.1021/jo000484h CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/03/2000

Oxidations of Hantzsch 1,4-Dihydropyridines
J . Org. Chem., Vol. 65, No. 24, 2000 8159
paper,2a,7 reported that Hantzsch 1,4-dihydropyridines
were oxidized by NO to give the corresponding Hantzsch
pyridine derivatives in quantitative yields, but the reac-
tion mechanism remains poorly understood, so it seemed
desirable that more and better mechanistic probing
compounds should be employed to reveal the full mecha-
nistic details of this important reaction. As a part of our
research in this field,8 we in this paper have examined
the reactions of a series of 4-substituted Hantzsch 1,4-
dihydropyridine derivatives and their close analogue
1-benzyl-1,4-dihydronicotinamide (BNAH) with NO, MNTS
(a good nitroso-transfer agent) and 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ, a good hydride accep-
tor), respectively. On the basis of the experimental
evidence collected, the oxidation mechanisms of Hantzsch
1,4-dihydropyridines by NO and MNTS can be clearly
elucidated.
Ta ble 1. Oxid a tion s of Ha n tzsch 1,4-Dih yd r op yr id in e
a n d Its Va r iou s Der iva tives by NO, MNTS, a n d DDQa
[O]
entry
R1
R2
NOb
MNTSc
DDQd
1
2
3
4
5
6
7
8
9
H
C2H5
n-C3H7
i-Pr
benzyl
C6H5
p-CH3-C6H5
p-Cl-C6H5
p-CN-C6H5
p-CH3O-C6H5
p-NO2-C6H5
p-NO2-C6H5
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
A (96)
B (86)
B (84)
A (88)
A (84)
B (76)
B (74)
B (76)
B (79)
B (75)
B (77)
B (70)
A (41)
B (45)
B (47)
A (51)
A (59)
B (67)
B (68)
B (70)
B (73)
B (79)
B (71)
B (91)e
A (80)
B (75)
B (78)
B (84)
B (75)
B (90)
B (85)
B (85)
B (83)
B (79)
B (73)
B (72)
11
10
12
Resu lts a n d Discu ssion
The treatment of Hantzsch 1,4-dihydropyridine (HEH)
with excess pure NO gas or MNTS in dry acetonitrile in
a closed system gave a sole aromatized Hantzsch pyridine
(A) in quantitative yield as shown in eq 2.
a
The yield of A or B are given in parentheses. Mixtures of
products A and B from the same reaction were never observed.
b The reactions were conducted in acetonitrile at room temperature
for 4 h. c The reactions were conducted in acetonitrile at room
d
temperature for 3 days except entry 12. The reactions were
conducted in THF at room temperature for 60 min. e The reaction
was conducted in acetonitrile at room temperature for 2 days.
From eq 2, it is clear to be found that two hydrogen
atoms at the 1 and 4 positions in HEH were lost during
the reaction process. To provide information necessary
to postulate the reaction mechanism, a series of 4-sub-
stituted Hantzsch 1,4-dihydropyridines and 2,6-diphenyl-
4-(p-nitrophenyl)-HEH were prepared and oxidized by
excess pure NO gas, MNTS, and DDQ, respectively, and
two different types of the final aromatized products (A
and B) were obtained. The full reaction results are
tabulated in Table 1.
Kinetic isotope effects on the reaction (eq 2) were
determined using N-deuterated Hantzsch 1,4-dihydro-
pyridine (N-d-HEH) and 4,4-dideuterated Hantzsch 1,4-
dihydropyridine (HEH-4,4-d2) instead of HEH to react
with NO (see Figure 1), MNTS and DDQ by the UV-vis
method, respectively. The experimental results gave the
observed kinetic isotope effects of 3.1 (kN-H/kN-D) and 1.4
(kC4-H/kC4-D) for NO, 1.2 and 1.1 for MNTS, and 2.1 and
1.1 for DDQ, respectively, which means that the three
reactions could be carried out by different reaction
mechanisms. When p-dinitrobenzene, a well-known elec-
F igu r e 1. Absorption spectra changes of HEH (a), HEH-4,4-
d2 (b) and N-d-HEH (c) from the reaction beginning (d) ([HEH]0
) [HEH-4,4-d2]0 ) [N-d-HEH]0 ) 2.5 × 10-4 M) at the constant
NO pressure (1 atm.) for 10 min.
tron-transfer inhibitor,9 was added into the three reaction
mixtures, no remarkable inhibitory effects were observed,
indicating no electron transfer occurrence in the three
reactions.
1-Benzyl-1,4-dihydronicotinamide (BNAH), another
NAD(P)H model, is a close analogue of HEH and was
used in place of HEH to react with NO, MNTS, and DDQ,
respectively, to compare the reactivities of HEH and
BNAH with NO and MNTS. The experimental results
are surprising in that no reactions of BNAH with NO
and MNTS except for DDQ were observed (see Table 2).
According to the structure of the Hantzsch 1,4-dihy-
dropyridines (1), the aromatizations of 1 by NO are
presumably involved in three alternative reaction path-
ways as shown in Scheme 1: (i) a direct hydride transfer
to NO to yield cations 3 and/or 4, which subsequently
take place by heterolysis to afford the final aromatized
products A and/or B, respectively (path a); (ii) an initial
one-electron transfer from HEH to NO to give radical
(7) Itoh, T.; Nagata, K.; Matsuya, Y.; Miyaki, M.; Ohsawa, A. J . Org.
Chem. 1997, 62, 3582.
(8) Zhu, X.-Q.; Liu, Y.-C.; Cheng, J .-P. J . Org. Chem. 1999, 64(25),
8980. (b) Zhu, X.-Q.; Liu, Y.-C.; Wang, H.-Y.; Wang, W. J . Org. Chem.
1999, 64(25), 8983. (c) Zhu, X.-Q.; Xian, M.; Wang, K.; Cheng, J .-P. J .
Org. Chem. 1999, 64, 4187. (d) Zhu, X.-Q.; Liu, Y.-C. J . Org. Chem.
1998, 63, 2786. (e) Zhu, X.-Q.; Liu, Y.-C.; Li, J .; Wang, H.-Y. J . Chem.
Soc., Perkin Trans. 2 1997, 2191. (f) Xian, M.; Zhu, X.-Q.; Li, Q.; Cheng,
J .-P. Tetrahedron Lett. 1999, 40, 19570. (g) Cheng, J .-P.; Xian, M.;
Wang, K.; Zhu, X.-Q.; Yin, Z.; Wang, P. G. J . Am. Chem. Soc. 1998,
10266. (h) Cheng, J .-P.; Lu, Y.; Zhu, X.-Q.; Mu, L.-J . J . Org. Chem.
1998, 63, 6108. (i) Zhao, B.; Zhu, X.-Q.; Lu, Y.; Xia, C.-Z.; Cheng, J .-P.
Tetrahedron Lett. 2000, 41, 257. (j) Wang, H.-Y.; Liu, Y.-C.; Zhu, X.-
Q.; Guo, Q.-X. Chin. J . Chem. 1999, 17, 1884. (k) Xian, M.; Zhu, X.-Q.;
Lu, J .-M.; Wen, Z.; Cheng, J .-P. Organic Lett. 2000, 2(3), 265. (l) Cheng,
J .-P.; Lu, Y.; Zhu, X.-Q.; Sun, Y.-K.; He, J .-Q. J . Org. Chem. 2000, 65,
3853-3857. (m) Zhu, X.-Q.; He, J .-Q.; Li, Q.; Xian, M.; Lu, J .; Cheng,
J .-P. J . Org. Chem. 2000, 65, 6729.
(9) Eynde, J . J . V.; Delfosse, F.; Mayence, A.; Haverbeke, Y. V.
Tetrohedron, 1995, 51, 6511.

8160 J . Org. Chem., Vol. 65, No. 24, 2000
Zhu et al.
Sch em e 1
Ta ble 2. Resu lts fr om th e Oxid a tion s of BNAH by NO,
MNTS a n d DDQ
had taken place by one-step direct hydride transfer
mechanism, the hydrogen atom at the C4 rather than N1
position on the 1,4-dihydropyridine ring would be re-
moved initially, B rather than A would be a sole final
aromatized product. Obviously, this hypothesis is in
sharp contrast to the experimental results of Entries 4
and 5 (Table 1) and also is not accordant with the small
observed kinetic isotope effect of 1.4 on the reaction of
HEH-4,4-d2 with NO.
[Ã]
T (°C)
time
yield (%)
NOa
rt
rt
rt
3 days
2 weeks
30 min
0
0
97
In Table 2, no reaction of BNAH with NO was detected
at room temperature for 3 days, which indicates that the
oxidation of HEH by NO could not be initiated by one-
electron transfer (path b). The reason is that BNAH has
a higher reactivity toward NO reduction than that of
HEH via one-electron-transfer on the basis of their redox
MNTSb
DDQc
a,b
The reactions were conducted in acetonitrile and were
monitored by TLC. The experimental conditions are identical to
those of the reactions of HEH with NO and MNTS, respectively.
c The reaction was conducted in THF, the yield was determined
by CG according to the consume of the corresponding reactant.
potentials (Eox ) 0.29V vs Fc+/Fc for BNAH and Eox
)
0.507V vs Fc+/Fc for HEH). If the oxidation of HEH by
NO had proceeded by an initial one-electron transfer, the
oxidation of BNAH by NO would have more easily
proceeded via the same way. The fact shows no reaction
occurrence of BNAH with NO. Furthermore, the in-
effectiveness of p-dinitrobenzene, an electron-transfer
inhibitor, on the reaction can also support it.
cations 2, which then undergo decomposition by alterna-
tive pathways to provide the final products A and/or B
(path b); and (iii) an initial H-atom abstraction with NO
to form radicals 5 and/or 6, which subsequently lose a
hydrogen atom or a radical R1 to give the final aromatized
products (path c).
Inspection of Table 1 reveals that in the case of NO as
the oxidant, when R1 is a secondary alkyl or benzyl group,
it was lost in the final aromatized products; in the case
of DDQ as the oxidant, however, it was well preserved
in the final aromatized products (entries 4 and 5). These
results indicated that the possibility of path a (an initial
direct hydride transfer pathway) in Scheme 1 may be
excluded, because it has been proved that the aromati-
zations of 1,4-dihydropyridine derivatives by quinones
were carried out via a direct hydride transfer mecha-
nism.10 If the aromatizations of 4-substituted (isopropyl,
benzyl) Hantzsch 1,4-dihydropyridines by NO had con-
ducted by one-step direct hydride transfer like the
mechanism of the reactions with DDQ, the 4-substituents
(i.e., isopropyl and benzyl groups) would have been well
preserved in the final aromatized products. In addition,
from Scheme 1 it is certain to conclude that the stability
of cation 4 is much larger than that of 3. If the reactions
It is seems most likely from the experimental results
described above that the oxidations of HEH and its
derivatives by NO commence with the abstraction of the
H-atom (path c in Scheme 1). On the basis of the facts
that sole dealkylated product was obtained from the
reactions of 4-isopropyl and benzyl Hantzsch 1,4-dihy-
dropyridines with NO and that when HEH was methy-
lated at the N1-position, no reaction was observed,7 it may
be inferred that the H-atom at the N1- rather than C4-
position on the 1,4-dihydropyridine ring was first trans-
ferred. Thus, we may propose a tentative mechanism for
the oxidations of HEH and its derivatives by NO as
depicted in Scheme 2.
In Scheme 2, the oxidation of 1 by NO was initiated
via hydrogen transfer from the N1-position on the 1,4-
dihydropyridine ring to NO to afford aminyl radical 5,11
which subsequently underwent homolysis of the C4-H
or C4-R1 bond to lose a hydrogen atom or a radical R1
(10) Kerber, R. C.; Urry, G. W.; Kornblum, N. J . Am. Chem. Soc.
1964, 86, 3904; 1965, 87, 4520; Korblum, N.; Michel, R. E.; Kerber, R.
C. J . Am. Chem. Soc. 1966, 88, 5560, 5562.
(11) Williams, D. L. H. Nitrosation; Cambridge University Press:
Cambridge, New York, 1988; p 77.

Oxidations of Hantzsch 1,4-Dihydropyridines
J . Org. Chem., Vol. 65, No. 24, 2000 8161
Sch em e 2
Sch em e 3
from the C4-position on the dihydropyridine ring to give
final aromatized product B or A, respectively. If R1 is
isopropyl or benzyl group, it was lost and A was obtained;
if R1 is a linear alkyl or aryl group, the hydrogen atom
was lost and B was the final product. The main reason
resulting in this result could be that the stability of the
isopropyl and benzyl radicals is much larger than that
of linear alkyl or aryl one, which, of course, are more
easily removed from the parent radical 5 than the linear
alkyl or aryl group. Whether dealkylation (A) or hydrogen
atom loss (B) will occur, in other words, is governed by
the stability of the potential leaving radical groups.
According to the large kinetic isotope effect of 3.1 on the
N-d-HEH reaction with NO and the small kinetic isotope
effect of 1.4 on the HEH-4,4-d2 reaction with NO, the
initial hydrogen transfer in Scheme 2 would be in the
rate-determining step, but the hydrogen atom transfer
from the C4-position would be in a non-rate-determining
step.
N-Methyl-N-nitrosotoluene-p-sulfonamide (MNTS) is
a good NO donor that can also oxidize 1 to give the
corresponding aromatized pyridine derivatives A or B
(see Table 1). From the fourth column in Table 1, it is
clear that the 4-substituent is an isopropyl or benzyl
group; it was lost, but the others were preserved. These
reaction results are the same as the results of the
reactions with NO as the oxidant, whereas kinetic isotope
effects on the two reactions are different from each other.
In the reaction with NMTS, the kinetic effect of the
deuterium at the N1-position of HEH is 1.1 (kN-H/kN-D),
but in the reaction with NO, the corresponding kinetic
isotope effect is 3.1. This result indicated that the NH
bond dissociation in the former reaction is not in the rate-
determining step, but in the latter reaction, it is in the
rate-determining step. In a word, the above results could
exclude the possibility of initial hydrogen transfer from
the N1-position of HEH to MNTS. To assess the possibil-
ity of initial one-electron transfer from HEH to MNTS,
we determined the redox potentials of MNTS and HEH
by CV electrochemical technique. The result shows that
the reduction potential of MNTS is quite negative (-1.32
V vs Fc+/Fc) and the oxidation potential of HEH is rather
positive (0.507 V vs Fc+/Fc). According to the redox
potentials of the two reactants, the free energy change
for the electron transfer from HEH to MNTS was
estimated to be a quite large positive value (+175.9 kJ
mol-1), which indicates that the oxidation of HEH by
MNTS initiated by one-electron transfer is extremely
unfavorable in thermodynamics. In addition, the inef-
fectiveness of p-dinitrobenzene on the reaction can also
support it. But on the basis of the previous papers,12 it
is well-known that MNTS is a good nitrosotransfer agent
and has been extensively used to nitrosate many com-
pounds containing NH group to form the corresponding
N-nitroso compounds via a direct nucleophilic substitu-
tion. Since all the 4-substituted Hantzsch 1,4-dihydro-
pyridines contain NH group on the 1,4-dihydropyridine
ring, it is conceivable that the aromatizations of 1 by
MNTS could be initiated by nitrosation to give N-
nitrosated Hantzsch 1,4-dihydropyridine (9) which then
were subjected to two-step decomposition to offer the final
aromatized products A or B. The mechanism of the
aromatizations is shown in Scheme 3. Unfortunately, no
N-nitroso Hantzsch 1,4-dihydropyridines were obtained
by using various nitrosation methods for all our best
efforts, which indicates that compound 9 is likely un-
stable in the reaction systems and readily decompose to
give radical 5, since vinyl nitroso-compound like vinyl
bromide is readily subjected to homolysis to release nitric
oxide. In Scheme 3, the aromatization of 1 by MNTS was
initiated via nitronium ion transfer from MNTS to HEH
by nucleophilic substitution to give cation 7 and anion
8, which subsequently took place by proton transfer to
complete the nitrosation process of HEH by MNTS.
(12) (a) Garcia-Rio, L.; Leis, J . R.; Iglesias, E. J . Org. Chem. 1997,
62, 4701. J . Org. Chem. 1997, 62, 4712. (b) Leis, J . R.; Rios, A. J . Chem.
Soc., Perkin Trans. 2 1996, 857. (c) Leis, J . R.; Pena, M. E.; Rios, A.
M. J . Chem. Soc., Perkin Trans. 2 1995, 587. (d) Garcia-Rio, L.; Iglesias,
E.; Leis, J . R.; Pena, M. E.; Rios, A. J . Chem. Soc., Perkin Trans. 2
1993, 29. (e) Leis, J . R.; Pena, M. E.; Rios, A. J Chem. Soc., Perkin
Trans. 2 1993, 1233. (f) Oh, M. N. Y. F.; Williams, D. L. H. J . Chem.
Soc., Perkin Trans. 2 1989, 755. (g) Castro, A.; Leis, J . R.; Pena, M. E.
J . Chem. Soc., Perkin Trans. 2 1989, 1861.

8162 J . Org. Chem., Vol. 65, No. 24, 2000
Zhu et al.
Obviously, the nucleophilic substitution during the nit-
rosation of HEH is in rate-determining step, since the
proton transfer is generally fast, which also can be
supported by the small kinetic effect of the deuterium at
the N1-position on the reaction (kN-H/kN-D ) 1.1).13
Comparing the product yields on the columns 4 and 5
in Table 1, it is worth noting that the reactivity of 1 is
decreased with the increase of the 4-substituent bulk for
the reactions with NO but increased with the substituent
bulk increase for the reaction with MNTS. Obviously, the
change is slight for the former reactions and remarkable
for the latter reactions. For example, in the case of the
reactions with NO, when the reductant was HEH (entry
1), the reaction took 3 h to provide the corresponding
aromatized product in 96% yield; when the reductant was
2,6-diphenyl-4-(p-nitrophenyl)-HEH (entry 12), the reac-
tion offered the corresponding aromatized product in 70%
yield for the same time; whereas, in the case of the
reactions with MNTS, when the reductant was HEH, the
reaction gave the corresponding aromatized product in
41% yield for 3 days, but when the reductant is 2,6-
diphenyl-4-(p-nitrophenyl)-HEH, the reaction time was
shortened 1 day from 3 days to yield the corresponding
aromatized product in 91% yield. These differences must
be related to the bulk and the conformation of the
reductants as well as the type of the reaction mecha-
nisms. For the reactions with NO, since NO is a small
molecule, which is considered to be volumeless, no large
changes of the product yields were observed from entry
1 to entry 12 in Table 1. But for the reactions with MNTS,
since the reactions were controlled by the rate of the
nucleophilic substitution in the first reaction step (see
Scheme 3), the bulk and the conformation of the reduc-
tants, of course, strongly affected the reaction rate. As
is well known, HEH adopts a plane-conformation,14 and
the lone electron pair on N1 is nearly vertically positioned
above the plane of dihydropyridine ring which makes the
lone pair orbital to delocalize to the dihydropyridine ring
resulting in a great disadvantage to accept NO+ from
MNTS. But for 2,6-diphenyl-4-(p-nitrophenyl)-HEH, it
was determined by X-ray structural analysis to show a
boatlike puckered conformation (see Figure 2): the atoms
N1 and C4 were located on the same side slightly away
from the plane consisted of the two double bonds on the
1,4-dihydropyridine ring, and the phenyl ring connected
to the C4-position adopts a twist-conformation with
respect to the central dihydropyridine ring. Partial
related bond lengths and dihedral angles of the crystal
structure are tabulated in Table 3. Obviously, in the
boatlike puckered conformation of 2,6-diphenyl-4-(p-
nitrophenyl)-HEH, the configuration of N1 has an axially
positioned lone electron pair that cannot delocalize to the
dihydropyridine ring to result in a great advantage to
accept NO+ from MNTS in a nucleophilic substitution
fashion. Since the puckered degrees of the dihydropyri-
dine ring conformation are strongly dependent on the
bulk of the substituents on it, larger substituents result
in larger puckered degrees of the dihydropyridine ring
and make the lone electron pair on N1 more facile to
accept NO+ from MNTS. These analyses may not only
be used to explain the change of the product yields with
F igu r e 2. X-ray crystal structure for 2,6-diphenyl-4-(p-
nitrophenyl)-HEH.
Ta ble 3. Select Bon d Len gth s a n d Dih ed r a l An gle of th e
Cr ysta l Str u ctu r e for 2,6-Dip h en yl-4-(p-n itr op h en yl)-HEH
bond
length, Å
planea
dihedral angle, deg
N(1)-C(15)
N(1)-C(11)
C(11)-C(12)
C(14)-C(15)
C(12)-C(13)
C(13)-C(14)
1.380(3)
1.378(3)
1.358(3)
1.342(3)
1.521(3)
1.526(3)
1-2
3-2
4-2
5-2
4-5
6-2
25.36
13.86
54.68
73.63
71.36
85.56
a
1: C12, C13, C14; 2: C11, C12, C14, C15; 3: N1, C11, C12; 4: C21
-
C
36; 5: C21-C26; 6: C41, C46, C42.
the substituents for the two reactions in Table 1 but also
lend to support the mechanisms of the title reactions
above-mentioned.
Su m m a r y a n d Con clu sion s
The present work indicates that the aromatizations of
NAD(P)H models Hantzsch 1,4-dihydropyridines by nitric
oxide (NO) or its donor MNTS are initiated by hydrogen
transfer from the N-position on the 1,4-dihydropyridine
ring to give the corresponding aminyl radicals or by
nitrosation of the NH group on the 1,4-dihydropyridine
ring to give the corresponding N-nitroso compounds,
which readily undergo the homolytic cleavage to provide
the final aromatized products. Other mechanistic pos-
sibilities for the reactions were ruled out. 1-Benzyl-1,4-
dihydronicotinamide (BNAH) is another NAD(P)H model,
but no reactions with NO and its donor MNTS were
observed. The reason could be that there is no NH group
on the 1,4-dihydropyridine ring in BNAH. This conclusion
suggests that NAD(P)H coenzyme could not be directly
oxidized by NO itself in vivo, since NAD(P)H coenzyme
like BNAH has not NH group on the 1,4-dihydropyridine
ring. The effects of NO on an NAD(P)H-dependent redox
reaction under physiological conditions presumably con-
tribute to the effects of metabolites of NO, such as
ONOO-, heme(FeIII-NO), etc., because NAD(P)H is readily
oxidized by ONOO- and heme(FeIII-NO) which are easily
(13) The reaction mechanism of MNTS with 1 initiated by decom-
position of MNTS to give NO can be ruled out, since experiment shows
that MNTS in acetonitrile at room temperature is quite stable.
(14) Leustra, A.; Petit, G.; Dommisse, R. Bull. Soc. Chim. Belg. 1979,
133.

Oxidations of Hantzsch 1,4-Dihydropyridines
J . Org. Chem., Vol. 65, No. 24, 2000 8163
•- 15
ambient temperature and refrozen to reiterate the evacuation-
Ar purge procedure. The series of operations was repeated
three times. 20 mL of pure NO gas was added into the reaction
vessel, and closed. The reaction mixture was allowed to react
with stirring for 4 h at room temperature. Then Ar gas was
bubbled to expel excess NO. The reaction mixture was con-
centrated under reduced pressure, the residue was purified
by column chromatography to give the pure aromatized
products, all of which are identified to be in complete accord
with those reported in the literature.7 Caution: nitric oxide is
toxic, and reactions should be performed in a well-vented fume
hood.
Gen er a l P r oced u r e for th e Oxid a tion of Ha n tzsch 1,4-
Dih yd r op yr id in es by MNTS. A suspension of the Hantzsch
1,4-dihydropyridine (0.5 mmol) and MNTS (2 mmol) in 20 mL
of deaerated dry acetonitrile was stirred at room temperature
for 3 days, the reaction mixture was worked up as depicted
above to yield pure products.
Typ ica l P r oced u r e for th e Oxid a tion of Ha n tzsch 1,4-
Dih yd r op yr id in es a n d BNAH by DDQ. To the solution of
the reductant (0.5 mmol) in THF was added DDQ (0.5 mmol).
The mixture was stirred for 30 min, and then some 0.1 N HCl
was added to give the corresponding oxidized products.
Red ox P oten tia ls of HEH, BNAH, a n d MNTS. All
electrochemical measurements were carried out in dry CH3-
CN solution under argon atmosphere. N-Bu4NPF6 (0.1M) was
employed as the supporting electrolyte. A standard three-
electrode assembly was consisted of a Pt disk as the working
electrode, AgNO3/Ag as reference, and a platinum wire as
counter electrode. All sample solutions were 1.5 mM. The
ferrocenium/ferrocene redox couple (Fc+/Fc) was taken as an
internal standard. Reproducibility was generally less than 10
mV.
derived from the reactions of NO with O2
and ferric
heme proteins16 under physiological conditions, respec-
tively. This and others aspects of these interesting
reactions are under current study.
Exp er im en ta l Section
Ma ter ia ls. All of the 4-substituted (including 4,4-dideuter-
ated) Hantzsch 1,4-dihydropyridines were prepared in the
same manner, using the appropriate aldehyde, ammonia, and
ethyl acetoacetate, and identified on the basis of their spectral
data and, eventually, their melting points. BNAH,17 N-me-
thylated and N-deuterated Hantzsch 1,4-dihydropyridines18
and 2,6-diphenyl-4-(p-nitrophenyl)-HEH19 were prepared ac-
cording to the literatures. N-Methyl-N-nitrosotoluene-p-sul-
fonamide (MNTS) and 2,3-dichloro-5,6-dicyano-1,4-benzoquino-
ne (DDQ) were Aldrich products, and used without further
purification. Pure dry NO gas was produced by the reaction
of NaNO2 with sulfuric acid in the absence of oxygen and
passing the stream of NO gas through 10% NaOH to remove
higher oxides of nitrogen and finally through the solid NaOH
dry tube to remove water. Reagent grade acetonitrile was
distilled from P2O5 being passed through a column of active
neutral alumina to remove water and protic impurities. The
other solvents were purified by standard methods before use.
Gen er al P r ocedu r e for th e Ar om atization of th e Han tz-
sch 1,4-Dih yd r op yr id in es b y NO. Hantzsch 1,4-dihydro-
pyridine (0.2 mmol) in dry acetonitrile (20 mL) was placed in
a two-necked flask equipped with a septum rubber and three-
way stopcock, one way of which was attached to an Ar balloon,
and another joined to a pump. The flask was cooled to -78 °C
to freeze the solvent and degassed under vacuo and filled with
Ar gas. Then the frozen solvent was allowed to warm to
Ackn owledgm en t. This work was possible by grants
from the Natural Science Foundation of China (NSFC
No. 29972028) and in part from the National Laboratory
of Applied Organic Chemistry, Lanzhou University.
X.-Q.Z. also thanks the reviewers for setting a high
standard and for good suggestions on this work.
(15) Wage, R. S.; Castro, C. E. Chem. Rew. Toxicol. 1990, 3, 289.
(16) Pryor, W. A.; Squadrito, G. L. Am. J . Physiol. 1995, 268, L699.
(17) Anderson, A. C.; Berkelnammer, G. J . Am. Chem. Soc. 1958,
80, 992.
(18) Kutz, J . L.; Hutton, R.; Westheimer, F. H. J . Am. Chem. Soc.
1961, 83, 584.
(19) (a) Horning, E. A. Organic Syntheses; Wiley: New York, 1941;
Collect. Vol. I, p 378. (b) Baumane, L.; Stradins, J .; Gavars, R.;
Cekavitcus, B. S.; Duburs, G. Khim. Geterofsikl. Soedin. 1991, 481.
J O000484H