Activation of pyridinium salts for electrophilic acylation: A method for conversion of pyridines into 3-acylpyridines

Article abstract of DOI:10.1023/B:COHC.0000040772.13427.0b

Cyanide adducts of N-MOM pyridinium salts react with strong acylating reagents to provide 3-acyl-4-cyano-1,4-dihydropyridines that can be aromatized to 3-acylpyridines using ZnCl₂ in refluxing ethanol.

Full text of DOI:10.1023/B:COHC.0000040772.13427.0b

Chemistry of Heterocyclic Compounds, Vol. 40, No. 6, 2004
ACTIVATION OF PYRIDINIUM SALTS
FOR ELECTROPHILIC ACYLATION:
A METHOD FOR CONVERSION OF
PYRIDINES INTO 3-ACYLPYRIDINES
A. Klapars1 and E. Vedejs2
Cyanide adducts of N-MOM pyridinium salts react with strong acylating reagents to provide 3-acyl-4-
cyano-1,4-dihydropyridines that can be aromatized to 3-acylpyridines using ZnCl2 in refluxing ethanol.
Keywords: dihydropyridines, pyridine acylation.
Electrophilic aromatic substitution reactions of pyridines are extraordinarily challenging. Instead of
C-substitution at the pyridine ring, the electrophile typically forms an adduct with the pyridine nitrogen, which
even further deactivates the already electron deficient pyridine ring toward electrophilic substitution. For
example, the direct nitration of pyridine may require a reaction temperature of 330°C to provide only a 15%
yield of 3-nitropyridine [1]. To the best of our knowledge, no direct, intermolecular C-acylations of pyridines
have been reported [2]. This seriously limits the choice of methods for the preparation of the ubiquitous
3-substituted pyridines [3, 4].
In a limited number of cases, the lack of reactivity of pyridines toward electrophiles has been addressed
by converting the recalcitrant pyridine into a temporarily activated 1,4-dihydropyridine [5-9]. In contrast to the
electron poor parent pyridine, the electron rich 1,4-dihydropyridine features strongly enhanced reactivity toward
electrophiles at the 3-position. Several steps are typically required including formation of the dihydropyridine,
the subsequent reaction with an electrophile, and rearomatization to the desired 3-substituted pyridine. A similar
concept has been ingeniously employed in a one pot nitration of pyridines in the presence of sulfite as the
nucleophile that temporarily activates the pyridine, and then acts as a leaving group in an aromatization step [10,
11]. In principle, mechanistically analogous nucleophilic activation-aromatization sequences may also be
possible with other combinations of nucleophiles and electrophiles, but other applications of this principle have
not been reported.
During our studies on indoloquinone synthesis involving the activation of oxazolium salts with cyanide
ion [12], we fortuitously encountered the conversion from pyridinium salts to Reissert-type 4-cyano-1,4-
dihydropyridines having the general structure 1. We decided to explore their reactivity with electrophiles in
anticipation that this might provide an indirect means for the introduction of a substituent into the 3-position of
the pyridine ring. The results of this work are presented here.
__________________________________________________________________________________________
1
Department of Process Research, Merck Research Laboratories, NJ 07065, USA; e-mail:
2
artis_klapars@merck.com. Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA;
e-mail: edved@umich.edu. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 887-894, June,
2004. Original article submitted June 01, 2004.
0009-3122/04/4006-0759©2004 Plenum Publishing Corporation
759

The cyanide ion and pyridinium salts form equilibrium mixtures containing labile 1,4-dihydropyridine
adducts 1 (Scheme 1) [13, 14]. We found that the treatment of these equilibrium mixtures with strong acylating
agents such as trifluoroacetic anhydride, trichloroacetyl chloride, or ethyl oxalyl chloride produces 3-acyl-1,4-
dihydropyridines 2. Due to stabilization by the electron withdrawing substituent in the 3-position, these acylated
dihydropyridines 2 are significantly more robust than the parent dihydropyridines 1 and can be purified by
aqueous extraction or even flash chromatography on silica gel. The deactivating effect of the 3-acyl group also
prevents the introduction of a second acyl group in the 5-position. However, treatment of 1 with weaker
acylating agents (AcCl, Ac2O, Ac2O–DMAP, EtOCOCl, PhCOCl, PhCOBr) or various other electrophiles
(TMSOTf, BnBr, MeOTf, TsCl) did not provide the desired dihydropyridines 2. Instead, products resulting from
the reaction with the cyanide ion, present in the equilibrium mixture, were observed.
Sheme 1
H CN
_
+
CN
N
R
N
R
+
1
Desired process
observed only with
strong acylating agents
Undesired process
observed with most
electrophiles
+
+
E
E
H CN
E CN
E
E
E
N
R
N
R
N
+
4
2
3
The extent of conversion from pyridinium salts to cyanide adducts is strongly affected by the pyridine
ring substituents (Scheme 2). For example, treatment of the 1,4-dimethylpyridinium salt 5 with
benzyltrimethylammonium cyanide gave an equilibrium mixture of 5 and the cyanide adduct 6 in a 95:5 ratio
according to NMR analysis, favoring the starting pyridinium salt 5. On the other hand, the presence of a
moderately electron withdrawing N-methoxymethyl (MOM) group in 7 reversed the ratio to favor the
dihydropyridine 8 over 7 (85:15). Neither of the dihydropyridines 6 nor 8 was reactive enough for acylation by
trichloroacetyl chloride.
When the same experiments were repeated with the 3-methylpyridinium salts 10 and 11, conversion to
the cyanide adducts 12 and 13 was strongly favored in both cases. Clearly, the diminished steric effect compared
to the 4-methyl analogues is responsible for this difference. More importantly, the reactivity of 12 and 13 with
strong electrophiles was also improved. Thus, treatment with trichloroacetyl chloride in the presence of a
hindered tertiary amine (Hünig's base) as HCl scavenger resulted in conversion to the acylated dihydropyridines
14 and 15. Initially, we regarded the N-allyl pyridinium salt 10 as the more desirable starting material compared
to the N-MOM analogue 11 in terms of the toxicity and cost issues. However, the N-allyl intermediate 12
consistently gave lower yields of the 3-acylated product 14 compared to the corresponding reaction from 13.
Therefore, the N-MOM pyridinium salts were chosen for further development.
The optimized procedure was applied to the acylation of several pyridine substrates (Scheme 3) using
the same sequence of N-alkylation with MeOCH2Cl (MOMCl), addition of the soluble cyanide source
BnMe3N+CN- to generate the activated dihydropyridine, and C-acylation in the presence of Hünig's base. Ethyl
oxalyl chloride gave generally good results with several pyridine substrates, so this reagent was used for
comparison studies with unsubstituted pyridine as well as with the 3-methyl and 2-methyl derivatives. In the
760

Sheme 2
Cl3CCOCl
NEtPr2-i
Me
O
Me CN
Me CN
BnMe3N+CN–
CDCl3
CCl3
N
R
N
R
N
R
+
5 R = Me
7 R = CH2OMe
95
15
:
5
6 R = Me
8 R = CH2OMe
9
:
85
O
H CN
H CN
Cl3CCOCl
NEtPr2-i
Me
BnMe3N+CN–
CDCl3
Me
Me
CCl3
N
R
N
R
+
N
R
14 R = Allyl
15 R = MOM (80%)
(39%)
10 R = Allyl
11 R = CH2OMe
12 R = Allyl
13 R = CH2OMe
15
:
85
99
1
:
case of the 3-methylpyridine, a single regioisomeric dihydropyridine intermediate 16b was observed, as also
seen in the trichloroacetylation experiments to give 12. However, the 2-methyl pyridine example afforded a 1:7
mixture of two separable regioisomers 18:19.
The aromatization of the dihydropyridines 16 and 19 to the desired 3-acyl pyridines 17 and 20 was
accomplished with the help of ZnCl2 as a mild cyanophile (Scheme 1). Upon addition of anhydrous ZnCl2 to 16a
or 19, a rapid decyanation to the pyridinium salt 3 (R = MOM) could be detected by NMR analysis of the
reaction mixture. Importantly, the formation of the pyridinium salt activated the MOM group toward
deprotection via nucleophilic displacement. This was accomplished by refluxing the crude pyridinium salt
mixture in ethanol, resulting in the formation of 17 and 20 in 67-70% overall yield from the starting pyridine.
When the same aromatization procedure was used with the trichloroacetylated dihydropyridine 15,
aromatization occurred as usual, but the refluxing ethanol deprotection step also cleaved the trichloroacetyl
group in a haloform-type reaction to give the corresponding picolinate ester 21 (54% overall). This
transformation proved quite useful because the alternative of preparing the picolinate via direct carboxylation of
the dihydropyridine adduct 13 with chloroformate esters was unsuccessful due to competing cyanide transfer to
the relatively unreactive electrophile.
Trifluoroacetylation of pyridine via the dihydropyridine 1 (R = MOM) salt was also possible, and gave
an isolable dihydropyridine 22. The aromatization of 22 using the zinc chloride procedure was viable according
to NMR assay of the reaction mixture. However, attempts to recover 3-trifluoroacetylpyridine 23 from the crude
product were not successful. The product proved to be highly polar, suggesting the formation of hydrates that
could not be purified by chromatography. Another unusual example was encountered in the quinoline series.
Quinoline activation and C-acylation with oxalyl chloride proceeded normally and gave the dihydroquinoline 24
in a typical 71% yield. However, 24 proved to be relatively stable, and the cyanide group resisted the usual zinc
chloride aromatization conditions. Aromatization did take place with the stronger cyanophile AgOTf, but the
quinoline product 25 was obtained in low (35%) yield.
In summary, an indirect method for the 3-acylation of pyridines is described via the intermediate
N-MOM pyridinium salts. This method utilizes the cyanide ion as a temporary nucleophile to form an activated,
electron rich dihydropyridine intermediate that is susceptible to acylation at the 3-position, and subsequent
aromatization. The cyanide can be permanently introduced in the pyridine ring if a pyridine N-oxide is treated
with TMSCN [15, 16]. Although the method is applicable only to highly reactive acylating agents and requires
the use of the toxic cyanide and MeOCH2Cl, no better alternatives for the 3-acylation of pyridines in a one-pot
procedure are currently available.
761

Sheme 3
O
CN
O
H
1. MeOCH2Cl
1. ZnCl2
MeCN, rt
2. BnMe3N+CN–
Me
OEt
R
R
OEt
O
3. EtO2CCOCl
NEtPr2-i
O
2. EtOH, reflux
N
N
N
CH2OMe
17a R = H (70%),
16b R = Me (67%)
16a R = H
16b R = Me
O
1. MeOCH2Cl
H
CN
2. BnMe3N+CN–
OEt
O
N
3. EtO2CCOCl
NEtPr2-i
Me
N
Me
CH2OME
18
O
CN
O
H
1. ZnCl2
MeCN, rt
OEt
OEt
1: 7 18 : 19
Me
O
O
2. EtOH, reflux
N
Me
N
CH2OMe
19
20 (55%)
O
CN
H
O
1. MeOCH2Cl
1. ZnCl2
MeCN, rt
Me
2. BnMe3N+CN–
Me
Me
CCl3
OEt
N
3. EtO2CCOCl
NEtPr2-i
2. EtOH, reflux
N
N
CH2OMe
15
21 (54%)
O
CN
O
H
1. MeOCH2Cl
2. BnMe3N+CN–
CF3
CF3
3. (F3CCO)2O
NEtPr2-i
N
N
N
CH2OMe
22 (90%)
23
O
H CN
1. MeOCH2Cl
O
2. BnMe3N+CN–
OEt AgOTf
EtOH, reflux
OEt
O
3. EtO2CCOCl
NEtPr2-i
N
O
N
N
CH2OMe
25 (36%)
24 (71%)
EXPERIMENTAL
General procedures. Solvents and reagents were purified as follows: acetonitrile was distilled from
P2O5; diisopropylethylamine was distilled from CaH2; methoxymethyl chloride (Aldrich, technical grade) was
distilled and sparged with nitrogen gas to remove the HCl impurity (CAUTION: methoxymethyl chloride and
particularly an impurity present in the reagent are strong carcinogens); the purified reagents and solvents were
762

stored under nitrogen. Chloroform (anhydrous, stabilized with amylenes) was obtained from Aldrich and used
immediately after opening of the bottle. Zinc chloride (Mallinckrodt, anhydrous) was used without further
purification. All reactions were performed under an atmosphere of nitrogen in glassware dried in an oven
(150°C) and cooled with a stream of nitrogen. All reaction mixtures were stirred magnetically. Flash
chromatography was performed with 230-400 mesh EM silica gel 60. Analytical TLC was performed on EM
glass plates coated with a 250 µm layer of silica gel 60 F254. Melting points were obtained on a Lab Devices
MelTemp apparatus and are uncorrected.
Benzyltrimethylammonium Cyanide (BnMe3N+CN–) was prepared according to a procedure reported
by Vedejs and Monahan [17] with some modifications (CAUTION: cyanide is a strong poison; sodium
hypochlorite bleach solution can be used to detoxify the cyanide residues). Benzyltrimethylammonium chloride
(Aldrich, 39.8 g, 0.214 mol) was dried at 0.5 mm Hg vacuum and dissolved in 50 ml of anhydrous MeOH. The
resulting solution was transferred via cannula into a stirred solution of NaCN (15.8 g, 0.322 mol) in 300 ml of
anhydrous MeOH. After stirring for 30 min at room temperature, the white suspension was carefully
concentrated (rotary evaporation followed by 0.5 mm Hg vacuum) with minimal exposure to the atmospheric
moisture. The residue was treated with 200 ml of hot anhydrous acetonitrile, and the resulting suspension was
filtered hot through a fritted glass filter under nitrogen. The filtrate was carefully concentrated by rotary
evaporation at room temperature to ca. 50 ml volume with minimal exposure to atmospheric moisture. The
resulting suspension was filtered under nitrogen, and the crystals were dried at 0.5 mm Hg vacuum to give 23.7 g
(63%) of the product as white hygroscopic crystals. The product was stable at room temperature for several years
if it was stored and handled in a dry box under nitrogen.
Ethyl 4-cyano-1-methoxymethyl-1,4-dihydro-3-pyridineglyoxylate (16a). To a solution of pyridine
(0.33 ml, 4.08 mmol) in 10 ml of CHCl3 at room temperature was added methoxymethyl chloride (0.34 ml,
4.48 mmol). After stirring at room temperature for 1 h, the colorless solution was transferred via cannula into a
50 ml round bottom flask charged with benzyltrimethylammonium cyanide (660 mg, 3.74 mmol). The resulting
clear solution was cooled to 0°C (ice bath), and diisopropylethylamine (0.80 ml, 4.59 mmol) was added followed
by ethyl oxalyl chloride (0.48 ml, 4.30 mmol). After stirring at 0°C for 1 h, the orange solution was poured into
50 ml of ether and washed with 2 × 20 ml of water. The bright yellow organic layer was dried (Na2SO4) and
concentrated by rotary evaporation to give the crude dihydropyridine 16a, which was used in the next step
without further purification. Analytical TLC on silica gel 60 F254, hexane–acetone (1:1), Rf 0.47. Molecular ion
calculated for C12H14N2O4 250.09530; found m/e 250.0943, error = 4 ppm. IR spectrum (neat) ν, cm-1: 2231
(C≡N), 1726 (C=O), 1678 (C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 7.97 (1H, d, J = 1.4); 6.27
(1H, dt, J = 7.9, 1.4); 5.20 (1H, dd, J = 7.9, 4.6); 4.65 (2H, s); 4.60 (1H, dd, J = 4.6, 1.4); 4.35 (2H, q, J = 7.2);
13
3.36 (3H, s); 1.39 (3H, t, J = 7.2). C NMR spectrum (76 MHz, CDCl3), δ, ppm: 180.1, 162.2, 146.6, 128.7,
118.4, 103.2, 102.3, 85.4, 62.2, 56.0, 24.0, 13.9.
Ethyl 3-Pyridineglyoxylate (17a). The crude dihydropyridine 16a prepared above was dissolved in
anhydrous acetonitrile (5 ml, including cannula washings), and the solution was transferred via cannula into a 25
ml round bottom flask charged with ZnCl2 (1.05 g, 7.70 mmol). After stirring at room temperature for 5 h, the
yellow suspension was filtered through Celite eluting with 2 × 2 ml of anhydrous acetonitrile. Anhydrous
ethanol (10 ml) was added to the filtrate, the resulting tan solution was refluxed for 15 h, cooled to room
temperature, and then poured into 10% aqueous solution of NaHCO3 (20 ml) at 0°C (ice bath). The resulting
suspension was filtered through Celite eluting with 2 × 10 ml of ethanol. The tan filtrate was extracted with
3 × 30 ml of CH2Cl2. The combined organic phase was dried (Na2SO4), concentrated by rotary evaporation, and
the residue was purified by flash chromatography on silica gel (2.5 × 20 cm, hexane–acetone (4:1) eluent, 15 ml
fractions). Fraction 9-15 gave 467 mg (70%) of the pyridine 17a as a light yellow liquid [18].
Ethyl 5-Methyl-3-pyridineglyoxylate (16b). To a solution of 3-methylpyridine (0.40 ml, 4.11 mmol) in
10 ml of CHCl3 at room temperature was added methoxymethyl chloride (0.34 ml, 4.48 mmol). After stirring at
room temperature for 1 h, the colorless solution was transferred via cannula into a 50 ml round bottom flask
charged with benzyltrimethylammonium cyanide (670 mg, 3.80 mmol). The resulting clear solution was cooled
763

to 0°C (ice bath), and diisopropylethylamine (0.80 ml, 4.59 mmol) was added followed by ethyl oxalyl chloride
(0.48 ml, 4.30 mmol). After stirring at 0°C for 1 h, the orange solution was poured into 50 ml of ether and
washed with 2 × 20 ml of water. The yellow organic layer was dried (Na2SO4) and concentrated by rotary
evaporation to give the crude dihydropyridine 16b, which was used in the next step without further purification.
The crude dihydropyridine 16b prepared above was dissolved in anhydrous acetonitrile (5 ml, including
cannula washings), and the solution was transferred via cannula into a 25 ml round bottom flask charged with
ZnCl2 (1.06 g, 7.78 mmol). After stirring at room temperature for 4 h, the orange suspension was filtered
through Celite eluting with 2 × 2 ml of anhydrous acetonitrile. Anhydrous ethanol (10 ml) was added to the
filtrate, the resulting tan solution was refluxed for 12 h, cooled to room temperature, and then poured into 10%
aqueous solution of NaHCO3 (20 ml) at 0°C (ice bath). The resulting suspension was filtered through Celite
eluting with 3 × 5 ml of ethanol. The tan filtrate was extracted with 3 × 30 ml of CH2Cl2. The combined organic
phase was dried (Na2SO4), concentrated by rotary evaporation, and the residue was purified by flash
chromatography on silica gel (2.5 × 20 cm, hexane–acetone (4:1) eluent, 15 ml fractions). Fractions 8-12 gave
492 mg (67%) of the pyridine 17b as a pale yellow liquid. Analytical TLC on silica gel 60 F254, hexane–acetone
(2:1), Rf 0.43. Molecular ion calculated for C10H11NO3 193.07390; found m/e 193.0744, error = 3 ppm. IR
spectrum (neat), ν, cm-1: 1734 (C=O), 1693 (C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 9.05
(1H, d, J = 2.0); 8.69 (1H, d, J = 2.0); 8.14 (1H, t, J = 2.0); 4.48 (2H, q, J = 7.1); 2.44 (3H, s); 1.44 (3H, t,
13
J = 7.1). C NMR spectrum (76 MHz, CDCl3), δ, ppm: 184.8, 162.3, 155.2, 148.6, 136.9, 133.5, 127.9, 62.5,
18.1, 13.9.
Ethyl 2-Methyl-3-pyridineglyoxylate (20). To a solution of 2-methylpyridine (0.42 ml, 4.25 mmol) in
10 ml of CHCl3 at room temperature was added methoxymethyl chloride (0.34 ml, 4.48 mmol). After stirring at
room temperature for 1 h, the colorless solution was transferred via cannula into a 50 ml round bottom flask
charged with benzyltrimethylammonium cyanide (670 mg, 3.80 mmol). The resulting colorless solution was
cooled to -40°C (dry ice-acetonitrile bath), and diisopropylethylamine (0.80 ml, 4.59 mmol) was added followed
by ethyl oxalyl chloride (0.48 ml, 4.30 mmol). After stirring at -40°C for 3 h, the cooling bath was removed. The
reaction mixture was allowed to warm to room temperature for 1 h, poured into 50 ml of ether and washed with
2 × 20 ml of water. The tan organic layer was dried (Na2SO4), concentrated by rotary evaporation to give a 7:1
ratio of the dihydropyridine isomers, and the residue was purified by flash chromatography on silica gel
(4 × 20 cm, hexane–acetone (3:1) eluent, 15 ml fractions). Fractions 45-70 gave the desired dihydropyridine
regioisomer 19, which was used in the next step without further purification.
The dihydropyridine 19 prepared above was dissolved in anhydrous acetonitrile (5 ml, including
cannula washings), and the solution was transferred via cannula into a 25 ml round bottom flask charged with
ZnCl2 (970 mg, 7.12 mmol). After stirring at room temperature for 4 h, the orange suspension was filtered
through Celite eluting with 2 × 1 ml of anhydrous acetonitrile. Anhydrous ethanol (10 ml) was added to the
filtrate, the resulting tan solution was refluxed for 12 h, cooled to room temperature, and then poured into 10%
aqueous solution of NaHCO3 (20 ml) at 0°C (ice bath). The resulting suspension was filtered through Celite
eluting with 3 × 10 ml of ethanol. The tan filtrate was extracted with 3 × 30 ml of CH2Cl2. The combined
organic phase was dried (Na2SO4), concentrated by rotary evaporation, and the residue was purified by flash
chromatography on silica gel (2.5 × 20 cm, hexane–acetone (3:1) eluent, 15 ml fractions). Fractions 8-14 gave
406 mg (55%) of the pyridine 20 as a colorless liquid. Analytical TLC on silica gel 60 F254, hexane–acetone
(2:1), Rf = 0.40. Molecular ion calculated for C10H11NO3 193.07390; found m/e 193.0742, error = 2 ppm. IR
spectrum (neat), ν, cm-1: 1734 (C=O), 1697 (C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 8.69
(1H, dd, J = 1.8, 4.9); 8.04 (1H, dd, J = 1.8, 8.0); 7.30 (1H, dd, J = 4.9, 8.0); 4.45 (2H, q, J = 7.2); 2.80 (3H, s);
13
1.42 (3H, t, J = 7.2. C NMR spectrum (76 MHz, CDCl3), δ, ppm: 187.4, 163.3, 160.1, 152.7, 139.0, 127.3,
120.8, 62.6, 24.4, 13.9.
4-Cyano-1-methoxymethyl-5-methyl-3-trichloroacetyl-1,4-dihydropyridine (15). To a solution of
3-methylpyridine (0.40 ml, 4.11 mmol) in 10 ml of CHCl3 at room temperature was added methoxymethyl
chloride (0.34 ml, 4.48 mmol). After stirring at room temperature for 1 h, the colorless solution was transferred
764

via cannula into a 50 ml round bottom flask charged with benzyltrimethylammonium cyanide (670 mg,
3.80 mmol). The resulting colorless solution was cooled to 0°C (ice bath), and diisopropylethylamine (0.80 ml,
4.59 mmol) was added followed by trichloroacetyl chloride (0.48 ml, 4.30 mmol). The cooling bath was
removed, and the reaction mixture was stirred at room temperature for 1 h. The yellow solution was poured into
100 ml of ethyl acetate and washed with 2 × 20 ml of water. The tan organic layer was dried (Na2SO4) and
concentrated by rotary evaporation to give the crude dihydropyridine 15, which was used in the next step
without further purification. Analytical TLC on silica gel 60 F254, hexane–acetone (2:1), Rf = 0.35. Pure material
was obtained by crystallization from chloroform, mp 160-161°C, decomposition, yellow plates. Molecular ion
calculated for C11H11Cl3N2O2 307.98861; found m/e 307.9857, error = 9 ppm. IR spectrum (KBr), ν, cm-1: 2225
(C≡N), 1695 (C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 7.98 (1H, s); 6.08 (1H, s); 4.70 (1H,
AB q, J = 10.5); 4.63 (1H, AB q, J = 10.5); 4.45 (1H, s); 3.34 (3H, s); 1.93 (3H, s); 7.98 (1H, s); 6.08 (1H, s);
4.70 (1H, AB q, J = 10.5); 4.63 (1H, AB q, J = 10.5); 4.45 (1H, s); 3.34 (3H, s); 1.93 (3H, s). 13C NMR spectrum
(76 MHz, DMSO-d6), δ, ppm: 177.9, 145.9, 124.1, 118.4, 111.5, 95.2, 92.6, 84.6, 55.1, 30.5, 17.9.
Ethyl 5-Methyl-3-pyridinecarboxylate (21). A solution of the crude dihydropyridine 15 prepared
above and ZnCl2 (2.04 g, 15.0 mmol) in 10 ml of anhydrous ethanol was refluxed for 24 h. After cooling to room
temperature, the reaction mixture was poured into 10% aqueous solution of NaHCO3 (40 ml) at 0°C (ice bath).
The resulting suspension was filtered through Celite eluting with 3 × 5 ml of ethanol, and the brown filtrate was
extracted with 2 × 60 ml of CH2Cl2. The combined organic phase was dried (Na2SO4), concentrated by rotary
evaporation, and the residue was purified by flash chromatography on silica gel (2.5 × 20 cm, hexane–acetone
(4:1) eluent, 7 ml fractions). Fractions 14-22 were concentrated by rotary evaporation and purified by another
flash chromatography on silica gel (2.5 × 20 cm, hexane–ethyl acetate (2:1) eluent, 7 ml fractions). Fractions 20-
27 gave 340 mg (54%) of the pyridine 21 as a pale tan liquid, identical with the literature report according to
NMR assay [19].
4-Cyano-1-methoxymethyl-3-trifluoroacetyl-1,4-dihydropyridine (22). To a solution of pyridine
(0.34 ml, 4.20 mmol) in 10 ml of CHCl3 at room temperature was added methoxymethyl chloride (0.34 ml, 4.48
mmol). After stirring at room temperature for 1 h, the colorless solution was transferred via cannula into a 50 ml
round bottom flask charged with benzyltrimethylammonium cyanide (675 mg, 3.82 mmol). The resulting
colorless solution was cooled to 0°C (ice bath), and diisopropylethylamine (0.80 ml, 4.59 mmol) was added
followed by trifluoroacetic anhydride (0.60 ml, 4.25 mmol). The yellow solution was stirred at 0°C for 1 h,
poured into 50 ml of ether, and washed with 2 × 20 ml of water. The tan organic layer was dried (Na2SO4),
concentrated by rotary evaporation, and the residue was purified by flash chromatography on silica
gel (2.5 × 20 cm, dichloromethane–ether (20:1) eluent, 15 ml fractions). Fractions 9-12 gave 852 mg (90%) of
the dihydropyridine 22 as yellow crystals. Analytical TLC on silica gel 60 F254, hexane–acetone (2:1), Rf 0.34.
o
Pure material was obtained by crystallization from ether–hexane, mp 49-50 C, yellow crystals. Molecular ion
calculated for C10H9F3N2O2 246.06160; found m/e 246.0609, error = 3 ppm. IR spectrum (neat), ν, cm-1: 2235
(C≡N), 1662 (C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 7.49 (1H, t, J = 1.2); 6.31 (1H, dt,
J = 7.9, 1.2); 5.24 (1H, dd, J = 7.9, 4.5); 4.69 (2H, s); 4.58 (1H, dt, J = 4.5, 1.2); 3.37 (3H, s). 13C NMR
spectrum (76 MHz, CDCl3), δ, ppm (J, Hz): 176.5 (q, J = 34.8); 145.5 (q, J = 4.7); 128.6, 118.1, 116.7 (q,
J = 290.4); 102.5; 99.6; 85.5; 56.0; 24.2. 19F NMR spectrum (282 MHz, CDCl3), δ, ppm: 69.9.
Ethyl 4-Cyano-1-methoxymethyl-1,4-dihydro-3-quinolineglyoxylate (24). To a solution of quinoline
(0.50 ml, 4.23 mmol) in 10 ml of CHCl3 at room temperature was added methoxymethyl chloride (0.34 ml,
4.48 mmol). After stirring at room temperature for 1 h, the light green solution was transferred via cannula into a
50 ml round bottom flask charged with benzyltrimethylammonium cyanide (680 mg, 3.86 mmol). After stirring
at room temperature for 4 h, the tan solution was cooled to 0°C (ice bath), and diisopropylethylamine (0.80 ml,
4.59 mmol) was added followed by ethyl oxalyl chloride (0.48 ml, 4.30 mmol). The cooling bath was removed,
the reaction mixture was stirred at room temperature for 1 h, the resulting brown solution was poured into 50 ml
of ether, and washed with 2 × 20 ml of water. The organic layer was dried (Na2SO4), concentrated by
765

rotary evaporation, and the residue was purified by flash chromatography on silica gel (2.5 × 20 cm, hexane–
ethyl acetate (1:1) eluent, 15 ml fractions). Fractions 13-21 gave 822 mg (71%) of the dihydroquinoline 24 as a
yellow solid. Analytical TLC on silica gel 60 F254, hexane–acetone (1:1), Rf 0.60. Pure material was obtained by
o
crystallization from ethanol, mp 118-119 C, yellow needles. Molecular ion calculatedd for C16H16N2O4
300.1110; found m/e 300.1116, error = 2 ppm. IR spectrum (KBr), ν, cm-1: 2233 (C N), 1736 (C=O), 1722
(C=O). NMR spectrum (300 MHz, CDCl3), δ, ppm (J, Hz): 8.27 (1H, s); 7.46-7.21 (4H, m); 5.24 (1H, s); 5.12
(2H, s); 4.36 (2H, q, J = 7.3); 3.41 (3H, s); 1.40 (3H, t, J = 7.3). 13C NMR spectrum (76 MHz, CDCl3), δ, ppm:
179.1, 162.3, 148.1, 134.7, 130.3, 129.8, 126.3, 118.9, 117.8, 115.8, 102.6, 84.1, 62.3, 55.9, 27.5, 13.9.
The research reported in this paper was performed at the Department of Chemistry, University of
Wisconsin, Madison, WI 53706, USA. Funding was provided by the National Institutes of Health (CA17918).
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