Synthesis of a new class of β-iodo N-alkenyl 2-pyridones

Article abstract of DOI:10.1021/ol202679t

A new method for the synthesis of β-iodo N-alkenyl 2-pyridones from substituted 2-propargyloxypyridines has been discovered. These compounds present a unique complement of orthogonal functionality and structural characteristics that are unavailable via ot

Full text of DOI:10.1021/ol202679t

ORGANIC
LETTERS
2011
Vol. 13, No. 23
6224–6227
Synthesis of a New Class of
β-Iodo N-Alkenyl 2-Pyridones
Sarah Z. Tasker,† Benjamin M. Brandsen,† Keun Ah Ryu,† Gregory S. Snapper,†
Richard J. Staples,‡ Roger L. DeKock,† and Carolyn E. Anderson*,†
Department of Chemistry and Biochemistry, Calvin College, Grand Rapids, Michigan
49546, United States, and Department of Chemistry, Michigan State University,
East Lansing, Michigan 48824, United States
Carolyn.Anderson@calvin.edu
Received October 5, 2011
ABSTRACT
A new method for the synthesis of β-iodo N-alkenyl 2-pyridones from substituted 2-propargyloxypyridines has been discovered . These
compounds present a unique complement of orthogonal functionality and structural characteristics that are unavailable via other routes.
The ready access to these compounds renders them an important entry point for the preparation of more complex N-alkyl pyridone-
containing targets.
N-Alkenyl pyridones, exemplified by compounds 1ꢀ4,
have been the object of much synthetic interest over the
past few decades (Figure 1). Frequently utilized as either
monomers for polymerization1 or substrates for Dielsꢀ
Alder reactions,2 simple N-vinyl and N-alkenyl pyridones
have been prepared from 2-pyridone in a variety of ways,
including additions to electron-deficient alkynes,3,4 addi-
tions to vinyl bromides,5 and aldol condensations after
CꢀN bond formation.6 While synthetically straightfor-
ward, each of these transformations is limited to a
particular archetype. Alternatively, R-bromo N-alkenyl
pyridone 3 was prepared while trying to access R-
haloenamides,7 and Cu-catalyzed Suzuki-type couplings
of alkenyl boronic acids or boronate salts have been
utilized to improve access to N-alkenyl pyridones.8 How-
ever, neither method appears to be general.
† Calvin College.
‡ Michigan State University.
(1) (a) Ueda, N.; Kondo, K.; Kono, M.; Takemoto, K.; Imoto, M.
Makromol. Chem. 1968, 120, 13–20. (b) Pitha, J.; Ts’o, P. O. P. J. Org.
Chem. 1968, 33, 1341–1344. (c) Kaye, H. J. Polymer Sci., Part B:
Polymer Lett. 1969, 7, 1–5.
Figure 1. Previously synthesized N-alkenyl pyridones.
(2) Review: Afarinkia, K.; Vinader, V.; Nelson, T. D.; Posner, G. H.
Tetrahedron 1992, 48, 9111–9171.
More complex N-alkenyl pyridone motifs are poised to
serve as important building blocks for the incorporation
of N-alkyl pyridones into complex scaffolds, including nat-
ural products9 and pharmacologically relevant targets.10
During our studies toward the synthesis of N-propargyl
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Acheson, R. M.; Tasker, P. A. J. Chem. Soc. (C) 1967, 1542–1543. (c)
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hedron Lett. 2003, 44, 4927–4931. (b) Bolshan, Y.; Batey, R. A. Angew.
Chem., Int. Ed. 2008, 47, 2109–2112. (c) Altman, R. A.; Buchwald, S. L.
Org. Lett. 2007, 9, 643–646.
r
10.1021/ol202679t
Published on Web 11/09/2011
2011 American Chemical Society

Table 1. Optimization of Reaction Conditions for the Forma-
tion of N-Alkenyl Pyridone 7a
Scheme 1. Observation of β-Iodo N-Alkenyl Pyridone 7a
pyridones 6a,11,12 β-iodo N-alkenyl pyridone 7a was observed
as a minor byproduct (Scheme 1). The unique complement of
orthogonal functionality found in compound 7a makes it an
optimal intermediate for the synthesis of other, more complex
pyridone scaffolds.
LiI
temp time
yield
entry (equiv)
additives
none
(°C)
(h)
6a:7a 7a (%)a
1
2
3
4
0.5
0.5
0.5
0.5
100
26
ꢀ
traceb
0.1 equiv butanol 100
72 1.0:1.1 20%
72 0:1.0 17%
0.1 equiv butanol
0.1 equiv butanol
10 wt % SiO2
butanol (5 M)
10 wt % SiO2
butanol (3 M)
10 wt % SiO2
10 wt % H2O
80
80
72 1.0:1.8 21%
72 1.0:4.2 21%
72 1.4:1.0 13%
Analysis of β-Iodo N-Alkenyl Pyridone 7a. The struc-
ture of byproduct 7a was initially determined by X-ray
crystallography.13 Further evaluation using 1H NMR
spectroscopy (CDCl3) showed that 7a possesses an unusually
strong coupling between the hydroxyl proton and adjacent
diastereotopic methylene protons (J = 4.8, 7.4 Hz).14
This interaction suggests that proton exchange with adventi-
tious water is not occurring on the NMR time scale and
supports the presence of an intramolecular H-bond between
the hydroxyl proton and the carbonyl of the pyridone.
Support for this constrained conformation was obtained
by applying standard computational methods to a truncated
version of 7a.15 In this way, the H-bound conformation was
found to be stabilized by 2.2 kcal/mol relative to the non-
bound structure at the B3LYP/6-31þG(d,p)/LANL2DZ
level of theory. Additional evidence for an intramolecular
H-bond was provided by calculating the 1H NMR chemical
shifts for these same conformations using the gauge inde-
pendent atomic orbital method (GIAO) at the B3LYP/
cc-pVTZ and B3LYP/6-311þþG(d,p) levels of theory.15
Using this method, a strong correlation was observed
between the calculated and experimental values for the
hydroxyl proton in the H-bound conformation (<0.5 ppm
difference), while a difference of more than 3 ppm exists in
the conformer without the H-bond.
5
6
0.5
1.0
80
80
a Isolated yield. b 81% yield of alkyne 6a isolated.
halogen bond (2.99 A).16 Such halogen bonds have been
observed in many halogenated materials and, of late, have
been utilized as a means for supramolecular organization
and crystal engineering.17
˚
Optimization of Reaction Conditions. Compound 7a was
initially observed upon treatment of pyridine 5a with 0.5
equiv of LiI at 100 °C in the absence of solvent, conditions
designed to generate the formal [1,3]-migration product 6a
(Table 1, entry 1).11 As the rate of formal [1,3]-migration is
sensitive to substrate concentration and temperature, 1-bu-
tanol was added to the reaction and the temperature was
decreased (entries 2 and 3). 1-Butanol was utilized as early
studies had shown that it provided improved product ratios
and yields relative to other solvents (e.g., CHCl3 or CH3CN).
Introduction of silica gel to the reaction was found to
increase the overall conversion to both alkyne 6a and alkene
7a, but even after 72 h the yield of compound 7a remained
undesirably low (entries 4 and 5).18
We postulate that alkene 7a is formed when iodine,
generated upon oxidation of LiI under the reaction condi-
tions, activates alkyne 5a, giving unsymmetrical iodenium
ion 8 (Scheme 2). Ring opening by the proximal pyridine
nitrogen in a 5-exo manner would then yield pyridinium
ion 9 with the proper geometry around the newly formed
alkene. Similar pyridinium rings have been observed in
related systems.3b,19 Opening of the five-membered ring
by a nucleophile, originally assumed to be adventitious
Unlike the solution phase structure of N-alkenyl pyridone
7a, the solid state structure does not contain an intramo-
lecular hydrogen bond.13 Rather, when the molecules
interact in the solid state, a network of intermolecular
H-bonds is observed. This change in preference seems to
be directed by the formation of a short iodineꢀoxygen
(9) Representative examples: (a) Camptothecin alkaloids: Wall,
M. E.; Wani, M. C. J. Ethnopharmacol. 1996, 51, 239–253. (b) Lupin
alkaloids: Gray, D.; Gallagher, T. Angew. Chem., Int. Ed. 2006, 45,
2419–2423. (c) Mappicine: Govindachari, T. R.; Ravindranath, K. R.;
Viswanathan, N. J. Chem. Soc., Perkin Trans. 1 1974, 1215–1217.
(10) Representative examples: (a) Huffman, J. W.; Lu, J.; Hynd, G.;
Wiley, J. L.; Martin, B. R. Bioorg. Med. Chem. 2001, 9, 2863–2870. (b)
Parlow, J. J.; South, M. S. Tetrahedron 2003, 59, 7695–7701.
(16) (a) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol.
Model. 2007, 13, 291–296. (b) Politzer, P.; Lane, P.; Concha, M. C.; Ma,
Y.; Murray, J. S. J. Mol. Model 2007, 13, 305–311.
(17) (a) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc.
Chem. Res. 2005, 38, 386–395. (b) Metrangolo, P.; Meyer, F.; Pilati, T.;
Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114–6127.
(18) For further evaluation of solid supports, see Supporting
Information.
(11) Lanni, E. L.; Bosscher, M. A.; Ooms, B. D.; Shandro, C. A.;
Ellsworth, B. A.; Anderson, C. E. J. Org. Chem. 2008, 73, 6425–6428.
(12) Ryu, K.-A.; Anderson, C. E. Unpublished work.
˚
(13) Structure was solved at 0.74 A resolution. For full details of the
structure and solid-state packing, see Supporting Information.
(14) The hydroxyl resonance undergoes exchange when the 1H NMR
spectrum is recorded in methanol-d4. See Supporting Information.
(15) Frisch, M. J. et al. Gaussian 03, revision E.02; Gaussian, Inc.:
Wallingford CT, 2004. See Supporting Information for full computational details.
(19) Dugstad, H.; Undheim, K. Synth. Commun. 2008, 38, 1846–
1854.
Org. Lett., Vol. 13, No. 23, 2011
6225

Table 2. Optimization of Atmospheric Conditions
Scheme 2. Proposed Mechanism
LiI
temp
time
yield
entry (equiv)
additives
(°C) atm (h) 6a:7a 7a (%)a
1
2
3
4
5
6
7
1.0
1.5
3.0
3.0
3.0
1.5
3.0
butanol (3 M)
10 wt % SiO2
butanol (3 M)
20 wt % SiO2
butanol (1 M)
30 wt % SiO2
butanol (1 M)
30 wt % SiO2
butanol (1 M)
30 wt % SiO2
butanol (3 M)
30 wt % SiO2
80 air
80 air
80 air
90 O2
90 O2
80 N2
72 1.0:1.4 29%
72 1.0:3.1 43%
72 1.0:8.0 56%
water, followed by proton transfer would then provide the
observed product 7a. To test this mechanistic hypothesis,
water was added to the reaction mixture; however, a decrease
in both the product ratio and isolated yield was observed
(Table 1, entry 6).
48
72
0:1.0 68%b
0:1.0 41%c
72 1.0:0
0%d
Given this result, a modified mechanism was proposed
in which molecular oxygen could be directly incorporated
to provide the newly formed allylic alcohol. To evaluate
this hypothesis, the reaction was conducted under differ-
ent atmospheric conditions (Table 2). In the presence of
air, alkene 7a was isolated in 29% yield (entry 1). Further
optimization of the amount of lithium iodide, silica gel,
and solvent facilitated the formation of the desired pro-
duct 7a in 56% isolated yield and an 8:1 ratio relative
to alkyne 6a (entries 2 and 3). The reaction was then
performed under pure oxygen, with and without mixing
(entries 4 and 5). In this environment, only 7a was
isolated, and the yield improved. However, while 7a
was isolated in 68% yield with agitation, only a 41%
yield was observed in the absence of mixing. When
oxygen was excluded entirely, the reaction yielded only
the undesired alkyne 6a in 67% yield (entry 6), lending
further support to the updated mechanism. At this time, it
was also discovered that increasing the temperature to
90 °C allowed comparable yields in only 48 h, without loss
of product selectivity (entry 4). Increasing the tempera-
ture to 100 °C, however, was found to severely erode the
yield and selectivity of the rearrangement (entry 7).
Finally, alternate sources of cationic iodide (NIS and I2)
were evaluated (Scheme 3). When NIS was used in place of
LiI, pyridine 5a could be recovered in significant quantities,
but no product or related species could be detected. Con-
versely, upon treatment with 1 equiv of iodine in the presence
of silica gel and oxygen at 90 °C, butyl ether 10 was isolated
in 37% yield. In this case, it is assumed that intermediate 9 is
intercepted by 1-butanol rather than reacting with oxygen.
This implies that the lithium cation must be playing an
important role in the ring opening by oxygen, as no butyl
ether has been observed in the presence of LiI.
butanol (1 M) 100 air
50 wt % SiO2
48 1.0:1.5 38%
a Isolated yield. b Mean value of duplicate experiments ((3%).
c Without agitation. d 67% isolated yield of compound 6a.
product in moderate to good yields. Alternate alkyl
chains, including those with increased steric crowding
on the alkyne terminus, are well tolerated (entries 1 and 2).
The reaction permits extended conjugation near the
alkyne, such as in phenyl substrate 5d, which was found
to undergo rearrangement in 67% yield (entry 3). The
inclusion of heteroatoms in the propargyl unit also pro-
ceeded without difficulty, giving silyl ether 7e in 67%
yield (entry 4). In addition, methyl substitution on the
pyridine ring at either C3 or C5 has no significant impact
on the overall reactivity or efficiency of the reaction
(entries 5 and 6). In contrast, substitution adjacent to
the nitrogen at C6 does reduce the reaction efficiency with
substituted pyridone 7h being produced in only 41% yield
(entry 7). Alternatively, methyl substitution at the pro-
pargylic position is well tolerated, allowing pyridone 7i,
containing a new chiral center at the allylic position, to be
isolated in 71% yield as a 2:1 mixture of diastereomeric
comformations as determined by 1H NMR analysis
(entry 8). This stereochemical outcome is imparted by
Scheme 3. Evaluation of Alternate Iþ Sources
Evaluation of Reaction Scope. Table 3 summarizes the
reaction of eight representative 2-propargyloxypyridines
5 under the optimized conditions (3.0 equiv of LiI,
30 wt % SiO2, 90 °C, O2 atmosphere, 48 h). In all cases the
reaction is robust, producing only the desired vinyl iodide
6226
Org. Lett., Vol. 13, No. 23, 2011

Table 3. Evaluation of Substrate Scopea
Scheme 4. Elaboration of Pyridone 7a
the free alcohol, however, proved to be more challen-
ging. To avoid this issue, compound 11 was treated with
CO, NEt3, and PdCl2(Ph3P)2 in methanol to give methyl
ester 13 cleanly in 84% yield.22 Alternatively, treatment
of alcohol 7a with tosyl chloride and NaH in DMF
readily provided access to allylic chloride 14 in 71%
yield.23
A new method for the synthesis of β-iodo N-alkenyl
pyridones has been developed and applied to a range of
substituted 2-propargyloxypyridine precursors 5 in moder-
ate to good yields (41ꢀ72%). These pyridones have been
shown to adopt different conformations depending on the
presence or absence of solvent. The dense core of orthogonal
functionality present in these compounds is unprecedented
and can be transformed into a variety of other functional
groups. Efforts to transform β-iodo N-alkenyl pyridones 7
into other complex N-alkyl pyridone-containing targets are
currently underway in our laboratories.
Acknowledgment. This work was supported by the
National Science Foundation under CHE-0911264, awards
from Research Corporation for Science Advancement,
Arnold and Mabel Beckman Foundation Scholars Pro-
gram, Donors of the American Chemical Society Petro-
leum Research Fund, and Calvin College. The CCD based
X-ray diffractometer at Michigan State University was
upgraded and/or replaced by departmental funds. We
thank B. Ooms and L. Schipper (Calvin College) for initial
observations and Dr. J. Greaves (UC-Irvine) for work in
support of this project.
a Conditions: LiI (3.0 equiv), SiO2 (30 wt %), 1-butanol (1 M), 90 °C,
O2 atmosphere, 48 h. b Isolated yields. Mean value from duplicate
experiments ((3%). c SiO2 (19 wt %). d SiO2 (35 wt %). e Ratio of
diastereomeric comformations = 2:1.
the transient axial chirality of the hindered CꢀN junc-
tion. Control of the stereochemical configuration at the
newly formed chiral center is currently under investiga-
tion in our laboratories.
Synthetic Utility of N-Alkenyl Pyridones. N-Alkyl pyr-
idones represent a motif of interest for pharmaceutical
targets due to their unique semiaromaticity20 and H-
bonding ability. To demonstrate the utility of these
species, conversion of the synthetic handles present in
alkene 7a to another functionality was pursued (Scheme 4).
Upon treatment with MeI and NaH, the allylic alcohol
could be converted into methyl ether 11 in 92% yield.21
Functionalization of the vinyl iodide in the presence of
Supporting Information Available. Representative ex-
perimental procedures, 1H and 13C NMR spectra for all
new compounds, further optimization, computational
details, and CIF file and solid state packing diagrams
for 7a. This material is available free of charge via the
Internet at http://pubs.acs.org.
(20) 2-Pyridone possesses 36% of the aromaticity of benzene; see:
Elvidge, J. A.; Jackman, L. M. J. Chem. Soc. 1961, 859–866.
(21) Okamoto, K.; Hayashi, T.; Rawal, V. H. Org. Lett. 2008, 10,
4387–4389.
(22) Nicolaou, K. C.; Tang, Y.; Wang, J. Angew. Chem., Int. Ed.
2009, 48, 3449–3453.
(23) Sureshan, K. M.; Shashidhar, M. S.; Praveen, T.; Gonnade,
R. G.; Bhadbhade, M. M. Carbohydr. Res. 2002, 337, 2399–2410.
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