Model Compounds of Caged Capsaicin: Design, Synthesis, and Photoreactivity

Article abstract of DOI:10.1021/jo034616t

Molecules were prepared with substituted nitrobenzyl groups covalently bonded to N-(4-hydroxy-3-methoxybenzyl)acetamide (2) by ether or carbonate linkages. These compounds decomposed under irradiation at 363 nm. Those with carbonate linkages decomposed at

Full text of DOI:10.1021/jo034616t

Mod el Com p ou n d s of Ca ged Ca p sa icin : Design , Syn th esis, a n d
P h otor ea ctivity
Alan R. Katritzky,* Yong-J iang Xu, and Anatoliy V. Vakulenko
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida,
Gainesville, Florida 32611-7200
Allan L. Wilcox and Keith R. Bley*
NeurogesX, Inc., 981F Industrial Road, San Carlos, California 94070
katritzky@chem.ufl.edu
Received May 9, 2003
Molecules were prepared with substituted nitrobenzyl groups covalently bonded to N-(4-hydroxy-
3-methoxybenzyl)acetamide (2) by ether or carbonate linkages. These compounds decomposed under
irradiation at 363 nm. Those with carbonate linkages decomposed at slower rates than those with
ether linkages. Molecules with dimethoxy-substituted benzyl groups decomposed more slowly than
monomethoxy-substituted benzyl groups due to the electronic characteristics of the benzylic carbon.
In tr od u ction
Capsaicin 110 and congeneric structures11 (Figure 1)
have attracted much attention because of their use as
analgesics,12 diagonistic aids,13 and vasodilators.14 Re-
cently, 4,5-dimethoxy-2-nitrobenzylcapsaicin has been
used as pharmacological tool for the photochemical gating
of heterologously expressed ion chanels.15 Capsaicin is a
natural product isolated from hot peppers and is struc-
turally related to vanillin, a natural product from the
vanilla plant. Capsaicin and related compounds are
classified as vanilloids and possess substructures related
to guaiacol (o-methoxyphenol), which is found in common
plants, such as rue and broom. Nelson first determined
the chemical structure of capsaicin,16 and the first total
synthesis was reported in 1930.17
It is well-known that the absorption of light by
molecules can cause a plethora of physical processes and
chemical reactions, including cleavage of chemical bonds.
Absorption of light and subsequent bond cleavage has
been used to protect functional groups in organic syn-
thesis1 and to mask the biological activity of compounds
for controlled introduction into biological systems. Bio-
logically “masked” molecules are referred to as caged
compounds and are used to study processes such as
biological energy regulation, neuronal responses, enzyme
structure, and function.
Caged compounds are useful in biologically studies
because the timing, location, and rate of cleavage can be
precisely controlled. Caged compounds mask features of
a structure important for biological recognition and
release biologically active molecules on photochemical
cleavage. Photolysis of caged compounds is an excellent
technique to examine fast kinetics and/or spatial hetero-
geneity of biochemical responses in cells, tissues, or
protein crystals.2 The technique has allowed study of
neurotransmitters,3 nucleotides,4 nucleosides,5 deoxyglu-
cose,6 glycine,7 peptides8 in biological systems, and
calcium uptake.9
Capsaicin is a small molecule with low melting point
and moderate hydrophobicity. These properties facilitate
penetration of skin stratum corneum by capsaicin from
topical applications. Capsaicin has been used for many
years as topical treatment for chronic pain conditions,
including postherpetic neuralgia, painful diabetic neur-
opathy, and osteoarthritis.10c,12 However, capsaicin is
classified as an irritant, causing localized burning sensa-
tion, erythema, or stinging, and aerosolized capsaicin can
induce coughing or sneezing.18 Therefore, capsaicin must
be handled with care.
* Corresponding author.
(1) Bochet, C. G. J . Chem. Soc., Perkin Trans. 1 2002, 125.
(2) Adams, S. R.; Tsien, R. Y. Annu. Rev. Physiol. 1993, 55, 755.
(3) Hess, G. P.; Grewer, C. In Methods in Enzymology; Marriott,
G., Ed.; Academic Press: New York, 1998; Vol. 291, pp 443-473.
(4) Raimbault, C.; Perraut, C.; Marcillat, O.; Buchet, R.; Vial, C.
Eur. J . Biochem. 1997, 250, 773.
(5) (a) Meldrum, R. A.; Chittock, R. S.; Wharton, C. W. In Methods
in Enzymology; Marriott, G., Ed.; Academic Press: New York, 1998;
Vol. 291, pp 483-495. (b) Cohen, B. E.; Stoddard, B. L.; Koshland, D.
E. Biochemistry 1997, 36, 9035.
(9) J ames, I. F.; Walpole, C. S. J .; Hixon, J .; Wood, J . N.; Wriggles-
worth, R. Mol. Pharmacol. 1988, 33, 643.
(10) (a) Buck, S. H.; Burks, T. F. Pharmacol. Rev. 1986, 38, 179. (b)
Holzer, P. Pharmacol. Rev. 1991, 43, 143. (c) Szallasi, A.; Blumberg,
P. M. Pharmacol. Rev. 1999, 51, 159.
(11) Bennett, D. J .; Kirby, G. W. J . Chem. Soc. C 1968, 442.
(12) USAN Council. Clin. Pharmacol. Ther. 1986, 40, 243.
(13) Molna´r, J .; Gyo¨rgy, L.; Unyi, G.; Kenyeres, J . Acta Physiol.
Acad. Sci. Hung. 1969, 35, 369.
(6) Watanabe, S.; Hirokawa, R.; Iwamura, M. Bioorg. Med. Chem.
Lett. 1998, 8, 3375.
(7) Grewer, C.; J a¨ger, J .; Carpenter, B. K.; Hess, G. P. Biochemistry
2000, 39, 2063.
(14) Martindale. The Extra Pharmacopoeia, 28th ed.; Reynolds, J .
E. F., Ed.; The Pharmaceutical Press: London, 1982; p 1629.
(15) Zemelman, B. V.; Nesnas, N.; Lee, G. A.; Miesenbo¨ck, G. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 1352.
(8) Odaka, M.; Furuta, T.; Kobayashi, Y.; Iwamura, M. Photochem.
Photobiol. 1996, 63, 800.
(16) Nelson, E. K. J . Am. Chem. Soc. 1919, 41, 1115.
(17) Spa¨th, E.; Darling, S. F. Ber. Dtsch. Chem. Ges. 1930, 63B, 737.
10.1021/jo034616t CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/23/2003
9100
J . Org. Chem. 2003, 68, 9100-9104

Model Compounds of Caged Capsaicin
serotonin,21 and phenylephrine.22 Caged tyrosine is cleaved
upon exposure to 350 nm light for 12 h. 4,5-Dimethoxy-
2-nitrobenzyl-caged serotonin releases the neurotrans-
mitter upon excitation by 308 or 337 nm laser pulses.
Phenylephrine derivatives are photolyzed by flashes from
a xenon arc lamp with a 300-350 nm filter or a frequency
doubled ruby laser at 347 nm in buffer solution and in
smooth muscle preparations. The latter wavelength of
light is suitable for biological samples because photo-
chemical damage to proteins and nucleotides is mini-
mized.23 The 4,5-dimethoxy groups in a caged nitrobenzyl
phenylephrine derivative increases the maximum UV
absorption wavelength from 272 to 330 nm and suggests
electron-rich nitrobenzyl groups can be cleaved from
caged compounds at longer wavelengths, an advantage
when working in biological systems. Dimethoxy substitu-
tion of caged nitrobenzyl phenylephrine also increases
the rate of photolysis relative to the unsubstituted
nitrobenzyl phenylephrine analogue.22a This observation
supports that electron-releasing benzyl substituents
promote photolytic cleavage of 2-nitrobenzyl phenolic
ethers.
QSAR analysis provides mechanistic details of the
photochemical cleavage of benzylic compounds useful in
the design of target structures.24 For example, photo-
chromic bleaching rates (k) of 2-(2-nitrobenzyl)pyridines25
are sharply increased by electron releasing substituents
as shown by eq 1.
F IGURE 1.
Studies of the effect of variation in molecular structure
on the activity of capsaicinoids demonstrate that blocking
the phenolic OH or a change in the size of the side chain
leads to loss of vanilloid receptor 1 (VR1, or TRPV1)
agonist activity.10c Activation of the TRPV1 receptor can
initiate calcium-induced neurotoxicity in sensory neur-
ons. A rational way to decrease the irritating properties
of capsaicin and increase the ease and safety of its use
is to photoactivate caged capsaicin near its site of action,
causing a rapid and full activation of TRPV1 receptors.
In theory, this should result in less irritation since
excessive depolarization and intracellular calcium ac-
cumulation will cause sensory neurons to become non-
functional.19 The target molecules of the present work
contain substituents on the phenolic group of N-(4-
hydroxy-3-methoxybenzyl)acetamide (2).
Targets based on 2 were designed to model properties
of capsaicin. The study of structures based on 2 provides
benefits of economics and safety. The parent 2 is easily
prepared from vanillin or vanillylamine and lacks the
noxious and irritating properties of capsaicin. Effects of
changes in structure on reactivity in derivatives of 2
should parallel those in capsaicin and synthetic methods
developed for preparation of derivatives of 2 should be
directly applicable to capsaicin. We now report our study
of the effects of changes in structure on hydroxyl-caged
derivatives of N-(4-hydroxy-3-methoxybenzyl)acetamide
2. Specifically, we describe the syntheses of the photo-
labile caged molecules 3-6 (Figure 1), using various
nitrobenzyl carbonates and nitrobenzyl ethers as caging
groups, and some initial investigations of their photolysis.
log k ) (-2.62 ( 0.40)σ - (1.69 ( 0.17)
(eq 1)
N ) 9, r2 ) 0.97, Q2 ) 0.95, s ) 0.20
The coefficient of σ in eq 1 describes a strong effect of
the electronic nature of the benzylic carbon on the
photolysis rates of 2-(2-nitrobenzyl)pyridines. Because
the photolysis of 2-nitrobenzyl compounds proceeds
through an aci-nitro intermediate,26 the effect of substit-
uents on the nitro group might be important in the rate-
determining step of the reaction. However, when 2-(2-
nitrobenzyl)pyridines in this dataset are parametrized
using σ for the effect of substituents on the nitro group,
the correlation is low (r2 ) 0.65) with photochromic
bleaching rates (log k). Therefore, eq 1 suggests the effect
of substituents on the benzylic carbon is more important
than that of substituents on the nitro group in determin-
ing rate of photolysis and is consistent with a positive
correlation of photolytic cleavage rate with more electron-
rich benzylic carbons.
Bochet recently showed the effects of substituents on
wavelength sensitivity and photoreactivity of 2-nitroben-
(20) Sreecumar, R.; Ikebe, M.; Fay, F. S.; Walker, J . W. In Methods
in Enzymology; Marriott, G., Ed.; Academic Press: New York, 1998;
Vol. 291, pp 78-94.
(21) Breitinger, H.-G. A.; Wieboldt, R.; Ramesh, D.; Carpenter, B.
K.; Hess, G. P. Biochemistry 2000, 39, 5500.
(22) (a) Walker, J . W.; Martin, H.; Schmitt, F. R.; Barsotti, R. J .
Biochemistry 1993, 32, 1338. (b) Boyle, W. A.; Muralidharan, S.; Maher,
G. M.; Nerbonne, J . M. J . Photochem. Photobiol. B 1997, 41, 233.
(23) McCray, J . A.; Trentham, D. R. Annu. Rev. Biophys. Biophys.
Chem. 1989, 18, 239; Chem. Abstr. 1989, 111, 53299w.
(24) QSAR equations in this paper are found in CQSAR 4.5
Database; Biobyte Corp.: Claremmont, CA.
Resu lts a n d Discu ssion
Design of Ta r get Str u ctu r es. Targets were designed
by consideration of the photolytic reactivity of related
structures, for example, 2-nitrobenzyl-caged tyrosine,20
(25) Weinstein, J .; Bluhm, A. L.; Sousa, J . A. J . Org. Chem. 1966,
31, 1983.
(26) Peng, L.; Goeldner, M. J . Org. Chem. 1996, 61, 185.
(18) Rains, C.; Bryson, H. M. Drugs. Aging 1995, 7, 317.
(19) Dray, A. Biochem. Pharmacol. 1992, 44, 611.
J . Org. Chem, Vol. 68, No. 23, 2003 9101

Katritzky et al.
zyl carbamates.27 Similar to photolysis of 2-(2-nitro-
benzyl)pyridines, electron-releasing benzyl substituents
increase the rate of photolysis (k) of 2-nitrobenzyl car-
bamates (eq 2) under 420 nm light
SCHEME 1
log k ) (-0.22 ( 0.08)σ+ - (1.59 ( 0.07)
N ) 5, r2 ) 0.96, Q2 ) 0.91, s ) 0.02
(eq 2)
Equation 2 supports the concept that substituents with
electron-releasing properties to the benzylic carbon in-
crease rates of photolysis of 2-nitrobenzyl carbamates.
Equation 2 further demonstrates the electronic nature
of the benzylic group is most important for the rate-
determining step because when the 2-nitrobenzyl car-
bamates are parametrized for the nitro group, the
correlation with rates of photolysis is poor (r2 ) 0.70).
Concern that the parent 2 might need optimization as
a leaving group from a photolytically activated interme-
diate led us to design molecules with the carbonate
linkage in order to modulate the rate of release of 2. A
2-nitrobenzyl group linked by a carbonate to the anti-
cancer drug 5-fluorodeoxyuridine (5-F-dU) is activated
by UV light. The structure is nontoxic to cells and upon
exposure to light (>350 nm) releases 5-F-dU. Photolysis
of 2-nitrobenzyl group linked by a carbonate to 5-F-dU
in vitro results in inhibition of cell growth.28
SCHEME 2
We sought to target nitrobenzyl-protected analogues
with electron-donating substituents to provide the most
rapid release of 2 upon photolysis. Four target molecules
(3-6) were selected to examine the effect of the electronic
nature of the benzyl group and two different types of
linkage groups bonded to 2, an ether (3 and 4), or a
carbonate (5 and 6) linkage (Figure 1). Two different
substitution patterns on the nitrobenzyl group were
selected to give a range of electronic properties of the
benzylic carbon atom. The 4-monomethoxy-substituted
nitrobenzyl has a more electron-rich benzylic carbon atom
(negative coefficient σ ) -0.27) than that of the 3,4-
dimethoxy-substituted nitrobenzyl compound (σ ) -0.15)
because the methoxy substituent in the meta position is
electron withdrawing (σ ) +0.12) with respect to the
benzyl carbon atom. On the basis of QSAR discussed
above, it was expected that monomethoxy-substituted
nitrobenzyl molecules 3 and 5 would decompose more
rapidly than their dimethoxy analogues under conditions
of photolysis.
Syn th eses. Syn th eses of Ta r get Molecu les 3 a n d
5. Target molecule 3 was synthesized by the coupling
reaction of compound 2 with 1-(bromomethyl)-4-methoxy-
2-nitrobenzene (8) in the presence of an equal amount of
potassium tert-butoxide to enhance the reactivity of the
phenolic group in 2 (Scheme 1). Compound 8 was
obtained by the bromination of commercially available
4-methyl-3-nitroanisole (7).29
The synthesis of molecule 5 was carried out using
coupling reaction of compound 2 with 1-[(chlorocarbonyl)-
oxy]methyl-4-methoxy-2-nitrobenzene (12) in the pres-
ence of anhydrous pyridine instead of potassium tert-
butoxide as a base. The key starting material 12 was
prepared in 83% yield by reaction of phosgene (11) with
4-methoxy-2-nitrobenzyl alcohol (10), which was obtained
from 8 by (i) oxidization of 8 by tetrabutylammonium
dichromate to get 4-methoxy-2-nitrobenzaldehyde (9)29
(75%) and (ii) reduction of 9 using NaBH4 to give 10 (72%)
(Scheme 1). The structures of 3, 5, and all the precursors
were fully characterized by 1H and 13C NMR and mi-
croanalysis (except 12, which is used in subsequent
reactions without purification).
Compound 2 was synthesized from 4-hydroxy-3-meth-
oxybenzylamine hydrochloride and acetic anhydride in
58% yield.30 Attempted chloroformylation of 2 to 1-[(acetyl-
amino)methyl]-4-[(chlorocarbonyl)oxy]-3-methoxybenzene
failed.
Syn th esis of Ta r get Molecu les 4 a n d 6. Target
molecule 4 was prepared in 58% yield by the similar
coupling reaction as that described for the synthesis of 3
using commercially available 1-(bromomethyl)-4,5-dimeth-
oxy-2-nitrobenzene (13) instead of compound 8 (Scheme
2).
Following a procedure similar to that described for the
preparation of 5, target molecule 6 was obtained in 42%
yield starting from 1-[(chlorocarbonyl)oxy]methyl-4,5-
dimethoxy-2-nitrobenzene (15),31 which was prepared in
84% yield by the reaction of phosgene (11) with 4,5-
dimethoxy-2-nitrobenzyl alcohol (14) (Scheme 3). The
structures of 4, 6, and 15 were also fully characterized
(27) Bochet, C. G. Tetrahedron Lett. 2000, 41, 6341.
(28) Wei, Y.; Yan, Y.; Pei, D.; Gong, B. Bioorg. Med. Chem. Lett.
1998, 8, 2419.
1
by H and 13C NMR and microanalysis.
(29) Mohan, R.; Katzenellenbogen, J . A. J . Org. Chem. 1984, 49,
1238.
(30) Nelson, E. K. J . Am. Chem. Soc. 1919, 41, 2121.
9102 J . Org. Chem., Vol. 68, No. 23, 2003

Model Compounds of Caged Capsaicin
F IGURE 2. UV-vis spectra of compound 3 (starting concen-
tration, 4.0 mg/mL).
F IGURE 3. UV-vis spectra of compound 4 (starting concen-
tration, 4.0 mg/mL).
SCHEME 3
that for 3. The spectral changes for 4 and 6 are similar
to those for 3 and 5. Spectra of 4 and 6 show a decreased
absorbance in the 200-247 nm and 308-367 nm regions
and an increased absorbance in the 247-308 nm and
above 367 nm region (Figure 3).
To summarize, four photolabile caged molecules were
synthesized and the behavior of each studied under UV
irradiation at 363 nm. Molecules with ether linkages (3
and 4) decomposed faster than those with carbonate
linkages (5 and 6). Molecules with an additional methoxy
group (3 and 5) decomposed more slowly than the des-
methoxy analogues (4 and 6), consistent with QSAR
analysis of related molecules which predicts the mono-
methoxy-substituted nitrobenzyl molecules 3 and 5 are
more sensitive than their dimethoxy analogues 4 and 6
to conditions of photolysis. The relative reactivity of
2-nitrobenzyl ether and carbonate-linked molecules sup-
port that compounds with more electron-rich benzylic
carbons 3 and 5 decompose more rapidly than their less
electron-rich analogues 4 and 6.
P h otolyses. Photolyses were carried out in the same
concentration (4.0 mg/mL) under 363 nm UV light for
1
each molecule. H NMR spectra and UV-vis absorption
data were used to monitor the photolysis characteristics
of compounds 3-6.
The 1H NMR spectra showed that all compounds
decomposed under irradiation. For 3, about 57% of the
ether linkage was cleaved after 20 min. In the same time
interval, 20% of 4 was transformed and less than 7% of
5 decomposed (from the 1H NMR spectrum). Similar
NMR spectra were recorded for 6 before and after 20 min
irradiation; the only difference was two very small new
peaks at 3.89 and 3.92 ppm.
After 80 min (4 × 20 min) of irradiation, the singlet at
Exp er im en ta l Section
5.62 ppm assigned to the ArCH2O- group disappeared
1
from the H NMR spectrum of 3, indicating that 3 had
THF was distilled from sodium benzophenone prior to use.
Melting points were determined using a Bristoline hot-stage
microscope and are uncorrected. 1H NMR and 13C NMR spectra
(300 and 75 MHz, respectively) were recorded on a Gemini
300 NMR spectrometer in CDCl3 (with TMS for 1H and
chloroform-d for 13C as the internal reference). The UV-vis
spectra were recorded on a Varian Cary 100 Conc spectropho-
tometer. Elemental analyses were performed on a Carlo Erba-
1106 instrument. HRMS were measured on an AEI-30 mass
spectrometer. Column chromatography was performed on
silica gel unless otherwise noted. All of the reactions were
carried out under N2.
decomposed completely. Similarly, 4 cleaved completely
after 140 min of irradiation. The H NMR spectrum of 5
showed some starting material after 380 min of irradia-
tion. J udging by the 1H NMR spectrum, about 80% 6 still
remained after 480 min of irradiation.
1
The color of the solutions also changed: after 20 min
of irradiation, the solution of 3 changed to medium
yellow, 4 changed to bright yellow, that of 5 was light
yellow, while the solution of 6 was nearly colorless. As
irradiation continued, the color of all the solutions
became progressively deeper. At the end of the irradia-
tion, all of the solutions were deep brown, including 6.
UV-vis spectra were studied before and after photoly-
sis. The spectra of 3 are shown in Figure 2. A decreased
absorbance in the 200-235 nm region and increased
absorbance in the 235-325 nm region is observed with
irradiation. Similar spectra were recorded with 5. A
difference between the spectra of 3 and 5 is that the
change of the absorbance in 5 is significantly slower than
P h otolysis. UV 450 W Immer. Lamp with 363 nm filter
was used as monochromatic light source. A quartz cuvette with
a 1-cm light-path length was used to measure absorption. The
intensity of the incident light, which was measured by the
standard method,32 is 5.25 × 10-9 Ein/s.
1-(Br om om eth yl)-4-m eth oxy-2-n itr oben zen e (8). A so-
lution of 2-nitro-4-methoxytoluene 7 (0.5 g, 3.03 mmol),
N-bromosuccinimide (0.539 g, 3.03 mmol), and benzoyl per-
oxide (1.5 mg) in 10 mL of carbon tetrachloride was refluxed
under strong illumination using a 400-W General Electric
sunlamp. After 7 h, the solution was cooled and the solid
(31) Amit, B.; Zehavi, U.; Patchornik, A. J . Org. Chem. 1974, 39,
192.
(32) Heller, H. G.; Langan, J . R. J . Chem. Soc., Perkin Trans. 2 1981,
341.
J . Org. Chem, Vol. 68, No. 23, 2003 9103

Katritzky et al.
succinimide was removed by filtration. The filtrate was
concentrated to yield brown oil. Purification by column chro-
matography (eluent: hexanes/benzene ) 1/1) afforded 0.49 g
(69%) of the product 8 as light yellow needles: mp 63-64 °C
(lit.29 mp 63-64 °C); 1H NMR δ 3.88 (s, 3H), 4.80 (s, 2H), 7.13
(dd, J ) 2.7, 8.7 Hz, 1H), 7.46 (d, J ) 8.4 Hz, 1H), 7.55 (d, J
) 2.7 Hz, 1H); 13C NMR δ 29.1, 55.9, 110.4, 119.9, 124.7, 133.6,
148.5, 160.0.
5.81 (br s, 2H), 6.77 (d, J ) 8.0 Hz, 1H), 6.81 (s, 1H), 6.86 (d,
J ) 8.0 Hz, 1H); 13C NMR δ 23.3, 43.7, 55.9, 110.8, 114.4,
120.8, 130.1, 145.2, 146.7, 169.8.
N-[3-Met h oxy-4-[(4-m et h oxy-2-n it r ob en zyl)oxy]b en -
zyl]a ceta m id e (3). To a solution of 2 (0.20 g, 1 mmol) in 10
mL of THF was added t-BuOK (1 mL, 1M in THF). Then 8
(0.246 g, 1 mmol) was added. The resulting mixture was stirred
for 24 h at rt. After removal of the solvent, the residue was
purified by column chromatography (eluent: EtOAc/hexanes
) 1/4). The product was recrystallized from CH2Cl2/Et2O to
give yellowish needles (0.26 g, 72%): mp 143-144 °C; UV
(C2H5OH) λ max [nm] (log ꢀ) 276 (34.9), 331 (11.4); 1H NMR δ
2.02 (s, 3H), 3.89 (s, 3H), 3.90 (s, 3H), 4.36 (d, J ) 5.8 Hz,
2H), 5.46 (s, 2H), 5.71 (br s, 1H), 6.75-6.82 (m, 2H), 6.87 (s,
1H), 7.20 (dd, J ) 2.6, 8.7 Hz, 1H), 7.66 (d, J ) 2.6 Hz, 1H),
7.78 (d, J ) 8.8 Hz, 1H); 13C NMR δ 23.3, 43.6, 55.9, 56.0,
67.8, 109.5, 111.9, 114.1, 120.1, 120.6, 125.8, 129.8, 132.0,
147.1, 147.5, 149.9, 159.2, 169.8. Anal. Calcd for C18H20N2O6:
C, 59.99; H, 5.59; N, 7.77. Found: C, 59.87; H, 5.67; N, 7.67.
4-Meth oxy-2-n itr oben za ld eh yd e (9). A solution of 8 (0.56
g, 2.26 mmol) and bis(tetrabutylammonium) dichromate (1.47
g, 2.0 mmol) in 5 mL of CHCl3 was heated under reflux for 8
h. TLC analysis showed the disappearance of 8. The reaction
was rapidly cooled in ice bath and filtered through a pad of
silica gel (2 g) to eliminate the inorganic and tetrabutylam-
monium salts. The silica was then washed with diethyl ether
(20 mL). Evaporation of the combined organic solvents afforded
the crude organic product. Purification by flash chromatogra-
phy (eluent: CHCl3) afforded 0.30 g (75%) 9 as yellowish
1
needles: mp 91-92 °C (lit.29 mp 91-92 °C); H NMR δ 3.98
(s, 3H), 7.24 (dd, J ) 2.3, 8.7 Hz, 1H), 7.52 (d, J ) 2.3 Hz,
1H), 7.98 (d, J ) 8.7 Hz, 1H), 10.29 (s, 1H); 13C NMR δ 56.3,
109.6, 119.1, 123.4, 131.4, 151.6, 163.7, 186.9.
N-[4-[(4,5-Dim et h oxy-2-n it r ob en zyl)oxy]-3-m et h oxy-
ben zyl]a ceta m id e (4). Following a procedure similar to that
used for 3, compound 2 (0.20 g, 1 mmol) and 4,5-dimethoxy-
2-nitrobenzyl bromide 13 (0.276 g, 1 mmol) were used. The
product was recrystallized from CH2Cl2/Et2O to give white
needles (0.22 g, 56%): mp 187-188 °C; UV (C2H5OH) λ max
(4-Meth oxy-2-n itr op h en yl)m eth a n ol (10). Sodium boro-
hydride (0.06 g, 1.65 mmol) was added in small portions to an
ice-cold solution of 9 (0.3 g, 1.65 mmol) in dry methanol (20
mL) with stirring. The mixture was left at 0-5 °C for 1 h.
Then the solvent was evaporated, and chloroform (30 mL) was
added to the residue obtained. The organic layer was washed
with sodium bicarbonate (5%, 20 mL) and H2O (30 mL) and
dried over MgSO4. The product obtained was recrystallized
from benzene/hexanes (0.22 g, 72%) as colorless needles: mp
79-81 °C; 1H NMR δ 2.63 (br s, 1H), 3.89 (s, 3H), 4.87 (s, 2H),
7.20 (dd, J ) 2.6, 8.5 Hz, 1H), 7.57-7.61 (m, 2H); 13C NMR δ
55.9, 62.3, 109.7, 120.4, 128.7, 131.5, 148.4, 159.4. Anal. Calcd
for C8H9NO4: C, 52.46; H, 4.95; N, 7.65. Found: C, 52.31; H,
4.93; N, 7.48.
1-[[(Ch lor ocar bon yl)oxy]m eth yl]-4-m eth oxy-2-n itr oben -
zen e (12). A solution of phosgene (11) in toluene (4.57 mL,
20% w/w, 8.78 mmol) was added to a stirred solution of 10
(0.67 g, 3.66 mmol) in 20 mL of dry THF. Stirring was
continued for 23 h at ambient temperature. The excess
phosgene was removed under low vacuum and trapped with
an aqueous NaOH. The remaining solvent was removed on a
rotary evaporator. The yellowish oil so formed was directly
used for the subsequent reaction without further purification
owing to its instability: 1H NMR δ 3.91 (s, 3H), 5.64 (s, 2H),
7.22 (dd, J ) 2.6, 8.7 Hz, 1H), 7.52 (d, J ) 8.7 Hz, 1H), 7.67
(d, J ) 2.6 Hz, 1H); 13C NMR δ 56.0, 69.5, 110.4, 120.1, 121.0,
131.3, 148.3, 150.4, 160.4.
1
[nm] (log ꢀ) 280 (32.5), 345 (27.0); H NMR 2.03 (s, 3H), 3.91
(s, 3H), 3.97 (s, 6H), 4.38 (d, J ) 5.6 Hz, 2H), 5.55 (s, 2H),
5.68 (br s, 1H), 6.78-6.89 (m, 3H), 7.49 (s, 1H), 7.76 (s, 1H);
13C NMR δ 23.3, 43.6, 56.0, 56.4 (2), 68.5, 107.9, 109.5, 111.9,
114.6, 120.3, 129.8, 132.3, 138.9, 147.1, 147.8, 149.9, 154.0,
169.8. Anal. Calcd for C19H22N2O7: C, 58.46; H, 5.68; N, 7.18.
Found: C, 58.83; H, 5.70; N, 7.12.
4-[(Acetyla m in o)m eth yl]-2-m eth oxyp h en yl-4-m eth oxy-
2-n itr oben zyl Ca r bon a te (5). To a stirred solution of 2 (0.58
g, 2.99 mmol) in 2.8 mL of anhydrous pyridine cooled to 0 °C
was added dropwise a solution of 12 (0.81 g, 3.3 mmol) in THF
(5 mL). The mixture was then stirred at rt for 41 h. The solvent
was evaporated to give the crude product, which was recrys-
tallized from chloroform/hexanes to afford yellowish microc-
rystals (0.62 g, 51%): mp 108-110 °C; UV (C2H5OH) λ max
[nm] (log ꢀ) 272 (36.8), 327 (12.4); 1H NMR (DMSO) δ 1.89 (s,
3H), 3.77 (s, 3H), 3.89 (s, 3H), 4.25 (d, J ) 5.8 Hz, 2H), 5.50
(s, 2H), 6.84 (d, J ) 8.2 Hz, 1H), 7.04 (s, 1H), 7.14 (d, J ) 8.1
Hz, 1H), 7.43 (dd, J ) 2.6, 8.7 Hz, 1H), 7.64-7.67 (m, 2H),
8.37 (t, J ) 5.8 Hz, 1H); 13C NMR (DMSO) δ 22.7, 42.1, 56.0,
56.3, 66.4, 110.2, 112.2, 119.3, 120.1, 122.0, 122.1, 131.7, 138.4,
139.3, 148.7, 150.6, 152.6, 159.8, 169.4. Anal. Calcd for
C
19H20N2O8: C, 56.43; H, 4.99; N, 6.93. Found: C, 56.55; H,
4.88; N, 6.78.
1-[[(Ch lor oca r bon yl)oxy]m eth yl]-4,5-d im eth oxy-2-n i-
tr oben zen e (15). Following a procedure similar to that used
for 12, the reaction of phosgene (11) in toluene (2.75 mL of
20% w/w, 5.28 mmol) and 4,5-dimethoxy-2-nitrobenzyl alcohol
14 (0.47 g, 2.2 mmol) went on for 36 h in 10 mL of dry THF.
After workup, the residue was recrystallized from benzene to
afford 0.51 g (84%) of yellowish needles: mp 125-127 °C (lit.31
mp 125-127 °C); 1H NMR δ 3.99 (s, 3H), 4.02 (s, 3H), 5.73 (s,
2H), 7.02 (s, 1H), 7.76 (s, 1H); 13C NMR δ 56.4, 56.5, 69.8,
108.3, 110.4, 124.1, 139.8, 148.9, 150.3, 153.7.
N-(4-Hyd r oxy-3-m eth oxyben zyl)a ceta m id e (2). A mix-
ture of 4-hydroxy-3-methoxybenzylamine hydrochloride (5 g,
26 mmol), acetic anhydride (9 mL, 88 mmol), and anhydrous
sodium acetate (2.46 g, 30 mmol) was heated at 130 °C for 6
h. Then an excess of sodium hydroxide was added to the cooled
mixture with shaking. The solution was thoroughly extracted
with chloroform (3 times). The residue formed after the
evaporation of the chloroform was saponified with cold alco-
holic potassium hydroxide, saturated with carbon dioxide, and
extracted with chloroform. After evaporation of the solvent,
the product was obtained as thick brown syrup which slowly
became crystalline (58%): mp 82-83 °C (lit.30 mp 84-85 °C);
1H NMR δ 2.01 (s, 3H), 3.88 (s, 3H), 4.33 (d, J ) 5.5 Hz, 2H),
4-[(Acetyla m in o)m eth yl]-2-m eth oxyph en yl-4,5-d im eth -
oxy-2-n itr oben zyl Ca r bon a te (6). Following a procedure
similar to that used for 5, the reaction of 2 (0.32 g, 1.64 mmol)
and 15 (0.5 g, 1.8 mmol) was carried out. After workup, the
precipitate was filtered off and washed with cool THF to give
yellowish microcrystals (0.30 g, 42%): UV (C2H5OH) λ max
[nm] (log ꢀ) 279 (35.2), 344 (38.9); 1H NMR (DMSO) δ 1.88 (s,
3H), 3.77 (s, 3H), 3.89 (s, 3H), 3.91 (s, 3H), 4.25 (d, J ) 5.8
Hz, 2H), 5.56 (s, 2H), 6.85 (d, J ) 8.1 Hz, 1H), 7.05 (s, 1H),
7.16 (d, J ) 8.1 Hz, 1H), 7.21 (s, 1H), 7.74 (s, 1H), 8.37 (t, J )
5.7 Hz, 1H); 13C NMR (DMSO) δ 22.6, 41.9, 55.8, 56.2, 56.3,
66.8, 108.3, 111.5, 112.1, 119.2, 122.0, 125.0, 138.2, 139.2,
139.8, 148.3, 150.5, 152.3, 153.2, 169.2. Anal. Calcd for
C
20H22N2O9: C, 55.30; H, 5.10; N, 6.45. Found: C, 55.50; H,
5.25; N, 6.39.
Ackn owledgm en t. We are indebted to Dr. K. Schan-
ze (University of Florida) for advice and help with the
photoreactions. A.W. thanks Corwin Hansch for access
to the database and useful discussions on QSAR analysis.
J O034616T
9104 J . Org. Chem., Vol. 68, No. 23, 2003