Photoamidation of N-acetyl-2-chlorotyrosine methyl ester and 3-chlorophenol

Full text of DOI:10.1021/jo960037v

3236
J . Org. Chem. 1996, 61, 3236-3237
Peptide linkages are inert toward chemical modifica-
P h otoa m id a tion of
tions under ordinary laboratory conditions, tyrosine often
plays an important role in the biochemistry of proteins,6
halotyrosines may be readily incorporated in lieu of
tyrosine into proteins by the method of molecular biol-
ogy,7 and Ar-X bonds are stable in the ground state.
Therefore, our next goal is to evaluate photoamidation
of 2-halotyrosine derivatives as a possible entry to
photoaffinity labels for tyrosine in protein chemistry.8
This communication deals with the photochemistry of
3-halophenols and derivatives of 2-halotyrosines.
N-Acetyl-2-ch lor otyr osin e Meth yl Ester a n d
3-Ch lor op h en ol†
Baowen Zhang,‡ J intao Zhang,
Ding-Djung H. Yang, and Nien-chu C. Yang*
Department of Chemistry, University of Chicago,
Chicago, Illinois 60637
Received J anuary 5, 1996 (Revised Manuscript Received April
2, 1996 )
When DL-N-acetyl-2-chlorotyrosine methyl ester9 (2 ×
10-3 M, 2a ) in HPLC grade methanol (100 mL) containing
1 equiv of sodium methanolate was irradiated with a
Hanovia 450 W Hg-arc through a Pyrex filter under
argon, about 40% of the substrate was consumed after 1
h. The products were isolated by semipreparative HPLC
using a silica gel column and chloroform:methanol (94/
6, v/v) as the eluent. Three new products were isolated:
3a , 4a , and 5a , in 35%, 11%, and 45% yield, respectively
(reaction 3). They were characterized unambiguously by
Aryl halides are stable in nucleophilic displacement
reactions. 4-Chlorophenols undergo a variety of photo-
chemical reactions to yield a plethora of products, while
3-chlorophenol undergoes photohydrolysis in water to
form resorcinol.1 Since the excited state of phenols is
more energetic than that of the corresponding phenox-
ides, excited phenols are known to ionize to excited
phenoxide.2 On the basis of our experience in the
photochemistry of 4-haloindoles3 and the charge distribu-
tions of the Lb excited state of benzene,4 the powerful
electron-releasing excited phenoxide ion will delocalize
a substantial portion of its charge to the meta-position.5
Localization of charge at the position bearing the halogen
substituent may lead to its activation and subsequent
displacement (see also Scheme 1).
We irradiated 3-fluoro- and 3-chlorophenol, 1, and
found that they undergo photomethanolysis to yield
resorcinol monomethyl ether in excellent yield (94%,
reaction 1). In examining the scope of photosubstitution
of 3-halophenols, we discovered that these 3-halophenols
undergo photoamidation by N-methylacetamide (reaction
2), a very weak nucleophile, to yield 3-(N-methyl-N-
acetylamino)phenol (77%) together with a small amount
of phenol (6%). A competitive study between methanol
and N-methylacetamide in intermolecular photosubsti-
tution indicates that methanol is more reactive than
N-methylacetamide by at least 1 order of magnitude.
elemental and spectroscopic analyses.10 The formation
of 4a has an induction period and was detected only after
an appreciable amount of 3a was present; 4a is thus
likely to be a secondary product formed from the irradia-
tion of 3a . The formation of 3a , methyl 1-acetyl-6-
hydroxy-2-indolinecarboxylate, is an intramolecular pho-
toamidation of an aryl halide, a novel chemical reaction.
This reaction is competitive vs the formation of 5a , the
intermolecular methanolysis with the solvent, methanol.
Since methanol is much more reactive than N-methylac-
etamide, a secondary amide in intermolecular photosub-
stitution, the formation of 3a vs 5a must be controlled
by a prevailing steric factor. We also studied the
photochemistry of DL-2-chlorotyrosine methyl ester, 2b,
under the same experimental conditions. The relative
yields of intramolecular amination products, 3b (19%)
and 4a (17%), to the intermolecular solvolysis product
5b (41%) are also approximately 1:1. Since amines are
much stronger nucleophiles than amides, the results
again indicate that the intramolecular process is con-
trolled by the stereochemistry of the substrate and not
† Dedicated to Professor Morris S. Kharasch on the centennial of
his birth.
by the nucleophilicity of the substituting reagent.
A
‡ On leave from the Institute of Photographic Chemistry, Academia
Sinica, Beijing, 1994.
parallel study of the photochemistry of L-2-fluorotyrosine
(1) Boule, P.; Guyon, C.; Tissot, A.; Lemaire, J . J . Chim. Phys. 1985,
82, 513. Oudjehani, K.; Boule, P. J . Photochem. Photobiol. A: Chem.
1992, 68, 363.
(2) See for example, Weller, A. Z. Phys. Chem. (Munich) 1958, 17,
224.
(3) Yang, N. C.; Huang, A.; Yang, D. H. J . Am. Chem. Soc. 1989,
111, 8060.
(6) See for examples: (a) Hunter, T.; Cooper, J . A. Annu. Rev.
Biochem. 1985, 54, 897. (b) Levitzki, A.; Gazit, A. Science 1995, 267,
1782.
(7) Ho, C.; Dowd, S. R.; Post, J . F. M. Curr. Top. Bioenerg. 1985,
14, 53. Nowak, M. W.; Davidson, N.; Schultz, P. G.; Dougherty, D. A.;
Lester, H. A. Science 1995, 268, 439.
(4) Platt, J . M. J . Chem. Phys. 1951, 19, 263.
(5) For substituent effects in photosubstitutions of aromatic com-
pounds, see Havinga, H.; de J ongh, R. O.; Kronenberg, M. E. Helv.
Chim. Acta 1967, 50, 2550. Zimmerman, H. E.; Sandel, V. R. J . Am.
Chem. Soc. 1963, 85, 915.
(8) For reviews on photoaffinity labeling, see (a) Chowdhry, V.;
Westheimer, F. H. Annu. Rev. Biochem. 1979, 48, 293. (b) Bayley, H.;
Knowles, J . R. Methods Enzymol. 1977, 46, 69. (c) Tometsko, A. M.;
Richards, F. M.; Eds. Ann. N.Y. Acad. Sci. 1980, 346.
(9) Bennett, E. L.; Niemann, C. J . Am. Chem. Soc. 1950, 72, 1806.
S0022-3263(96)00037-0 CCC: $12.00 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 10, 1996 3237
Sch em e 1
methyl ester 2c indicates that the fluoro compound is less
reactive than the 2-chloro compound in the intramolecu-
lar amination; i.e., the yields of 3b, 4a , and 5b were 13%,
15%, and 58%, respectively.
Our experimental results suggest that the reactions
proceed via a highly reactive electron-deficient interme-
diate. When the photolyses of 3-chlorophenol was carried
out in acetonitrile containing 1-2% of water or methanol,
arylation of acetonitrile occurred and (N-acetylamino)-
phenol was isolated as the major product (45-75% yield).
N-Alkylamides may be prepared from the alkylation of
nitriles under acidic conditions via the carbocation
intermediate, the Ritter reaction.11 Photoarylations of
amides and nitriles suggest that these reactions proceed
via a cationic intermediate 7 (or a solvent separated ion-
pair). This intermediate may be formed from either the
excited phenol or the excited phenoxide (Scheme 1). The
photoamidation is novel because the reaction takes place
under neutral conditions, and it is an arylation.
biology. These photoaffinity labels cannot be incorpo-
rated into aromatic amino acids in peptides and proteins,
and are thus incompatible in the study of protein-
biopolymer interactions. Aryl-halogen bonds are stable
in the ground state, yet halogens meta to the phenol in
2-halotyrosines readily undergo photosubstitutions. De-
pending on their steric environment, they may be sub-
stituted competitively by a polar OH or NH group and
by a non-nucleophilic amide, the linkage of amino acids
in proteins. We had synthesized two insulins in which
the B-25 phenylalanine, a position crucial in the receptor
binding, was substituted by 3-fluorotyrosine and by
4-fluorophenylalanine.12 These modified insulins were
found to be as active as insulin in both the hormone assay
and the receptor-binding assay.12 Photosubstitution of
2-halotyrosines, therefore, offers a novel approach to
photoaffinity labeling by the interactions of these ha-
loaromatic amino acids with all neighboring amino acids,
polar and apolar, in proteins. The photochemical inter-
actions of 2-halotyrosyl derivatives and 3-halophenols
with other weak nucleophiles will be explored.
Photoaffinity labeling is a powerful technique in ex-
ploring protein:substrate interactions.8 However, com-
mon photoaffinity labels such as azides and diazo com-
pounds are chemically labile to acidic reagents used in
peptide and polynucleotide syntheses,8 and many are
toxic to common bacteria and cell lines used in molecular
(10) Analytical data: Compound 3a : mp 207-8 °C. Anal. Calcd
for C12H13NO4: C, 61.28; H, 5.53; N, 5.96. Found: C, 61.13; H, 5.50;
N, 5.88. MS (EI): 235(19), 193(17), 134(100). 1H NMR (methanol-d4,
500 MHz, 22 °C): two conformers (A:B ) 1:3): conformer A: δ 6.97
(d, J ) 8 Hz, 1H), 6.72 (s, 1H), 6.42(d, J ) 8 Hz, 1H), 5.05 (d, J ) 11
Hz, 1H), 3.67 (s, 3H), 3.38 (dd, J ) 11 and 15.5 Hz, 1H), 2.91 (d, J )
15.5 Hz, 1H), 2.42 (s, 3H); conformer B: δ 6.92 (d, J ) 8 Hz, 1H), 6.70
(s, 1H), 6.42 (d, J ) 8 Hz, 1H), 5.13 (d, J ) 11 Hz, 1H), 3.72 (s, 3H),
3.50 (dd, J ) 11 and 15.5 Hz, 1H), 3.12 (d, J ) 15.5 Hz, 1H), 2.41 ppm
(s, 3H). At 55 °C, all peaks broaden, and the COOCH peaks at δ 3.7
and the CH3CON peaks at δ 2.4 coalesce into single peaks. Compound
3b: It resisted all efforts for recrystallization so far and was readily
converted into 3a by acetylation. MS (EI): 193(31), 134(100), 116-
(11). 1H NMR (methanol-d4, 500 MHz): δ 6.75 (d, J ) 8 Hz, 1H), 6.12
(d, J ) 2.5 Hz, 1H), 6.07 (dd, J ) 8 and 2.5 Hz, 1H), 4.29 (dd, J ) 10.5
and 6 Hz, 1H), 3.64 (s, 3H), 3.18 (dd, J ) 10.5 and 15.5 Hz, 1H), 3.02
(dd, J ) 6 and 15.5 Hz). Compound 4a : mp 165-8 °C. Anal. Calcd
for C10H9NO3: C, 62.83; H, 4.71; N, 7.33. Found: C, 62.83; H, 4.81;
Ack n ow led gm en t. The authors wish to thank Pro-
fessor Kan Agarwal and the late Professor Howard
Tager for their collaborations and suggestions, and the
National Institute of General Medical Sciences and the
National Science Foundation for their financial support.
N, 7.22. MS (EI): 191(79), 159(100), 131(53), 105(29), 51(15). λmax
-
(methanol), 319 nm. Fluorescence, Emax(methanol), 381 nm. 1H NMR
(methanol-d4, 500 MHz), δ 7.33 (d, J ) 9 Hz, 1H), 6.98 (s, 1H), 6.70 (d,
J ) 2 Hz, 1H), 6.57 (dd, J ) 9 and 2 Hz), 3.81 (s, 3H). Compound 5a :
mp 155-7 °C. Anal. Calcd for C13H17NO5: C, 58.42; H, 6.37; N, 5.24.
Found: 57.98; H, 6.18; N, 5.21. MS (EI): 267(8), 208(74), 177(16), 137-
(100), 107(38). 1H NMR (methanol-d4, 500 MHz): δ 6.77 (d, J ) 8 Hz,
1H), 6.31 (d, J ) 2 Hz, 1H), 6.20 (dd, J ) 8 and 2 Hz), 4.54 (dd, J )
8.5 and 6.5 Hz, 1H), 3.77 (s, 3H), 3.62 (s, 3H), 3.02 (dd, J ) 13 and 8.5
Hz, 1H), 2.77 (dd, J ) 13 and 8.5 Hz), 1.89 (s, 3H). Compound 5b:
Due to the difficulty involved in the isolation of 5b by HPLC, it was
first acetylated with acetic anhydride to 5a which was isolated and
characterized as such.
Su p p or tin g In for m a tion Ava ila ble: Additional informa-
tion (3 pages).
J O960037V
(12) Tager, H. S.; Nakagawa, S.; Chin, T. M.; Yang, N. C., unpub-
lished results (supporting information). See also: Nakagawa, S. H.;
Tager, H. S. J . Biol. Chem. 1986, 261, 7332. Nakagawa, S. H.; Tager,
H. S. J . Biol. Chem. 1987, 262, 12054. Mirmira, R. G.; Tager, H. S. J .
Biol. Chem. 1989, 264, 6349.
(11) Krimen, L. I.; Cota, D. J . Org. React. 1969, 17, 213.