Photochemical cleavage and release of carboxylic acids from α-keto amides

Article abstract of DOI:10.1021/ol020227u

(Matrix presented) Carboxylate groups incorporated at the position α to the keto carbonyl of α-keto amides 1 were photochemically cleaved in aqueous media to give carboxylic acids in 70-90% yields with quantum yields of 0.3. The cleavage coproducts were diastereomeric hemiacetals 2. Prompt release of acetate and γ-aminobutyrate (GABA) in buffer was observed by difference FT-IR spectroscopy upon 355 nm laser flash photolysis. The time-constant for release of GABA was <30 ms.

Full text of DOI:10.1021/ol020227u

ORGANIC
LETTERS
2003
Vol. 5, No. 1
71-74
Photochemical Cleavage and Release of
Carboxylic Acids from r-Keto Amides
,†
Chicheng Ma,† Mark G. Steinmetz,* Qing Cheng,‡ and Vasanthi Jayaraman‡
Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201-1881,
and Department of IntegratiVe Biology and Pharmacology, UniVersity of Texas Health
Science Center at Houston, Houston, Texas 77030
mark.steinmetz@marquette.edu
Received November 4, 2002
ABSTRACT
Carboxylate groups incorporated at the position r to the keto carbonyl of r-keto amides 1 were photochemically cleaved in aqueous media
to give carboxylic acids in 70−90% yields with quantum yields of 0.3. The cleavage coproducts were diastereomeric hemiacetals 2. Prompt
release of acetate and γ-aminobutyrate (GABA) in buffer was observed by difference FT-IR spectroscopy upon 355 nm laser flash photolysis.
The time-constant for release of GABA was <30 ms.
Numerous applications exploit photocleavage reactions to
deprotect biologically active substrates,1-3 to release synthetic
biooligomers from resin,4 and for photolithographic pattern-
ing of surfaces.5,6 Such applications largely rely on a few
basic types of photocleavable groups such as various
derivatives of the o-nitrobenzyl group,7 the benzoin group,8
and the p-hydroxyphenacyl group.9 Since each type of
photocleavable group has specific advantages and limitations
for use in a given application, there has continued to be
considerable interest in the development of new photocleav-
able groups, which may offer additional versatility.
† Marquette University.
‡ University of Texas Health Science Center at Houston.
(1) Givens, R. S.; Weber, J. F. W.; Jung, A. H.; Park, C.-H. New
Photoprotecting Groups: Desyl and p-Hydroxyphenacyl Phosphate and
Carboxylate Esters. In Methods Enzymology: Caged Compounds; Marriott,
G., Ed.; 1998; Vol. 291, pp 1-29.
In this communication we report on R-keto amides 1a-
d, which photochemically release carboxylate leaving groups
(LG) that are attached to the position R to the keto carbonyl.
(2) (a) Gee, K. R.; Carpenter, B. K.; Hess, G. P. Synthesis, Photochem-
istry, and Biological Characterization of Photolabile Protecting Groups for
Carboxylic Acids and Neurotransmitters. In Methods Enzymology: Caged
Compounds; Marriott, G., Ed.; Academic Press: New York, 1998; Vol.
291, pp 30-50. (b) Hess, G. P.; Grewer, C. Development and Application
of Caged Ligands for Neurotransmitter Receptors in Transient Kinetic and
Neuronal Mapping Studies. In Methods Enzymology: Caged Compounds;
Marriott, G., Ed.; Academic Press: New York, 1998; Vol. 291, pp 30-50.
(3) Corrie, J. E. T.; Trentham, D. R. Caged Nucleotides and Neurotrans-
mitters. In Biorganic Photochemistry: Biological Applications of Photo-
chemical Switches; Morrison, H., Ed.; Wiley & Sons, Inc.: New York,
1993; Vol. 2, 243-305.
The impetus for our studies of 1a-d was the possibility that
leaving group release could occur via heterolytic cleavage
of photogenerated intermediates 3, since such reactivity was
(4) (a) Kahl, J. D.; Greenberg, M. M. J. Am. Chem. Soc. 1999, 121,
597-604. (b) Pirrung, M. C.; Fallon, L.; Lever, D. C.; Shuey, S. W. J.
Org. Chem. 1996, 61, 2129-2136. (c) Lloyd-Williams, P.; Albericio, F.;
Giralt, E. Tetrahedron 1993, 49, 11065-11133.
(5) (a) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. T.;
Solas, D. Science 1991, 251, 767-773. (b) Reichmanis, E.; Smith, B. C.;
Gooden, R. J. Polym. Sci.: Polym. Chem. Ed. 1985, 23, 1-8.
(6) (a) McGall, G. H.; Barone, A. D.; Diggelmann, M.; Fodor, S. P. A.;
Gentalen, E.; Ngo, N. J. Am. Chem. Soc. 1997, 119, 5081-5090. (b) Pease,
A. C.; Solas, D.; Sullivan, E. J.; Cronin, M. T.; Holmes, C. P.; Fodor, S. P.
A. Proc. Nat. Acad. Sci. U.S.A. 1994, 91, 5022-5026.
(7) (a) Cheng, Q.; Steinmetz, M. G.; Jayaraman, V. J. Am. Chem. Soc.
2002, 124, 7676-7677. (b) Walker, J. W.; Reid, G. P.; McCray, J. A.;
Trentham, D. R. J. Am. Chem. Soc. 1988, 110, 7170-7177. (c) Barth, A.;
Hauser, K.; Mantele, W.; Corrie, J. E. T.; Trentham, D. R. J. Am. Chem.
Soc. 1995, 117, 10311-10316. (d) Barth, A.; Corrie, J. E. T.; Gradwell,
M. J.; Maeda, Y.; Mantele, W.; Meier, T.; Trentham, D. R. J. Am. Chem.
Soc. 1997, 119, 4149-4159.
10.1021/ol020227u CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/14/2002

expected to reflect a high degree of zwitterionic character
in the intermediates due to charge stabilization by nitrogen
and oxygen at the sites shown.10
of 2 was labeled with one deuterium. The NMR analyses
also showed the presence of minor amounts (<5%) of
oxazolidinone 4.17
Analogous zwitterionic intermediates had been previously
postulated to account for the photochemistry of other R-keto
amides.11,12 Other potentially applicable mechanisms would
be solvolytic ionization of the excited state8a,9 and cleavage
to radical pairs that undergo electron transfer.13
Photolyses (>300 nm) of R-keto amides 1 in nitrogen-
saturated solutions of 50% D2O in CD3CN (1a,b) or air-
saturated D2O containing 25 mM phosphate buffer at pD
5-6 (1c,d) resulted in the formation of benzoic acid,
phenylacetic acid, acetic acid, and GABA, respectively,
according to 1H and 13C NMR spectroscopy of the photoly-
sates.14,15 In each case, a cleavage coproduct was also formed,
which was identified as a ca. 1.5:1 mixture of diastereomeric
hemiacetals 2.16 The CH3 group R to the carboxamide group
A preparative direct photolysis of 1a was conducted in 50%
aqueous CH3CN, and the hemiacetals 2 were isolated by
lyophilization of the aqueous phase that remained after
repeated extractions with ethyl acetate. To improve the
efficiency of the extractions, the two-phase mixture was
frozen in dry ice before removing the ethyl acetate phase.
The chemical yields of the carboxylic acids produced from
direct photolyses of 1a-d with Pyrex-filtered light were 70-
90% at high conversions (Table 1). Quantum yields, deter-
Table 1. Photochemical Yields of Released Carboxylic Acids
and Hemiacetal 2 in 50% D2O in CD3CN or D2O Containing 25
mM Phosphate Buffer at pD 5-6
(8) (a) Rajesh, C. S.; Givens, R. S.; Wirz, J. J. Am. Chem. Soc. 2000,
122, 611-618. (b) Corrie, J. E. T.; Trentham, D. R. J. Chem. Soc., Perkin
Trans. 1 1992, 2409-2417.
yield, %a,b
reactant
additive
N2
N2
N2
buffer, air
buffer, air
unreacted 1
RCO2H
2
(9) (a) Conrad, P. G.; Givens, R. S.; Hellrung, B.; Rajesh, C. S.; Ramseier,
M.; Wirz, J. J. Am. Chem. Soc. 2000, 122, 9346-9347. (b) Park, C.-H.;
Givens, R. S. J. Am. Chem. Soc. 1997, 119, 2453-2463.
1a
1a
1b
1c
1d
7
11c
12
24
20
93
89c
86
69
75
88
ndd
66e
77
(10) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11,
92-111.
(11) Chesta, C. A.; Whitten, D. G. J. Am. Chem. Soc. 1992, 114, 2188-
2197.
83
(12) (a) Aoyama, H.; Sakamoto, M.; Kuwabara, K.; Yoshida, K.; Omote,
Y. J. Am. Chem. Soc. 1983, 105, 1958-1964. (b) Aoyama, H.; Sakamoto,
M.; Omote, Y. J. Chem. Soc., Perkin Trans. 1 1981, 1357-1359. (c)
Aoyama, H.; Hasegawa, T.; Omote, Y. J. Am. Chem. Soc. 1979, 101, 5343-
5347. (d) Zehavi, U. J. Org. Chem. 1977, 42, 2821-2825. (e) Johansson,
N. G.; Akermark, B.; Sjoberg, B. Acta Chem. Scand. B 1976, 30, 383-
390.
a Yields determined by NMR spectroscopy; runs in buffer used glycine
as an internal standard, whereas DMSO was the standard for 1b. b Yields
of 4 were <5%. c Determined by HPLC analysis using an internal standard
and 254 nm UV detection. d Not determined. e Yield of 4 ) 10%.
(13) (a) Pincock, J. A. Acc. Chem. Res. 1997, 30, 43-49. (b) Lipson,
M.; Deniz, A. A.; Peters, K. S. J. Am. Chem. Soc. 1996, 118, 2992-2997.
(14) (a) Photolyses used an air-cooled 450 W medium-pressure mercury
lamp inside a Pyrex filter sleeve with each 25 mL sample contained in a
quartz tube mounted beside the water-jacketed apparatus. For the NMR
studies, 0.5 mL samples of 1a,b in 50% D2O in CD3CN and 1c,d in
phosphate buffer (25 mM) in D2O at pD 5-6 were photolyzed for 1-2 h.
(b) Quantum yield determinations used a 200 W high-pressure mercury
lamp, monochromator, and associated optics as described previously,14c with
ferrioxalate as the actinometer.14d (c) Steinmetz, M. G.; Luo, C.; Liu, C. J.
Org. Chem. 1999, 64, 2057-2065. (d) Hatchard, C. G.; Parker, C. A. Proc.
R. Soc. London 1956, 235, 518.
(15) Photochemical reactants 1a-d, BocGABA derivative 7d, and
compounds 6a (R ) Ph), 6b (R ) PhCH2), 6c (R ) CH3), and 6d (R )
Boc-NHCH2CH2CH2) gave satisfactory C, H, and N elemental analyses.
Trifluoroacetate salt 1d was not analyzed. Slow decomposition of 1d in
pD 5-6 buffer was observed by NMR in the dark with a half-life of 38 h.
Samples of 1c in buffer and 1a-c in 50% D2O:CD3CN were stable for
days.
mined at 310 nm using ferrioxalate actinometry, were 0.31
( 0.03 and 0.37 ( 0.02 for 1a and 1b, respectively, in
nitrogen-saturated 50% aqueous acetonitrile. The quantum
yields for 1c were 0.28 ( 0.03 and 0.32 ( 0.03 in
air-saturated 50% aqueous acetonitrile and 25 mM phosphate
buffer at pH 5.4, respectively. For 1c, only the quantum
yields of disappearance of reactant could be obtained by
HPLC analyses of photolysates with 254 nm detection.
Nevertheless, these values should equal the efficiency for
acetate release, because acetic acid, hemiacetal 2, and traces
of 4 were the only products according to NMR spectroscopy
(vide supra). Decarboxylation of a potential acyloxy radical
intermediate13a was not evidenced by an observed decrease
in chemical yield or quantum yield for formation of phenyl-
acetic acid from 1b.
(16) Major diastereomer of hemiacetals 2 (undeuterated): 1H NMR
(CDCl3) δ 1.11 (t, J ) 7 Hz, 3 H), 1.35 (d, J ) 5.5 Hz, 3 H), 1.52 (s, 3 H),
3.11 (dq, J ) 14, 7 Hz, 1 H), 3.45 (dq, J ) 14, 7 Hz, 1 H), 5.30 (q, J )
5.5 Hz, 1 H). 13C NMR (CDCl3) δ 12.71, 20.12, 23.52, 34.64, 83.56, 98.38,
169.37. Minor diastereomer of hemiacetals 2: 1H NMR (CDCl3) δ 1.11 (t,
J ) 7 Hz, 3 H), 1.44 (d, J ) 5.5 Hz, 3 H), 1.47 (s, 3 H), 3.09 (dq, J ) 14,
7 Hz, 1 H), 3.51 (dq, J ) 14, 7 Hz, 1 H), 5.10 (q, J ) 5.5 Hz, 1 H). 13C
NMR (CDCl3) δ 12.56, 21.34, 22.83, 34.75, 83.93, 98.97, 169.02. These
spectra are for hemiacetals 2 that were isolated by careful lyophilization.
The presence of some water was unavoidable. Anal. Calcd for C7H13NO3:
C, 52.82; H, 8.23; N, 8.80. Calcd for 26.3% water content: C, 50.76; H,
8.35; N, 8.46. Found: C, 50.76; H, 8.28; N, 8.66.
To determine whether acetate and GABA were released
rapidly from 1c,d in pD 6 phosphate buffer (25 mM) in D2O,
(17) Oxazolidinone 4: 1H NMR (CDCl3) δ 1.19 (t, J ) 7 Hz, 3 H),
1.47 (d, J ) 5.5 Hz, 3 H), 3.22 (dq, J ) 14, 7 Hz, 1 H), 3.69 (dq, J ) 14,
7 Hz, 1 H), 4.56 (d, J ) 2 Hz, 1 H), 4.90 (d, J ) 2 Hz, 1 H), 5.45 (q, J )
5.5 Hz, 1 H).
72
Org. Lett., Vol. 5, No. 1, 2003

355 nm laser photolyses were monitored directly by differ-
ence FT-IR spectroscopy using the system described
previously.7a The difference spectra represent the difference
between the spectra obtained after photolysis and the
spectrum before photolysis. In the case of 1c, one laser shot
resulted in the prompt appearance of a 1560 cm-1 feature,
which corresponded to the asymmetric carboxylate vibration
of the released acetate (Figure 1). This feature increased with
Figure 2. Time evolution of the difference feature at 1564 cm-1
(area integrated between 1540 and 1600 cm-1) in the FTIR
difference spectrum (inset) for pulsed laser photolysis of 1d. The
increase in intensity of the feature at 1564 cm-1 could be well
represented with a single exponential (dotted line) with a time
constant of 30 ( 5 ms.
GABA derivative 1d was synthesized by DCC coupling
of diol 5 with N-Boc-γ-aminobutyric acid, PCC oxidation
of secondary alcohol 6d, and deprotection of 7d using TFA
(Scheme 1). Sephadex LH-20 chromatography and lyo-
philization then gave the trifluoroacetate salt of 1d. The
synthesis of acetate 1c in 45% overall yield followed a
similar route, whereas for 1a and 1b, diol 5 was instead
acylated using benzoic anhydride or phenylacetyl chloride
followed by oxidation to give 25 and 52% overall yields of
these keto amides after two steps.
Figure 1. Difference FT-IR spectra obtained for a series of 355
nm laser flash photolyses of 0.18 M 1c in 25 mM phosphate buffer.
additional laser shots, along with a 1681 cm-1 feature, which
was confirmed by FT-IR spectroscopy to be due to formation
of hemiacetals 2. The negative features in the difference FT-
IR spectrum at 1735 cm-1 and 1630 cm-1 resulted from
photolytic depletion of the reactant 1c. However, the time
constant for the release of the acetate could not be deter-
mined, because of the low intensity of absorption of the
acetate produced after a single laser pulse.
Scheme 1
For the trifluoroacetate salt of “caged” GABA derivative
1d (LG ) H3N+CH2CH2CH2CO2), the FT-IR difference
spectra showed a prominent feature at 1564 cm-1 after one
laser shot. This feature, which could be assigned to the
asymmetric carboxylate vibration of the released GABA, was
of sufficient intensity that its time evolution could be
followed using the rapid-scan technique. The time constant
for photolytic release of the GABA was determined to be
30 ( 5 ms (Figure 2).7a Since this time constant is at the
lower limit of the time resolution of the rapid-scan method,
work is underway to synthesize the large quantities of the
caged GABA 1d needed for determination of the time
constant on the microsecond time scale by the step-scan
technique, which requires a flowing sample.
Org. Lett., Vol. 5, No. 1, 2003
73

In conclusion, our experimental results are the first to
demonstrate photorelease of leaving groups from R-keto
amides. Carboxylates are released efficiently in high chemical
yields upon exposure to λ > 300 nm light, and the
efficiencies are comparable to those of other photoremovable
protecting groups for carboxylates.1,2 The photorelease of
GABA occurs on the sub-30 ms time scale. The quantum
yields are similar for a variety of carboxylate leaving groups
in aqueous acetonitrile and buffer, which is consistent with
a mechanism involving release of carboxylates via cleavage
of photogenerated zwitterionic intermediates. Nevertheless,
further work is needed to elucidate the mechanism of the
photocleavages, to determine the release time of GABA on
a shorter time scale and to explore the photorelease of other
functional groups from R-keto amides.
Acknowledgment. We thank Mr. Stanley Luberda for
technical assistance and the National Science Foundation
(Grant NSF-0096635) (V.J.) for financial support of this
research.
Supporting Information Available: FT-IR spectra of
1c,d, hemiacetals 2, acetate, and γ-aminobutyrate (GABA)
in 25 mM phosphate buffer, 1H and 13C NMR data for 1a-
1
d, and H and 13C NMR spectra of 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL020227U
74
Org. Lett., Vol. 5, No. 1, 2003