Photodecarboxylative additions of N-protected α-amino acids to N-methylphthalimide

Article abstract of DOI:10.1016/j.tetlet.2010.05.020

Photoreactions involving N,N-dimethylated α-amino acid salts and N-methylphthalimide are dominated by photoreduction and acetone trapping. Only, N-phenyl glycinate underwent photodecarboxylative addition in a moderate yield of 30%. In contrast, N-acylated α-amino acid salts readily gave addition products in fair to high yields of 20-95%. Comparison experiments with N,N-dimethylacetamide and amino-/amido-containing phthalimides revealed the origin of the crucial electron-transfer step and the reactivity order NR ₃ RCO2- ≥ RCONR₂ was established.

Full text of DOI:10.1016/j.tetlet.2010.05.020

Tetrahedron Letters 51 (2010) 3639–3641
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photodecarboxylative additions of N-protected
N-methylphthalimide
a-amino acids to
Sonia Gallagher a, Fadi Hatoum a, Nicolai Zientek b, Michael Oelgemöller a,b,
*
a Dublin City University, School of Chemical Sciences, Glasnevin, Dublin 9, Ireland
b James Cook University, School of Pharmacy and Molecular Sciences, Townsville, Queensland 4811, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Photoreactions involving N,N-dimethylated
by photoreduction and acetone trapping. Only, N-phenyl glycinate underwent photodecarboxylative
addition in a moderate yield of 30%. In contrast, N-acylated -amino acid salts readily gave addition prod-
ucts in fair to high yields of 20–95%. Comparison experiments with N,N-dimethylacetamide and amino-/
a
-amino acid salts and N-methylphthalimide are dominated
Received 29 March 2010
Revised 28 April 2010
Accepted 7 May 2010
Available online 12 May 2010
a
amido-containing phthalimides revealed the origin of the crucial electron-transfer step and the reactivity
À
order NR3 » RCO2 P RCONR2 was established.
Keywords:
Photodecarboxylation
Phthalimides
Ó 2010 Elsevier Ltd. All rights reserved.
Photochemistry
Amino acids
Photoinduced electron transfer
The photochemistry of phthalimides has attracted considerable
attention over the last two decades, and a number of efficient and
selective transformations have been realized.1 Photodecarboxyla-
similar to 3 were only obtained in low yields with simple amines
as reaction partners.6,7
Photoreductions by amines are known to be sensitive towards
the presence of water.8,9 The reaction involving 2-(dimethyl-
amino)acetic acid was thus carried out using dry acetonitrile as
solvent. The reaction again proceeded rather sluggishly, but after
2 h the photoreduction product 4 was identified as the main prod-
tive additions of carboxylates,
a-keto carboxylates and hetero-
atom-substituted carboxylates to phthalimides, for example, have
been developed as versatile alternatives to Grignard additions.2
To explore further the scope of this attractive application, we be-
came interested in using amino acid derivatives as starting materi-
als. Three nitrogen-containing carboxylates 2a–c were thus
irradiated at 300 nm in aqueous acetone in the presence of N-
methylphthalimide (1, Scheme 1).3 With N,N-dimethylated amino
acids 2a and 2c, only photoreduction to 4 and 5, and acetone trap-
ping to 6 was observed (Table 1). Conversion rates and product ra-
tios varied depending on the irradiation time as demonstrated for
N,N-dimethylglycine (2a). The desired photoaddition product 3
could only be obtained when the potassium salt of N-phenyl gly-
cine (2b) was used as the starting material.4 The reaction was slug-
gish, but the addition product 3b was isolated in a yield of 30%
after column chromatography. Due to the low oxidation potentials
of tertiary amines (NR3: EOx = 0.7–1.3 vs SCE5), photoinduced elec-
tron transfer (PET) reactions involving phthalimides are highly
exergonic. This suggests that electron transfer is followed by rapid
O
R3
R1
N
CH3
+
N
CO2K
R2
O
hν
1
2a-c
acetone/H2O
R2
R1
N
HO
R3
H
HO
+
+
N
CH3
N
CH3
O
3a-c
O
4
H
HO
H
OH
photoreduction through hydrogen abstraction,
a commonly
N
CH3
N
CH3
observed side reaction of amines. In fact, photoaddition products
O
O
5
6
* Corresponding author. Tel.: +61 (0)7 4781 4543; fax: +61 (0)7 4781 6078.
E-mail address: michael.oelgemoeller@jcu.edu.au (M. Oelgemöller).
Scheme 1. Photoreaction of 1 with nitrogen-containing carboxylates 2a–c.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.020

3640
S. Gallagher et al. / Tetrahedron Letters 51 (2010) 3639–3641
Table 1
For related photocyclizations of phthaloyl peptides, different
scenarios for the crucial photoinduced electron transfer (PET)12
step have been proposed. Whereas Yoon and Mariano suggested
an electron transfer from the amide linker,13 Griesbeck and
Oelgemöller postulated an electron transfer from the carboxylate
function instead.14 The latter scenario is supported by successful
Product compositions and experimental details for photoadditions of 1 with 2a–c
2
R1
R2
R3
Time (h)
Product compositiona (%)
1
3
4
5
6
a
a
b
c
Me
Me
Ph
Me
Me
H
H
H
H
Bn
1
18
4
—
—
25
33
—
37
4
19
36
34
45
5
29
51
10
10
—
41b
—
macrocyclization of dipeptides with terminal
x-amino acids. In or-
Me
Me
3
21
der to establish the origin of the crucial photoinduced electron
transfer step, the phthalimides 10a–c carrying potential donor sub-
stituents on the N-side chain were irradiated in the presence of
potassium propionate 11 (Scheme 4; Table 3).
a
Determined by 1H NMR spectroscopy of the crude reaction mixture.
Isolated yield 30%.
b
The amine-derived phthalimide 10a completely prevented
photodecarboxylative addition and showed extensive photode-
composition, as noticeable from its poor recovery of 23%. Unselec-
tive photodegradation of 10a was also observed in the absence of
11. In contrast, the incorporation of an amide group into the N-side
chain had no influence on the ethylation and the corresponding
addition products 12b and 12c were obtained in moderate yields
of 43% and 51%, respectively.15 Cyclization products arising from
competing intramolecular CH activations or PET reactions were
not detected.16,17
uct. Hence, the outcome of the photoreactions involving nitrogen-
containing carboxylic acids or carboxylates predominantly de-
pends on the structure of the acid, in particular the substituents
at the nitrogen, and not necessarily on the presence of water.
Likewise, a set of N-acylated glycine salts 7a–h was irradiated
in the presence of 1 under the conditions of the photodecarboxyla-
tive addition (Scheme 2; Table 2).10 In contrast to their N-alkylated
counterparts all the experiments proceeded readily and the corre-
sponding addition products 8a–h were collected in moderate to
excellent yields of 20–95% after just 1–4 h of irradiation.11 Only
in the case of Fmoc-protected glycine 7f were larger amounts of
unidentified by-products detected in the crude NMR spectrum,
but no attempt was made to isolate these products.
Since phthalimides are known to react via H-abstraction reac-
tions,1a N-methylphthalimide (1) was furthermore irradiated in
the presence of 5 equiv of N,N-dimethylacetamide (9) (Scheme
3). After irradiation for 5 h, no addition (8b) or photoreduction
products (4/5) were observed. Instead, 1 was reisolated in 81%
yield. Hence, photoadditions via H-abstraction do not contribute
to the formation of the addition products 8.
The general mechanistic scenario for photoreactions of amino
acids with N-methylphthalimide is depicted in Scheme 5. For the
amino-carboxylates 2, electron transfer from the nitrogen gives
the corresponding radical ion pair (path A). For phenyl glycinate,
subsequent
a-decarboxylation and carbon bond formation yields
the addition product 3b. For the dialkylated amino acids, stepwise
photoreduction is observed instead.18 N-acylation of the amino
acids 7 restored photoreactivity. This suggests that the crucial elec-
tron transfer step now occurs primarily from the carboxylate func-
O
HO
R1
X
X
hν
R2
N
N
N
+
CO2K
O
R3
R3
HO
R3 R3
CO2K
acetone/H2O
hν
R1
O
12a-c
O
10a-c
+
N
CH3
N
CH3
N
11
acetone/H2O
R2
7a-h
Scheme 2. Additions of N-protected glycine salts 7a–h to 1.
O
O
8a-h
Scheme 4. Addition of propionate 11 to phthalimides 10a–c.
1
Table 3
Yields and experimental details for photoadditions of 10a–c with 11
12
X
Time (h)
Conversiona (%)
Yield (%)
Table 2
Yields and experimental details for photoadditions of 1 with 7a–h
a
b
c
NMe
CONH
CONMe
4
4
4
—
95
95
(23b)
43 (45c)
51 (54c)
8
R1
R2
R3
Time (h)
Yield (%)
a
b
c
d
e
f
Ac
Ac
Ac
Boc
Cbz
Fmoc
Ac
H
H
H
H
H
H
H
Me
(CH2)5
2
1
1
1
4
2
4
3
71 (75a)
73
Determined by 1H NMR spectroscopy of the crude reaction mixture.
Reisolated 10a.
Yields based on conversion.
a
b
c
Me
Ph
H
H
H
95
74
56
20b
83
g
h
H
H
R2
N
R2
N
O
Ac
80
3Sens., PETCO
-
CO2
R3
2
a
R1
R1
N CH3
+
Yield based on a conversion of 1 of 95%.
Larger amounts of unknown by-products detected.
1 +
b
R3 R3
2, 7
R3
- CO2
B
1. H+
3Sens.,
PETN
2. C-C
A
O
N
R1
R2
HO
O
HO
O
N
hν
R2
O
R3
R3
CH3
N
CH3
+
N
CH3
N
N
CO2
1. - CO2
2. C-C
R1
+
acetone/H2O
N
CH3
N
R3 R3
O
O
1
9
8b
3 / 8
Scheme 3. Attempted addition of N,N-dimethylacetamide (9) to 1.
Scheme 5. Mechanistic scenario.

S. Gallagher et al. / Tetrahedron Letters 51 (2010) 3639–3641
3641
4. Photosensitised decarboxylations of N-phenyl glycine have been described
earlier, see: (a) Rajesh, C. S.; Thanulingam, T. L.; Das, S. Tetrahedron 1997, 53,
16817; (b) Davidson, R. S.; Harrison, K.; Steiner, P. R. J. Chem. Soc. C 1971, 3480;
(c) Davidson, R. S.; Steiner, P. R. J. Chem. Soc. C 1971, 1682.
5. Pienta, N. J. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.;
Elsevier: Amsterdam, 1988; pp 421–486.
tion (path B) as supported by the oxidation potentials of the com-
peting electron donors (EOx RCONR2 P EOx RCO2À).19 Subsequent
decarboxylation of the resulting carboxy radical to the correspond-
ing carbon-centred radical and carbon bond formation furnish the
observed addition products 8.20
6. Kanaoka, Y.; Sakai, K.; Murata, R.; Hatanaka, Y. Heterocycles 1975, 3, 719.
An additional argument for the mechanistic scenario comes from
the attempted photoaddition of dimethylacetamide to 1. Since no
addition product could be detected, electron transfer from the amide
function (similar to path A in Scheme 5) appears energetically not
feasible.21,22 Likewise, amide groups within the N-side chain, as in
compounds 10b and 10c, did not prevent photodecarboxylative
ethylation. If electron transfer would operate from the amide-linker,
complete deactivation could be expected as, for example, is known
for thioether-derived phthalimides (10; X = S).23
7. Better results were achieved with
1 and N-trimethylsilylmethyl-N,N-
diethylamine, but the outcome of this transformation was critically
dependent on the solvent used. When irradiated in acetonitrile, the
corresponding addition product was isolated in 41% yield, but the primary
photoreduction product 4 was also obtained as a by-product in 22% yield. See:
Yoon, U. C.; Kim, H. J.; Mariano, P. S. Heterocycles 1989, 29, 1041.
8. (a) Cossy, J.; Belotti, D. Tetrahedron 2006, 62, 6459; (b) Hoffmann, N. Pure Appl.
Chem. 2007, 79, 1949; (c) Cohen, S. G.; Parola, A.; Parsons, G. H., Jr. Chem Rev.
1973, 73, 141.
9. Cohen, S. G.; Stein, N. M. J. Am. Chem. Soc. 1971, 93, 6542.
10. N-Acylated amino acids, if not commercially available, were synthesised
according to: Paulmann, W. Arch. Pharm. 1894, 232, 601.
In conclusion, N,N-dialkylated amino acid salts only undergo
unselective photoreductions and acetone trapping. In contrast,
11. Selected physical and spectral data for N-[(1-hydroxy-2-methyl-3-oxoisoindolin-
1-yl) methyl]-N-phenylacetamide (8c): yellowish solid, mp 158–160 °C. 1H NMR
(400 MHz, CDCl3): d (ppm) = 1.63 (s, 3H, CH3), 2.57 (s, 3H, NCH3), 4.18 (d,
2J = 14.5 Hz, 1H, CH2), 4.51 (s, 1H, OH), 4.66 (d, 2J = 14.5 Hz, 1H, CH2), 6.69 (dd,
3J = 7.8, 4J = 2.3 Hz, 2H, Harom), 7.19 (m, 3H, Harom), 7.36 (m, 2H, Harom), 7.44 (m,
1H, Harom), 7.56 (m, 1H, Harom). 13C NMR (100 MHz, CDCl3): d (ppm) = 22.9 (s,
1C, CH3), 23.9 (s, 1C, NCH3), 52.5 (s, 1C, CH2), 90.0 (s, 1C, COH), 122.9 (s, 1C,
CHarom), 123.6 (s, 1C, CHarom), 127.6 (s, 2C, CHarom), 128.1 (s, 1C, CHarom), 129.8
(s, 2C, CHarom), 130.0 (s, 1C, CHarom), 131.7 (s, 1C, CHarom), 132.0 (s, 1C, Cq),
143.1 (s, 1C, Cq), 145.5 (s, 1C, Cq), 167.7 (s, 1C, C@O), 172.2 (s, 1C, C@O). IR
N-phenyl glycine and N-acylated
a-amino acid salts undergo
photodecarboxylative addition to 1. The simple protocol makes this
transformation interesting for ‘micro-photochemistry’, that is, pho-
tochemistry in micro-structured reactors.24
Acknowledgements
(KBr):
m
= 3249, 2935, 2345, 1656, 1691, 1631, 703 cmÀ1. MS (EI, 70 eV): m/z
(%) = 310 (M+, 4), 292 (M+ÀH2O, 35), 77 (C6H5, 51). MS (ESI, positive ions): m/
z = 311 (M+H)+, 621 (M2+H)+. HR-MS (ESI, positive ions): Calcd [M+H]+:
311.13902 for C18H18N2O3 + H+. Found [M+H]+: 311.13944. Calcd [M+Na]+:
333.12096 for C18H18N2O3 + Na+. Found [M+Na]+: 333.12141.
This research project was financially supported by Science
Foundation Ireland (SFI, 07/RFP/CHEF817 and 06/RFP/CHO028)
and Dublin City University (Research Career Start Award 2006).
The authors also thank Professor J. Mattay and Dr. M. C. Letzel
(University of Bielefeld, Germany) for providing MS analyses.
12. (a) Hoffmann, N. J. Photochem. Photobiol., C: Photochem. Rev. 2008, 9, 43; (b)
Oelgemöller, M.; Bunte, J.-O.; Mattay, J. In Synthetic Organic Photochemistry;
Griesbeck, A. G., Mattay, J., Eds.; Marcel Dekker: New York, 2004; pp 267–295.
Chapter 10; (c) Mattay, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 825.
13. Yoon, U. C.; Jin, Y. X.; Oh, S. W.; Park, C. H.; Park, J. H.; Campana, C. F.; Cai, X.;
Deusler, E. N.; Mariano, P. S. J. Am. Chem. Soc. 2003, 125, 10664.
References and notes
14. (a) Griesbeck, A. G.; Heinrich, T.; Oelgemöller, M.; Molis, A.; Heidtmann, A.
Helv. Chem. Acta 2002, 85, 4561; (b) Griesbeck, A. G.; Heinrich, T.; Oelgemöller,
M.; Molis, A.; Lex, J. J. Am. Chem. Soc. 2002, 124, 10972; (c) Oelgemöller, M.;
Griesbeck, A. G.; Kramer, W.; Nerowski, F. J. Inf. Rec. 1998, 24, 87; (d) Griesbeck,
A. G.; Henz, A.; Kramer, W.; Lex, J.; Nerowski, F.; Oelgemöller, M.; Peters, K.;
Peters, E.-M. Helv. Chim. Acta 1997, 80, 912.
1. (a) McDermott, G.; Yoo, D. J.; Oelgemöller, M. Heterocycles 2005, 65, 2221; (b)
Oelgemöller, M.; Griesbeck, A. G. In CRC Handbook of Organic Photochemistry
and Photobiology; Horspool, W. M., Lenci, F., Eds., 2nd ed.; CRC Press: Boca
Raton, 2004; pp 1–19. Chapter 84; (c) Oelgemöller, M.; Griesbeck, A. G. J.
Photochem. Photobiol., C: Photochem. Rev. 2002, 3, 109; (d) Yoon, U. C.; Mariano,
P. S. Acc. Chem. Res. 2001, 34, 523; (e) Kramer, W.; Griesbeck, A. G.; Nerowski,
F.; Oelgemöller, M. J. Inf. Rec. 1998, 24, 81; (f) Coyle, J. D. In Synthetic Organic
Photochemistry; Horspool, W. M., Ed.; Plenum Press: New York, 1984; pp 259–
284; (g) Mazzocchi, P. H. Org. Photochem. 1981, 5, 421; (h) Kanaoka, Y. Acc.
Chem. Res. 1978, 11, 407.
15. Selected physical and spectral data for 2-(1-ethyl-1-hydroxy-3-oxoisoindolin-2-
yl)-N-methylacetamide (12b): yellow solid, mp 95–97 °C. 1H NMR (400 MHz,
CDCl3): d (ppm) = 0.46 (t, 3J = 7.3 Hz, 3H, CH3), 2.15 (m, 2H, CH2), 2.72 (br s, 3H,
CH3), 3.81 (d, 2J = 16.4 Hz, 1H, CH2), 4.35 (d, 2J = 16.4 Hz, 1H, CH2), 5.67 (s, 1H,
OH), 7.02 (br s, 1H, NH), 7.46 (ddd, 3J = 7.3, 3J = 7.3, 4J = 1.0 Hz, 1H, Harom), 7.53
(d, 3J = 7.3 Hz, 1H, Harom), 7.57 (ddd, 3J = 7.3, 3J = 7.3, 4J = 1.0 Hz, 1H, Harom), 7.70
(d, 3J = 7.3 Hz, 1H, Harom). 13C NMR (100 MHz, CDCl3): d (ppm) = 8.1 (s, 1C, CH3),
26.7 (s, 1C, NCH3), 29.6 (s, 1C, CH2), 42.7 (s, 1C, CH2), 92.1 (s, 1C, COH), 122.3 (s,
1C, CHarom), 123.5 (s, 1C, CHarom), 129.7 (s, 1C, CHarom), 130.7 (s, 1C, Cq), 133.2
(s, 1C, CHarom), 147.5 (s, 1C, Cq), 169.0 (s, 1C, C@O), 170.6 (s, 1C, C@O). IR (KBr):
2. (a) Hatoum, F.; Gallagher, S.; Oelgemöller, M. Tetrahedron Lett. 2009, 50, 6593;
(b) Hatoum, F.; Gallagher, S.; Baragwanath, L.; Lex, J.; Oelgemöller, M.
Tetrahedron Lett. 2009, 50, 6335; (c) Kim, A. R.; Lee, K.-S.; Lee, C.-W.; Yoo, D.
J.; Hatoum, F.; Oelgemöller, M. Tetrahedron Lett. 2005, 46, 3395; (d)
Oelgemöller, M.; Cygon, P.; Lex, J.; Griesbeck, A. G. Heterocycles 2003, 59,
669; (e) Griesbeck, A. G.; Oelgemöller, M.; Lex, J. Synlett 2000, 1455; (f)
Griesbeck, A. G.; Oelgemöller, M. Synlett 2000, 71; (g) Griesbeck, A. G.; Gudipati,
M. S.; Hirt, J.; Lex, J.; Oelgemöller, M.; Schmickler, H.; Schouren, F. J. Org. Chem.
2000, 65, 7151; (h) Griesbeck, A. G.; Oelgemöller, M. Synlett 1999, 492; (i)
Griesbeck, A. G.; Kramer, W.; Oelgemöller, M. Green Chem. 1999, 1, 205.
3. General procedure for irradiation: N-methylphthalimide (1.5 mmol) was
dissolved in acetone (50 mL). A solution of the potassium carboxylate (4.5
mmol) in H2O (50 mL) was added, and the mixture was irradiated (Rayonet
Photochemical Reactor RPR-200; k = 300 20 nm) at 15–20 °C in a Pyrex tube
(k P 300 nm) while purging with a slow stream of nitrogen. The progress of the
reaction was monitored by TLC analysis or by passing the departing gas stream
through a saturated barium hydroxide solution until precipitation of barium
carbonate had ceased. Most of the acetone was evaporated and the remaining
solution was extracted with CH2Cl2 (3 Â 50 mL). The combined organic layer
was washed with 5% NaHCO3 (50 mL) and brine (50 mL), dried over MgSO4 and
evaporated. The products were purified by column chromatography (eluent: n-
hexane/EtOAc = 1:1). In some cases, the pure product precipitated upon
evaporation of acetone and was isolated by vacuum filtration and drying in
vacuo instead. Selected physical and spectral data for 3-hydroxy-2-methyl-3-
[(phenylamino)methyl] isoindolin-1-one (3b): colourless solid, mp 152–155 °C.
1H NMR (400 MHz, CDCl3): d (ppm) = 3.02 (s, 3H, NCH3), 3.67 (dd, 2J = 12.8,
3J = 6.2 Hz, 1H, CH2), 3.90 (dd, 2J = 12.8, 3J = 6.2 Hz, 1H, CH2), 4.60 (dd, 3J = 6.2,
3J = 6.2 Hz, 1H, NH), 5.54 (s, 1H, OH), 6.58 (m, 1H, Harom), 6.68 (d, 2J = 7.6 Hz, 2H,
Harom), 7.06 (m, 2H, Harom), 7.51 (dd, 3J = 7.2, 4J = 1.0 Hz, 1H, Harom), 7.59 (dd,
3J = 7.2, 4J = 1.0 Hz, 1H, Harom), 7.66 (d, 3J = 7.6 Hz, 1H, Harom), 7.79 (d,
m
= 3291, 2929, 2346, 1690, 1663, 1617, 1260, 760 cmÀ1
.
16. For photocyclizations of amino-derived phthalimides, see: (a) Machida, M.;
Takechi, H.; Kanaoka, Y. Chem. Pharm. Bull. 1982, 30, 1579; (b) Coyle, J. D.;
Smart, L. E.; Challiner, J. F.; Haws, E. J. J. Chem. Soc., Perkin Trans. 1 1985, 1, 121.
17. For photocyclizations of N-acyl-containing phthalimides, see: Machida, M.;
Takechi, H.; Shishido, Y.; Kanaoka, Y. Synthesis 1982, 1078.
18. Stepwise photoreductions have been reported for irradiations involving other
imide chromophores, see: (a) Kubo, Y.; Imaoka, T.; Egusa, C.; Araki, T. Chem.
Express 1989, 4, 527; (b) Kubo, Y.; Egusa, C.; Araki, T. Chem. Lett. 1985, 1213.
19. Oxidation potential of N-methylacetamide: EOx = 1.81 V in MeCN versus SCE,
see: (a) Siegerman, H.. In Technique of Electroorganic Synthesis (Techniques of
Chemistry); Weinberg, N. L., Ed.; J. Wiley & Sons: New York, 1975; Vol. 5, pp
667–1056; Calculated oxidation potential of acetate: EOx = 1.54 V in MeCN
versus SCE, see: (b) Eberson, L.. In Electron Transfer Reactions in Organic
Chemistry (Reactivity and Structure-Concepts in Organic Chemistry); Hafner, K.,
Ed.; Springer: Berlin, 1987; Vol. 25, pp 39–66.
20. (a) Görner, H.; Oelgemöller, M.; Griesbeck, A. G. J. Phys. Chem. A 2002, 106,
1458; (b) Görner, H.; Griesbeck, A. G.; Heinrich, T.; Kramer, W.; Oelgemöller, M.
Chem. Eur. J. 2001, 7, 1530.
21. Photocleavages arising from electron transfer from the amide function have
been reported by Hill and co-workers for N-p-toluenesulfonyl peptides, see:
Hill, R. R.; Moore, S. A.; Roberts, D. R. Photochem. Photobiol. 2005, 81, 1439.
22. Yoshimi, Y.; Masuda, M.; Mizunashi, T.; Nishikawa, K.; Maeda, K.; Koshida, N.;
Itou, T.; Morita, T.; Hatanaka, M. Org. Lett. 2009, 11, 4652.
3J = 7.6 Hz, 1H, Harom). 13C NMR (100 MHz, CDCl3):
d (ppm) = 23.9 (s, 1C,
23. Griesbeck, A. G.; Oelgemöller, M.; Lex, J.; Haeuseler, A.; Schmittel, M. Eur. J. Org.
Chem. 2001, 1831.
24. (a) Coyle, E. E.; Oelgemöller, M. Photochem. Photobiol. Sci. 2008, 7, 1313; (b)
Coyle, E. E.; Oelgemöller, M. Chem. Technol. 2008, 5, T95; (c) Matsushita, Y.;
Ichimura, T.; Ohba, N.; Kumada, S.; Sakeda, K.; Suzuki, T.; Tanibata, H.; Murata,
T. Pure Appl. Chem. 2007, 79, 1959.
NCH3), 49.0 (s, 1C, CH2), 90.2 (s, 1C, COH), 113.9 (s, 2C, CHarom), 117.8 (s, 1C,
CHarom), 123.3 (s, 1C, CHarom), 123.7 (s, 1C, CHarom), 129.8 (s, 2C, CHarom), 130.4
(s, 1C, CHarom), 130.7 (s, 1C, CHarom), 133.4 (s, 1C, Cq), 147.7 (s, 1C, Cq), 149.6 (s,
1C, Cq), 167.5 (s, 1C, C@O). IR (KBr):
m = 3393, 2925, 1670, 1601, 1529, 1498,
1070, 741, 691 cmÀ1
.