Effective synthesis of optically active 3-phenyl-3-(3-trifluoromethyl) diazirinyl bishomophenylalanine derivatives

Article abstract of DOI:10.3987/COM-08-S(D)30

Effective incorporation of phenyldiazirine moiety on the acyl residue of _L- and _D- glutamic acid by Friedel-Crafts reactions with triflic acid developed simple preparation of bishomophenylalanine (bhPhe) for aromatics, which added a

Full text of DOI:10.3987/COM-08-S(D)30

HETEROCYCLES, Vol. 79, 2009
359
HETEROCYCLES, Vol. 79, 2009, pp. 359 - 364. © The Japan Institute of Heterocyclic Chemistry
Received, 25th September, 2008, Accepted, 23rd October, 2008, Published online, 31st October, 2008.
DOI: 10.3987/COM-08-S(D)30
EFFECTIVE SYNTHESIS OF OPTICALLY ACTIVE 3-PHENYL-3-
(
3-TRIFLUOROMETHYL) DIAZIRINYL BISHOMOPHENYLALANINE
DERIVATIVES
a
b
c
Yuta Murai, Yasumaru Hatanaka, Yuichi Kanaoka, and Makoto
a,
Hashimoto *
a
Department of Agricultural and Life Science, Obihiro University of Agriculture
and Veterinary Medicine, Inada-cho, Obihiro 080-8555, Hokkaido, Japan
b
Graduate School of Medicine and Pharmaceutical Sciences, University of
Toyama, 2630 Sugitani, Toyama 930-0194, Japan
c
Toyama College, 444 Gankaiji, Toyama, 930-0193 Japan
E-mail: hasimoto@obihiro.ac.jp
Abstract – Effective incorporation of phenyldiazirine moiety on the acyl residue
of - and - glutamic acid by Friedel-Crafts reactions with triflic acid developed
L
D
simple preparation of bishomophenylalanine (bhPhe) for aromatics, which added
a versatile and a reliable access to photoreactive peptide probes.
Photoaffinity labeling is a useful biochemical method to reveal structural and functional relationships
1
between low molecular weight bioactive compounds and biomolecules ; and for this, various photophores,
such as phenyldiazirine, arylazide and benzophenone, are used. Although comparative irradiation studies
of these three photophores in living cells indicated that
a
carbene precursor
1
c
3
-phenyl-3-(3-trifluoromethyl)-3H-diazirine is the most promising photophore, the many steps for the
synthesis of the 3-phenyl-3-(3-trifluoromethyl)diazirinyl ring has resulted in fewer applications in
2
biomolecular studies than other photophores. Elucidations of side chain effects in amino acid are
attractive subjects in peptide chemistry and photoaffinity labeling is one of the suitable methods for the
purpose. Bishomophenylalanine (bhPhe, 2-Amino-5-phenylpentanoic acid) elongates two carbon atoms
in a side chain of phenylalanine (Phe). Although a few of these 3-phenyl-3-(3-trifluoromethyl)diazirinyl
bhPhe has been reported, the synthetic routes require condensation of corresponding photoreactive
3
bromide and acetoamidemalonate, followed by an enzymatic resolution to isolate -form in low yield.
L
Dedication to the great contribution to bioorganic chemistry of the late scientist emeritus Dr. John Daly.

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HETEROCYCLES, Vol. 79, 2009
Asymmetric synthesis of a bhPhe derivative has been reported using several methodologies including
4
5
6
hydrogenation of cyclic dehydrodipeptides, Suzuki-coupling, diastereoselective substitution and
7
enzymatic reductive amination. These methods require the preparation of special reagents or precursors
for the asymmetric synthesis of both enantiomers. Glutamic acid is one of the precursors for the effective
synthesis of bhPhe, because Friedel-Crafts (F-C) acylation with γ-carboxyl, followed by reduction of
introduced carbonyl moiety is one of the techniques used for retrosynthesis. It has been reported that L-
N-phthaloyl glutamic anhydride was reacted F-C reaction in excess benzene (acyl acceptor) at 80 °C
8
under AlCl . It is too hard to apply the synthetic condition for precious and heat sensitive aromatics. In
3
this paper we describe the synthesis of bhPhe derivatives including 3-trifluoromethyldiazirinyl moiety via
asymmetric F-C reaction with stoichiometric amounts of glutamic acid analogues and aromatics. Only a
few studies have reported F-C reactions with various aromatics and a side chain of glutamic acid
derivatives, because many α-amino acids were less soluble in many organic solvent. For example, a F-Cꢀ
reaction of aspartic acid derivative, with a shorter side chain of glutamic acid and aromatics, was
9
performed in excess aromatics as a solvent at high temperatures during long reaction times. The synthetic
strategies were not applied to 3-phenyl-3-(3-trifluoromethyl)diazirinyl compounds, which are not stable
1
0
in the presence of Lewis acid at temperatures above rt. Triflic acid was one of the good solvents for the
α-amino acid and a promoter for F-C reactions. We have already reported catalytic amounts of triflic acid
did not affect 3-trifluoromethyldiazirinyl moiety and promote F-C alkylation to a benzene ring with
2e
dichloromethyl methyl ether at low temperatures. Furthermore, it is preferable that the reaction proceed
under the condition of stoichiometric amounts of glutamic acid derivatives and aromatics, and produce a
homogeneous mixture. It is appropriate that triflic acid was used as a catalyst for the F-C reactions and a
solvent for glutamic acid derivatives, as in accordance with previous information. N-TFA-Glu(Cl)-OMe
1
1
4
( - or - form) was selected as an acyl donor, because the compound had only one acylation site and
L D
both protective groups acquired a tolerance at the F-C condition. Several aromatics were subjected to the
F-C reaction (Scheme 1). Triflic acid was added to the mixture of stoichiometric amounts of benzene 1
and 4 at 0 °C. The reaction mixture immediately became a clear solution and was stirred for an hour at
0
°C and then poured into ice-water and ethyl acetate to afford F-C products 5a over 90% yield. Further
1
purification was not needed by H-NMR analysis after the reaction mixture was subjected to partition (1N
HCl, saturated NaHCO . and saturated NaCl). Toluene 2 afforded only p-substituted product 6a, however,
3
anisole 3 afforded both p- (7a) and o- substituted (8a) products (p- : o- = 9 : 1) in high yield. These F-C
products were subjected to hydrogenation of benzyl carbonyl group with Pd/C in acetic acid (5b-7b,
9
5-98%), followed by deprotection of trifluoroacetamide and methyl ester with 6N HCl at 80 °C in good
yield to afford - or - bhPhe analogues (5c-7c, 98-99%). The stereochemistry of these compounds was
L
D
1
2
determined [α]D and chiral HPLC (CHIROBIOTIC T; eluted with 10% EtOH – H O; flow rate 1.0
2

HETEROCYCLES, Vol. 79, 2009
361
ml/min; UV detection at 210 nm) for both enantiomers. Enantiomeric excess of all of the deprotected
compounds was calculated >99% in order to succeed with asymmetric synthesis of bhPhe derivatives.
The stereochemistry of the starting glutamic acid was retained for the each bhPhe (5c-7c) as expected.
The synthetic routes were very useful because overall yields from F-C acylation to deprotection were very
high.
R1
R1
R1
R
COCl
O
6N HCl aq.
H , Pd/C
TfOH
rt, 1 h
2
R2
R2
R2
AcOH
rt,1 h
8
0 °C
6 h
TFAHN
CO Me
2
1
2
3
R = H
4
R = Me
R = OMe
TFAHN
CO Me
TFAHN
CO Me
H N
2
CO Me
2
2
2
5
6
7
a (L- 94%, D- 90%)
a (L- 94%, D- 92%)
a, 8a (L- 88%, 9%)
5b (L- 99%, D- 98%)
6b (L- 98%, D- 99%)
7b (L- 95%, D- 85%)
5c (L- 99%, D- 99%)
6c (L- 99%, D- 99%)
7c (L- 99%, D- 99%)
(
D- 83%, 9%)
5
R , R = H, 6 R = Me, R = H, 7 R = OMe, R = H, 8 R = H, R = OMe
1
2
1
2
1
2
1
2
Scheme 1. Asymmetric Friedel-Crafts reactions for synthesis of bishomophenylalanine derivatives.
The synthetic route applied to 3-phenyl-3-(3-trifluoromethyl)-3H-diazirine derivatives (Scheme 2).
3
-Phenyl-3-(3-trifluoromethyl)-3H-diazirine was less reactive for nucleophilic substitutions than benzene,
because quaternary carbon in 3-trifluoromethyldiazirine was polarizes slightly positive by two nitrogen
13
and trifluoromethyl groups. 3-Phenyl-3-(3-trifluoromethyl)-3H-diazirine, which has no substituent on
the benzene ring, did not react with - or - 4 in TfOH at 0°C and the decomposed diazirinyl ring at higher
L
D
temperatures. But we have already reported that 3-phenyl-3-(3-trifluoromethyl)-3H-diazirine is able to
2e
react to F-C alkylation in TfOH with dichloromethyl methyl ether in moderate yield. The results
indicated that the reactivity of the acyl donor is also plays critical role.
-(3-Methoxyphenyl)-3-(3-trifluoromethyl)-3H-diazirine 9 will be a good acyl acceptor because it has a
a
14
3
1
5
methoxy substituent.
The mixture of stoichiometric amounts of 4 and 9 was dissolved in TfOH at 0 °C and maintained at the
1
6
same temperature for 1h to afford only one F-C acylated product 10 in moderate yield (76%). NOESY
measurement of a methoxy group and an aromatic proton in 10 revealed that the F-C reaction proceeded
at o-position against the methoxy substituent specifically, because steric hindrance between the diazirinyl
ring and the substitute at p-position against the methoxy substituent may have contributed to the
observation. It has already been reported that the diazirinyl N=N double bond was unstable under
1
7
H -Pd/C conditions. Specific reduction of benzyl carbonyl to methylene in a diazirinyl compound was
2
1
8
performed with triethylsilane - TFA in good yield. The reduction of 10 with the condition for an hour at
19
rt afforded 11 in good yield. No decomposition of the diazirinyl ring was observed during the reaction.
Deprotection of both protecting groups for 11 was performed using an alkaline condition to afford
2
0
diazirinyl bhPhe 12 in good yield (90%). Deprotection of 11 with an acidic condition (6N HCl at 80 °C

362
HETEROCYCLES, Vol. 79, 2009
in 6 h) afforded byproducts, in which diazirinyl moiety was broken, slightly. The stereochemistry of
compound 12 was determined to be chiral HPLC (CHIROBIOTIC T; eluted with 10% EtOH – H O; flow
2
rate 1.0 ml/min; UV detection at 210 nm) for both enantiomers and the stereochemistry of the starting
glutamic acid was retained for the each of the compounds.
F3C
F3C
F3C
N
N
N
N
N
Et SiH
F3C
3
NaOH
MeOH
N
N
N
TfOH
TFA
COCl
O
0
°C, 3 h
rt, 1 h
rt, 2 h
OMe
OMe
OMe
OMe
TFAHN
9
CO Me
2
4
TFAHN
CO2Me
TFAHN
CO Me
H N
2
CO Me
2
2
1
0 (L- 74%, D- 71%)
11 (L- 95%, D- 91%)
12 (L- 90%, D- 86%)
Scheme 2. Asymmetric synthesis of diazirinyl bishomophenylalanine derivatives.
We have demonstrated effective asymmetric synthesis of bhPhe from enantiopure glutamic acid
derivative and aromatics in TfOH at low temperature, followed by reduction and deprotection in high
yield. The protocol was applied to 3-(3-methoxyphenyl)-3-(3-trifluoromethyl)-3H-diazirine, which is one
of the most useful photophores for photoaffinity labeling and is reported as unstable compounds for the
F-C reaction at high temperatures. The results will be contributed to a better understanding of
structure-activity relationships for the side chain of aromatic α-amino acids.
ACKNOWLEDGEMENTS
This research was partially supported by a Ministry of Education, Science, Sports and Culture
Grant-in-Aid for Scientific Research on a Priority Area, 18032007 and for Scientific Research (C),
1
9510210. The authors would also like to thank Professor Masaru Hashimoto at Hirosaki University for
the mass spectrometry measurements.
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6. 10 H -NMR (CDCl ) δ:7.69 (1H, d, J = 8.0 Hz), 6.80 (1H, d, J = 8.0 Hz), 6.64 (1H, s), 4.60 (1H, td,
3
J = 8.0, 4.6 Hz), 3.87 (3H, s), 3.74 (3H, s), 3.14-3.01 (2H, m), 2.36-2.29 (1H, m), 2.17-2.13 (1H, m).
1
3
2
C-NMR (CDCl ) δ: 200.10, 170.96, 158.59, 157.06 (q, J = 37.6 Hz), 134.86, 130.94, 128.09,
3
CF
1
1
1
2
2
21.73 (q, J = 273.9 Hz), 118.75, 115.60 (q, J = 288.3 Hz), 109.47, 55.60, 52.80, 52.41, 39.67,
CF CF
2
+
8.33 (q, J = 40.8 Hz), 25.77. ESI-MS 456 (MH ). [α] - ＋25.00 (c 2.0 CHCl ), [α] - 26.50 (c
CF
D
3
D
.0 CHCl3).
1
7. Y. Ambroise, C. Miskowski, G. Djega-Mariadassou, and B. Rousseau, J. Org. Chem., 2000, 65,
183.
7
1
1
8. M. Hashimoto, Y. Hatanaka, and K. Nabeta, Heterocycles, 2003, 59, 395.
1
9. 11 H -NMR (CDCl ) δ: 7.08 (1H, d, J = 8.0 Hz), 6.71 (1H, d, J = 8.0 Hz), 6.57 (1H, s), 4.65 (1H, td,
3
J = 7.7, 5.3 Hz), 3.78 (3H, s), 3.74 (3H, s), 2.65-2.56 (2H, m), 1.96-1.92 (1H, m), 1.76-1.73 (1H, m).
1
3
2
1
.61-1.56 (2H,m). C-NMR (CDCl ) δ: 171.58, 157.72, 156.97 (q, J = 37.6 Hz), 131.86, 130.33,
3 CF

364
HETEROCYCLES, Vol. 79, 2009
1
1
1
3
28.35, 122.31 (q, J = 274.7 Hz), 118.89, 115.79 (q, J = 287.5 Hz), 108.16, 55.32, 52.90, 52.55,
CF
CF
2
+
1.60, 29.45, 28.60 (q, JCF = 40.4 Hz), 25.04. ESI-MS 442 (MH ). [α] -＋37.00 (c 1.0 CHCl ),
D
3
[
α] - 37.00 (c 1.0 CHCl ).
D
3
1
2
0. 12 H-NMR (CD OD) δ: 7.21 (1H, d, J = 8.6 Hz), 6.75 (1H, d, J = 6.9 Hz), 6.64 (1H, s), 3.79 (3H, s),
3
1
3
3
1
3
.52 (1H, dd, J = 6.6, 5.4 Hz), 2.65 (2H, t, J = 7.2 Hz), 1.89-1.64 (4H, m). C-NMR (CD OD) δ:
3
1
74.54, 159.26, 133.75, 131.68, 128.94, 123.62 (q, J = 273.1 Hz), 119.78, 109.04, 56.13, 55.92,
CF
2
+
2.08, 30.52, 29.54 (q, J = 40.0 Hz), 26.24. ESI-MS 332 (MH ). [α] -‐6.0 (c 1.0 MeOH), [α] -
CF
D
D
＋
5.5 (c 1.0 MeOH).