Synthesis of nonpolar peptide nucleic acid monomers containing fluoroaromatics

Article abstract of DOI:10.1039/b103170h

A general strategy for the synthesis of nonpolar peptide nucleic acid monomers containing fluoroaromatics (F-PNA) is described. These compounds have been designed as hybrid analogues of the difluorotoluene nucleoside, F (1) with PNA. Fluorophenylacetic ac

Full text of DOI:10.1039/b103170h

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Synthesis of nonpolar peptide nucleic acid monomers containing
fluoroaromatics
Norio Shibata,* Biplab Kumar Das, Hiroshi Honjo and Yoshio Takeuchi
1
Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University,
Sugitani 2630, Toyama 930-0194, Japan. E-mail: nozshiba@ms.toyama-mpu.ac.jp
Received (in Cambridge, UK) 6th April 2001, Accepted 25th May 2001
First published as an Advance Article on the web 19th June 2001
A general strategy for the synthesis of nonpolar peptide nucleic acid monomers containing fluoroaromatics (F-PNA)
is described. These compounds have been designed as hybrid analogues of the difluorotoluene nucleoside, F (1)
with PNA. Fluorophenylacetic acid derivatives 9 were coupled to the Boc-protected pseudopeptide backbone 8 by
a standard peptide coupling reaction using DhbtOH and DCC in the presence of triethylamine to afford the doubly
protected F-PNA monomers 14 in moderate to good yields. The ethyl esters 14a, 14c and 14e underwent hydrolytic
cleavage under basic conditions to generate N-protected F-PNA monomers 15 in good yields. The tert-butyl esters
14b, 14d were treated with TFA in dichloromethane to produce the free F-PNA monomers 16 in good to excellent
yields. The β-F-PNA monomers designed based on the structure of 2Ј,5Ј-linked isoDNA were also synthesized in
a similar fashion to the preparation of F-PNA monomers in moderate to good yields as both N-protected and free
monomers.
Introduction
Aryl C-nucleosides have received considerable attention due
to their ability for aromatic stacking while relinquishing their
hydrogen bonding potentials.1 DNA–duplex stability and
sequence specificity are founded on the complementary
Watson–Crick hydrogen bonding patterns of adenine with
thymine and cytosine with guanine. However, recent investi-
gations have demonstrated that the aromatic stacking and
hydrophobicity are major driving forces in DNA structure and
synthesis. Nonpolar DNA base analogues such as aryl C-
nucleosides stabilize significantly the helices even when the
bases in question do not undergo pairing.2 Among the most
Fig. 1
important analogues in this field is difluorotoluene nucleoside
F (1) which has been developed by Kool et al.3 Compound F
is designed as a nonpolar shape mimic for natural thymidine. It
serves as a template for DNA synthesis even though it lacks
standard hydrogen bonding (Fig. 1).
Another interesting class of substrates derived from DNA
is peptide nucleic acids (PNAs). PNAs have a pseudopeptide
backbone composed of N-(2-aminoethyl)glycine where only
nucleobases are left of the original DNA structure. PNAs are
one of the most successful mimetics of nucleic acids, which
exhibit a remarkable affinity for both complementary DNA and
Fig. 2
RNA, which is greater than their natural counterparts.4 They
DNA hybrid duplexes.8 Although several analogues of PNA
hold the prestigious position of being potential replacements of
oligonucleotides for use in antisense therapeutics,5 molecular
biology reagents and chemical diagnostics. With these points as
a background, the design and synthesis of a new class of PNA
analogues having aromatics in either their backbone6 or side
chain7,8 have received much interest because of their ability
to demonstrate π–π stacking interactions. Woski and Challa7
recently demonstrated the synthesis of PNA analogues contain-
ing aromatic moieties as a replacement of natural nucleobases
to highlight the roles of base–base stacking and hydrogen bond-
ing and also to examine the sequence specificity to form a PNA–
DNA double helical structure.7,8 Oligomers containing phenyl
residue 2 could form PNA–DNA double helices although this
showed little sequence specificity. PNA monomers containing
pyrene 3 and anthraquinone 4 have also been synthesized for
their potential ability to intercalate into DNA to form PNA–
having aromatics have been synthesized,7,8 to the best of our
knowledge, there is no report published yet about the synthesis
of nonpolar PNA analogues having fluoroaromatics (F-PNA)
which could be much more interesting as a probe to study the
role of hydrophobic and stacking interactions (Fig. 2).
In this paper we describe the first synthesis of F-PNA
monomers as well as the β-F-PNA monomers designed based
on 2Ј,5Ј-linked isoDNA 5 (Fig. 3).
Results and discussion
Synthesis of F-PNA monomers
We recently reported the one-pot synthesis of N-fluoroarylalkyl
pendented α,α- and α,β-dipeptides 6, 7 by the Ugi four-
DOI: 10.1039/b103170h
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611
This journal is © The Royal Society of Chemistry 2001
1605

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Table 1 Synthesis of protected F-PNA monomers 14
Entry
8
R1
9
R2
14
Yield (%)a
1
2
3
4
5
8a
8b
8a
8b
8a
Et
9a
9a
9b
9b
9c
2,4-Difluorophenyl
2,4-Difluorophenyl
Pentafluorophenyl
Pentafluorophenyl
2,4-Difluoro-5-methylphenyl
14a
14b
14c
14d
14e
51
50
58
55
52
But
Et
But
Et
a Yields were determined based on 8 employed.
Fig. 3
Scheme 1 Reagents and conditions: i, CuCN, DMF, 160 ЊC, 50%;
ii, conc. H2SO4, H2O, reflux, 100%; iii, ClCO2Et, Et3N, CH2N2, 98%;
iv, MeOH, PhCOOAg, 60%; v, 1 M LiOH, THF, 58%.
The coupling of the acid 9 to the backbone 8 using peptide
coupling agents 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzo-
triazine (DhbtOH) and DCC in DMF in the presence of tri-
ethylamine proceeded nicely to afford the protected F-PNA
monomers 14a–e in 50–58% yields (Table 1).
Compound 14e provides considerable interest as the 2,4-
difluorotoluene moiety directly acts as an isostere of thymine
which might mimic the natural PNA analogue having a thymine
base moiety.
Selective removal of the C-terminus of ethyl esters 14a, 14c
and 14e was accomplished by basic hydrolysis with 1 M LiOH
in THF to afford the N-protected F-PNA monomers 15a–c in
54–70 yields (Scheme 2).
Fig. 4
component condensation.9 This method, however, cannot be
applied for the preparation of F-PNA because the backbone
of the products obtained is not pseudopeptide but peptide
(Fig. 4).
Therefore we synthesized F-PNA monomers using con-
ventional stepwise methods. One of the key intermediates for
the synthesis of F-PNA monomers is N-(2-Boc-aminoethyl)-
glycinates 8 which have been very often used as the backbone
for PNA monomer synthesis. The backbone 8 was prepared
according to the literature procedure.10 Other key moieties are
the fluoroarylacetic acid derivatives 9. 2,4-Difluorophenylacetic
acid (9a) and pentafluorophenylacetic acid (9b) are com-
mercially available and 2,4-difluoro-5-methylphenylacetic acid
(9c) was prepared from 2,4-difluoro-5-methylbromobenzene
(10)3a in four steps as follows. Compound 10 was first converted
into the corresponding cyanide 11 by treatment with copper()
cyanide11 in DMF at 160 ЊC in 50% yield. The cyanide under-
went acidic hydrolysis12 to give the 2,4-difluoro-5-methyl-
benzoic acid (12) in quantitative yield. The acid 12 was then
treated with ethyl chlorocarbonate and diazomethane to give
the diazoketone 13 in 98% yield, which was then converted
to the methyl ester through Arndt–Eistert rearrangement by
treating with methanol catalyzed with silver benzoate in 60%
yield. The methyl ester underwent basic hydrolysis with 1 M
LiOH in THF13 to give 9c in 58% yield (Scheme 1).
Scheme 2 Reagents and conditions: i, 1 M LiOH, THF, rt, 1 h.
The tert-butyl esters of 14b and 14d underwent hydrolytic
cleavage by the action of TFA in dichloromethane to produce
the free F-PNA monomers 16a and 16b respectively, in 77–95%
yields (Scheme 3).
Synthesis of ꢀ-F-PNA monomers
We next focused our attention on the synthesis of β-F-PNA
monomers, that were designed based on the structure of PNA
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Table 2 Synthesis of protected β-F-PNA monomers 20
Entry
17
R1
Acid
R2
20
Yield (%)a
1
2
3
4
5
17a
17b
17a
17b
17a
Et
19a
19a
19b
19b
12
2,4-Difluorophenyl
2,4-Difluorophenyl
Pentafluorophenyl
Pentafluorophenyl
2,4-Difluoro-5-methylphenyl
20a
20b
20c
20d
20e
60
50
But
Et
22 (51b)
24 (50b)
65
But
Et
a Yields were determined based on 17 employed. b The reaction was carried out in the absence of Et3N.
Scheme 3 Reagents and conditions: i, TFA, DCM, rt, 2 h.
Scheme 5 Reagents and conditions: i, 1 M LiOH, THF, rt, 1 h.
and 2Ј,5Ј-linked isoDNA 5 (Fig. 3). 2Ј,5Ј-Linked isoDNA
has recently drawn much interest due to its ability to form a
heteroduplex with RNA that is as stable as the comparable
normal DNA–RNA duplexes.14 The β-F-PNA backbone 17 is
composed of N-(3-aminopropyl)glycine, which is one carbon
longer than the N-(2-aminoethyl)glycine backbone in PNA.
Therefore, the β-F-PNA consists of a repeating unit of 7 atoms.
Fluoroaromatics are attached to the new backbone through
carbonyl linkers. This should be a suitable structural analogue
of 2Ј,5Ј-linked isoDNA.
Scheme 6 Reagents and conditions: i, TFA, DCM, rt, 2 h.
PNA monomers 22a and 22b respectively, in 77–97% yields
(Scheme 6).
The new backbone 17 was easily prepared following the same
procedure as applied to 810 by direct alkylation of N-Boc-
diaminopropane (18) instead of N-Boc-diaminoethane using
ethyl chloroacetate or tert-butyl chloroacetate (Scheme 4).
Conclusion
We have demonstrated the synthesis of a novel class of non-
polar peptide nucleic acid monomers containing fluoroarom-
atics namely F-PNA and β-F-PNA by the standard peptide
coupling reaction. Of particular interest to the synthesis,
we have incorporated the 2,4-difluorotoluene moiety into the
F-PNA analogues as an isosteric replacement for thymine
which might confer unique properties to the compounds.
Conversion of the F-PNA monomers into oligomers is under
investigation.
Scheme 4 Reagents and conditions: i, ClCO2R1 (R1 = Et or But) Et3N,
KI, 50 h.
Experimental
Another part of β-F-PNA is fluorobenzoic acids such as 19a,
b and 12. 2,4-Difluorobenzoic acid (19a) and pentafluoro-
benzoic acid (19b) are commercially available and 2,4-difluoro-
5-methylbenzoic acid (12) was already prepared in two steps as
shown in Scheme 1. We first attempted the coupling of acids 19
and 12 to the backbone 17 using standard peptide coupling
agents DhbtOH and DCC in DMF in the presence of triethyl-
amine. The yields are moderate except in the case of coupling
for 20c and 20d (22–24%) presumably due to the high acidity of
19b. After optimization of the reaction conditions, the coupling
reaction was performed nicely with the peptide coupling agents
DhbtOH and DCC in the absence of triethylamine to furnish
20c and 20d in 51 and 50% yield, respectively (Table 2).
Completion of the synthetic studies was accomplished by
deprotection of 20 to produce N-protected and free β-F-PNA
monomers intended to be used for oligomerization. The ethyl
esters 20a, 20c and 20e were deprotected selectively by treating
with 1 M LiOH in THF to give N-Boc-β-F-PNA monomers
21a–21c in 52–57% yields (Scheme 5).
Melting points were determined on a Yanagimoto micro-
melting point apparatus and are uncorrected.
IR spectra (cmϪ1) were recorded on a Perkin-Elmer 1600
spectrometer.
1H-NMR spectra were measured as solutions in CDCl3 or
CD3OD and chemical shifts are expressed in ppm relative to
internal Me4Si (0.00 ppm) and were recorded on a JEOL GX-
270 (270 MHz) spectrometer. 19F-NMR spectra were measured
with CFCl3 as an internal standard and were taken with a
JEOL GX-270 (254 MHz) spectrometer. Upfield shifts are
quoted as negative δ values. The following abbreviations are
used: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br,
broad. J Values are given in Hz.
Electron ionization (EI) mass spectra were taken with a
JEOL JMS-D300 spectrometer. Column chromatography and
preparative TLC were performed on BW-200 (Fuji Silysia) and
Kieselgel 60 (Merck, art. 7748), respectively.
All reactions were carried out under a dry N2 atmosphere.
Unless otherwise noted, reagents were added by syringe. Com-
mercially available dehydrated THF [stabilized with butylated
Likewise, the tert-butyl esters 20b and 20d underwent
acidolysis with TFA in dichloromethane to afford the β-F-
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611
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hydroxytoluene (BHT)] was used for reaction. DMF was
distilled over CaH2 immediately prior to use.
the methyl ester (409 mg, 2.04 mmol) in THF (3 ml), 1 M LiOH
(3 ml) was added and stirred overnight at room temperature.
Then the reaction mixture was diluted with ether (20 ml). Water
(10 ml) was added and extracted to take the aqueous portion.
The aqueous portion was acidified with 1 M HCl up to pH 2.
A solid precipitate was observed which was collected by
filtration and on subsequent drying yielded 9c (220 mg, 58%) as
a white amorphous solid; mp 119–120 ЊC (dichloromethane–
hexane); νmax (KBr)/cmϪ1 2980, 1705 (C᎐O), 1143, 1087; δ (270
2,4-Difluoro-5-methylbenzonitrile 11
To a well-stirred solution of 103a (900 mg, 4.34 mmol) in dry
DMF (40 ml), copper() cyanide (487 mg, 5.43 mmol) was
added in one portion and the mixture was heated at 160 ЊC for 3
h. Then the reaction mixture was cooled and poured into water
(20 ml), whereupon 20% aqueous FeCl3 (9 ml) was added. The
mixture was extracted with ether (2 × 100 ml) and the ether
portion was washed with brine, dried over anhydrous Na2SO4
and concentrated in vacuo. The residue was chromatographed
on silica gel (hexane–ethyl acetate 98 : 2) to afford 11 (335 mg,
50%) as a white crystalline solid; mp 55–56 ЊC (ethyl acetate–
hexane); νmax (KBr)/cmϪ1 3054, 2932, 2232 (CN), 1535, 1089,
1004; δH (270 MHz, CDCl3) 2.28 (3H, s, ArCH3), 6.92 (1H, t,
J 9.1, ArH), 7.47 (1H, t, J 7.5, ArH); m/z (EI) 153 (Mϩ), 126
(Mϩ Ϫ HCN); HRMS found Mϩ 153.0365. C8H5F2N requires
153.0342. Found: C, 62.80; N, 8.96; H, 3.11. Calc. for C8H5F2N:
C, 62.75; N, 9.15; H, 3.29%.
᎐
H
MHz, CDCl3) 2.28 (3H, s, ArCH3), 3.67 (2H, s, ArCH2), 6.86
(1H, t, J 9.9 ArH), 7.88 (1H, t, J 8.2, ArH); δF (254 MHz,
CDCl3) Ϫ107.6 (m), Ϫ103.7 (m); m/z (EI) 186 (Mϩ), 141
(Mϩ Ϫ COOH); HRMS found Mϩ 186.0470. C9H8F2O2
requires 186.0493.
General experimental procedure for the synthesis of protected
F-PNA monomers 14: {N-[2-(tert-butoxycarbonylamino)ethyl]-
N-[2-(2,4-difluorophenyl)acetyl]amino}acetic acid ethyl ester 14a
To a stirred solution of 8a (200 mg, 0.813 mmol), DhbtOH (132
mg, 0.813 mmol) and 9a (139 mg, 0.813 mmol) in DMF (5 ml)
were added triethylamine (164 mg, 1.62 mmol) and DCC (168
mg, 0.813 mmol) at 0 ЊC. The ice-bath was removed after 1 h
and stirring was continued for another 2–3 h at room temper-
ature. The precipitated DCU was removed by filtration and
washed twice with dichloromethane. To the combined filtrates
was added dichloromethane (100 ml) and the resulting mixture
was successively washed with dilute aqueous NaHCO3, dilute
aqueous KHSO4, and brine. The organic phases were com-
bined, dried over anhydrous Na2SO4, and concentrated to pro-
duce an oily residue which was chromatographed on silica gel
(hexane–ethyl acetate 6 : 4) to give 14a (165 mg, 51%) as a
mixture of rotamers; pale yellow oil; νmax (neat)/cmϪ1 3429
2,4-Difluoro-5-methylbenzoic acid 12
A solution of 11 (1.0 g, 6.53 mmol) in a mixture of concen-
trated H2SO4 (3.8 ml) and H2O (2.8 ml) was stirred under reflux
for 1 h. Then the mixture was poured into ice–water (20 ml).
The precipitate obtained was filtered under reduced pressure
and dried to yield 12 (1.74 g, 100%) as a white crystalline solid;
mp 122–123 ЊC (dichloromethane–hexane); νmax (KBr)/cmϪ1
2981, 1699 (C᎐O); δH (270 MHz, CDCl3) 2.26 (3H, s, ArCH3),
᎐
6.82 (1H, t, J 9.9, ArH), 7.83 (1H, t, J 8.2, ArH); δF (254 MHz,
CDCl3) Ϫ109.3 (m), Ϫ105.7 (m); m/z (EI) 172 (Mϩ), 155
(Mϩ Ϫ OH), 127 (Mϩ Ϫ COOH); HRMS found Mϩ 172.0371.
C8H6F2O2 requires 172.0386. Found: C, 55.86; H, 3.52. Calc.
for C8H6F2O2: C, 55.82; H, 3.51%.
(NH), 2981, 1741, 1704, 1651 (C᎐O), 1508, 1250, 1171; δH (270
᎐
MHz, CDCl3) 1.29 (3H, t, J 7.2, OCH2CH3), 1.43 (9H, s, But),
3.28–3.57 (4H, m, N(CH2)2N ), 3.72, 4.01 (total 2H, each s,
ArCH2), 3.74, 4.03 (total 2H, each s, NCH2CO), 4.21 (2H, q,
J 7.2, 14.1, OCH2CH3), 4.95, 5.49 (total 2H, each br s,
NH × 2), 6.83 (2H, m, ArH), 7.27 (1H, m, ArH); δF (254 MHz,
CDCl3) Ϫ111.9, Ϫ112.2 (each m), Ϫ113.2, Ϫ114.1 (each m);
m/z (EI) 400 (Mϩ), 401 (Mϩ ϩ 1), 343 (Mϩ Ϫ But); HRMS
found Mϩ 400.1780. C19H26F2N2O5 requires 400.1804. Found:
C, 56.66; N, 6.62; H, 6.21. Calc. for C19H26F2N2O5: C, 56.99; N,
7.00; H, 6.54%.
2-Diazo-1-(2,4-difluoro-5-methylphenyl)ethanone 13
To a solution of 12 (500 mg, 2.92 mmol) in THF (15 ml) were
added triethylamine (0.4 ml) and ethyl chlorocarbonate (0.27
ml) at Ϫ15 ЊC and stirred for 15 minutes. Then the mixture was
allowed to warm to 0 ЊC and diazomethane in ether solution
was added dropwise until a yellowish colour persisted for a long
period. The whole mixture was stirred for 3 hours at room tem-
perature. The reaction mixture was diluted with hexane–ethyl
acetate (1 : 1; 100 ml) and washed successively with saturated
NaHCO3, H2O and brine, dried over Na2SO4, filtered and con-
centrated to obtain 13 (710 mg, 98%) as a yellowish solid mass
which was used for the next step without further purification;
νmax (neat)/cmϪ1 2986, 2109 (CH᎐N ), 1735 (C᎐O); δ (270
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(2,4-difluoro-
phenyl)acetyl]amino}acetic acid tert-butyl ester 14b. This com-
pound was prepared analogously to 14a from 8b (200 mg, 0.729
mmol), 9a (125 mg, 0.728 mmol), DhbtOH (119 mg, 0.728
mmol), triethylamine (147 mg, 1.45 mmol) and DCC (150 mg,
0.728 mmol) in DMF (5 ml) to give 14b (157 mg, 50%) as a
mixture of rotamers; pale yellow oil; νmax (neat)/cmϪ1 3315, 3302
᎐
᎐
2
H
MHz, CDCl ) 2.28 (3H, s, ArCH ), 4.34 (1H, s, CH᎐N ), 6.87
᎐
3
3
2
(1H, t, J 9.9, ArH), 7.85 (1H, t, J 8.2, ArH); δF (254 MHz,
CDCl3) Ϫ106.6 (m), Ϫ101.8 (m); m/z (EI) 196 (Mϩ); HRMS
found Mϩ 196.0427. C9H6N2F2O2 requires 196.0449.
(NH), 2980, 1735, 1704, 1670 (C᎐O), 1508, 1247, 1160; δH (270
᎐
MHz, CDCl3) 1.38, 1.39 (total 9H, each s, But), 1.41, 1.44 (total
9H, each s, But), 3.27–3.71 (6H, m, N(CH2)2N ϩ ArCH2),
3.88, 3.98 (total 2H, each s, NCH2CO), 5.04, 5.47 (total 2H,
each br s, NH × 2), 6.83 (2H, m, ArH), 7.27 (1H, m, ArH);
δF (254 MHz, CDCl3) Ϫ112.2, Ϫ112.5 (each m), Ϫ113.3, Ϫ114
(each m); m/z (EI) 429 (Mϩ ϩ 1), 428 (Mϩ), 371 (Mϩ Ϫ But);
HRMS found Mϩ ϩ 1 429.2220. C21H31F2N2O5 requires
429.2201. Found: C, 58.50; N, 6.55; H, 6.79. Calc. for C21H30-
F2N2O5: C, 58.87: N, 6.54; H, 7.06%.
(2,4-Difluoro-5-methylphenyl)acetic acid 9c
To a solution of 13 (668 mg, 3.40 mmol) in methanol (15 ml)
under N2 at Ϫ25 ЊC with the exclusion of light was added silver
benzoate (85.6 mg, 0.373 mmol) dissolved in triethylamine (1.37
ml, 9.85 mmol) and the whole mixture was allowed to warm to
room temperature within 3 h. The reaction mixture was diluted
with ethyl acetate (200 ml) and successively washed with satur-
ated NaHCO3, NH4Cl and brine, dried over Na2SO4, filtered
and concentrated to obtain the methyl ester as a white solid
(409 mg, 60%) which was characterized by NMR and mass
spectral data; δH (270 MHz, CDCl3) 2.26 (3H, s, ArCH3), 3.71
(2H, s, ArCH2), 3.91 (3H, s, OCH3), 6.78 (1H, t, J 9.9, ArH),
7.81 (1H, t, J 8.2, ArH); δF (254 MHz, CDCl3) Ϫ109.1 (m),
Ϫ115.1 (m); m/z (EI) 200 (Mϩ), 185 (Mϩ Ϫ Me); HRMS found
Mϩ 200.0638. C10H10F2O2 requires 200.0649. To a solution of
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(pentafluoro-
phenyl)acetyl]amino}acetic acid ethyl ester 14c. This compound
was prepared from 8a (200 mg, 0.813 mmol), 9b (184 mg, 0.812
mmol), DhbtOH (133 mg, 0.812 mmol), triethylamine (164 mg,
1.62 mmol) and DCC (168 mg, 0.812 mmol) in DMF (5 ml) by
a procedure analogous to 14a. A similar purification procedure
gave 14c (214 mg, 58%) as a mixture of rotamers; pale yellow
oil; νmax (neat)/cmϪ1 3434, 3321 (NH), 3006, 1743, 1703, 1661
1608
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611

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(C᎐O), 1512, 1248, 1171, 1009; δ (270 MHz, CDCl ) 1.24, 1.28
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(pentafluoro-
᎐
H
3
(total 3H, each t, J 7.2, OCH2CH3), 1.44 (9H, s, But), 3.33–3.67
(4H, m, N(CH2)2N ), 3.74, 4.02 (total 2H, each s, ArCH2),
3.83, 4.07 (total 2H, each s, NCH2CO), 4.21 (2H, q, J 7.2,
14.1, OCH2CH3), 5.02, 5.62 (total 2H, each br s, NH × 2); δF
(254 MHz, CDCl3) Ϫ142.6, Ϫ143.2 (each m), Ϫ156.3, Ϫ156.8
(each m), Ϫ163.3, Ϫ163.6 (each m); m/z (EI) 454 (Mϩ), 398
(Mϩ Ϫ OC2H5), 397 (Mϩ Ϫ But), 381 (Mϩ Ϫ ButO); HRMS
found (Mϩ Ϫ But) 397.1156. C15H14F5N2O4 requires 397.1177.
phenyl)acetyl]amino}acetic acid 15b. Compound 15b (26 mg,
55%) was prepared analogously to 15a from the selective basic
hydrolysis of 14c (50 mg, 0.11 mmol) as a mixture of rotamers;
white solid; mp 123–125 ЊC; νmax (KBr)/cmϪ1 3415 (NH), 2990,
1740, 1654 (C᎐O), 1442, 1369, 1183, 1132; δ (270 MHz, CDCl )
᎐
H
3
1.42 (9H, s, But), 3.33–3.82 (6H, m, N(CH2)2N ϩ ArCH2),
3.82, 4.04 (total 2H, each s, NCH2CO), 4.18, 5.52 (total 2H,
each br s, NH × 2); δF (254 MHz, CDCl3) Ϫ142.7 (m), Ϫ156.2,
Ϫ156.6 (each m), Ϫ162.8, Ϫ163.3 (each m); m/z (EI) 426 (Mϩ),
425 (Mϩ Ϫ 1), 409 (Mϩ Ϫ OH), 369 (Mϩ Ϫ But); HRMS
found Mϩ 426.1232. C17H19F5N2O5 requires 426.1214. Found:
C, 47.62; N, 6.54; H, 4.38. Calc. for C17H19F5N2O5: C, 47.89; N,
6.57; H, 4.49%.
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(pentafluoro-
phenyl)acetyl]amino}acetic acid tert-butyl ester 14d. This com-
pound was prepared from 8b (200 mg, 0.729 mmol), 9b (165
mg, 0.728 mmol), DhbtOH (119 mg, 0.728 mmol), triethyl-
amine (147 mg, 1.45 mmol) and DCC (150 mg, 0.728 mmol)
in DMF (5 ml) by a procedure analogous to 14a. A similar
chromatographic purification gave 14d (193 mg, 55%) as a
mixture of rotamers; pale yellow oil; νmax (neat)/cmϪ1 3340
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(2,4-difluoro-5-
methylphenyl)acetyl]amino}acetic acid 15c. Compound 15c (25
mg, 54%) was obtained analogously to 15a from the selective
basic hydrolysis of 14e (59 mg, 0.12 mmol) as a mixture of
rotamers; white powder; mp 68–69 ЊC; νmax (KBr)/cmϪ1 3388,
(NH), 2981, 1703, 1657 (C᎐O), 1512, 1245, 1160, 1008; δH (270
᎐
MHz, CDCl3) 1.43, 1.44 (total 9H, each s, But), 1.44, 1.45 (total
9H,eachs,But),3.29–3.72(6H,m, N(CH2)2N ϩ ArCH2),3.92,
4.13 (total 2H, each s, NCH2CO), 4.95, 5.53 (total 2H, each br
s, NH × 2); δF (254 MHz, CDCl3) Ϫ139.1, Ϫ139.7 (each m),
Ϫ152.7, Ϫ153.3 (each m), Ϫ159.6, Ϫ159.8 (each m); m/z (EI)
482 (Mϩ), 481(Mϩ Ϫ 1), 425 (Mϩ Ϫ But), 368 (Mϩ Ϫ But × 2);
HRMS found Mϩ 482.1847. C21H27F5N2O5 requires 482.1840.
Found: C, 52.49; N, 5.66; H, 5.29. Calc. for C21H27F5N2O5: C,
52.28; N, 5.81: H, 5.64%.
3158 (NH), 2980, 1715, 1664 (C᎐O), 1512, 1174; δH (270 MHz,
᎐
CDCl3) 1.43, 1.49 (total 9H, each s, But), 2.19, 2.28 (total 3H,
eachs,ArCH3),3.17–3.69(6H,m, N(CH2)2N ϩ ArCH2),4.01,
4.06 (total 2H, each s, NCH2CO), 4.23, 5.45 (total 2H, each br
s, NH × 2), 6.71 (1H, m, ArH), 7.52 (1H, m, ArH); δF (254
MHz, CDCl3) Ϫ114.8, Ϫ118.4 (each m), Ϫ120.1, Ϫ121.7 (each
m); m/z (EI) 386 (Mϩ), 387 (Mϩ ϩ 1), 329 (Mϩ Ϫ But); HRMS
found Mϩ 386.1672. C18H24F2N2O5 requires 386.1653.
{N-[2-(tert-Butoxycarbonylamino)ethyl]-N-[2-(2,4-difluoro-5-
methylphenyl)acetyl]amino}acetic acid ethyl ester 14e. This
compound was prepared from 8a (200 mg, 0.813 mmol), 9c (151
mg, 0.813 mmol), DhbtOH (133 mg, 0.813 mmol), triethyl-
amine (164 mg, 1.62 mmol) and DCC (168 mg, 0.813 mmol) in
DMF (5 ml). A similar work-up and purification procedure
yielded compound 14e (175 mg, 52%) as a mixture of rotamers;
colourless oil; νmax (neat)/cmϪ1 3447 (NH), 2981, 1743, 1704,
General procedure for the full deprotection of 14: {N-(2-amino-
ethyl)-N-[2-(2,4-difluorophenyl)acetyl]amino}acetic acid 16a
A clear solution of 14b (50 mg, 0.116 mmol) in a mixture of
dichloromethane (2 ml) and trifluoroacetic acid (1 ml) was
stirred at room temperature for 2 h. The reaction mixture was
dried in vacuo and the residual trifluoroacetic acid was removed
by co-evaporation twice with toluene. The residue was tritur-
ated with diethyl ether, filtered and dried to yield 16a (24 mg,
77%) as a mixture of rotamers; amorphous solid; mp 118–
1644 (C᎐O), 1507, 1247, 1174, 1109; δ (270 MHz, CDCl ) 1.21,
᎐
H
3
1.33 (total 3H, each t, J 7.2, OCH2CH3), 1.44, 1.38 (total 9H,
each s, But), 2.21, 2.24 (total 3H, each s, ArCH3), 3.19–3.71
(4H, m, N(CH2)2N ϩ ArCH2), 3.98, 4.17 (total 2H, each s,
NCH2CO), 4.26 (2H, q, J 7.2, 14.5, OCH2CH3), 5.02, 5.29 (2H,
each br s, NH × 2), 6.78 (1H, t, J 9.2, ArH), 7.22 (1H, m,
ArH); δF (254 MHz. CDCl3) Ϫ110.2, Ϫ110.4 (each m), Ϫ115.1,
Ϫ115.7 (each m); m/z (EI) 414 (Mϩ), 413 (Mϩ Ϫ 1), 357
(Mϩ Ϫ But); HRMS found Mϩ 414.1967. C20H28F2N2O5
requires 414.1974.
120 ЊC; νmax (KBr)/cmϪ1 3088 (NH3ϩ), 1727 (C᎐O), 1512, 1325
᎐
(COOϪ), 1194, 1142; δH (270 MHz, CD3OD) 3.13–3.70 (4H, m,
N(CH2)2N ), 3.70, 3.87 (total 2H, each s, ArCH2), 4.12, 4.31
(2H, each s, NCH2CO), 6.89 (2H, m, ArH), 7.26 (1H, m, ArH);
δF (254 MHz, CD3OD) Ϫ111.6, Ϫ112.1 (each m), Ϫ112.7 (q,
J 8.7, 17.9); m/z (EI) 272 (Mϩ), 271 (Mϩ Ϫ 1), 255 (Mϩ Ϫ OH),
254 [(Mϩ Ϫ 1) Ϫ OH]; HRMS found [(Mϩ Ϫ 1) Ϫ OH]
254.0867. C12H12F2N2O2 requires 254.0855.
General procedure for the selective deprotection of 14: {N-[2-(tert-
butoxycarbonylamino)ethyl]-N-[2-(2,4-difluorophenyl)acetyl]-
amino}acetic acid 15a
{N-(2-Aminoethyl)-N-[2-(pentafluorophenyl)acetyl]amino}-
acetic acid 16b. Compound 16b (32 mg, 95%) was obtained by a
similar treatment of 14d (50 mg, 0.103 mmol) with trifluoro-
acetic acid in dichloromethane as a mixture of rotamers;
amorphous solid; mp 129–130 ЊC; νmax (KBr)/cmϪ1 3127
(NH3ϩ), 1633 (C᎐O), 1514, 1334 (COOϪ), 1193, 1138; δ (270
MHz, CD3OD) 3.12 (2H, t, J 5.8, N(CH2CH2N ), 3.66 (2H, t,
J 5.8, NCH2CH2N ), 3.79, 3.98 (total 2H, each s, ArCH2),
4.02, 4.21 (2H, each s, NCH2CO); δF (254 MHz, CD3OD)
Ϫ142.6, Ϫ142.8 (each dd, J 8.4, 21.2), Ϫ157.4, Ϫ157.7 (each t,
J 20.3), Ϫ164.3, Ϫ164.6 (each m); m/z (EI) 326 (Mϩ), 309
(Mϩ Ϫ OH); HRMS found (Mϩ Ϫ OH) 309.0659. C12H10F5-
N2O2 requires 309.0662.
Compound 14a (50 mg, 0.125 mmol) was suspended in THF (2
ml) and 1 M LiOH (2 ml) was added. The mixture was stirred
for 1 h at room temperature. Water (2 ml) was added to the
solution and it was washed with dichloromethane (10 ml). Add-
itional water (2 ml) was added and the solution was washed
once more with dichloromethane (5 ml). The aqueous solution
was cooled to 0 ЊC and the pH was adjusted to 2 by dropwise
addition of 1 M HCl. A white precipitate was formed which
was collected by decantation or filtration. The precipitate was
dried to afford 15a (33 mg, 70%) as a mixture of rotamers;
amorphous solid; mp 134–135 ЊC; νmax (KBr)/cmϪ1 3441 (NH),
᎐
H
General procedure for the preparation of 17: {[3-(tert-butoxy-
carbonylamino)propyl]amino}acetic acid ethyl ester 17a
2931, 1705, 1602 (C᎐O), 1509, 1206, 1174; δ (270 MHz, CDCl )
᎐
H
3
1.43 (9H, s, But), 3.30–3.73 (6H, m, N(CH2)2N ϩ ArCH2),
3.73, 4.06 (total 2H, each s, NCH2CO), 4.14, 5.34 (total 2H,
each br s, NH × 2), 6.83 (2H, m, ArH), 7.26 (1H, m, ArH);
δF (254 MHz, CDCl3) Ϫ112.0, Ϫ112.4 (each m), Ϫ113.2,
Ϫ114.0 (each m); m/z (EI) 372 (Mϩ), 355 (Mϩ Ϫ OH), 315
(Mϩ Ϫ But); HRMS found Mϩ 372.1518. C17H22F2N2O5
requires 372.1497.
To a stirred solution of 18 (2.08 g, 11.4 mmol) in dichlorometh-
ane (10 ml) were added triethylamine (1.16 g, 11.4 mmol) and
potassium iodide (1.9 g, 11.4 mmol). Ethyl chloroacetate (1.17
ml, 11.4 mmol) was then added dropwise with stirring and the
mixture was stirred at 40 ЊC for 50 h. Water (50 ml) was added
and the mixture was agitated vigorously. The organic phase was
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611
1609

View Article Online
separated and the aqueous phase was extracted with dichloro-
methane (2 × 100 ml). The organic phases were combined,
washed with brine, dried over anhydrous Na2SO4, filtered and
concentrated in vacuo. The resulting crude product was
chromatographically purified on silica gel (ethyl acetate–
methanol 95 : 5) to yield 17a (1.25 g, 42%) as a pale yellow oil;
δH (270 MHz, CDCl3) 1.29 (3H, t, J 7.1, OCH2CH3), 1.44 (9H,
s, But), 1.67 (2H, m, CH2CH2CH2) 2.66 (2H, t, J 6.7,
CH2CH2N), 3.22 (2H, br d, J 4.3, NCH2CH2CO), 3.53 (2H, s,
NCH2CO), 4.17 (2H, q, J 7.1, 14.2, OCH2CH3), 4.85 (1H, br s,
NH), 5.05 (1H, br s, NH); m/z (EI) 260 (Mϩ), 261 (Mϩ ϩ 1), 204
(Mϩ Ϫ But); HRMS found Mϩ 260.1682. C12H24N2O4 requires
260.1706.
prepared from 17a (200 mg, 0.769 mmol), 19b (163 mg, 0.768
mmol), DhbtOH (125 mg, 0.768 mmol), DCC (159 mg, 0.768
mmol) in DMF (3 ml) and dichloromethane (2 ml) by a pro-
cedure analogous to 14a. A similar work-up and purification
gave 20c (189 mg, 51%) as a mixture of rotamers; colourless oil;
νmax (neat)/cmϪ1 3364 (NH), 2981, 1744, 1703, 1660 (C᎐O),
᎐
1503, 1251, 1171; δH (270 MHz, CDCl3) 1.81 (3H, t, J 7.2,
OCH2CH3), 1.39, 1.44 (total 9H, each s, But), 1.76 (2H, m,
CH2CH2CH2), 3.03–3.68 (4H, m, NCH2CH2CH2N ), 3.94,
4.17 (total 2H, each s, NCH2CO), 4.16 (2H, q, J 7.2, 14.2,
OCH2CH3), 4.47, 5.11 (total 2H, each br s, NH × 2); δF (254
MHz, CDCl3) Ϫ140.2, Ϫ140.5 (each br d, J 15.7), Ϫ151.3,
Ϫ151.9 (each t, J 19.4), Ϫ159.7 (m); m/z (EI) 454 (Mϩ), 397
(Mϩ Ϫ But), 409 (Mϩ Ϫ OC2H5); HRMS found Mϩ 454.1505.
C19H23F5N2O5 requires 454.1527.
{[3-(tert-Butoxycarbonylamino)propyl]amino}acetic acid tert-
butyl ester 17b. This compound was prepared analogously to
17a from 18 (2.25 g, 12.9 mmol), triethylamine (1.29 g, 12.9
mmol), potassium iodide (2.1 g, 12.9 mmol), tert-butyl chloro-
acetate (1.95 g, 12.9 mmol) in dichloromethane (10 ml). A simi-
lar work-up and purification procedure yielded 17b (1.7 g, 45%)
as a pale yellow oil; δH (270 MHz, CDCl3) 1.41 (9H, s, But), 1.44
(9H, s, But), 1.72 (2H, m, CH2CH2CH2), 2.66–3.21 (4H, m,
NCH2CH2CH2N),3.31(2H,s, NCH2CO),4.97(1H,brs,NH),
5.02 (1H, br s, NH); m/z (EI) 288 (Mϩ), 289 (Mϩ ϩ 1), 215
(Mϩ Ϫ But × 2); HRMS found Mϩ 288.2086. C14H28N2O4
requires 288.2065.
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(pentafluoro-
benzoyl)amino}acetic acid tert-butyl ester 20d. This compound
was prepared from 17b (200 mg, 0.694 mmol), 19b (147 mg,
0.694 mmol), DhbtOH (113 mg, 0.694 mmol), DCC (143 mg,
0.698 mmol) in DMF (3 ml) and dichloromethane (2 ml) by a
procedure analogous to 14a. A similar work-up and purifi-
cation procedure gave 20d (158 mg, 50%) as a mixture of
rotamers; pale yellow oil; νmax (neat)/cmϪ1 3451, 3340 (NH),
2980, 1706, 1659 (C᎐O), 1504, 1247, 1162; δ (270 MHz, CDCl )
᎐
H
3
1.40, 1.41 (total 9H, each s, But), 1.44, 1.46 (total 9H, s, But),
1.72 (2H, m, CH2CH2CH2), 3.03–3.65 (4H, m, NCH2CH2-
CH2N ), 3.82, 4.14 (total 2H, each s, NCH2CO), 4.34, 5.13
(total 2H, each br s, NH × 2); δF (254 MHz, CDCl3) Ϫ139.9,
Ϫ140.5 (each m), Ϫ151.6, Ϫ152.1 (each t, J 19.4), Ϫ159.9 (m);
m/z (EI) 482 (Mϩ), 481 (Mϩ Ϫ 1), 425 (Mϩ Ϫ But); HRMS
found Mϩ 482.1805. C21H27F5N2O5 requires 482.1830.
General procedure for the preparation of 20: {N-[3-(tert-butoxy-
carbonylamino)propyl]-N-(2,4-difluorobenzoyl)amino}acetic acid
ethyl ester 20a
This compound was prepared from 17a (200 mg, 0.769 mmol),
19a (121 mg, 0.768 mmol), DhbtOH (125 mg, 0.768 mmol),
triethylamine (156 mg, 1.53 mmol) and DCC (159 mg, 0.768
mmol) in DMF (5 ml) by a procedure analogous to 14a. A
similar work-up and purification yielded 20a (185 mg, 60%) as
a mixture of rotamers; pale yellow oil; νmax (neat)/cmϪ1 3356
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(2,4-difluoro-5-
methylbenzoyl)amino}acetic acid ethyl ester 20e. This com-
pound was prepared from 17a (200 mg, 0.769 mmol), 12 (132
mg, 0.768 mmol), DhbtOH (125 mg, 0.768 mmol), triethyl-
amine (156 mg, 1.53 mmol) and DCC (159 mg, 0.768 mmol)
in DMF (5 ml). A similar work-up and purification yielded 20e
(209 mg, 65%) as a mixture of rotamers; colourless oil; νmax
(NH), 2981, 1744, 1704, 1642 (C᎐O), 1510, 1369, 1267, 1171,
᎐
1101; δH (270 MHz, CDCl3) 1.21, 1.28 (total 3H, each t, J 7.2,
OCH2CH3), 1.39, 1.44 (total 9H, each s, But), 1.78 (2H, m,
CH2CH2CH2), 2.88–3.64 (4H, m, NCH2CH2CH2N ), 3.93,
4.21 (total 2H, each s, NCH2CO), 4.21 (2H, q, J 7.2, 14.2,
OCH2CH3), 4.95, 5.34 (total 2H, each br s, NH × 2), 6.88 (2H,
m, ArH), 7.37 (1H, m, ArH); δF (254 MHz, CDCl3) Ϫ106.9,
Ϫ107.2 (each m), Ϫ110.7, Ϫ111.4 (each q, J 8.3, 16.5); m/z (EI)
400 (Mϩ), 401 (Mϩ ϩ 1), 343 (Mϩ Ϫ But); HRMS found Mϩ
400.1830. C19H26F2N2O5 requires 400.1810. Found: C, 57.06; N,
7.09; H, 6.51. Calc. for C19H26F2N2O5: C, 56.99; N, 7.00; H,
6.54%.
(neat)/cmϪ1 3453 (NH), 2982, 1744, 1704, 1641 (C᎐O), 1507,
᎐
1248, 1176, 1026; δH (270 MHz, CDCl3) 1.21, 1.31 (total 3H,
each t, J 7.1, OCH2CH3), 1.39, 1.44 (total 9H, each s, But), 1.78
(2H, m, CH2CH2CH2), 2.22, 2.25 (total 3H, each s, ArCH3),
2.99–3.64 (4H, m, NCH2CH2CH2N ), 3.94, 4.17 (2H, each s,
NCH2CO), 4.27 (2H, q, J 7.1, 14.3, OCH2CH3), 4.43, 5.32
(total 2H, each br s, NH × 2), 6.82 (1H, m, ArH), 7.22 (1H, m,
ArH); δH (254 MHz, CDCl3) Ϫ110.2, Ϫ110.5 (each d, J 8.3),
Ϫ115.1, Ϫ115.7 (each q, J 8.3, 15.6); m/z (EI) 414 (Mϩ), 415
(Mϩ ϩ 1), 358 [(Mϩ ϩ 1) Ϫ But]; HRMS found Mϩ 414.1961.
C20H28F2N2O5 requires 414.1967. Found: C, 57.94; N, 6.75; H,
6.54. Calc. for C20H28F2N2O5: C, 57.96; N, 6.76; H, 6.81%.
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(2,4-difluoro-
benzoyl)amino}acetic acid tert-butyl ester 20b. This compound
was prepared analogously to 14a from 17b (200 mg, 0.694
mmol), 19a (110 mg, 0.693 mmol), DhbtOH (113 mg, 0.693
mmol), triethylamine (140 mg, 1.38 mmol) and DCC (143 mg.
0.693 mmol) in DMF (5 ml). A similar work-up and purific-
ation procedure yielded 20b (150 mg, 50%) as a mixture of
rotamers; colourless oil; νmax (neat)/cmϪ1 3344 (NH), 2980,
Selective deprotection of 20: {N-[3-(tert-butoxycarbonylamino)-
propyl]-N-(2,4-difluorobenzoyl)amino}acetic acid 21a
Compound 21a (29 mg, 52%) was obtained by a procedure
analogous to 15a from the selective basic hydrolysis of 20a (60
mg, 0.15 mmol) as a mixture of rotamers; white amorphous
solid; mp 69–70 ЊC; νmax (KBr)/cmϪ1 3356 (NH), 2981, 2936,
1739, 1704, 1642 (C᎐O), 1510, 1247, 1160; δ (270 MHz,
᎐
H
CDCl3) 1.38, 1.39 (total 18H, each s, But × 2), 1.77 (2H, m,
CH2CH2CH2), 2.82–3.63 (4H, m, NCH2CH2CH2N ), 3.81,
4.13 (total 2H, each s, NCH2CO), 4.68, 5.31 (total 2H, each
br s, NH × 2), 6.87 (2H, m, ArH), 7.37 (1H, t, J 7.4, ArH); δF
(254 MHz, CDCl3) Ϫ107.2 (m), Ϫ110.7, Ϫ111.3 (each m); m/z
(EI) 428 (Mϩ), 427 (Mϩ Ϫ 1), 371 (Mϩ Ϫ But); HRMS found
Mϩ 428.2123. C21H30F2N2O5 requires 428.2138. Found: C,
58.52; N, 6.48; H, 6.89. Calc. for C21H30F2N2O5: C, 58.87;
N, 6.54; H, 7.06%.
1744, 1702, 1642 (C᎐O), 1269, 1170; δ (270 MHz, CDCl ) 1.39,
᎐
H
3
1.44 (total 9H, each s, But), 1.72 (2H, m, CH2CH2CH2), 3.01–
3.74 (4H, m, NCH2CH2CH2N ), 3.95, 4.26 (total 2H, each s,
NCH2CO), 4.48, 5.31 (total 2H, each br s, NH × 2), 6.91 (2H,
m, ArH), 7.38 (1H, m, ArH); δF (254 MHz, CDCl3) Ϫ106.6,
Ϫ106.8 (each m), Ϫ110.5, Ϫ111.4 (each m); m/z (EI) 372 (Mϩ),
371 (Mϩ Ϫ 1), 355 (Mϩ Ϫ OH), 315 (Mϩ Ϫ But); HRMS found
Mϩ 372.1498. C17H22F2N2O5 requires 372.1497.
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(pentafluoro-
benzoyl)amino}acetic acid ethyl ester 20c. This compound was
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(pentafluoro-
benzoyl)amino}acetic acid 21b. Compound 21b (30 mg, 56%)
1610
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611

View Article Online
was obtained by a procedure analogous to 15a from the selec-
tive basic hydrolysis of 20c (60 mg, 0.124 mmol) as a mixture
of rotamers; white powder; mp 62–63 ЊC; νmax (KBr)/cmϪ1 3455
MHz, CD3OD) 1.98 (2H, m, CH2CH2CH2), 3.04–3.72 (4H, m,
NCH2CH2CH2N ), 4.08, 4.17 (total 2H, each s, NCH2CO);
δF (254 MHz, CD3OD) Ϫ140.6, Ϫ141.5 (each m), Ϫ152.9,
Ϫ153.1 (each m), Ϫ160.8, Ϫ161.1 (each m); m/z (EI) 326 (Mϩ),
309 (Mϩ Ϫ OH); HRMS found Mϩ 326.0724. C12H11F5N2O3
requires 326.0720.
(NH), 2930, 1708, 1658 (C᎐O), 1503, 1368, 1246, 1166; δH (270
᎐
MHz, CDCl3) 1.40, 1.44 (total 9H, each s, But), 1.73 (2H, m,
CH2CH2CH2), 3.06–3.67 (4H, m, NCH2CH2CH2N ), 3.97,
4.29 (total 2H, s × 2, NCH2CO), 4.56, 5.15 (total 2H, each
br s, NH × 2); δF (254 MHz, CDCl3) Ϫ140.1 (m), Ϫ151.6 (m),
Ϫ159.8 (m); m/z (EI) 426 (Mϩ), 425 (Mϩ Ϫ 1), 369 (Mϩ Ϫ But);
HRMS found Mϩ 426.1263. C17H19F5N2O5 requires 426.1245.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Science, Sports
and Culture, Japan. N. S. wishes to thank the Suntory Institute
for Bioorganic Research for support.
{N-[3-(tert-Butoxycarbonylamino)propyl]-N-(2,4-difluoro-5-
methylbenzoyl)amino}acetic acid 21c. Compound 21c (32 mg,
57%) was obtained by a procedure analogous to 15a from the
selective basic hydrolysis of 20e (60 mg, 0.144 mmol) as a mix-
ture of rotamers; white solid; mp 81–82 ЊC; νmax (KBr)/cmϪ1
References
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10 G. Breipohl, D. W. Will, A. Peyman and E. Uhlmann, Tetrahedron,
1997, 53, 14671.
1009; δH (270 MHz, CDCl3) 1.39, 1.44 (total 9H, each s, But),
1.76 (2H, m, CH2CH2CH2), 2.22, 2.26 (total 3H, each s,
ArCH3), 3.03–3.75 (4H, m, NCH2CH2CH2N ), 3.97, 4.26
(total 2H, each s, NCH2CO), 4.44, 5.31 (total 2H, each br s,
NH × 2), 6.82 (1H, m, ArH), 7.24 (1H, m, ArH); δF (254 MHz,
CDCl3) Ϫ112.2, Ϫ112.4 (each m), Ϫ117.3, Ϫ118.6 (each m);
m/z (EI) 386 (Mϩ), 387 (Mϩ ϩ 1), 329 (Mϩ Ϫ But); HRMS
found Mϩ 386.1680. C18H24F2N2O5 requires 386.1704. Found:
C, 55.56; N, 7.26; H, 6.11. Calc. for C18H24F2N2O5: C, 55.95;
N, 7.25; H, 6.26%.
Full deprotection of 20: [N-(3-aminopropyl)-N-(2,4-difluoro-
benzoyl)amino]acetic acid 22a
Compound 22a (37 mg, 97%) was obtained by a acid treatment
of 20b (60 mg, 0.14 mmol) similar to 16a as a mixture of
rotamers; sticky solid; νmax (neat)/cmϪ1 3066 (NH3ϩ), 1634
(C᎐O), 1471, 1430 (COOϪ), 1200, 1138; δ (270 MHz, CD OD)
᎐
H
3
1.88 (2H, m, CH2CH2CH2), 3.04–3.71 (4H, m, NCH2CH2-
CH2N ), 3.98, 4.24 (total 2H, each s, NCH2CO), 7.08 (2H,
m, ArH), 7.44 (1H, m, ArH); δF (254 MHz, CD3OD) Ϫ106.6,
Ϫ106.8 (each m), Ϫ110.9, Ϫ111.4 (each q, J 8.3, 17.6); m/z
(EI) 272 (Mϩ), 255 (Mϩ Ϫ OH); HRMS found Mϩ 272.0973.
C12H14F2N2O3 requires 272.0995.
11 I. L. Knunyants and G. G. Yakobson, Synthesis of Fluoroorganic
Compounds, Springer-Verlag, Berlin, New York, 1995.
12 R. Adams and A. F. Thal, Organic Synthesis, vol. 2, John Wiley &
Sons. Inc., London, 1992, pp. 63–65.
13 J. Podlech and D. Seebach, Liebigs Ann., 1995, 1217.
14 (a) T. L. Sheppard and R. C. Breslow, J. Am. Chem. Soc., 1996, 118,
9810; (b) T. L. Sheppard, A. T. Rosenblatt and R. C. Breslow, J. Org.
Chem., 1994, 59, 7243; (c) J. P. Dougherty, C. J. Rizzo and R. C.
Breslow, J. Am. Chem. Soc., 1992, 114, 6254.
[N-(3-Aminopropyl)-N-(pentafluorobenzoyl)amino]acetic acid
22b. Compound 22b (31 mg, 75%) was obtained by a similar
acid treatment of 20d (60 mg, 0.124 mmol) as a mixture of
rotamers; amorphous solid; mp 94–95 ЊC; νmax (KBr)/cmϪ1 3116
(NH3ϩ), 1658 (C᎐O), 1504, 1320 (COOϪ), 1201, 1142; δ (270
᎐
H
J. Chem. Soc., Perkin Trans. 1, 2001, 1605–1611
1611