The synthesis of novel heterocyclic substituted α-amino acids; further exploitation of α-amino acid alkynyl ketones as reactive substrates

Article abstract of DOI:10.1039/b001825m

A series of novel non-proteinogenic heterocyclic substituted α-amino acids derived from L-aspartic acid have been synthesised using the alkynyl ketone functionality as a versatile building block. Condensation of (S)-2-tert-butoxycarbonylamino-4-oxohex-5-y

Full text of DOI:10.1039/b001825m

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The synthesis of novel heterocyclic substituted ꢀ-amino acids;
further exploitation of ꢀ-amino acid alkynyl ketones as reactive
substrates
1
Robert M. Adlington, Jack E. Baldwin,* David Catterick, Gareth J. Pritchard† and
Lam T. Tang
The Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford,
UK OX1 3QY. E-mail: jack.baldwin@chem.ox.ac.uk
Received (in Cambridge, UK) 7th March 2000, Accepted 2nd May 2000
Published on the Web 30th June 2000
A series of novel non-proteinogenic heterocyclic substituted α-amino acids derived from -aspartic acid have
been synthesised using the alkynyl ketone functionality as a versatile building block. Condensation of (S)-2-tert-
butoxycarbonylamino-4-oxohex-5-ynoic acid tert-butyl ester 4 with enamines 5, 6, phenylhydrazine, hydroxylamine
and phenyl azide has led to the generation of pyridines 9, 10, pyrazolines 11a/b, isoxazoles 12a/b, and triazole 13,
respectively in moderate to excellent yields. Acid deprotection of the initial adducts afforded the desired hetero-
cyclic substituted α-amino acids as their TFA salts or in the form of the zwitterions themselves after ion-exchange
chromatography. The enantiomeric purity of a representative selection of these products were greater than 98% ee
19
as verified by derivatisation to the corresponding Mosher’s amides and subsequent F NMR spectroscopy.
Introduction
There has recently been an increasing interest in the develop-
ment of novel α-amino acids due to their diverse range of
biological and toxicological properties. Their importance is
exemplified not only by their role as constituents of peptides,
proteins, and peptidoglycans in bacterial cell-walls but also as
1
mediators of neuronal signal transduction. In particular the
2
heterocyclic α-amino acids azatyrosine 1, mimosine 2, and
lathyrine 3 have been observed to display antibiotic, antitumor,
3
wool growth and pollen growth inhibition activities (Fig. 1).
As part of an on going research program which focuses on
the harnessing of new reactive cores using a parallel synthesis
approach, we have developed a novel route to potentially bio-
4,5
logically active α-amino acid substrates. Through a retrosyn-
thetic analysis we identified that pyridinyl, triazolyl, isoxazolyl
and pyrazolyl heterocycles could all be derived from a common
5b
alkynyl ketone precursor, thus by application of a parallel
synthesis strategy we envisaged that these core structures could
all be accessed readily subsequent to the generation of the
common reactive species. A disconnection of these heterocycles
by our method led to three simple components comprised of a
dinucleophilic amphiphile, an acetylene and a carboxylic acid
Scheme 1
ment of a highly reactive alkynyl ketone intermediate and
its cyclocondensation reactions with selected bifunctional
nucleophiles.
(
Scheme 1). The obvious advantage of utilising this strategy is
that many variants of such α-amino acid heterocycles could be
easily accessible, by selecting the appropriately substituted
fragments, leading to the rapid generation of structurally
related compound libraries in a controlled manner.
Herein we fully describe a versatile stereoselective synthetic
route to a wide range of heterocyclic α-amino acids by involve-
Results and discussion
Initially
(S)-2-tert-butoxycarbonylamino-4-oxohex-5-ynoic
acid tert-butyl ester 4 was synthesised from the corresponding
Weinreb amide as described previously.
4
Bohlmann and Rahtz reported that cyclocondensations
between alkynyl ketones and suitable enamines allowed the
6
generation of functionalised pyridines in excellent yields. From
this evidence we believed that by reacting 4 with the appro-
priate enamines we would be able to access pyridine substituted
α-amino acids, which would bear relation to the naturally
occurring amino acid -azatyrosine. The stabilised enamines
methyl (Z)-3-aminobut-2-enoate 5 and (Z)-4-aminopent-3-
en-2-one 6 were thus prepared by condensation of ammonia
with methyl acetoacetate and acetoacetone respectively, in an
Fig. 1
†
Present address: The Department of Chemistry, Loughborough
University, Loughborough, Leicestershire, UK LE11 2NH.
7,8
attempt to investigate this possibility. Trial reactions were
DOI: 10.1039/b001825m
J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316 2311
This journal is © The Royal Society of Chemistry 2000

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carried out on 4 with either 5 or 6 in ethanol at room temper-
ature leading to the formation of single products. Proton NMR
spectroscopy of these products revealed that instead of the
expected cyclocondensations occurring to afford the desired
pyridines, the disubstituted trans-alkenes 7 and 8 had been
reaction of 4 with hydroxylamine hydrochloride in an ethanolic
10
solution with solid sodium carbonate allowed isolation of the
isoxazole 12a in poor yield (13%) as the only isoxazole isomer.
Variation of the solvent and base employed, to ethanol and 1.2
11
equivalents of pyridine, subsequently allowed the regio-
3
formed (olefinic J = 15.5, 16 Hz compared to ca. 8.0 Hz
isomeric isoxazoles 12a/b to be obtained in a moderate yield
14
for pyridines) by means of singular Michael additions to the
alkynyl ketone. The observation of trans addition to the triple
bond of the alkynyl ketone was consistent with that reported by
(51%), as a 1:3 separable mixture of regioisomers. A literature
search however revealed that Giacomelli and co-workers had
reported a successful cyclocondensation of hydroxylamine
upon an enamino ketone, albeit in low yield, by a simple reac-
9
Bromidge, with the Z-geometry of the enamine double bond
15
expected due to the possibility of hydrogen bonding. Bohlmann
tion with the hydrochloride salt in refluxing methanol. We
therefore investigated the possibility of cyclocondensations
under these conditions. Refluxing 4 with hydroxylamine hydro-
chloride in ethanol gratifyingly led to the formation of
isoxazole 12a exclusively in a respectable yield (62%) (Scheme
4). Assignment of the two regioisomers 12a/b were made by
6
and Rahtz had however carried out their cyclocondensations
at elevated temperatures and it was subsequently found that
by stirring an ethanolic solution of 4 with either enamine 5
or 6 at reflux, a high conversion to the pyridin-6-yl substituted,
protected amino acids 9 and 10 was observed (Scheme 2).
Scheme 2 Reagents and conditions: i, 5 or 6, EtOH, RT, 89% (7), 91%
Scheme 4 Reagents and conditions: i, H N-OHؒHCl, EtOH, pyridine,
2
(
8); ii, 5 or 6, EtOH, reflux, 81% (9), 91% (10).
reflux, 51%; ii, H N-OHؒHCl, EtOH, reflux, 62%.
2
Literature evidence also suggested that cyclocondensation of
1
13
1
comparison of their H and C NMR whereby the H and
4
with hydrazines and hydroxylamine should allow access to
13
C chemical shifts for C (H) of 12a were shown to occur at a
characteristically lower field than the corresponding C (H)
10,11
5
both pyrazole and isoxazole substituted amino acids.
There-
16
3
fore the reaction of (S)-2-tert-butoxycarbonylamino-4-oxohex-
-ynoic acid tert-butyl ester 4 with hydrazines was investigated
by refluxing an ethanolic solution of 4 and phenylhydrazine
hydrochloride in the presence of solid sodium carbonate.
This has encouragingly allowed the desired pyrazolyl substi-
tuted protected α-amino acids 11a/b to be isolated in satisfac-
tory yield, as an inseparable 1:1 mixture of regioisomers
resonances for 12b.
5
To diversify the scope of our compound library even further,
it was next decided to attempt the formation of a 1,2,3-triazolyl
substituted α-amino acid by a cycloaddition of 4 with an azide.
Furthermore, it had been shown that reaction of azides with
alkynyl ketones had led to exclusive addition at the acetylene.
Thus reaction of 4 with phenyl azide was carried out in reflux-
17
(
Scheme 3).
17
ing diethyl ether to generate the triazol-4-yl, protected amino
acid 13, in high yield (91%) (Scheme 5). Proton NMR spectro-
Scheme 5 Reagents and conditions: i, Ph-N , Et O, reflux, 91%.
3
2
scopy (CD OD) indicated that only a single regioisomer 13 was
3
generated with a high chemical shift value (δ 9.11 ppm) for the
H
C-5 aromatic proton. Assignment of the structure to the 1,4-
disubstituted regioisomer 13 is consistent with the preferred
mode of azide addition to acetylenic ketones as demonstrated
1
7
by Vereshchagin et al. This regiochemistry also accounts for
Scheme 3 Reagents and conditions: i, H N-NHPhؒHCl, Na CO , H O,
2
2
3
2
1
EtOH, reflux, 67%.
the H NMR spectrum of 13 in which the phenyl substituent
interacts with the triazole ring splitting the ortho protons with
respect to the meta-para protons. A singlet would have been
more likely expected if the phenyl substituent were in the more
sterically hindered position adjacent to the carbonyl group,
i.e. 1,5-disubstituted adduct.
Isoxazoles were the next class of compounds to be targeted
using our strategy since it is known that certain non-protein
α-amino acids such as ibotenic acid behave effectively as con-
18
12,13
formationally restricted glutamic acid analogues.
Initially
2
312 J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316

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The extent of racemisation during the syntheses of these
compounds was then determined by the formation of their
diastereomeric Mosher’s amides. This was achieved by selective
Boc-deprotection involving azeotropic distillation of the fully
whereas full deprotection of 9 had to be carried out by refluxing
in 1 M HCl. The free amino acids 14 (80%) and 15 (85%) were
then obtained, by ion-exchange chromatography, as solids in
high yields and the amino acids 16a/b (98%), 17a/b (96%) and
18 (100%) were isolated as their TFA salts, after diethyl ether
trituration, due to their low solubilities (Scheme 7).
protected products with TsOHؒH O–toluene, which gave the
2
corresponding free amines after a sodium bicarbonate wash.
The N-deprotected form were thereafter converted into both
Mosher’s amides by coupling with both (R)- and (S)-Mosher’s
acid chlorides in dichloromethane with pyridine and catalytic
DMAP (Scheme 6). Fluorine NMR analysis of these amides
then proved the enantiomeric purity to be greater than 98% ee
Conclusions
In conclusion we have shown that our methodology
4a,b
can be
readily expanded to other novel heterocyclic systems using a
parallel synthesis strategy. Further exploitation of the alkynyl
ketone reactive group has allowed rapid and efficient construc-
tion of a range of representative heterocyclic non-protein-
ogenic amino acids including analogues of the natural product
19
in a representative range of cases.
Facile deprotection of the amino acids 10, 11a/b, 12a/b and
3 was readily performed by their dissolution in TFA–anisole,
1

-azatyrosine. This has further highlighted the flexibility and
power of our approach with pyridinyl, pyrazolyl, isoxazolyl and
,2,3-triazolyl substituted amino acids being generated in
1
approximately 30% yield from -aspartic acid. Further routes to
novel heterocyclic non-protein amino acids, will be reported in
due course.
Experimental
Standard general procedures and techniques as described
4b
previously were employed.
(
S,5E,7Z)-8-Amino-2-(tert-butoxycarbonylamino)-4-oxo-7-
Scheme 6 Reagents and conditions: i, TsOHؒH O (1 equiv.), toluene,
azeotropic distillation; ii, NaHCO3 (aq), EtOAc; iii, (R)- or (S)-2-
methoxy-2-(trifluoromethyl)phenylacetyl chloride (1 equiv.), pyridine
methoxycarbonylnona-5,7-dienoic acid tert-butyl ester 7
2
To a stirred solution of 4 (130 mg, 0.44 mmol) in ethanol (5 ml),
(Z)-methyl 3-aminobut-2-enoate 5 (51 mg, 0.44 mmol) was
(
excess), DMAP (cat.), DCM.
Scheme 7 Reagents and conditions: i, TFA, anisole, DCM; ii, 1 M HCl, reflux; iii, Dowex 50X8-100 ion-exchange resin.
J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316
2313

View Article Online
added and the mixture was stirred at room temperature for 48
hours. The solvent was then concentrated in vacuo and the resi-
due was subjected to purification by flash chromatography
(Ar-CH), 132.50 (Ar-C), 138.77 (Ar-CH), 155.50 (Ar-C),
159.42, 160.33, (2 × C᎐O), 166.50 (Ar-C), 170.62 (C᎐O); m/z
ϩ
ϩ
(APCI, CI ) 417 [MNa , 5%], 283 [100], 239 [90] and 193 [23]
ϩ
(
SiO , EtOAc–petroleum ether (PE) 40–60; 1:1) to afford the
(Found: MH 395.2187, C H N O requires 395.2182)
2
20 31
2
6
title compound 7 (160 mg, 89%) as a pale yellow oil (Found: C,
(Found: C, 60.90; H, 7.84; N, 7.20. C H N O requires C,
60.90; H, 7.67; N, 7.10%).
20 30 2 6
5
8.24; H, 7.82; N, 6.79. C H N O requires C, 58.09; H, 8.00;
20
32
2
7
22
N, 6.57%) (R 0.2, EtOAc–PE 40–60; 1:1); [α] ϩ66.0 (c 0.98 in
CHCl ); νmax(thin film)/cm 3371br (N–H str), 3220w, 2979m
f
D
Ϫ1
(S)-2-tert-Butoxycarbonylamino-3-(5-acetyl-6-methylpyridin-
2-yl)propanoic acid tert-butyl ester 10
3
(
C–H str), 2934w, 1717s (C᎐O str), 1670m (C᎐O str), 1624m,
᎐ ᎐
1
1
560s, 1496s, 1457m, 1438m, 1393m, 1368s, 1319m, 1287s,
To a stirred solution of 4 (185 mg, 0.62 mmol) in ethanol (10
ml) was added (Z)-4-aminopent-3-en-2-one 6 (61.7 mg, 0.62
mmol). The reaction was set to reflux and stirred for 16 hours
before the solvent was concentrated in vacuo to afford a yellow
oil. The residue was taken up into ethyl acetate, washed with
saturated aqueous sodium bicarbonate (2 × 20 ml) and brine
252m, 1211s, 1156s; δH (200 MHz; CDCl ) 1.44 (9H, s,
3
C(CH ) ), 1.46 (9H, s, C(CH ) ), 2.31 (3H, s, 8-CH ), 2.99 (1H,
dd, J 4.0 and 17.5 Hz, 3-C(H)H), 3.25 (1H, dd, J 4.0 and 17.5
Hz, 3-C(H)H), 3.82 (3H, s, CH O), 4.40–4.47 (1H, m, 2-CH),
3
3
3
3
3
3
5
7
.62 (1H, d, J 9.0 Hz, NH), 6.52 (1H, d, J 15.5 Hz, CH᎐CH),
.64 (1H, d, J 15.5 Hz, CH᎐CH), 9.6–9.8 (1H, br, NH); δC
᎐
᎐
(
2 × 20 ml), dried over MgSO and concentrated in vacuo to
4
(
50.31 MHz; CDCl ) 22.03 (CH ), 27.75 (C(CH ) ), 28.18
3 3 3 3
give the crude product. Purification by flash column chromato-
graphy (SiO , 3:2; Et O–PE 40–60) yielded the title com-
(
C(CH ) ), 42.77 (3-CH ), 50.57 (2-CH), 50.83 (CH O), 79.54
3 3 2 3
2
2
(
C(CH ) ), 81.72 (C(CH ) ), 94.23 (C᎐C), 118.82, 140.14
3 3 3 3
pound 10 as a yellow oil (215 mg, 91%) (R 0.3, Et O–PE 40–60;
f
2
(
1
3
CH = CH), 156.02 (NC᎐O), 167.49 (C᎐C), 170.23, 171.29,
᎐ ᎐
ϩ ϩ
23
Ϫ1
3
:2); [α]D ϩ27.65 (c 1.15 in CHCl ); νmax(thin film)/cm 3360br
3
98.04 (3 × C᎐O); m/z (APCI, CI ) 395 [MH Ϫ H O, 100%],
᎐
2
(
N–H str), 2978m (C–H str), 1715s (C᎐O str), 1689s (C᎐O),
᎐ ᎐
40 [31], 296 [38], 295 [91] and 239 [26].
1
8
586s (C᎐C str), 1497s, 1392m, 1367s, 1257s, 1155s, 1057w,
᎐
48w; δ (200 MHz; CDCl ) 1.30 (9H, s, C(CH ) ), 1.34 (9H, s,
H
3
3 3
(
S,5E,7Z)-7-Acetyl-8-amino-2-(tert-butoxycarbonylamino)-4-
C(CH ) ), 2.49 (3H, s, ArCH ), 2.63 (3H, s, COCH ), 3.10–3.33
3
3
3
3
oxonona-5,7-dienoic acid tert-butyl ester 8
(
2H, m, 3-CH ), 4.45–4.55 (1H, m, 2-CH), 5.71 (1H, d, J 8.0
2
To a solution of 4 (0.120 g, 0.40 mmol) in ethanol (5 ml) was
added (Z)-4-aminopent-3-en-2-one 6 (0.040 g, 0.40 mmol)
and the resulting mixture was stirred for 48 hours at RT. Flash
Hz, NH), 7.02 (1H, d, J 8.0 Hz, Ar-CH), 7.85 (1H, d, J 8.0 Hz,
Ar-CH); δ (50.31 MHz; CDCl ) 24.63 (ArCH ), 27.67, 28.10
C
3
3
(2 × C(CH ) ), 29.05 (COCH ), 39.27 (3-CH ), 52.98 (2-CH),
3
3
3
2
chromatography (SiO , EtOAc–PE 40–60; 1:1) of the residue
79.39, 81.55 (2 × C(CH ) ), 120.88 (Ar-H), 130.58 (Ar-C, ipso),
2
3 3
left after removal of the solvent in vacuo yielded the title com-
pound 8 (0.145 g, 91%) as a pale yellow oil (Found: C, 60.59;
H, 8.13; N, 7.07. C H N O requires C, 60.44; H, 8.29; N,
137.51 (Ar-CH), 155.60, 157.98, 159.94 (2 × Ar-C, NHCO ),
2
ϩ ϩ
170.84 (CO ); m/z (APCI, CI ) 401 [MNa , 5%], 267 [100], 245
2
ϩ
[18] and 223 [44] (Found: MH 379.2218, C H N O requires
20
32
2
6
20 31
2
5
22
7
.01%) (R 0.25, EtOAc–PE 40–60; 1:1); [α] ϩ52.0 (c 0.95 in
379.2233) (Found: C, 63.71; H, 8.01; N, 7.22. C H N O
f
D
20 30 2 5
Ϫ1
CHCl ); νmax(thin film)/cm 3339br (N–H str), 2979m (C–H
requires C, 63.47; H, 7.99; N, 7.40%).
3
str), 1715s (C᎐O str), 1558s, 1368s, 1280m, 1155s, 1107m,
᎐
1
027w, 983m, 733w; δH (200 MHz; CDCl ) 1.42 (9H, s,
(S)-2-tert-Butoxycarbonylamino-3-(1-phenylpyrazol-3-yl)-
propanoic acid tert-butyl ester 11a and (S)-2-tert-butoxy-
3
C(CH ) ), 1.44 (9H, s, C(CH ) ), 2.25 (3H, s, CH ), 2.33 (3H, s,
3
3
3
3
3
CH ), 2.98 (1H, dd, J 4.5 and 17.5 Hz, 3-C(H)H), 3.23 (1H, dd,
carbonylamino-3-(2-phenylpyrazol-3-yl)propanoic acid tert-butyl
3
J 4.5 and 17.5 Hz, 3-C(H)H), 4.38–4.46 (1H, m, 2-CH), 5.57
ester 11b
(
1H, d, J 9.0 Hz, NH), 5.98 (1H, d, J 16.0 Hz, CH᎐CH), 7.74
᎐
To a stirred solution of 4 (148.5 mg, 0.50 mmol) in ethanol
(
1H, d, J 16.0 Hz, CH᎐CH); δ (50.31 MHz; CDCl ) 23.13
᎐
C
3
(
(
3 ml) was added water (18 µl), phenyl hydrazine hydrochloride
72.3 mg, 0.50 mmol) and sodium carbonate (64 mg, 0.60
(
(
1
1
CH ), 27.78 (C(CH ) ), 28.21 (C(CH ) ), 29.86 (CH ), 42.65
3 3 3 3 3 3
3-CH ), 50.53 (2-CH), 79.64 (C(CH ) ), 81.77 (C(CH ) ),
2
3
3
3
3
mmol). The reaction was then heated to reflux and stirred for
four hours before being concentrated in vacuo. The residue was
taken into ethyl acetate, washed with saturated aqueous sodium
bicarbonate (2 × 20 ml) and brine (2 × 20 ml), dried over
04.98 (C᎐C), 120.83 (᎐CH), 142.65 (᎐CH), 156.00 (NC᎐O),
᎐ ᎐ ᎐ ᎐
67.07 (C᎐C), 171.08, 197.13, 198.11 (3 × C᎐O); m/z (APCI,
᎐
᎐
ϩ
ϩ
CI ) 379 [MH Ϫ H O, 100%], 324 [45], 279 [15], 267 [17] and
2
2
23 [12].
MgSO and concentrated in vacuo to give a crude product.
4
Purification by flash column chromatography (SiO , from 3:1
2
(
S)-2-tert-Butoxycarbonylamino-3-(5-methoxycarbonyl-6-
to 2:1; Et O–PE 40–60) yielded the title compounds 11a and
2
methylpyridin-2-yl)propanoic acid tert-butyl ester 9
1
1b (overall 130 mg, 67%, as a 1:1 inseparable mixture of
Ϫ1
To a stirred solution of 4 (160 mg, 0.54 mmol) in ethanol (10
ml) was added methyl (Z)-3-aminobut-2-enoate 5 (62 mg, 0.54
mmol). The reaction was set to reflux and stirred for 16 hours
before the solvent was concentrated in vacuo to afford a yellow
oil. The residue was taken up into ethyl acetate, washed with
saturated aqueous sodium bicarbonate (2 × 20 ml) and brine
regioisomers) as a brown oil; νmax (thin film)/cm 3438w, 3281w
(NH), 2978s, 2933w (CH), 1715br s (C᎐O), 1603s, 1505s, 1367s,
1253s, 1155s, 1056m, 847w; δH (200 MHz; CDCl both regio-
3,
isomers corresponding to 58H) 1.42 (18H, s, 2 × C(CH ) ), 1.43
3
3
(18H, s, 2 × C(CH ) ), 2.8–3.0 (2H, m, CH ), 3.19 (2H, br d,
3
3
2
J 5.0, CH ), 4.45–4.60 (2H, br m, 2 × CH), 5.38 (1H, br d, J 8.5
2
(
2 × 20 ml), dried over MgSO and concentrated in vacuo to give
Hz, NH), 5.46 (1H, br d, J 8.5 Hz, NH), 6.29 (1H, d, J 2.5 Hz,
ArH), 6.85–6.92 (1H, m, ArH), 7.02–7.06 (2H, m, ArH), 7.22–
7.30 (3H, m, ArH), 7.39–7.46 (3H, m, ArH), 7.6–7.7 (2H, m,
ArH), 7.83 (1H, d, J 2.5 Hz, ArH), 8.39 (1H, br s, ArH);
δC (125.8 MHz; CDCl both regioisomers) 27.93 (C(CH ) ),
4
the crude product. Purification by flash column chromato-
graphy (SiO , 2:3; Et O–PE 40–60) yielded the title compound
2
2
9
as a yellow oil (173 mg, 81%) (R 0.25, Et O–PE 40–60; 2:3);
f 2
23 Ϫ1
D 3
[α] ϩ26.13 (c 0.75 in CHCl ); νmax(thin film)/cm 2978 (C–H
3
3 3
str), 1726s (C᎐O str), 1591m (C᎐C str), 1497m, 1393m, 1367s,
28.30 (C(CH ) ), 31.04 (CH ), 38.04 (CH ), 51.85 (CH), 53.32
᎐
᎐
3 3 2 2
1
278s, 1154s, 1085s, 849w; δH (200 MHz; CDCl ) 1.35 (9H, s,
(CH), 79.47 (C(CH ) ), 79.64 (C(CH ) ), 81.69 (C(CH ) ), 81.98
3 3 3 3 3 3
3
C(CH ) ), 1.40 (9H, s, C(CH ) ), 2.77 (3H, s, ArCH ), 3.15–3.37
(C(CH ) ), 107.56, 113.01, 118.72, 120.74 (4 × Ar-CH), 121.55
3
3
3
3
3
3 3
(
5
2H, m, 3-CH ), 3.88 (3H, s, OCH ), 4.50–4.60 (1H, m, 2-CH),
.75 (1H, d, J 8.5 Hz, NH), 7.04 (1H, d, J 8.0 Hz, Ar-CH), 8.09
(Ar-C, ipso), 126.08, 127.25, 129.25, 129.32 (4 × Ar-CH),
139.90, 143.20, 151.00 (3 × Ar-C, ipso), 155.27, 170.54, 170.84
2
3
ϩ
ϩ
(
(
(
1H, d, J 8.0 Hz, Ar-CH); δC (50.31 MHz; CDCl ) 24.78
ArCH ), 27.85, 28.28 (2 × C(CH ) ), 39.46 (3-CH ), 52.12
OCH ), 53.03 (2-CH), 79.48, 81.60 (2 × C(CH ) ), 120.89
(3 × C᎐O); m/z (APCIϩ) 388 (MH , 35%), 332 [MH Ϫ
3
᎐
ϩ
(C H ), 30] and 276 [MH Ϫ 2 × (C H ), 100]; HRMS found
MH 388.2236; C H N O requires 388.2236.
3
3
3
2
4
8
ϩ
4
8
3
3
3
21 30 3 4
2
314
J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316

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(
S)-2-tert-Butoxycarbonylamino-3-(isoxazol-3-yl)propanoic
129.83, 130.19 (4 × Ar-CH), 136.48, 147.97 (2 × Ar-C, ipso),
ϩ
acid tert-butyl ester 12a
155.75, 170.71, 192.73 (3 × C᎐O); m/z (APCIϩ) 417 (MH ,
᎐
ϩ
ϩ
7
1
4
%), 361 [MH Ϫ (C H ), 25] and 305 (MH Ϫ 2 × (C H ),
4 8 4 8
ϩ
00); HRMS found MH 417.2138; C H N O requires
To a stirred solution of 4 (59.4 mg, 0.20 mmol) in ethanol (2 ml)
was added hydroxylamine hydrochloride (13.9 mg, 0.20 mmol).
The reaction was then heated to reflux and stirred overnight
before being concentrated in vacuo. The residue was taken into
ethyl acetate, washed with saturated aqueous sodium bicarb-
21
29
4
5
17.2138.
General procedure for selective Boc deprotection and Mosher’s
amide formation
onate (2 × 10 ml) and brine (2 × 10 ml), dried over MgSO and
4
concentrated in vacuo to give a crude product. Purification by
Typically, to a solution of the protected amino acid (1 equiv.),
in toluene, was added toluene-p-sulfonic acid monohydrate
(1 equiv.). The toluene was then gradually removed in vacuo. To
the residue was added toluene and the process repeated
approximately 10 further times. The resulting residue was taken
into ethyl acetate before being washed with saturated aqueous
sodium bicarbonate solution and brine, dried over MgSO4
and concentrated in vacuo. To a solution of the resulting free
amine in DCM was added either (R)- or (S)-2-methoxy-2-
(trifluoromethyl)phenylacetyl chloride (1 equiv.), excess pyri-
dine and catalytic DMAP. After being stirred overnight the
reaction mixture was concentrated in vacuo, taken into ethyl
acetate, washed with saturated aqueous sodium bicarbonate
solution and brine, dried over MgSO4 and concentrated in
flash column chromatography (SiO , from 1:3; EtOAc–PE 40–
2
6
0) yielded the title compound 12a (38.5 mg, 62%) as a white
26
solid; mp 95–98 ЊC; [α] ϩ23.1 (c 0.5 in CHCl ); ν (thin
D
3
max
Ϫ1
disc)/cm 3362br w (NH), 2980m, 2934w (CH), 1715br s
C᎐O), 1503m, 1368s, 1251m, 1155s, 1054w, 848w; δH (200
(
᎐
MHz; CDCl ) 1.43 (9H, s, C(CH ) ), 1.44 (9H, s, C(CH ) ), 3.20
3
3
3
3 3
(
2H, br d, J 5.5 Hz, CH ), 4.49–4.59 (1H, br m, CH), 5.30 (1H,
2
br d, J 6.0 Hz, NH), 6.28 (1H, d, J 1.5 Hz, ArH), 8.35 (1H,
d, J 1.5 Hz, ArH); δC (125.8 MHz; CDCl ) 27.85 (C(CH ) ),
3
3 3
2
8
8.26 (C(CH ) ), 29.06 (CH ), 52.54 (CH), 79.90 (C(CH ) ),
3
3
2
3
3
2.58 (C(CH ) ), 104.66 (Ar-CH), 155.00 (HNC᎐O), 158.34
3
3
ϩ
(
Ar-CH), 170.04 (C᎐O); m/z (APCIϩ) 313 (MH , 4%), 257
᎐
ϩ ϩ
[
[
MH Ϫ (C H ), 10], 213 [MH Ϫ (CO ϩC H ), 70], 157
4 8 2 4 8
ϩ ϩ
19
MH Ϫ (CO ϩ2 × C H ), 100]; HRMS found MH 313.1761;
vacuo to yield the crude product for F NMR analysis.
2
4
8
C H N O requires 313.1763.
1
5
25
2
5
(
S)-3-(5-Methoxycarbonyl-6-methylpyridin-2-yl)-2-amino-
propanoic acid tert-butyl ester. δ (200 MHz; CDCl ) 1.39 (9H,
(
S)-2-tert-Butoxycarbonylamino-3-(isoxazol-3-yl)propanoic acid
tert-butyl ester 12a and (S)-2-tert-butoxycarbonylamino-3-
isoxazol-5-yl)propanoic acid tert-butyl ester 12b
H
3
s, C(CH ) ), 2.79 (3H, s, ArCH ), 3.13 (1H, dd, J 7.5 and 15.0
3
3
3
Hz, 3-C(H)H), 3.31 (1H, dd, J 4.5 and 15.0 Hz, 3-C(H)H), 3.91
3H, s, OCH ), 4.01 (1H, dd, J 4.5 and 7.5 Hz, 2-CH), 7.08 (1H,
(
(
3
To a stirred solution of 4 (50.5 mg, 0.17 mmol) in ethanol (2 ml)
was added hydroxylamine hydrochloride (11.8 mg, 0.17 mmol)
and pyridine (19.5 µl, 0.24 mmol). The reaction was then heated
to reflux and stirred overnight before being concentrated in
vacuo. The residue was taken into ethyl acetate, washed with
saturated aqueous sodium bicarbonate (2 × 10 ml) and brine
d, J 8.0 Hz, Ar-CH), 8.13 (1H, d, J 8.0 Hz, Ar-CH).
δF of (R,S)-Mosher’s amide using (R)-Mosher’s acid chloride
Ϫ71.03 ppm, δF of (SS)-Mosher’s amide using (S)-Mosher’s
acid chloride Ϫ69.79 ppm.
(
S)-3-(5-Acetyl-6-methylpyridin-2-yl)-2-aminopropanoic
(
2 × 10 ml), dried over MgSO and concentrated in vacuo to
4
acid tert-butyl ester. δH (200 MHz; CDCl ) 1.42 (9H, s,
3
give a crude product. Purification by flash column chromato-
C(CH ) ), 2.58 (3H, s, ArCH ), 2.72 (3H, s, COCH ), 3.07 (1H,
3
3
3
3
graphy (SiO , from 2:7; EtOAc–PE 40–60) yielded the title
2
dd, J 7.5 and 14.5 Hz, 3-C(H)H), 3.27 (1H, dd, J 5.0 and 14.5
Hz, 3-C(H)H), 3.91 (1H, dd, J 5.0 and 7.5 Hz, 2-CH), 7.12 (1H,
d, J 8.0 Hz, Ar-CH), 7.92 (1H, d, J 8.0 Hz, Ar-CH).
δF of (RS)-Mosher’s amide using (R)-Mosher’s acid chloride
Ϫ71.01 ppm, δF of (SS)-Mosher’s amide using (S)-Mosher’s
acid chloride Ϫ69.77 ppm.
compounds 12a and 12b (overall 27.1 mg, 51%, as an insepar-
able 1:3 mixture of regioisomers) as a white solid; data for 12b,
δH (200 MHz; CDCl ) 1.42 (9H, s, C(CH ) ), 1.44 (9H, s,
3
3 3
C(CH ) ), 3.28–3.34 (2H, br m, CH ), 4.47–4.55 (1H, br m,
3
3
2
CH), 5.24 (1H, br d, J 8.0 Hz, NH), 6.09 (1H, d, J 1.5 Hz,
ArH), 8.17 (1H, d, J 1.5 Hz, ArH); δC (125.8 MHz; CDCl3)
2
8
1
7.84 (C(CH ) ), 28.25 (C(CH ) ), 29.71 (CH ), 52.46 (CH),
3 3 3 3 2
(S)-4-(1-Phenyl-1,2,3-triazol-4-yl)-4-oxo-2-aminobutanoic
acid tert-butyl ester, toluene-p-sulfonate salt. δH (200 MHz;
CDCl ) 1.31 (9H, s, C(CH ) ), 2.10 (3H, s, CH ), 3.85–4.01 (2H,
0.07 (C(CH ) ), 82.94 (C(CH ) ), 101.97, 150.21 (2 × Ar-CH),
3
3
3 3
55.01, 158.71, 169.58 (Ar-C, ipso; 2 × C᎐O).
᎐
3
3
3
3
m, CH ), 4.54–4.62 (1H, m, CH), 6.87 (2H, d, J 8.0 Hz, ArH),
2
(
S)-2-tert-Butoxycarbonylamino-4-(1-phenyl-1,2,3-triazol-4-
7.32–7.44 (3H, m, ArH), 7.58 (2H, d, J 8.0 Hz, ArH), 7.65–7.70
ϩ
yl)-4-oxobutanoic acid tert-butyl ester 13
(2H, m, ArH), 8.21 (3H, br s, NH ), 8.89 (1H, s, ArH).
3
δF of (RS)-Mosher’s amide using (R)-Mosher’s acid chloride
Ϫ69.24 ppm, δF of (SS)-Mosher’s amide using (S)-Mosher’s
acid chloride Ϫ69.74 ppm.
To a stirred solution of 4 (148.5 mg, 0.50 mmol) in Et O (5 ml)
2
was added phenyl azide (59.5 mg, 0.5 mmol) and the reaction
mixture heated to reflux for 24 hours. The solvent was then
removed in vacuo to generate a crude product. Purification by
flash column chromatography (SiO , 4:5; Et O–PE 40–60)
General procedure for the amino acid deprotection and
purification
2
2
yielded the title compound 13 (189 mg, 91%) as a white crystal-
22
line solid; mp 116–117 ЊC; [α]D 32.9 (c 1.74 in CHCl ) (Found: C,
Typically, to a stirred solution of the protected compounds, in
trifluoroacetic acid and dichloromethane was added anisole (ca.
3% v/v). The reaction mixture was stirred at room temperature
overnight before being concentrated in vacuo and triturated
3
6
0.43; H, 6.83; N, 13.35. C H N O requires C, 60.57; H, 6.78;
21
28
Ϫ1
4
5
N, 13.45%); νmax (thin disc)/cm 3372br w (NH), 2979m (CH),
708s (C᎐O), 1506s, 1368s, 1257m, 1155s, 1020m, 847w; δ (200
1
᎐
H
MHz; CDCl ) 1.45 (18H, s, 2 × C(CH ) ), 3.66 (1H, dd, J 4.5
with Et O to give a crude TFA salt. This could then be purified
by ion-exchange chromatography using Dowex 50X8-100 ion-
exchange resin. The crude TFA salts were loaded in aqueous
solution and eluted using 2 M aqueous ammonia solution.
3
3
3
2

Hz, 18.0, CH(H)), 3.86 (1H, dd, J 4.5 and 18.0 Hz, CH(H)),
.63–4.72 (1H, m, CH), 5.53 (1H, d, J 8.0 Hz, NH), 7.51–7.64
3H, m, ArH), 7.74–7.80 (2H, m, ArH), 8.52 (1H, s, ArH);
δH (200 MHz; CD OD) 1.42 (18H, s, 2 × C(CH ) ), 3.42–3.68
4
(
3
3 3
(
2H, m, CH ), 4.61 (1H, t, J 6.0 Hz, CH), 7.51–7.63 (3H, m,
(S)-3-(5-Carboxy-6-methylpyridin-2-yl)-2-aminopropanoic
acid 14. When the usual procedure was operated on 9 (100 mg,
0.25 mmol), 52 mg of a (2:1; α-amino acid ester–amino-diacid)
mixture was obtained. Thus the mixture was further subjected
2
ArH), 7.86–7.91 (2H, m, ArH), 9.11 (1H, s, ArH); δC (50.3
MHz; CDCl ) 27.74 (C(CH ) ), 28.18 (C(CH ) ), 42.00 (CH ),
5
3
3
3
3
3
2
0.05 (CH), 79.75 (C(CH ) ), 82.14 (C(CH ) ), 120.95, 123.81,
3 3 3 3
J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316
2315

View Article Online
ϩ
ϩ
to reflux in 1 M HCl (4 ml) aqueous solution for 24 hours to
give 57 mg of the amino-diacid hydrochloride salt on evapor-
ation of the solvent under vacuum. The free amino-diacid was
liberated using ion-exchange chromatography to afford the title
compound 14 as a white powder (45 mg, 80%); νmax(thin disc)/
(APCIϩ) 157 (MH , 20%) and 141 (100); HRMS found MH
157.0617; C H N O requires 157.0613.
6
9
2
3
(
S)-4-(1-Phenyl-1,2,3-triazol-4-yl)-4-oxo-2-aminobutanoic
acid trifluoroacetate 18. Compound 18 was prepared from 13
(25 mg, 0.06 mmol), TFA (0.5 ml), anisole (50 µl) and DCM (2
ml), followed by ether tituration to yield the TFA salt 18 (22.4
mg, 100%) as a white solid; mp 156–158 ЊC (decomp.); [α] ϩ7.3
c 0.22 in 1 M NaOH); νmax (KBr)/cm 3520–2400br m (NH/
OH), 2958m (CH), 1735s, 1697s (C᎐O), 1655s, 1538m, 1507s,
393m, 1209s, 1183s, 1136s, 1024w, 850w; δH (500 MHz; D O)
.75–3.85 (2H, m, CH ), 4.43–4.45 (1H, m, CH), 7.48–7.54 (3H,
m, ArH), 7.68–7.69 (2H, m, ArH), 8.99 (1H, s, ArH); δ (125.8
Ϫ1
cm 3043br, 1588s (C᎐O str), 1398s, 1104m, 969w, 864w, 825w,
7
89w, 534w; δH (200 MHz; D O) 2.34 (3H, s, CH ), 2.98 (1H,
2
3
2
2
dd, J 8.0 and 15.0 Hz, 3-C(H)H), 3.17 (1H, dd, J 5.0 and 15.0
Hz, 3-C(H)H), 3.87 (1H, dd, J 5.0 Hz and 8.0 Hz, 2-CH), 7.01
D
Ϫ1
(
(
(
1H, d, J 8.0 Hz, Ar-CH), 7.55 (1H, d, J 8.0 Hz, Ar-CH); δC
50.31 MHz; D O) 21.65 (CH ), 37.12 (3-CH ), 54.80 (2-CH),
᎐
1
2
2
3
2
3
1
1
22.19 (Ar-CH), 134.31 (Ar-C), 137.29 (Ar-CH), 154.45,
54.77 (2 × Ar-C), 173.79 (CO H), 176.50 (CO H); m/z (APCI,
2
C
2
2
ϩ
ϩ
MHz; D O), 39.01 (CH ), 48.48 (CH), 121.29, 126.87, 129.93,
30.10 (4 × Ar-CH), 135.67, 145.66 (2 × Ar-C, ipso), 171.39,
91.24 (2 × C᎐O); m/z (APCIϩ) 261 (MH , 20%), 172 (70) and
CI ) 225 [MH , 35%] and 152 [100].
2
2
1
1
1
ϩ
(
S)-3-(5-Acetyl-6-methylpyridin-2-yl)-2-aminopropanoic
᎐
02 (100).
acid 15. Carrying out the TFA–anisole procedure followed by
ion exchange on 10 (100 mg, 0.26 mmol) afforded 15 (50 mg,
Ϫ1
8
1
1
2
5%) as a yellow powdery solid; νmax(thin disc)/cm 3168br,
Acknowledgements
We thank the EPSRC and F. Hoffmann La Roche for student-
ships to D. C. and L. T. respectively and the EPSRC mass spec-
trometry service (Swansea) for high resolution mass spectra.
684s (C᎐O str), 1587s (C᎐O str), 1500s, 1420s, 1360s, 1295m,
264s, 1146m, 1097w, 1061m, 972m, 934w; δ (200 MHz; D O)
.39 (6H, s, 2 × CH ), 3.00–3.22 (2H, m, 3-CH ), 3.90 (1H, dd,
J 5.5 and 7.0 Hz, 2-CH), 7.08 (1H, d, J 8.0 Hz, Ar-CH), 7.92
᎐
᎐
H
2
3
2
(
1H, d, J 8.0 Hz, Ar-CH); δ (50.31 MHz; D O) 23.46 (ArCH ),
C 2 3
References
2
1
1
5
9.40 (COCH ), 37.24 (3-CH ), 54.64 (2-CH), 122.00 (Ar-CH),
3
2
32.17 (Ar-C), 139.60 (Ar-CH), 157.76, 158.61 (2 × Ar-C),
1
R. O. Duthaler, Tetrahedron, 1994, 50, 1539.
ϩ
ϩ
73.68 (CO H), 206.17 (COCH ); m/z (APCI, CI ) 223 [MH ,
2 A. G. Myers and J. L. Gleason, J. Org. Chem., 1996, 61, 813; for a
2
3
7%], 177 [100] and 149 [82].
recent synthesis and references cited therein.
3
G. A. Rosenthal, Plant Nonprotein Amino and Imino Acids
Biological, Biochemical and Toxicological Properties, Academic
Press, New York, 1982, p. 78 for mimosine and p. 117 for lathyrine.
(a) R. M. Adlington, J. E. Baldwin, D. Catterick and G. J. Pritchard,
Chem. Commun., 1997, 1757; (b) R. M. Adlington, J. E. Baldwin,
D. Catterick and G. J. Pritchard, J. Chem. Soc., Perkin Trans. 1,
1999, 855.
(
S)-3-(1-Phenylpyrazol-3-yl)-2-aminopropanoic
acid
tri-
fluoroacetate 16a and (S)-3-(2-phenylpyrazol-3-yl)-2-amino-
propanoic acid trifluoroacetate 16b. Compounds 16a and 16b
were prepared from 11a/b (as a 1:1 mixture of regioisomers)
(
(
(
νmax (KBr)/cm 3600–2400br m (NH/OH), 3060m (CH), 1676s
(
MHz; D O, 20H for both 16a/b) 3.21–3.31 (4H, s, CH (16a and
1
Hz, ArH), 7.28–7.59 (12H, m, ArH), 8.03 (1H, d, J 2.5 Hz,
ArH); δ (125.8 MHz; D O), 28.59 (CH ), 53.74 (CH), 107.57,
1
1
8
4
31 mg, 0.08 mmol), TFA (0.5 ml), anisole (50 µl) and DCM
2 ml), followed by ether tituration to yield the TFA salt 16a/b
5
(a) R. M. Adlington, J. E. Baldwin, D. Catterick and G. J. Pritchard,
J. Chem. Soc., Perkin Trans. 1, 2000, 299; (b) R. M. Adlington,
J. E. Baldwin, D. Catterick, G. J. Pritchard and L. T. Tang, J. Chem.
Soc., Perkin Trans. 1, 2000, 303.
27 mg, 98%) as a dark brown solid; mp 75–81 ЊC (decomp.);
Ϫ1
C᎐O), 1600s, 1501s, 1400w, 1319w, 1202s, 1054w, 800w; δ (500
᎐
H
6 F. Bohlmann and D. Rahtz, Chem. Ber., 1957, 90, 2265.
7 O. Mumm and E. Gottschaldt, Chem. Ber., 1922, 55, 2064;
C. Mentzer, M. Billet, D. Motho and D. Xuong, Bull. Soc. Chim.
Fr., 1945, 12, 161.
2
2
6b)), 4.11–4.13 (2H, m, CH(16a and 16b)), 6.40 (1H, d, J 2.5
8
A. G. H. Wee, A. Y. L. Shu, E. Bunnenberg and C. Djerassi, J. Org.
Chem., 1984, 49, 3327.
S. M. Bromidge, D. A. Entwhistle, J. Goldstein and B. S. Orlek,
Synth. Commun., 1993, 23, 487.
C
2
2
14.91, 119.65, 123.93, 125.53, 127.15, 129.39, 129.51, 129.60,
9
ϩ
30.32, 139.20, 148.44, 172.65 (C᎐O); m/z (APCIϩ) 232 (MH ,
᎐
ϩ
0%), 197 (50) and 94 (100); HRMS found MH 232.1084;
10 K. Bowden and E. R. H. Jones, J. Chem. Soc., 1946, 953.
C H N O requires 232.1086.
11 K. M. Johnston and R. G. Shotter, J. Chem. Soc. (C), 1968, 1774.
12
14
3
2
1
1
1
2 U. Madsen, M. A. Dumpis, H. Brauner-Osborne and L. B.
Piotrovsky, Bioorg. Med. Chem. Lett., 1998, 8, 1563.
3 M. Falorni, G. Giacomelli and E. Spanu, Tetrahedron Lett., 1998,
(
S)-3-(Isoxazol-3-yl)-2-aminopropanoic acid trifluoroacetate
7a and (2S)-3-(isoxazol-5-yl)-2-aminopropanoic acid trifluoro-
acetate 17b. Compounds 17a and 17b were prepared from 12a/b
1
3
9, 9241.
4 The communication, reference 5b, incorrectly states the ratio of
(
(
as a 1:3 mixture of regioisomers) (15.6 mg, 0.05 mmol), TFA
0.5 ml), anisole (50 µl) and DCM (2 ml), followed by ether
12a/b as 3:1.
15 M. Falorni, G. Giacomelli and A. M. Spanedda, Tetrahedron:
Asymmetry, 1998, 9, 3039.
16 P. Grünanger and P. Vita-Finzi, The Chemistry of Heterocyclic
Compounds, Vol 49: Isoxazols, part one, J. Wiley and Sons, New
York, 1991, pp. 24–54.
17 L. I. Vereshchagin, L. G. Tikhonova, A. V. Maksikova, E. S.
Serebryakova, A. G. Proidakov and T. M. Filippova, Zh. Org.
Khim., 1980, 16, 730, (English edition, 1980, 16, 641); Chem. Abstr.,
tituration to yield the TFA salt 17a/b (13 mg, 96%) as a white
solid; mp 205–209 ЊC (decomp.); νmax (KBr)/cm 3540–2480br
m, 3504m (c H, O H, NH), 1735s, 1690s (C᎐O), 1594m, 1454w,
1
3
4
ArH(17a)), 6.39 (1H, d, J 1.5 Hz, ArH(17b)), 8.30 (1H, d, J 2.0
Hz, ArH(17a)), 8.50 (1H, d, J 1.5 Hz, ArH(17b)); δC (125.8
MHz; D O) 26.80, 27.38 (CH ), 52.95, 53.09 (CH), 103.40,
Ϫ1
᎐
177m, 1053w, 818w; δH (500 MHz; D O 17a:17b (1:3)) 3.20–
2
.30 (2H, m, CH (17a)), 3.32–3.42 (2H, m, CH (17b)), 4.01–
2
2
.03 (2H, m, CH(17a and 17b)), 6.29 (1H, d, J 2.0 Hz,
9
3: 114405m.
1
8 P. Ykman, G. L. Abbé and G. Smets, Tetrahedron, 1971, 27, 5623;
G. Garcia-Muñoz, R. Madroñero, M. Rico and M. C. Saldaña,
J. Heterocycl. Chem., 1969, 6, 921.
2
2
1
04.92, 151.47 (3 × Ar-CH), 158.51 (Ar-C, ipso), 160.35 (Ar-
1
9 J. A. Dale, D. L. Dull and H. S. Mosher, J. Org. Chem., 1969, 34,
2543.
CH), 166.79, 172.47, 172.75 (Ar-C, ipso, 2 × C᎐O); m/z
᎐
2
316
J. Chem. Soc., Perkin Trans. 1, 2000, 2311–2316