Synthesis of novel proline-thiazole based cyclic hexa- and octapeptides

Article abstract of DOI:10.1016/S0040-4020(03)00992-X

Novel proline-thiazole based cyclopeptides were produced by cyclooligomerisation of an L-proline thiazole amino acid HCl in the presence of pentafluorophenyl diphenylphosphinate (FDPP) or diphenyl phosphorazidate (DPPA).

Full text of DOI:10.1016/S0040-4020(03)00992-X

Tetrahedron 59 (2003) 6637–6646
Synthesis of novel proline–thiazole based cyclic
hexa- and octapeptides
*
Sarva Jayaprakash, Gerald Pattenden, Murray S. Viljoen and Claire Wilson
School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
Received 20 March 2003; revised 27 May 2003; accepted 19 June 2003
Abstract—Novel proline–thiazole based cyclopeptides were produced by cyclooligomerisation of an L-proline thiazole amino acid HCl in
the presence of pentafluorophenyl diphenylphosphinate (FDPP) or diphenyl phosphorazidate (DPPA).
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
which exercise profound effects on the conformational
preferences these molecules can assume.9 A striking
example is the case of ceratospongamide which has been
isolated as two stable cis,cis- and trans,trans- confor-
mational isomers, 4 and 5 respectively.4 Not unexpectedly
the two isomers 4 and 5 have different biological activities
and each can be produced from the other on heating (5:1
equilibrium mixture in favour of the trans,trans-isomer 5).10
As part of our ongoing interests in the biological activity of
proline thiazole based cyclic peptides and poly oxazole
natural products, e.g. telomestatin 2, we have evaluated the
scope for cyclooligomerisation of the proline thiazole amino
acid 10 to synthesise novel cyclic constructs similar to the
natural products 1–5. These studies and their outcomes are
described here.
Nature is a rich source of structurally novel thiazole and
oxazole based cyclopeptidic structures, several of which are
beginning to show scope for development as potential
chemotherapeutic agents. Some representative examples
include the lissoclinums, e.g. 1, from ascidians (sea
squirts),1 telomestatin 2 isolated from Streptomyces,2
wewakazole 3 from the marine cyanobacterium Lyngbya
majuscula3 and the ceratospongamides 4 and 5 produced by
a marine alga/sponge symbiont.4 In earlier studies we have
demonstrated the scope for cyclooligomerisation and metal-
templated assembly of unusual cyclopeptidic constituents
from heterocyclic based amino acids.5,6 We have also
evaluated the metal ion chelating properties and cell-
membrane transport phenomena of a range of natural and
non-natural thiazole based cyclopeptides.7,8
2. Results and discussion
A feature of many bioactive cyclic peptides is the presence
of proline, in particular, and also N-methylated amino acids,
The known N-Boc protected L-proline thiazole amino acid
Keywords: cyclopeptides; amino acid; thiazole.
*
Corresponding author. Tel.: þ44-115-951-3530; fax: þ44-115-951-3564; e-mail: gp@nottingham.ac.uk
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00992-X

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S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
ester 7 was first synthesised starting from commercially
available Boc-L-proline following conversion to the corre-
sponding thioamide 6, and a modified Hantzsch reaction
with ethyl bromopyruvate.10,11 Saponification of the ethyl
ester 7, followed by N-Boc deprotection of the resulting
carboxylic acid 9 then produced the proline thiazole acid
hydrochloride 10 as fine colourless crystals (Scheme 1).
structures and stereochemistry were confirmed by X-ray
crystallography, Figures 1 and 2 respectively. The 1H NMR
spectrum of 11 showed only one set of resonances for its
three proline thiazole dipeptide units reflecting its C3
symmetry in analogy with the X-ray crystal structure. The
tetramer 12, however, displayed multiple resonances for its
four dipeptide units in the H NMR spectrum, suggesting
interaction of its nitrogen donor atoms with the NMR
solvent.
1
Treatment of the amino acid HCl 10 with benzotriazol-1-
yloxytris(pyrrolidino)phosphonium hexafluorophosphate
(PyBOP) and N,N-diisopropylethylamine (DIPEA) in
DMF at room temperature for 3 days led only to the cyclic
trimer 11 but in poor yield (5%). However, treatment of 10
with either of the coupling reagents pentafluorophenyl
diphenylphosphinate (FDPP) or diphenyl phosphorazidate
(DPPA), under similar conditions, produced both the trimer
11 and the corresponding tetramer 12, in a 3:1 ratio and in
similar overall yields of 15–20%. Both the trimer 11 and the
tetramer 12 were obtained as crystalline solids, and their
In order to obtain larger amounts of each of the trimer 11
and the tetramer 12 it was found to be expedient to
synthesize each of them in a linear fashion from 9, and from
the amino ester 8 produced from 7, following straight-
forward N-Boc deprotection. Thus, a coupling reaction
between 9 and 8 in the presence of 1-(3-dimethylamino-
propyl)-3-ethylcarbodiimide hydrochloride (EDCI) and
1-hydroxy benzotriazole (HOBt) first gave the proline
thiazole ‘dimer’ 13.
Scheme 1. Reagents: (i) KHCO3, DME, ethyl bromopyruvate, 2128C, then TFAA, collidine, 2128C, 61%; (ii) 4 M HCl in 1,4-dioxane, 97%; (iii) NaOH,
THF–H2O (3:1), 93%; (iv) 4 M HCl in 1,4-dioxane, 91%.

S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
6639
Figure 1. X-Ray crystal structure of the cyclic trimer 11.
Saponification of 13 to 14, followed by a second coupling
reaction with 8, next produced the linear ‘trimer’ 15
(Scheme 2). Saponification and N-Boc deprotection of 15
then gave the amino acid 17, which underwent macro-
lactamisation with FDPP and DIPEA to give, finally, the
target cyclic trimer 11. In a similar sequence of ordered
deprotections and coupling reactions the proline thiazole
dimer 13 was elaborated to 14 and 18 and hence to the linear
tetramer precursors 19 and 21 to the cyclic tetramer 12
(Scheme 3).
3. Conclusion
The aforementioned study has demonstrated that proline
thiazole amino acids can be cyclooligomerised in reason-
able yields leading to novel cyclopeptides, e.g. 11 and 12.
Similar to the naturally occurring telomerase inhibitor
telemostatin 2 the cyclic octapeptide 12 has eight nitrogen
donor centres inside its macrocycle cavity. This feature,
together with the metal chelating capacities of these
nitrogen donor ligands, suggest that the novel proline–
thiazole based cyclic peptides 11 and 12 could reveal
unusual ionophoric and biological properties.† These
features are now being examined as part of our continuing
studies of synthesis, metal-chelation, and ion transport
phenomena with unusual heterocyclic based cyclic peptides.
†
Interestingly when the cyclooligomerisation of 10 was carried out in the
presence of LiBF4 or NaBF4 increased amounts of the trimer 11 (ratio 2:1
of 11 to 12) were produced, and in the presence of AgBF4 the trimer 11
was formed almost exclusively in a 21% yield.
Figure 2. X-Ray crystal structure of the cyclic tetramer 12 (with
encapsulation of CH3CN).

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S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
Scheme 2. Reagents: (i) EDCI, HOBt, 8, DIPEA, DMF, 08C to room temperature, 8.5 h, 91%; (ii) NaOH, THF–H2O (3:1), room temperature, 7 h, 85%;
(iii) EDCI, HOBt, 8, DIPEA, DMF, 08C to room temperature, 14 h, 87%; (iv) NaOH, THF–H2O (3:1), room temperature, 9 h, 97%; (v) 4 M HCl in 1,2-
dioxane, room temperature, 6 h, 94%; (vi) FDPP, DIPEA, DMF (2.1 mM), room temperature, 5 days, 67%.
Scheme 3. Reagents: (i) NaOH, THF–H2O (3:1), room temperature, 7 h, 85%; (ii) 4 M HCl in 1,4-dioxane, room temperature, 6 h 96%; (iii) EDCI, HOBt, 18,
DIPEA, DMF, 08C to room temperature, 15 h, 81%; (iv) NaOH, THF–H2O (3:1), 30 h, room temperature, 87%; (v) 4 M HCl in 1,4-dioxane, 92%; (vi) DPPA,
DIPEA, DMF (2.0 mM), room temperature, 5 days, 79%.

S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
6641
4. Experimental
68.8 mmol) and collidine (21.0 g, 173.7 mmol) was added
to the mixture over 4 min, and the resulting orange
suspension was stirred for a further 15 min at 2128C. Ice
(40 ml) was added and the yellow suspension was then
extracted with CHCl3 (3£50 ml). The combined organic
extracts were washed successively with 2 M hydrochloric
acid (2£50 ml), a saturated solution of copper sulphate
(2£50 ml), water (2£50 ml) and brine (100 ml), and then
dried over MgSO4. The filtrate was concentrated in vacuo to
leave an oil which was purified by flash chromatography on
silica gel with petrol–ethyl acetate (9:1)!(1:1) as eluent to
give the thiazole (4.1 g, 61%) which crystallised as light
yellow crystals; mp 102–1048C (diethyl ether–hexane) (the
lit.11d quotes mp 1808C). (Found: C, 54.7; H, 6.7; N, 8.3%;
calcd for C15H22N2O4S: C, 55.2; H, 6.8; N, 8.6%);
[a]D20¼296.7 (c 1.69, DMF) [the lit.11d quotes [a]2D0¼38.1
(c 1, DMF)]; nmax (CHCl3) 3125, 1716, 1694, 1388 and
1098 cm21; dH (360 MHz, CDCl3, T¼298 K) two rotomers
4.1. General details
All melting points were determined on a Kofler hot stage
apparatus and are uncorrected. Optical rotations were
recorded in spectroscopic grade chloroform or methanol
on a Jasco DIP-370 polarimeter, [a]D values are recorded in
units of 1021 deg cm2 g21. Infrared spectra were obtained
using a Perkin–Elmer 1600 series FT-IR instrument as
liquid films or as dilute solutions in spectroscopic grade
chloroform. Proton NMR spectra were recorded on either a
Bruker DPX 360 or a Bruker DPX 500 spectrometer as
dilute solutions in either deuterochloroform or deutero-
methanol unless otherwise stated. The chemical shifts are
quoted in parts per million (ppm) relative to residual
chloroform (d 7.27) or residual methanol (d 3.35) as the
internal standard and the multiplicity of each signal is
designated by the following abbreviations: s, singlet; d,
doublet; t, triplet; q, quartet; quin, quintet; b, broad; m,
multiplet; app., apparent. All coupling constants are quoted
in Hertz. Carbon-13 NMR spectra were recorded on either a
Bruker DPX 360 or a Bruker DPX 500 spectrometer as
dilute solutions in either deuterochloroform or deutero-
methanol unless otherwise stated. Chemical shifts are
recorded relative to internal chloroform (d 77.2) or residual
methanol (d 49.05) as standard on a broad band decoupled
mode, and the multiplicities determined using a DEPT
sequence. Mass spectra were recorded on a VG micromass
7070E instrument. Microanalytical data were obtained on a
Perkin–Elmer 204B elemental analyzer.
t
(2:1) 1.41 (9H, s, Bu), 1.43 (3H, t, J¼7.1 Hz, OCH2CH3),
1.85–1.96 (2H, m, NCH2CH2), 2.19–2.39 (2H, m, CH2-
CH2CH), 3.43–3.65 (2H, m, NCH2), 4.41 (2H, q, J¼7.1 Hz,
OCH2CH3), 5.19 (1H, bs, NCH), 8.06 (1H, s, Ar H); dC
(90.5 MHz; CDCl3, T¼298 K) 14.4 (q, CH3), 23.1 (t, CH2),
28.2 (q, C(CH3)3), 34.2 (t, CH2), 46.7 (t, NCH2), 59.6 (d,
NCH), 61.3 (t, OCH2), 80.4 (s, C(CH3)3), 126.7 (d, CH–S),
147.2 (s, CvC–CvO), 154.1 (s, CH–CvN), 161.4 (s,
NCO–O), 177.0 (CO2Et); m/z (FAB) 327.1382 (MþHþ,
C15H23N2O4S requires 327.1378).
4.1.2. 2-[N-tert-Butoxycarbonyl-2,4-pyrrolidinyl]thia-
zole-4-carboxylic acid (9). Solid sodium hydroxide
(0.6 g, 15.0 mmol) was added in one portion to a stirred
solution of 7 (0.7 g, 1.9 mmol) in a mixture (3:1) of THF
and water (8 ml). The mixture was stirred at room
temperature for 5 h, then cooled to 08C and acidified to
pH 2 with dilute hydrochloric acid (2 M). The mixture was
diluted with DCM (50 ml), then washed thoroughly with
brine (2£40 ml) and dried over MgSO4. The filtrate was
concentrated in vacuo to leave the thiazole carboxylic acid
(0.55 g, 93%) which crystallised as fine colourless needles;
mp 187–1898C (petrol–dichloromethane) (lit.11d mp
188.68C); [a]D21¼298.0 (c 1.07, DMF) [lit.11d [a]D20¼
217.3 (c 1, DMF)]; nmax (CHCl3) 3431, 1761, 1694, 1391
and 1368 cm21; dH (360 MHz, CD3OD, T¼298 K) 1.35
Flash chromatography was performed on Merck silica gel
60 as the stationary phase and the solvents employed were
either of analytical grade or were distilled before use. All
reactions were monitored by TLC using Merck silica gel 60
F254 precoated aluminium backed plates, which were
visualized with UV-light and then with either acidic
ninhydrin solution or basic potassium permanganate
solution.
Routinely, dry organic solvents were stored under nitrogen
˚
on 3 A molecular sieves or purchased as solvents stored
over molecular sieves. Other organic solvents were dried by
distillation from the following: THF (potassium benzo-
phenone ketyl), dichloromethane (calcium hydride), and
methanol (magnesium methoxide). All organic extracts
were either dried over magnesium sulphate or sodium
sulphate. Solvent was removed on a Bu¨chi rotary evaporator
and reactions requiring anhydrous conditions were per-
formed with flame-dried apparatus under nitrogen or argon
atmosphere as stated.
t
(9H, s, Bu), 1.98–2.09 (2H, m, CH2CH2CH2), 2.16–2.28
(1H, m, CH2CH2CH), 2.40–2.55 (1H, m, CH2CH2CH),
3.48–3.59 (1H, m, NCH2), 3.62–3.70 (1H, m, NCH2),
5.14–5.22 (1H, m, NCH), 8.18–8.32 (1H, m, ArH); dC
(90.5 MHz, CD3OD, T¼298 K) 24.2 (t, CH2), 28.5 (q,
C(CH3)3), 34.1 (t, CH2), 47.9 (t, NCH2), 60.5 (d, NCH), 81.9
(s, C(CH3)3), 128.7 (d, CH–S), 148.4 (s, C–CO2H), 157.1
(s, NCO–O), 164.0 (s, CH–CvN), 177.8 (s, CO); m/z
(FAB) 321.0833 (MþNaþ, C13H18N2O4SNa requires
321.0884).
4.1.1. Ethyl 2-(N-tert-butoxycarbonyl-2,4-pyrroli-
dinyl)thiazole-4-carboxylate (7). Flame-dried potassium
hydrogen carbonate (21.7 g, 217.0 mmol) was added in one
portion to a stirred solution of the thioprolinamide 611c
(5.0 g, 21.7 mmol) in a mixture (95:5) of 1,2-dimethoxy-
ethane and DMF (90 ml) at room temperature under a
nitrogen atmosphere. The suspension was stirred at room
temperature for 10 min, cooled to 2128C, and then ethyl
bromopyruvate (12.7 g, 65.1 mmol) was added dropwise
over 5 min. A mixture of trifluoroacetic anhydride (18.2 g,
4.1.3. 2-Amino-(2,4-pyrrolidinyl)thiazole-4-carboxylic
acid (10). A solution of hydrochloric acid (4 M) in 1,4-
dioxane (15 ml) was added in one portion to the N-Boc-
proline thiazole amino acid 9 (1.7 g, 5.7 mmol) at room
temperature under a nitrogen atmosphere. The resulting off
white suspension was stirred at room temperature for 2.5 h,
until TLC analysis indicated the complete consumption of
starting material. The dioxane–toluene azeotrope was

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S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
removed in vacuo by repeatedly adding toluene (4£30 ml)
to leave the amine hydrochloric acid salt (1.21 g, 91%) as a
colourless solid; mp 255–2568C (ethyl acetate–ethanol).
(Found: C, 40.8; H, 4.6; N, 11.9%; C8H11N2O2SC requires
C, 40.9; H, 4.7; N, 11.9%); [a]2D1¼219.3 (c 1.08, MeOH);
nmax (MeOH) 2331, 2130 and 1959 cm21; dH (360 MHz,
CD3OD, T¼298 K) 2.24–2.35 (3H, m, CH2CH2CH2 and
CH2CH2CH), 2.67–2.69 (1H, m, CH2CH2CH), 3.50–3.66
(2H, m, NCH2), 4.99 (1H, bs, NH), 5.23 (1H, t, J¼7.3 Hz,
NCH), 8.53 (1H, s, Ar H); dC (90.5 MHz, CD3OD,
T¼298 K) 24.7 (t, CH2), 32.6 (t, CH2), 46.9 (t, CH2), 60.7
(d, NCH)), 131.3 (d, CH–S), 148.6 (s, CHvC–CvO),
163.8 (s, CH–CvN), 166.3 (s, CO); m/z (FAB) 221.0343
(MþNaþ, C8 H10O2N2SNa requires 221.0360).
[a]2D0¼259.8 (c 1.03, CHCl3); nmax (CHCl3) 3697, 3122,
2982, 2888, 2358, 1622, 1485, 1393 and 1325 cm21; dH
(360 MHz, CDCl3, T¼298 K) 1.77–1.93 (3H, m, CH2CH2-
CH2), 1.95–2.07 (3H, m, CH2CH2CH2), 2.08–2.17 (3H, m,
CHCH2CH2) 2.50–2.64 (3H, m, CHCH2CH2), 3.72 (3H,
ddd, J¼12.1, 9.9, 7.4 Hz, NCH2), 4.08 (3H, ddd, J¼12.2,
9.8, 6.8 Hz, NCH2), 6.82 (3H, d, J¼7.9 Hz, NCH), 7.68
(3H, s, Ar H); dC (90.5 MHz; CDCl3, T¼298 K) 21.2 (t,
CH2), 35.1 (t, CH2), 47.7 (t, NCH2), 60.0 (d, NCH), 124.5
(d, CH–S), 151.5 (s, CHvC–CvO), 162.1 (s, CH–CvN),
174.5 (s, CO); m/z (FAB) 563.0968 (MþNaþ, C24H24N6-
S3O3Na requires 563.0969); and (ii) cyclic-tetra-
(S),(S),(S),(S)-proline thiazole 12 (eluted second) which
crystallised as colourless crystals; mp 238–2398C (aceto-
nitrile–methanol); [a]2D0¼2108.3 (c 1.01, CHCl3); nmax
(CHCl3) 3686, 3124, 2980, 2889, 2255, 1621, 1488, 1460,
1393 and 1311 cm21; dH (360 MHz, CDCl3, T¼298 K)
1.88–2.69 and 2.90–3.00 (16H, m, CH2CH2CH2 and
CH2CH2CH), 3.65–4.20 (8H, m, NCH2), 5.34 (1H, d,
J¼7.4 Hz, NCH), 5.35 and 5.68–5.73 (1H, m, NCH), 6.42
(1H, d, J¼7.3 Hz, NCH), 6.62 (1H, t, J¼7.7 Hz, NCH), 7.70
(1H, s, Ar H), 8.06–8.25 (3H, m, Ar H); dC (90.5 MHz;
CDCl3, T¼298 K) 20.9 (t, CH2), 21.2 (t, CH2), 21.5 (t,
CH2), 23.3 (t, CH2), 26.1 (t, CH2), 30.1 (t, CH2), 32.1 (t,
CH2), 33.9 (t, CH2), 34.6 (t, CH2), 35.0 (t, CH2) 47.3 (t,
CH2), 48.0 (t, CH2), 58.7 (d, CH), 60.4 (d, CH), 61.0 (d,
CH), 61.7 (d, CH), 122.5 (d, CH), 123.2 (d, CH), 127.3 (d,
CH), 127.6 (d, CH), 148.8 (s, CHvC–CvO), 149.9 (s,
CHvC–CvO), 150.7 (s, CHvC–CvO), 151.0 (s,
CHvC–CvO), 160.4 (s, CH–CvN), 161.4 (s, CH–
CvN), 162.1 (s, CH–CvN), 162.5 (s, CH–CvN), 168.7
(s, CO), 170.6 (s, CO), 172.2 (s, CO), 175.4 (s, CO); m/z
(FAB) 721.1547 (MþHþ, C32H33N8O4S4 requires
721.1507).
4.1.4. Ethyl 2-[amino-2,4-pyrrolidinyl]thiazole-4-car-
boxylate (8). A solution of hydrochloric acid (4 M) in
1,4-dioxane (7.5 ml) was added to the N-Boc proline 7
(0.9 g, 2.8 mmol) at room temperature and the mixture was
then stirred at room temperature for 6 h under a nitrogen
atmosphere. The dioxane was removed in vacuo by twice
forming an azeotrope with toluene to leave a waxy yellow
solid which was stirred with a mixture (2:3) of ethyl acetate
and petrol (20 ml) for 1 h, and then filtered. The residue was
again treated with toluene to leave the amine hydrochloride
salt (0.7 g, 97%) which crystallised as colourless crystals;
mp 169–1718C (ethyl acetate–dichloromethane) (lit.11d mp
1708C). (Found: C, 45.4; H, 5.6; N, 10.4%; Calc. for
C10H15N2O2SCl: C, 45.7; H, 5.8; N, 10.7%); [a]2D2¼221.2
(c 1.02, DMF) [lit.11d [a]D20¼235 (c 1, DMF)]; nmax
(CHCl3) 2963, 2675, 1722, 1340 and 1098 cm21; dH
(360 MHz, CD3OD, T¼298 K) 1.43 (3H, t, J¼7.2 Hz,
OCH2CH3), 2.20–2.40 (3H, m, CH2CH2CH and CH2CH2-
CH2), 2.62–2.73 (1H, m, CH2CH2CH), 3.48–3.66 (2H, m,
NHCH2), 4.44 (2H, q, J¼7.2 Hz, OCH2), 5.21 (1H, t,
J¼7.4 Hz, NCH), 8.56 (1H, s, Ar H); dC (90.5 MHz,
CD3OD, T¼298 K) 14.6 (q, CH3), 24.6 (t, CH2), 32.6 (t,
CH2), 46.9 (t, OCH2), 60.6 (d, NCH), 62.8 (t, NCH2), 131.3
(d, CH–S), 148.0 (s, CHvC–CO2H), 162.4 (s, CH–CvN),
166.5 (s, CO); m/z (FAB) 227.0848 (MþHþ, C10H15N2O2S
requires 227.0854).
Cyclooligomerisation of 10 using PyBOP produced only 11
(5%), and DPPA produced a 3:1 mixture of 11 and 12 in an
overall 20% yield.
X-Ray crystal structure determination of 11 and 12. For
each compound data were collected at 150 K on a Bruker
SMART CCD area detector diffractometer equipped with an
Oxford Cryosystem open-flow nitrogen cryostat, 11 on a
SMART1000 and 12 on an APEX. The structures was
solved by direct methods (SHELXS-97) and refined
using full-matrix least squares refinement against F 2.
All non-atoms were refined with anisotropic atomic
displacement parameters (adps) and H atoms placed in
geometrically calculated positions and refined as part of a
riding model, with U(H)iso¼1.5Ueq(C) unless otherwise
stated.
4.1.5. Cyclic trimer (11) and cyclic tetramer (12) by
cyclooligomerisation of (10). N,N-Diisopropylethylamine
(0.07 ml, 0.4 mmol) was added dropwise over 5 min to a
stirred solution of the proline thiazole amino acid
hydrochloride 10 (0.03 g, 0.13 mmol) in N,N-dimethyl-
formamide (30 ml, 5.0 mM) at room temperature under a
nitrogen atmosphere. Pentafluorophenyl diphenylphos-
phinate (0.1 g, 0.26 mmol) was added in one portion, and
the mixture was stirred at room temperature for 72 h. The
solvent was removed in vacuo to leave a light brown oil.
Chloroform (100 ml) was added and the mixture was then
washed successively with 2 M HCl (2£50 ml), 2 M NaOH
(2£50 ml), and brine (2£50 ml). The combined organic
extracts were dried (MgSO4) and concentrated in vacuo to
leave a colourless oil (11 mg, 15%) consisting of a 3:1
mixture of 11 and 12. Purification by flash chromatography
(SiO2, 5 % methanol in chloroform) gave: (i) cyclic-tris-
(S),(S),(S)-proline thiazole 11 (eluted first) which crystal-
lised as colourless crystals; mp .3208C (chloroform,
decomp.). (Found: C, 51.4; H, 4.5; N, 14.5%;
C24H26N6O4S3 requires C, 51.6; H, 4.7; N, 15.0%);
Colourless crystals of 11 were obtained by slow crystal-
lization (CDCl3) at 0–48C over 1 week. A crystal of
dimensions 0.15£0.12£0.10 mm3 was selected and found to
crystallize in the orthorhombic space group P212121 with
3
˚
˚
a¼6.446(7), b¼18.756(7), c¼20.357(7) A, V¼2461(3) A ,
Z¼4, Dcalcd¼1.411 g/cm3, T¼150 K. 15012 reflections
were measured, 5604 unique (Rint¼0.19), 5601 of which
were used in all calculations. The final wR(F 2) was 0.152
for all data, R1(F) was 0.084 for 2803 observed data where
I.2s(I). The Flack parameter refined to 20.06(16)
suggesting that the absolute configuration reported is correct

S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
6643
although there may be some ambiguity due to the slightly
large standard uncertainty.
(t, CH2), 34.5 (t, CH2), 46.2 (t, CH2), 46.4 (t, CH2), 47.4 (t,
CH2), 49.5 (t, CH2), 57.9 (d, CH), 58.8 (d, CH), 59.5 (d,
CH), 60.8 (t, 2£CH2), 61.1 (d, CH), 79.8 (s, C(CH3)3) 110.1
(d, CH), 117.8 (d, CH), 126.5 (d, CH), 126.6 (d, CH), 147.0
(s, CHvC–CvO), 148.9 (s, CHvC–CvO), 149.3 (s,
CHvC–CvO), 149.9 (s, CHvC–CvO), 153.5 (s, CH–
CvN), 154.2 (s, CH–CvN), 160.9 (s, NCO–O), 161.4 (s,
CH–CvN), 161.8 (s, CH–CvN), 173.2 (s, CO), 173.4 (s,
CO), 173.9 (s, CO), 176.6 (s, CO); m/z (FAB) 530.1653
(MþHþNaþ, C23H31N4O5S2Na requires 530.1633).
Colourless crystals of 12 were obtained by crystallization
(CH3CN–MeOH) at 0–48C. A crystal of dimensions
0.36£0.28£0.06 mm3 was selected for the study and also
found to crystallize in the orthorhombic space group
˚
P212121 with a¼13.568(3), b¼14.631(3), c¼18.062(3) A,
3
˚
V¼3586(2) A , Z¼4, T¼150 K. 21581 reflections were
measured, 8075 unique (Rint¼0.12), 8043 of which were
used in all calculations. The final wR(F 2) was 0.143 for all
data, R1(F) was 0.058 for 6367 observed data where
I.2s(I). The Flack parameter refined to 20.09(8) indicat-
ing the correct assignment of the absolute configuration.
Disorder was observed in C7a and C8a and these were
modelled over two sites with occupancies 0.60 and 0.40,
suitable distance restraints were applied and the atoms
refined with isotropic adps, as were those of the included
MeCN molecule.
4.1.7. 2-[(N-tert-Butoxycarbonyl-2,4-pyrrolidinyl)-
thiazole]2-4-carboxylic acid (14). Solid sodium hydroxide
(0.38 g, 9.5 mmol) was added in one portion to a stirred
solution of the proline thiazole dimer 13 (0.6 g, 1.2 mmol)
in a mixture (3:1) of THF and water (12 ml), and the
resulting milky suspension was stirred at room temperature
for 7 h. The mixture was acidified to pH 2 with dilute
hydrochloric acid (2 M) and then extracted thoroughly with
ethyl acetate (3£40 ml). The combined organic extracts
were washed with brine (2£50 ml), dried over MgSO4, and
then evaporated in vacuo to leave the carboxylic acid
(0.52 g, 93%) which crystallised as colourless crystals; mp
196–1978C (petrol–dichloromethane); [a]2D4¼2113.9 (c
1.05, CHCl3); nmax (CHCl3) 3698, 2980, 1710, 1620, 1391
and 1367 cm21; dH (360 MHz, CD3OD, T¼298 K) 1.33
(9H, s, tBu), 1.94–2.50 (8H, m, CH2CH2CH2 and CH2CH2-
CH), 3.46–4.15 (4H, m, NCH2), 5.18 (1H, d, J¼7.3 Hz,
NCH), 5.64 (1H, m, NCH), 6.27–6.42 (1H, m, NCH), 8.13–
8.30 (2H, m, Ar H); dC (90.5 MHz, CD3OD, T¼298 K) 22.2
(t, CH2), 24.1 (t, CH2), 28.6 (q, C(CH3)3), 33.1 (t, CH2),
34.9 (t, CH2), 47.8 (t, CH2), 51.5 (t, CH2), 60.4 (d, CH), 62.9
(d, CH), 81.8 (s, C(CH3)3), 128.0 (d, CH–S), 128.9 (d, CH–
S), 148.2 (s, CHvC–CvO), 148.9 (s, CHvC–CvO),
156.0 (s, NCO–O), 164.0 (s, CH–CvN), 164.3 (s, CH–
CvN), 175.7 (s, CO), 178.0 (s, CO); m/z (FAB) 501.1281
(MþNaþ, C21H26N4O5S2Na requires 501.1242).
Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publicationnumbersCCDC200095and200096.Copiesofthe
data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge, CB2 1EZ [fax: þ44(0)-1223-
336033 or email: deposit@ccdc.cam. ac.uk].
4.1.6. Ethyl 2-[(N-tert-butoxycarbonyl-2,4-
pyrrolidinyl)thiazole]2-4-carboxylate (13). 1-Hydroxy-
benzotriazole (0.23 g, 1.7 mmol) was added in one portion
to a stirred solution of the proline thiazole amino acid 9
(0.42 g, 1.41 mmol) in dry DMF (10 ml) at 08C under a
nitrogen atmosphere. After 10 min, 1-(3-dimethyl-
aminopropyl)-3-ethylcarbodiimide hydrochloride (0.32 g,
1.7 mmol) was added, and the mixture was stirred at room
temperature for a further 5 min. A solution of the amino
ester 8 (0.4 g, 1.5 mmol) in dry DMF (5 ml) was added at
08C, followed 5 min later by N,N-diisopropylethylamine
(0.49 ml, 2.8 mmol) in one portion at the same temperature.
The mixture was stirred at 08C for 2.5 h, then at 108C for 3 h
and finally at room temperature for a further 3 h. The DMF
was removed in vacuo and the residue was diluted with ethyl
acetate (100 ml). The ethyl acetate extract was washed with
a saturated aqueous solution of ammonium chloride
(2£40 ml), and brine (2£40 ml), and then dried (MgSO4).
The solvent was removed in vacuo to leave a light brown
residue which was purified by flash chromatography on
silica gel using petrol–ethyl acetate (9:1)!(1:4) as eluent to
give the linear proline thiazole dimer (0.65 g, 91%) as a
colourless foam (1:1 mixture of two rotamers). (Found: C,
54.0; H, 5.8; N, 11.2%; C23H30N4O5S2 requires C, 54.5; H,
6.0; N, 11.1%); [a]2D4¼278.9 (c 1.41, CHCl3); nmax
(CHCl3) 3125, 1716, 1693, 1621, 1387, 1368 and
1098 cm21; dH (360 MHz, CDCl3, T¼298 K) 1.15 (3H, t,
J¼7.3 Hz, OCH2CH3), 1.22 (9H, s, tBu), 1.55–2.34 (8H, m,
CH2CH2CH2 and CH2CH2CH), 3.10–3.94 (4H, m, NCH2),
4.19 (2H, q, J¼7.3 Hz, OCH2CH3), 4.65 and 4.98 (1H, dd,
J¼31.0, 6.4 Hz, NCH), 5.52 (1H, d, J¼4.3 Hz, NCH), 6.14
(1H, dd, J¼40.4, 6.4 Hz, NCH), 7.82–7.97 (2H, m, Ar H);
dC (90.5 MHz; CDCl3, T¼298 K) 14.0 (q, CH3), 14.1 (q,
CH3), 20.6 (t, CH2), 22.8 (t, CH2), 23.5 (t, CH2), 24.7 (t,
CH2), 28.0 (q, C(CH3)3), 31.2 (t, CH2), 32.2 (t, CH2), 32.8
4.1.8. Ethyl 2-[(N-tert-butoxycarbonyl-2,4-
pyrrolidinyl)thiazole]3-4-carboxylate (15). 1-Hydroxy-
benzotriazole (0.16 g, 1.2 mmol) was added in one portion
to a stirred solution of the proline thiazole dimer acid 14
(0.47 g, 0.97 mmol) in DMF (10 ml) at 08C under a
nitrogen atmosphere. The mixture was stirred at 08C for
5 min and then 1-(3-dimethylaminopropyl)-3-ethylcarbo-
diimide hydrochloride (EDCI) (0.22 g, 1.2 mmol), the
amino ester 8 (0.3 g, 1.0 mmol) and N,N-diisopropylethyl-
amine (0.34 ml, 1.9 mmol) were added consecutively over
5 min intervals. The mixture was allowed to reach room
temperature, stirred for 14 h and then the DMF was removed
in vacuo. Ethyl acetate (40 ml) was added to produce a
milky brown suspension which was washed with a saturated
aqueous solution of ammonium chloride (2£40 ml). The
separated aqueous solution was again extracted with ethyl
acetate (2£40 ml) and the combined organic extracts were
then washed with brine (3£40 ml), dried over MgSO4, and
evaporated to dryness in vacuo to leave an oily residue.
Purification by chromatography on silica gel with petrol–
ethyl acetate (1:1)!(0:100) and then ethyl acetate–
methanol (100:0)!(92:8) gave the proline thiazole trimer
(0.6 g, 87%) as colourless crystals; mp 92–948C. (Found: C,
53.3; H, 5.5; N, 12.1%; C32H41N6O7S3 requires C, 53.5; H,
5.8; N, 11.7%); [a]2D4¼2135.9 (c 1.06, CHCl3); nmax

6644
S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
(CHCl3) 3124, 2980, 1720, 1692, 1620, 1389 and
1323 cm21; dH (360 MHz, CDCl3, T¼298 K) 1.24 (3H, t,
m, NCH2), 5.10–5.30 (1H, m, NCH), 5.60–5.90 (1H, m,
NCH), 6.18–6.24 (1H, m, NCH), 8.20–8.40 (3H, m, Ar H);
dC (90.5 MHz, CD3OD, T¼298 K) 22.7 (t, CH2), 24.6 (t,
CH2), 25.8 (t, CH2), 32.8 (t, CH2), 33.4 (t, CH2), 35.9 (t,
CH2), 47.1 (t, CH2), 48.7 (t, CH2), 51.1 (t, CH2), 60.4 (d,
CH), 60.9 (d, CH), 63.2 (d, CH), 129.0 (d, CH), 129.3 (d,
CH), 129.6 (d, CH), 148.8 (s, CHvC–CvO), 150.0 (s,
CHvC–CvO), 151.5 (s, CHvC–CvO), 162.9 (s, CH–
CvN), 163.3 (s, CH–CvN), 163.9 (s, CH–CvN), 165.3
(s, CO), 172.4 (s, CO), 179.2 (s, CO); m/z (FAB) 559.1221
(MþHþ, C24H27N6O4S3 requires 559.1255).
t
J¼7.1 Hz, OCH2CH3), 1.33 (9H, s, Bu), 1.94–2.40 (12H,
m, CH2CH2CH2 and CH2CH2CH), 3.30–4.08 and 4.25–
4.48 (6H, m, NCH2), 4.10 (2H, q, J¼7.1 Hz, OCH2CH3),
4.75–5.19 (1H, m, NCH), 5.39 and 5.70 (1H, bs, NCH),
6.00–6.43 (1H, m, NCH), 7.98–8.10 (3H, m, Ar H); dC
(90.5 MHz; CDCl3, T¼298 K) 14.3 (q, CH3), 21.2 (t, CH2),
25.3 (t, CH2), 28.4 (q, C(CH3)3), 30.0 (t, CH2), 31.7 (t,
CH2), 35.0 (t, CH2), 47.0 (t, CH2), 48.0 (t, CH2), 49.8 (t,
CH2), 58.9 (d, CH), 60.0 (d, CH), 60.5 (t, CH2), 61.5 (t,
CH2), 61.8 (d, CH), 80.5 (s, C(CH3)3), 126.7 (d, CH–S),
127.1 (d, CH–S), 128.0 (d, CH–S), 146.9 (s, CHvC–
CvO), 147.5 (s, CHvC–CvO), 148.9 (s, CHvC–CvO),
154.2 (s, NCO–O), 161.5 (s, CH–CvN), 162.2 (s, CH–
CvN), 162.5 (s, CH–CvN), 171.3 (s, CO), 173.7 (s, CO),
177.4 (s, CO); m/z (FAB) 710.1990 (MþHþNaþ, C31H39-
N6O6S3Na requires 710.1990).
4.1.11. Cyclic-tris-(S),(S),(S)-proline thiazole (11). N,N-
Diisopropylethylamine (0.12 ml, 0.7 mmol) and penta-
fluorophenyl diphenylphosphinate (0.14 g, 0.36 mmol)
were added sequentially in one portion to a stirred solution
of the amino acid 17 (0.10 g, 0.17 mmol) in dry DMF
(80 ml), and the mixture was stirred at room temperature for
5 days under nitrogen atmosphere. The DMF was removed
in vacuo to leave a semi-crystalline residue. Ethyl acetate
(120 ml) was added and the mixture was washed thoroughly
with a 2 M sodium hydroxide solution (5£40 ml), to remove
any pentafluorophenyl diphenylphosphinic acid. The com-
bined organic extracts were washed successively with 2 M
hydrochloric acid (3£40 ml) and brine (2£40 ml), then
dried (MgSO4), and evaporated to dryness in vacuo. The
residue was purified by flash chromatography on silica gel
using ethyl acetate–methanol (100:0)!(92:8) as eluent to
give the cyclic trimer (61 mg, 67%) as orthorhombic
crystals (from CHCl3), whose spectroscopic data were
identical to those described earlier.
4.1.9. 2-[(N-tert-Butoxycarbonyl-2,4-pyrrolidinyl)-
thiazole]3-4-carboxylic acid (16). Solid sodium hydroxide
(0.18 g, 0.6 mmol) was added in one portion to a stirred
solution of the linear trimer 15 (0.4 g, 0.6 mmol) in a
mixture (3:1) of THF and water (12 ml), and the milky
suspension was stirred at room temperature for 9 h. The
mixture was acidified to pH 2 by the addition of 2 M
hydrochloric acid, and then extracted with chloroform
(3£40 ml). The combined chloroform extracts were washed
with brine (2£40 ml), dried over MgSO4, and then
concentrated in vacuo to leave a viscous residue. The
residue was triturated with ether to leave the trimer
carboxylic acid (0.37 g, 97%) as fine colourless crystals;
mp 120–1228C; [a]2D4¼2134.4 (c 1.03, CHCl3); nmax
(CHCl3) 3124, 2979, 1760, 1693, 1620, 1391, 1368 and
1113 cm21; dH (360 MHz, CD3OD, T¼298 K) 1.29–1.52
(9H, m, tBu), 1.87–2.55 (12H, m, CH2CH2CH2 and
CH2CH2CH), 3.44–3.97 (6H, m, NCH2), 4.25 and 5.15
(1H, m, NCH), 5.40–5.70 (1H, m, NCH), 6.19–6.30 (1H,
m, NCH), 8.15–8.30 (3H, m, Ar H); dC (90.5 MHz,
CD3OD, T¼298 K) 22.3 (t, CH2), 24.83 (t, CH2), 25.99 (t,
CH2), 28.6 (q, C(CH3)3), 32.1 (t, CH2), 33.1 (t, CH2), 35.7
(t, CH2), 47.7 (t, CH2), 48.1 (t, CH2), 51.5 (t, CH2), 60.4 (d,
CH), 61.6 (d, CH), 63.1 (d, CH), 81.8 (s, C(CH3)3), 124.4 (d,
CH), 128.6 (d, CH), 128.9 (d, CH), 148.8 (s, CHvC–
CvO), 150.1 (s, CHvC–CvO), 151.1 (s, CHvC–CvO),
155.9 (s, NCO–O), 163.3 (s, CH–CvN), 164.0 (s, CH–
CvN), 164.3 (s, CH–CvN), 172.4 (s, CO), 173.0 (s, CO),
178.2 (s, CO); m/z (FAB) 681.1583 (MþNaþ, C29H34N6-
O6S3Na requires 681.1599).
4.1.12. Ethyl 2-[(amino-2,4-pyrrolidinyl)thiazole]2-4-
carboxylate (18). A solution of hydrochloric acid (4 M)
in 1,4-dioxane (8.5 ml) was added in one portion to the
proline thiazole dimer 13 (1.24 g, 2.5 mmol) and the
resulting light yellow suspension was stirred at room
temperature for 6 h under a nitrogen atmosphere. The
mixture was treated with toluene (5£20 ml), forming an
azeotrope with 1,4-dioxane, to leave the amine hydrochloric
acid salt (1.04 g, 96%) as colourless crystals; mp 187–
1898C (ethyl acetate–dichloromethane). (Found: C, 48.3;
H, 5.2; N, 12.4%; C18H23N4O3S2Cl requires C, 48.8; H, 5.2;
N, 12.7%); [a]D21¼239.6 (c 1.04, CHCl3); nmax (CHCl3)
3698, 3605, 2958, 2526, 2304, 1723, 1626, 1458, 1387 and
1100 cm21; dH (360 MHz, CDCl3, T¼298 K) 1.38 (3H, t,
J¼7.2 Hz, OCH2CH3); 1.81–2.62 (8H, m, CH2CH2CH2
and CH2CH2CH), 3.44–4.10 (4H, m, NCH2), 4.36 (2H, q,
J¼7.2 Hz, OCH2CH3); 4.98–5.11 (1H, m, NCH), 5.65 and
6.67 (1H, d, J¼7.5 Hz, NCH), 8.00–8.20 (2H, m, Ar H),
9.20 (1H, bs, NH); dC (90.5 MHz; CDCl3, T¼298 K) 14.5
(q, CH3), 21.6 (t, CH2), 24.2 (t, CH2), 31.6 (t, CH2), 35.4 (t,
CH2), 45.6 (t, NCH2), 48.2 (t, NCH2), 59.0 (d, NCH), 61.3
(d, NCH), 61.8 (t, OCH2), 127.3 (d, CH–S), 128.5 (d, CH–
S), 146.7 (s, CHvC–CvO), 150.2 (s, CHvC–CvO),
160.9 (s, CH–CvN), 161.5 (s, CH–CvN), 163.5 (s, CO),
177.8 (s, CO–O); m/z (FAB) 407.1133 (MþHþ,
C18H23N4O3S2 requires 407.1211).
4.1.10. 2-Amino-[(2,4-pyrrolidinyl)thiazole)]3-4-car-
boxylic acid (17). A solution of hydrochloric acid (4 M)
in 1,4-dioxane (9 ml) was added in one portion to the (Boc)-
protected amino acid 16 (0.42 g, 0.63 mmol), and the
resulting suspension was stirred at room temperature under
a nitrogen atmosphere for 6 h. The solvent was removed in
vacuo, using toluene as an azeotrope, to leave a foam which
was triturated with ether to give the amino acid hydro-
chloride salt (0.35 g, 94%) as colourless crystals; mp 196–
1988C (decomp.); [a]2D0¼257.8 (c 1.10, MeOH); nmax
(CHCl3) 3696, 3631, 2944, 2838, 1602, 1392 and
1015 cm21; dH (360 MHz, CD3OD, T¼298 K) 1.63–2.70
(12H, m, CH2CH2CH2 and CH2CH2CH), 3.61–4.40 (6H,
4.1.13. Ethyl 2-[(N-tert-butoxycarbonyl-2,4-
pyrrolidinyl)thiazole]4-4-carboxylate (19). 1-Hydroxy-
benzotriazole hydrate (0.42 g, 3.2 mmol) and 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride

S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
6645
(0.60 g, 3.2 mmol) were added sequentially over a 5 min
period to a stirred solution of the amino acid 14 (1.26 g,
2.6 mmol) in dry DMF (25 ml) at 08C under a nitrogen
atmosphere. The mixture was stirred at 08C for 15 min and
then a solution of the ethyl ester 18 (1.37 g, 3.1 mmol) in dry
DMF (10 ml) was added in one portion. The mixture was
stirred at 08C for 5 min before adding N,N-diisopropylethyl-
amine (0.9 ml, 5.3 mmol) over 2 min. The mixture was
stirred from 08C to room temperature for 15 h, then the
DMF was removed in vacuo to leave a residue which was
diluted with chloroform (80 ml). The organic extract was
washed successively with a saturated aqueous solution of
ammonium chloride (2£50 ml) and brine (2£50 ml), then
dried over MgSO4 and concentrated in vacuo to leave an
oily residue. Purification by chromatography on silica gel
using chloroform–methanol (100:0)!(92:8) gave the
acyclic tetramer (1.83 g, 81%) as a colourless powder; mp
87–898C. (Found: C, 52.4; H, 5.6; N, 12.8%; C39H48N6O8S3
requires C, 52.9; H, 5.5; N, 12.7%); [a]2D1¼2167.5 (c 1.21,
CHCl3); nmax (CHCl3) 2980, 2358, 1724, 1692, 1619, 1487,
1390 and 1097 cm21; dH (360 MHz, CDCl3, T¼298 K) 1.26
CH2), 59.1 (d, CH), 59.3 (d, CH), 59.6 (d, CH), 60.5 (t,
CH2), 61.7 (d, CH), 80.6 (s, C(CH3)3), 126.6 (d, CH–S),
126.9 (d, CH–S), 127.7 (d, CH–S), 128.2 (d, CH–S), 146.6
(s, CHvC–CvO), 147.3 (s, CHvC–CvO), 148.9 (s,
CHvC–CvO), 149.6 (s, CHvC–CvO), 154.3 (s, NCO–
O), 161.9 (s, CH–CvN), 162.2 (s, CH–CvN), 162.3 (s,
CH–CvN), 163.1 (s, CH–CvN), 171.3 (s, CO), 173.6 (s,
CO), 173.8 (s, CO), 176.9 (s, CO); m/z (FAB) 861.1970
(MþNaþ, C37H42N8O7S4 requires 861.1957).
4.1.15. 2-Amino-[(2,4-pyrrolidinyl)thiazole]4-4-car-
boxylic acid (21). A solution of hydrochloric acid (4 M)
in 1,4-dioxane (14 ml) was added in one portion to the linear
tetramer amino acid 20 (1.5 g, 1.7 mmol) and the mixture
was stirred at room temperature for 1.5 h under a nitrogen
atmosphere. The dioxane was removed in vacuo by forming
an azeotrope with toluene to leave the amine as a sticky
colourless foam. Trituration with ethyl acetate left the
amino acid (1.21 g, 92%) as colourless crystals; mp 180–
2118C (slow decomp.); [a]2D4¼273.9 (c 1.06, CHCl3); nmax
(CHCl3) 3696, 2958, 1715, 1616 and 1391 cm21; dH
(360 MHz, CD3OD, T¼298 K) 1.82–2.41 (16H, m, CH2-
CH2CH2 and CH2CH2CH), 3.43–4.20 (8H, m, NCH2),
4.94–5.10 (1H, m, NCH), 5.26–5.41 (1H, m, NCH), 5.61–
5.69 (1H, m, NCH), 6.08–6.50 (1H, m, NCH), 8.15–8.50
(4H, m, Ar H); dC (90.5 MHz, CD3OD, T¼298 K) 22.2 (t,
CH2), 24.6 (t, CH2), 26.1 (t, CH2), 26.2 (t, CH2), 32.5 (t,
CH2), 32.7 (t, CH2), 32.8 (t, CH2), 35.7 (t, CH2), 47.0 (t,
CH2), 51.2 (t, CH2), 51.5 (t, CH2), 54.9 (t, CH2), 60.4 (d,
CH), 60.7 (d, CH), 60.8 (d, CH), 62.4 (d, CH), 126.9 (d,
CH–S), 127.2 (d, CH–S), 127.4 (d, CH–S), 128.2 (d, CH–
S), 148.0 (s, CHvC–CvO), 148.8 (s, CHvC–CvO),
150.7 (s, CHvC–CvO), 151.4 (s, CHvC–CvO), 163.5
(s, CH–CvN), 163.9 (s, CH–CvN), 164.8 (s, CH–CvN),
165.3 (s, CH–CvN), 172.4 (s, CO), 175.7 (s, CO), 176.2 (s,
CO), 178.4 (s, CO); m/z (FAB) 739.1644 (Mþ,
C32H35N8O5S4 requires 739.1613).
t
(3H, t, J¼7.2 Hz, OCH2CH3), 1.39 (9H, s, Bu), 1.74–2.43
(16H, m, CH2CH2CH2 and CH2CH2CH), 3.38–4.20 (8H, m,
NCH2), 3.43 (2H, q, J¼7.2 Hz, OCH2CH3), 4.90–5.52 (2H,
m,NCH),6.10–6.45(2H, m,NCH), 7.90–8.14(4H, m,ArH);
dC (90.5 MHz; CDCl3, T¼298 K) 14.6 (q, CH3), 21.4(t, CH2),
25.4 (t, CH2), 28.5 (q, C(CH3)3), 29.9 (t, CH2), 31.7 (t, CH2),
32.7(t, CH2), 34.4(t, CH2), 35.3(t, CH2), 47.7(t, CH2), 48.0(t,
CH2), 49.9 (t, CH2), 59.0 (d, CH), 59.3 (d, CH), 60.0 (t, CH2),
60.2(d, CH), 60.6(t, CH2), 61.0 (d, CH), 61.5 (t, CH2), 80.5 (s,
C(CH3)3), 125.9 (d, CH–S), 126.5 (d, CH–S), 126.7 (d, CH–
S), 127.1 (d, CH–S), 147.0 (s, CHvC–CvO), 147.6 (s,
CHvC–CvO), 148.9 (s, CHvC–CvO), 150.0 (s,
CHvC–CvO), 154.8 (s, NCO–O), 161.6 (s, CH–CvN),
162.2 (s, CH–CvN), 162.6 (s, CH–CvN), 162.7 (s, CH–
CvN), 171.3 (s, CO), 171.5 (s, CO), 174.5 (s, CO), 177.4 (s,
CO); m/z (FAB) 889.2221 (MþHþ, C39H46N8S4O7 requires
889.2270).
4.1.16. Cyclic-tetra-(S),(S),(S),(S)-proline thiazole (12).
N,N-Diisopropylethylamine (0.08 ml, 0.45 mmol) and
diphenylphosphoryl azide (0.07 ml, 0.32 mmol) were added
sequentially in one portion to a solution of the linear tetramer
amino acid hydrochloride salt 21 (0.10 g, 0.13 mmol) in dry
DMF (65 ml), and the resulting suspension was stirred at room
temperature for 120 h under a nitrogen atmosphere. The
mixture was evaporated to dryness in vacuo and the residue
was then diluted with chloroform (70 ml). The chloroform
solution was stirred with a saturated aqueous solution of
sodium hydrogen carbonate (70 ml) for 4 h, and the separated
chloroform extract was then washed with a saturated aqueous
solution of ammonium chloride (3£40 ml) followed by brine
(2£30 ml). The dried extract was concentrated in vacuo to
leavearesiduewhichwaspurifiedbyflashchromatographyon
silica gel, using CHCl3–MeOH (100:0)!(95:5) as eluent to
give the cyclic tetramer (73.1 mg, 79%) as colourless crystals
whose spectroscopic data were identical to those described
earlier.
4.1.14. 2-[(N-tert-Butoxycarbonyl-2,4-pyrrolidinyl)-
thiazole]4-4-carboxylic acid (20). Solid sodium hydroxide
(0.7 g, 16.75 mmol) was added in one portion to a stirred
solution of the tetramer 19 (1.8 g, 2.1 mmol) in a mixture
(3:1) of THF and water (40 ml) at room temperature, and the
mixture was then stirred at room temperature for 30 h. The
mixture was diluted with water (50 ml) and then washed
with ethyl acetate (60 ml) to remove the unreacted starting
material. The aqueous layer was acidified to pH 2 with a
2 M hydrochloric acid solution and then extracted with
chloroform (3£50 ml). The combined organic extracts were
washed with brine (2£50 ml), dried over MgSO4 and then
concentrated in vacuo to leave the carboxylic acid (1.54 g,
87%) as a colourless solid; mp 125–1278C; [a]2D1¼2155.4
(c 1.14, CHCl3); nmax (CHCl3) 3696, 2979, 2886, 1731,
1693, 1620, 1488, 1391, 1346 and 1115 cm21; dH
t
(360 MHz, CD3OD, T¼298 K) 1.47 (9H, s, Bu), 1.82–
2.35 (16H, m, CH2CH2CH2 and CH2CH2CH), 3.45–4.10
(8H, m, NCH2), 4.20 and 4.89 (1H, m, NCH), 5.24–5.42
(1H, m, NCH), 5.52–5.72 (1H, m, NCH), 6.02–6.45 (1H,
m, NCH), 8.01–8.13 (4H, m, Ar H); dC (90.5 MHz; CDCl3,
T¼298 K) 21.2 (t, CH2), 21.5 (t, CH2), 25.2 (t, CH2), 25.3 (t,
CH2), 28.4 (q, C(CH3)3), 30.6 (t, CH2), 31.3 (t, CH2), 31.5
(t, CH2), 35.0 (t, CH2), 46.7 (t, CH2), 47.8 (t, CH2), 50.1 (t,
Acknowledgements
We thank the University of Nottingham for financial
support.

6646
S. Jayaprakash et al. / Tetrahedron 59 (2003) 6637–6646
Gahan, L. R.; Hanson, G. R.; Fairlie, D. P. J. Am. Chem. Soc.
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