Trifluoroacetyl as an Orthogonal Protecting Group for Guanidines

Article abstract of DOI:10.1021/jo0348874

The trifluoroacetyl moiety has been used as a new protecting group for guanidine functionality. The protecting group is easily cleaved under mild basic conditions and is complementary to the Boc, Cbz, and Ddpe protecting groups. The protecting group can be applied to peptide synthesis in solution as well as on a solid phase as it is orthogonal to a Boc and Cbz strategy and semiorthogonal to an Fmoc strategy.

Full text of DOI:10.1021/jo0348874

Tr iflu or oa cetyl a s a n Or th ogon a l P r otectin g Gr ou p for Gu a n id in es
Sandra Bartoli, Kim B. J ensen, and J eremy D. Kilburn*
Department of Chemistry, University of Southampton, Southampton SO17 1BJ , U.K
jdk1@soton.ac.uk.
Received J une 24, 2003
The trifluoroacetyl moiety has been used as a new protecting group for guanidine functionality.
The protecting group is easily cleaved under mild basic conditions and is complementary to the
Boc, Cbz, and Ddpe protecting groups. The protecting group can be applied to peptide synthesis in
solution as well as on a solid phase as it is orthogonal to a Boc and Cbz strategy and semiorthogonal
to an Fmoc strategy.
In tr od u ction
Investigations were therefore performed to find alter-
native protecting groups, which are orthogonal to pro-
tecting groups used in peptide synthesis. Our attention
has focused on the trifluoroacetyl group, which is a well-
known protecting group for amine functionality and is
easily cleaved6 but, to date, has not been used as a
protecting group for guanidines.7 The scope and limita-
tion of the trifluoroacetyl group as a protecting group for
guanidines has therefore been investigated, and the
experimental details are reported herein.
The guanidine moiety, often found in natural and
pharmaceutical compounds,1 plays an essential role in
biological systems2 and is often used in molecular rec-
ognition.3 Consequently, procedures that allow the gen-
eration of highly substituted guanidines in good yields
and the selective removal of the guanidine protecting
group under mild conditions are of great interest in
medicinal chemistry, and much effort has been directed
toward developing efficient synthetic routes for the
preparation of these compounds.4
In our research we have developed tweezer receptors5
with a guanidinium “head” group as peptide receptors.
We have prepared both split-and-mix libraries of such
receptors3h and individual tweezer compounds,3d using
solid-phase and solution methodology, but to date, in all
cases, the choice of protecting group for the guanidine
moiety has been the tosyl group (Figure 1). Cleavage of
the tosyl group, however, requires harsh conditions
(hydrofluoric acid), which presents practical difficulties
and has limited the scope of the chemistry.
Resu lts a n d Discu ssion
Various methods exist for the synthesis of guanidines
from different starting materials and reagents.4 One of
the common methods involves the guanidinylation of a
thiourea. In the protocol we have followed the thiourea
was activated as its thiouronium salt before guanidin-
ylation.3h,8
To gauge the applicability of the trifluoroacetamide
guanidinylation procedure, thioureas 3a -f with different
protecting groups were prepared, and conversion to the
corresponding guanidines was investigated. Thioureas
3a -f were synthesized from the corresponding amine 1
and isothiocyanate 2 (Scheme 1).
Alkylation of all the thioureas 3a -f with methyl iodide
and counterion exchange gave the thiouronium hexafluo-
rophosphate, which was guanidinylated in the presence
of DBU and trifluoroacetamide to give the protected
guanidine 4.
Guanidinylation of thiouroniums proceeded with very
good yields for 4a -d and 4f (Table 1, entries 1-4 and
6), whereas a moderate yield was obtained in the case of
4e (Table 1, entry 5). The reduced yield of 4e is due to
(1) (a) Greenhill, J . V.; Lue, L. In Progress in Medicinal Chemistry;
Ellis, G. P., Luscombe, D. K., Eds.; Elsevier Science: New York, 1993;
Vol. 30, Chapter 5. (b) Yamamoto, Y., Kojima, S. In Synthesis and
Chemistry of Guanidine Derivatives; Patai, S., Rappoport, Z., Eds.; J ohn
Wiley & Sons, Ltd.: New York, 1991; Vol. 2, pp 485-526.
(2) Hannon, C. L.; Anslyn, E. V. Bioorganic Chemistry Frontiers;
Springer: Berlin, 1993; Vol. 3, pp 193-255.
(3) For a review see: (a) Fitzmaurice, R. J .; Kyne, G. M.; Douheret,
D.; Kilburn, J . D. J . Chem. Soc., Perkin Trans. 1 2002, 841-864. See
also, for example: (b) Gala´n, A.; Andreu, D.; Echavarren, A.; Prados,
P.; de Mendoza, J . J . Am. Chem. Soc. 1992, 114, 1511-1512. (c)
Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609-1646. (d)
Davis, M.; Bonnat, M.; Guillier, F.; Kilburn, J . D.; Bradley, M. J . Org.
Chem. 1998, 63, 8696-8703. (e) Linton, B.; Hamilton, A. D. Tetrahe-
dron 1999, 55, 6027-6038. (f) Schneider, S. E.; O’Neil, S.; Anslyn, E.
V. J . Am. Chem. Soc. 2000, 122, 542-543. (g) Cabell, L. A.; Best, M.
D.; Lavigne, J . J .; Schneider, S. E.; Perreault, D. M.; Monahan, M. K.;
Anslyn, E. V. J . Chem. Soc., Perkin Trans. 2 2001, 315-323 and
references therein. (h) J ensen, K. B.; Braxmeier, T. M.; Demarcus, M.;
Kilburn, J . D. Chem.sEur. J . 2002, 8, 1300-1309.
(4) (a) Orner, B. P.; Hamilton, A. D. J . Inclusion Phenom. Macro-
cyclic Chem. 2001, 41, 141-147, (b) Manimala, J . C.; Anslyn, E. V.
Tetrahedron Lett. 2002, 43, 565-567 and references therein.
(5) For original work on tweezer receptors, see: (a) Chen, C.-W.;
Whitlock, H. W., J r. J . Am. Chem. Soc. 1978, 100, 4921-4922. (b)
Zimmerman, S. C.; Wu, W.; Zeng, Z. J . Am. Chem. Soc. 1991, 113,
196-201.
(6) Boger, D. L.; Yohannes, D. J . Org. Chem. 1989, 54, 2498-2502.
(7) Reaction of guanidino-R-amino acids (e.g., arginine) with tri-
fluoroacetic anhydride has been described: Klein, C.; Schulz, G.;
Steglich, W. Liebigs Ann. Chem. 1983, 1623-1637.
(8) See, for example: (a) Kurzmeier, H.; Schmidtchen, F. P. J . Org.
Chem. 1990, 55, 3749-3755. (b) Kneeland, D. M.; Ariga, K.; Lynch, V.
M.; Huang, C.-Y.; Anslyn, E. V. J . Am. Chem. Soc. 1993, 115, 10042-
10055. (c) Bonnat, M.; Bradley, M.; Kilburn, J . D. Tetrahedron Lett.
1996, 37, 5409-5412.
10.1021/jo0348874 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/24/2003
9416
J . Org. Chem. 2003, 68, 9416-9422

Trifluoroacetyl as a Protecting Group for Guanidines
F IGURE 1. Synthesis of tweezer receptors.
SCHEME 1a
a
Reagents and conditions: (a) MeOH/CH2Cl2, 20 °C; (b) CH3I, acetone; (c) NH4PF6, MeOH/CH2Cl2; (d) CF3CONH2, DBU, toluene/
CHCl3, reflux; (e) K2CO3, CH3OH/H2O; (f) NH4PF6, CH3OH.
TABLE 1. Syn th esis of Tr iflu or oa cetylgu a n id in es 5
entry
3/4/5
R1
R2
m
n
yield of 3 (%)
yield of 4 (%)
yield of 5 (%)
1
2
3
4
5
6
a
b
c
d
e
f
Boc
Boc
Boc
Cbz
Ddpe
Ddpe
Boc
Boc
Aloc
Boc
Boc
Boc
2
1
1
2
1
2
2
1
1
1
1
1
75
75
72
72
69
78
86
84
92
85
45
75
90
88
83
90
75
76
both the partial cleavage of the Ddpe protecting group9
(R1) and the formation of a cyclic byproduct with pre-
sumed structure 6, formed via nucleophilic attack of the
Ddpe nitrogen onto the thiocarbonyl.
Ddpe protecting group is observed. The guanidinylation
was not compatible with the Fmoc group which was
cleaved under the basic conditions used, and Fmoc-
protected variants of 4 could not be prepared directly by
this route. However, treatment of 4c with 9-fluorenyl-
oxycarbonyl chloride and tributyltin hydride in the
presence of a catalytic amount of Pd(0)10 gave guanidine
4g in 72% yield (Scheme 3).
The resulting guanidines 4 could in each case be
converted to the corresponding guanidinium hexafluoro-
phosphate 5 (Scheme 1) when treated with potassium
carbonate in a mixture of MeOH and water, without loss
of any of the accompanying protecting groups, except,
again, in the case of the Fmoc group. However, all of the
amine protecting groups, including the Fmoc protecting
group, could be cleaved with standard methods [(Aloc)
Pd(PPh)3, tributyltin hydride (1.2 equiv);10 (Boc) 20%
TFA, CH2Cl2; (Ddpe) aqueous hydrazine (5%);11 (Fmoc)
piperidine, CH2Cl2), giving quantitative yields of the
The corresponding byproduct is not formed when the
longer propylene spacer is used to separate the Ddpe and
the guanidine moieties, and consequently 4f is formed
in a much better yield, although some cleavage of the
(10) Roos, E. C.; Bernabe, P.; Hiemstra, H.; Speckamp, W. N.;
Kaptein B.; Boesten, W. H. J . J . Org. Chem. 1995, 60, 1733-1740.
(11) Kellam, B.; Chan, W. C.; Chhabra, S. R.; Bycroft, B. W.
Tetrahedron Lett. 1997, 38, 5391-5394.
(9) Chhabra, S. R.; Hothi, B.; Evans, D. J .; White, P. D.; Bycroft, B.
W.; Chan, W. C. Tetrahedron Lett. 1998, 39, 1603-1606.
J . Org. Chem, Vol. 68, No. 24, 2003 9417

Bartoli et al.
SCHEME 2a
a
Reagents and conditions: (a) 30% CF3CO2H, CH2Cl2; (b) N-Boc-Gly-L-Val-L-Ala-OH, EDC, HOBt, DIPEA, DMF (45%); (c) K2CO3,
MeOH/H2O.
correspondent free amines and without affecting the
trifluoroacetyl protecting group.
as on a solid support. We believe this protecting group
may find widespread use in the synthesis of both simple
and highly complex guanidine-containing compounds.
To show the utility of the new protecting group in
peptide chemistry, we chose to synthesize two tweezers,
in which the guanidine has been incorporated. The
peptide arms of the tweezer could be synthesized either
in solution using a Boc-coupling strategy (Scheme 2) or
on a solid phase using an Fmoc-coupling strategy (Scheme
3). For the peptide coupling in solution, guanidine 4a was
used as starting material. After Boc deprotection of 4a
and coupling with N-Boc-Gly-L-Val-L-Ala-OH using EDC/
HOBt, the guanidine derivative 7 was obtained in 45%
yield. Cleavage of the trifluoroacetyl protecting group
afforded the hydrogen carbonate salt of the guanidinium
tweezer 8 as a white powder in quantitative yield.
To prepare a guanidinium tweezer on a solid phase,
the ester derivative 9 was synthesized in two steps from
the guanidine derivative 4g. After Boc deprotection of 4g
and coupling with N-Fmoc-L-Glu(OtBu)-OH using Py-
BOP/HOBt, the guanidine derivative 9 was obtained in
80% yield. Deprotection of the tert-butyl group provided
10, suitably functionalized with a carboxylic acid moiety
to allow attachment to a solid support. Coupling of acid
10 to Rink amide resin and Fmoc cleavage were followed
by sequential coupling of Fmoc-Gly and Fmoc-Val. The
resin was treated with potassium carbonate in a mixture
of MeOH and water to remove the trifluoroacetyl group,
and finally, cleavage from the Rink amide resin gave the
guanidinium tweezer 11 as a white TFA salt without
notable impurities.
Exp er im en ta l Section
2-{1-[(2-Am in oet h yl)a m in o]-2-p h en ylet h ylid en e}-5,5-
d im eth yl-1,3-cycloh exa n ed ion e (1e). 2-(1-Hydroxy-2-phe-
nylethylidene)-5,5-dimethylcyclohexane-1,3-dione9 (2.3 g, 8.9
mmol) in CH2Cl2 (200 mL) was added dropwise to a solution
of ethylenediamine (2.25 g, 37.4 mmol) and TFA (70 µL, 0.91
mmol) in CH2Cl2 (10 mL), over a period of 10 h, with vigorous
stirring. The stirring was continued for a further 36 h at room
temperature. The solution was washed with a 2 M solution of
sodium carbonate (2 × 100 mL) and water (2 × 100 mL). The
organic layer was dried over magnesium sulfate and the
solvent evaporated under reduced pressure to afford 1b as a
pale yellow oil (2.70 g, 100%): IR νmax (film) ) 2961 (w), 2927
(w), 2858 (w), 2364 (w), 2339.96 (w), 1647 (s), 1556 (s), 1496
(m), 1449 (m), 1404 (m), 1036 (m) cm-1; H NMR (300 MHz,
1
CDCl3) δ 1.05 (s, 6H), 1.50 (br s, 2H), 2.38 (m, 4H), 2.89 (m,
2H), 3.39 (q, J ) 5.7 Hz, 2H), 4.60 (s, 2H), 7.20 (m, 5H), 12.32
(br s, 1H); 13C NMR (75 MHz, CDCl3) δ 28.5, 30.2, 35.4, 40.8,
45.9, 53.5, 111.8, 126.7, 128.1, 128.9, 136.1, 174.0, 197.6; MS
(ES+) m/z (rel intens) ) 601 (13) [2M + H]+.
P r oced u r e A for th e Syn th esis of Th iou r ea s 3c, 3d , 3e,
a n d 3f. Monoprotected diamine 1 (7.6 mmol) was added to
isothiocyanate 2 (6.9 mmol) in a 1:1 solvent mixture of MeOH
(10 mL) and CH2Cl2 (10 mL). After the mixture was stirred at
room temperature for 48 h, the solvent was removed under
reduced pressure. The crude product was purified by FC on
silica gel (eluting with EtOAc/CH2Cl2, 50:50 for 3c and 3f, 30:
70 for 3d , and EtOAc/petroleum ether, 80:20 for 3e).
{2-[3-(2-Vin yloxycar bon ylam in oeth yl)th iou r eido]eth yl}-
ca r ba m ic Acid ter t-Bu tyl Ester (3c). This compound was
synthesized according to procedure A on a 6.9 mmol scale:
yield 1.72 g (72%); pale yellow oil; Rf ) 0.16 (EtOAc/CH2Cl2,
50:50); IR (neat) νmax ) 3300 (m), 2964 (m), 2930 (m), 2883
In summary, a new protecting group for guanidines is
described. The protecting group is orthogonal to acid-
cleavable protecting groups as well as Cbz and Ddpe, and
is easily cleaved under mild basic conditions. The utility
of the protecting group for the guanidine moiety is shown
with examples from peptide synthesis in solution as well
(w) 1669 (s), 1521 (s), 1245 (s), 1160 (s) cm-1; H NMR (400
1
MHz, CDCl3) δ 1.45 (s, 9H), 3.31 (q, J ) 5.7 Hz, 2H), 3.42 (q,
9418 J . Org. Chem., Vol. 68, No. 24, 2003

Trifluoroacetyl as a Protecting Group for Guanidines
SCHEME 3a
a
Reagents and conditions: (a) FmocCl, Pd(PPh3)4, Bu3SnH, CH2Cl2 (72%); (b) 20% CF3CO2H, CH2Cl2; (c) Fmoc-L-Glu(OtBu)-OH, PyBOP,
HOBt, DIPEA, CH2Cl2 (80%); (d) 60% CF3CO2H, CH2Cl2; (e) Fmoc-deprotected Rink amide resin (sub 0.45 mmol/g), HOBt, PyBOP, DIPEA,
DMF/CH2Cl2; (f) acetic anhydride, DMAP, CH2Cl2; (g) 20% piperidine in DMF; (h) Fmoc-Gly-OH, DIC, HOBt, DIPEA, DMF; (i) Fmoc-
L-Val-OH, DIC, HOBt, DIPEA, DMF; (j) K2CO3, MeOH/H2O; (k) 15-30% CF3CO2H, TIS, CH2Cl2.
J ) 5.7 Hz, 2H), 3.54 (br s, 2H), 3.62 (br s, 2H), 4.57 (d, J )
5 Hz), 5.09 (br s, 1H), 5.22 (dd, J ) 10.6, 1.3 Hz, 1H), 5.30
(dd, J ) 17.1, 1.3 Hz, 1H), 5.45 (br s, 1H), 5.90 (m, 1H), 6.83
(br s, 2H); 13C NMR (100 MHz, CDCl3) δ 28.4, 39.7, 40.3, 44.9,
45.2, 65.9, 80.4, 117.9, 132.7, 157.3, 182.3; MS (ES+) m/z (rel
intens) ) 347 (5) [M + H]+, 693 (5) [2M + H]+, 715 (27) [2M
+ Na]+. All structural assignments were in agreement with
foam; Rf ) 0.20 (EtOAc/CH2Cl2, 50:50); IR (neat) νmax ) 3288
(m), 2954 (m), 2931 (m), 2869 (w) 1688 (m), 1630 (m), 1565
(s), 1496 (s), 1450 (s), 1341 (s), 1272 (s), 1248 (s), 1163 (s) cm-1
;
1H NMR (400 MHz, CDCl3) δ 0.98 (s, 6H), 1.36 (s, 9H), 1.82
(quint, J ) 7 Hz, 2H), 2.34 (s, 4H), 3.19 (t, J ) 6 Hz, 2H), 3.38
(q, J ) 7 Hz, 2H), 3.43 (br s, 4H), 4.50 (s, 2H), 7.15 (m, 5H),
13.53 (br s, 1H); 13C NMR (100 MHz, CDCl3) δ 28.1, 28.2, 28.5,
29.9, 35.0, 38.3, 39.7, 40.8, 52.8, 80.2, 108.0, 126.5, 128.0, 128.8,
136.2, 157.9, 174.3, 182.5, 197.8; MS (ES+) m/z (rel intens) )
517 (80) [M + H]+.
1
MS and H and 13C NMR data available from the literature.3h
{3-[3-(2-ter t-Bu t oxyca r b on yla m in oet h yl)t h iou r eid o]-
p r op yl}ca r ba m ic Acid Ben zyl Ester (3d ). This coumpound
was synthesized according to procedure A on a 2.53 mmol
scale: yield 750 mg (72%); white foam; Rf ) 0.19 (EtOAc/CH2-
Cl2, 30:70); IR (neat) νmax ) 3302 (m), 2961 (m), 2932 (m), 2883
P r oced u r e B for th e Syn th esis of Th iou r ea s 3a a n d
3b. 1-(tert-Butyloxycarbonyl)propyldiamine (1.2 g, 6.9 mmol)
was dissolved in chloroform (50 mL), and triethylamine (0.9
mL, 6.8 mmol) was added to the solution, which was cooled to
0 °C. Thiophosgene (264 µL, 3.4 mmol) was added dropwise
and the solution allowed to stir at room temperature for 36 h.
The solvents were evaporated under reduced pressure to give
a yellow oil. Further purification by column chromatography
(EtOAc/CH2Cl2, 25:75) afforded the pure products as white
foams.
1
(w) 1679 (s), 1521 (s), 1245 (s), 1161 (s) cm-1; H NMR (400
MHz, CDCl3) δ 1.35 (s, 9H), 1.70 (quint, J ) 6 Hz, 2H), 3.19
(m, 4H), 3.45 (br s, 4H), 5.02 (s, 3H), 5.32 (br s, 1H), 6.76 (br
s, 2H), 7.27 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 28.8, 29.8,
38.2, 40.2, 41.5, 67.2, 80.6, 128.4, 128.6, 128.9, 136.9, 157.5,
157.6, 182.3; MS (ES+) m/z (rel intens) ) 411 (20) [M + H]+,
433 (100) [M + Na]+, 449 (40) [M + K]+, 843 (25) [2M + Na]+.
[2-(3-{2-[1-(4,4-Dim eth yl-2,6-d ioxocycloh exylid en e)-2-
ph en yleth ylam in o]eth yl}th iou r eido)eth yl]car bam ic Acid
ter t-Bu tyl Ester (3e). This compound was synthesized ac-
cording to procedure A on a 8.3 mmol scale: yield 3.0 g (69%);
white foam; Rf ) 0.18 (EtOAc/petroleum ether, 80:20); IR
(neat) νmax ) 3118 (m), 2909 (w), 2250 (w), 1659 (s), 1596 (s),
1475 (s), 1406 (s), 1304 (m), 1185 (s) cm-1; 1H NMR (300 MHz,
CDCl3) δ 1.04 (s, 6H), 1.43 (s, 9H), 2.40 (s, 4H), 3.29 (m, 2H),
3.54 (br s, 2H), 3.70 (br s, 4H), 4.59 (s, 2H), 7.19 (m, 5H), 12.46
(br s, 1H); 13C NMR (100 MHz, CDCl3) δ 28.4, 28.5, 30.2, 35.4,
39.1, 42.5, 44.0, 53.0, 81.0, 108.5, 127.7, 128.3, 128.9, 141.9,
158.9, 166.1, 179.3, 198.4; MS (ES+) m/z (rel intens) ) 503
(79) [M + H]+, 1005 (20) [2M + H]+.
{3-[3-(3-ter t-Bu toxyca r bon yla m in op r op yl)th iou r eid o]-
p r op yl}ca r ba m ic Acid ter t-Bu tyl Ester (3a ). This com-
pound was synthesized according to procedure B on a 3.4 mmol
scale: yield 1.00 g (75%); Rf ) 0.28 (EtOAc/CH2Cl2, 25:75); IR
(neat) νmax ) 3295 (m), 2978 (m), 2935 (m), 1686 (m), 1625 (s),
1
1520 (s), 1251 (s), 1163 (s), cm-1; H NMR (400 MHz, CDCl3)
δ 1.37 (s, 18H, (CH3)3C), 1.68 (quint, J ) 6 Hz, 4H, CH2CH2-
CH2), 3.13 (q, J ) 6 Hz, 4H, CH2NHCO), 3.49 (br s, 4H, CH2-
NHCS), 4.87 (br s, 2H, NHBoc), 6.70 (br s, 2H, NH); 13C NMR
(100 MHz, CDCl3) δ 27.4, 28.7, 36.3, 39.8, 78.7, 155.9, 180.5;
MS (ES+) m/z (rel intens) ) 391 (100) [M + H]+, 413 (20) [M
+ Na]+.
{3-[3-(3-ter t-Bu t oxyca r b on yla m in oet h yl)t h iou r eid o]-
eth yl}ca r ba m ic Acid ter t-Bu tyl Ester (3b). This compound
was synthesized according to procedure B on a 3.0 mmol
scale: yield 821 mg (75%); Rf ) 0.25 (EtOAc/CH2Cl2, 25:75);
IR (neat) νmax ) 3334 (m), 3256 (m), 2974 (m), 2935 (m), 1676
[2-(3-{3-[1-(2,6-Dioxocycloh exilid en e)-2-p h en ylet h yl-
a m in o]p r op yl}th iou r eid o)eth yl]ca r ba m ic Acid ter t-Bu -
tyl Ester (3f). This compound was synthesized according to
procedure A on a 1 mmol scale: yield 400 mg (78%); white
J . Org. Chem, Vol. 68, No. 24, 2003 9419

Bartoli et al.
(m), 1624 (s), 1532 (s), 1437 (m), 1365 (m), 1275 (m), 1237 (s),
1160 (s), cm-1 1H NMR (400 MHz, CDCl3) δ 1.37 (s, 18H),
3.26 (q, J ) 6 Hz, 4H), 3.48 (br s, 4H), 5.09 (br s, 2H), 6.82 (br
s, 2H); 13C NMR (100 MHz, CDCl3) δ 28.8, 36.3, 40.1, 80.5,
157.5, 182.5; MS (ES+) m/z (rel intens) ) 363 (18) [M + H]+,
385 (100) [M + Na]+.
white hygroscopic foam; Rf ) 0.16 (EtOAc/CH2Cl2, 40:60); IR
(neat) νmax ) 3326 (m), 2978 (m), 1690 (m), 1630 (s), 1520 (s),
;
1
1438 (m), 1370 (m), 1257 (m), 1139 (m) cm-1; H NMR (400
MHz, CDCl3) δ 1.42 (s, 9H), 3.26 (br s, 6H), 3.47 (br s, 2H),
4.54 (d, J ) 5.5 Hz, 2H), 5.20 (d, J ) 8.0 Hz, 1H), 5.30 (d, J )
17.6 Hz, 1H), 5.68 (br s), 5.93 (m, 1H), 6.07 (br s), 6.47 (br s),
6.64 (br s), 6.96 (br s), 9.32 (br s, NHCNR); 13C NMR (100 MHz,
CDCl3) δ 27.3, 38.1, 38.6, 38.9, 39.9, 40.2, 40.9, 41.3, 41.7, 65.1,
79.1, 79.2, 115.8 (q, J ) 285 Hz), 116.5, 133.0, 156.7, 157.0,
160.7, 165.2 (q, J ) 35.0 Hz); MS (ES+) m/z (rel intens) 426
(95) [M + H]+, 448 (100) [M + Na]+. Anal. Calcd for
Gen er a l P r oced u r e for th e Syn th esis of Gu a n id in e 4.
Methyl iodide (200 µL, 2.1 mmol) was added to a stirred
solution of thiourea 3 (1.3 mmol) in acetone (20 mL), and the
reaction mixture was stirred for 18 h at room temperature.
The solvent and all volatile compounds were removed under
reduced pressure to give a slightly yellow foam. The foam was
redissolved in a 1:1 mixture of CH2Cl2 (15 mL) and MeOH (15
mL). Ammonium hexafluorophosphate (424 mg, 2.6 mmol) was
added and the resulting solution stirred for 18 h at room
temperature. The solvents were evaporated, and the yellow
oily residue was redissolved in CH2Cl2 (100 mL) and washed
with water (80 mL). The organic solution was dried over
magnesium sulfate and the solvent removed under reduced
pressure to afford the methylthiouronium hexafluorophosphate
as a white foam in quantitative yield. The hexafluorophosphate
was redissolved in a 4:1 mixture of toluene (20 mL) and
chloroform (5 mL), and neat DBU (450 µL, 3.0 mmol) and
trifluoroacetamide (542 mg, 4.8 mmol) were added. The
mixture was refluxed for 8-18 h under vigorous stirring. The
solvents were removed under reduced pressure to give an oil.
The crude product was purified by FC on silica gel using the
eluent mixture EtOAc/CH2Cl2. All the NMR spectra show that
the trifluoroacetylguanidines exist in solution as more than
one isomer/conformer; the major conformers are in all cases
the ones containing a hydrogen bond between the CO of the
trifluoroacetyl and one of the guanidine NH groups. Thus, the
symmetrical compounds 4a and 4b have only one major form,
but the unsymmetrical ones show two major isomers. 13C NMR
spectra are therefore very complex; signals for the major
isomers/conformers are reported.
{3-[N′-(3-ter t-Bu toxyca r bon yla m in op r op yl)-N′′-(2,2,2-
tr iflu or oa cetyl)gu a n id in o]p r op yl}ca r ba m ic Acid ter t-
Bu tyl Ester (4a ). This compound was synthesized according
to the general procedure on a 1.3 mmol scale: yield 524 mg
(86%); white hygroscopic foam; Rf ) 0.28 (EtOAc/CH2Cl2, 40:
60); IR (neat) νmax ) 3328 (m), 2975 (m), 2939 (m), 1692 (m),
1629 (s), 1520 (s), 1438 (m), 1371 (m), 1257 (m), 1163 (s), 1171
(s) cm-1; 1H NMR (400 MHz, CD3CN) δ 1.34 (s, 18H), 1.55 (br
s, 2H), 1.67 (br s, 2H), 3.03 (m, 4H), 3.15 (m, 2H), 3.35 (m,
2H), 5.37 (br s, 1H), 5.46 (br s, 1H), 6.37 (br s, 1H), 9.29 (br s,
1H); 13C NMR (100 MHz, CD3CN) δ 28.3, 29.6, 30.5, 37.3, 38.1,
38.4, 39.1, 79.1, 117.0 (q, J ) 285 Hz), 156.1, 156.2, 160.2,
165.3 (q, J ) 35 Hz); MS (ES+) m/z (rel intens) ) 470 (100) [M
+ H]+, 492 (35) [M + Na]+, 939 (20) [2M + Na]+, 961 (15) [2M
+ Na]+. Anal. Calcd for C19H34F3N5O5: C, 48.61; H, 7.30; N,
14.92. Found: C, 48.38; H, 7.41; N, 14.80.
C
16H26F3N5O5: C, 45.17; H, 6.16; N, 16.45. Found: C, 45.22;
H, 6.30; N, 16.32.
{3-[N′-(2-t er t -Bu t oxyca r b on yla m in oet h yl)-N′′-(2,2,2-
tr iflu or oa cetyl)gu a n id in o]p r op yl}ca r ba m ic Acid Ben zyl
Ester (4d ). This compound was synthesized according to to
the general procedure on a 1.22 mmol scale: yield 510 mg
(85%); white hygroscopic foam; Rf ) 0.28 (EtOAc/CH2Cl2, 30:
70); IR (neat) νmax ) 3329 (m), 2973 (m), 2939 (m), 1690 (m),
1629 (s), 1438 (m), 1371 (m), 1257 (m), 1163 (s), 1171 (s), 1139
1
(m) cm-1; H NMR (400 MHz, CD3CN) δ 1.42 (s, 9H), 1.60-
1.90 (m, 4H), 3.10-3.60 (m, 8H), 5.09 (s, 2H), 5.69 (br s,), 5.81
(br s), 5.89 (br s), 7.36 (m, 5H), 6.67 (br s), 9.42 (br s); 13C NMR
(100 MHz, CD3CN) δ 27.2, 28.4, 29.1, 37.0, 37.5, 37.7, 38.1,
38.3, 39.8, 40.4, 41.8, 65.6, 78.3, 78.8, 117.0 (q, J ) 286 Hz),
127.4, 127.5, 128.1, 137.1, 156.4, 160.5, 165.1 (q, J ) 35.0 Hz);
MS (ES+) m/z (rel intens) ) 490 (100) [M + H]+, 512 (35) [M
+ Na]+, 979 (10) [2M + H]+, 1001 (5) [2M + Na]+. Anal. Calcd
for C21H30F3N5O5: C, 51.53; H, 6.18; N, 14.31. Found: C, 51.29;
H, 6.43; N, 14.08.
{2-[N′-{3-[1-(2,6-Dioxocycloh exylid en e)-2-p h en ylet h -
ylam in o]eth yl}-N′′-(2,2,2-tr iflu or oacetyl)gu an idin o]eth yl}-
ca r ba m ic Acid ter t-Bu tyl Ester (4e). This compound was
synthesized according to the general procedure on a 0.12 mmol
scale: yield 31 mg (45%); white hygroscopic foam; Rf ) 0.25
(EtOAc/CH2Cl2, 75:25); IR (neat) νmax ) 3636 (m), 3568 (m),
2982 (w), 1625 (m), 1566 (s), 1520 (s), 1367 (m), 1190 (m), 1140
(s) cm-1 1H NMR (400 MHz, CDCl3) δ 1.05 (s, 6H), 1.46 (s,
;
9H), 2.41 (s, 4H), 3.00-3.80 (m, 8H), 4.52 (s, 2H), 5.12 (br s,
1H), 7.55 (br s, 1H), 7.02 (m, 2H), 7.13 (m, 1H), 7.19 (m, 2H),
9.54 (br s, 1H), 13.66 (br s, 1H); MS (ES+) m/z (rel intens) )
582 (55) [M + H]+, 604 (100) [M + Na]+, 1185 (20) [2M + Na]+;
HRMS m/z calcd for C28H39N5O5F3 [M + H]+ 582.2887, found
582.2898.
{2-[N′-{3-[1-(2,6-Dioxocycloh exylid en e)-2-p h en ylet h -
yla m in o]p r op yl}-N′′-(2,2,2-t r iflu or oa cet yl)gu a n id in o]-
eth yl}ca r ba m ic Acid ter t-Bu tyl Ester (4f). This compound
was synthesized according to the general procedure on a 0.72
mmol scale: yield 321 mg (75%); white hygroscopic foam; Rf
) 0.28 (EtOAc/CH2Cl2, 70:30); IR (neat) νmax ) 3637 (m), 3569
(m), 2982 (w), 1625 (m), 1566 (s), 1520 (s), 1367 (m), 1190 (m),
1
1140 (s), 825 (s) cm-1; H NMR (400 MHz, CD3CN) δ 0.98 (s,
6H), 1.42 (s, 9H), 1.82 (m, 2H), 2.37 (s, 4H), 3.10-3.40 (m, 8H),
4.60 (s, 2H), 6.93 (br s, 1H), 7.06 (br s, 1H), 7.21 (m, 2H), 7.30
(m, 1H), 7.38 (m, 2H), 7.60 (br s, 1H), 9.12 (br s, 1H); 13C NMR
(100 MHz, CD3CN) δ 27.0, 27.3, 28.6, 29.3, 34.2, 38.3, 38.4,
40.2, 40.4, 52.2, 79.2, 107.2, 117.0 (q, J ) 286 Hz), 126.1, 127.7,
128.3, 136.1, 156.1, 156.6, 160.5, 173.1; MS (ES+) m/z (rel
intens) ) 596 (100) [M + H]+, 1181 (10) [2M + H]+. Anal. Calcd
for C29H40N5O5F3: C, 58.48; H 6.77; N 11.76. Found: C, 58.23;
H, 6.96; N, 11.54.
Gen er a l P r oced u r e for th e Dep r otection of th e Tr i-
flu or oa ceta m id e a n d Exch a n ge of th e Cou n ter ion to th e
Hexa flu or op h osp h a te 5. Dicarbamate 4 (0.092 mmol) was
dissolved in a mixture of MeOH and water (5:2, 3.5 mL), and
potassium carbonate (100 mg, 0.725 mmol) was added. The
mixture was stirred for 3 h at room temperature.12 Solvents
were removed under reduced pressure, and the residue was
suspended in EtOAc (3 × 10 mL) and filtered. The organic
{3-[N′-(3-t er t -Bu t oxyca r b on yla m in oet h yl)-N′′-(2,2,2-
tr iflu or oa cetyl)gu a n id in o]eth yl}ca r ba m ic Acid ter t-Bu -
tyl Ester (4b). This compound was synthesized according to
the general procedure on a 0.56 mmol scale: yield 207 mg
(84%); white hygroscopic foam; Rf ) 0.16 (EtOAc/CH2Cl2, 40:
60); IR (neat) νmax ) 3328 (m), 2975 (m), 2939 (m), 1692 (m),
1629 (s), 1520 (s), 1438 (m), 1371 (m), 1257 (m), 1163 (s), 1171
1
(s), 1139 (m), cm-1; H NMR (400 MHz, (CD3)2SO) δ 1.43 (s,
18H), 3.10-3.20 (m, 6H), 3.30-3.40 (m, 2H), 5.50 (br s, 1H),
5.60 (br s, 1H), 6.60 (br s, 1H), 9.29 (br s, 1H); 13C NMR (100
MHz, (CD3CN) δ 13C NMR (100 MHz, CD3CN) δ 27.3, 38.2,
39.9, 40.5, 41.8, 78.1, 78.8, 116.8 (q, J ) 285 Hz), 156.4, 160.9,
165.2 (q, J ) 32 Hz); MS (ES+) m/z (rel intens) ) 442 (100) [M
+ H]+, 464 (30) [M + Na]+, 883 (10) [2M + H]+, 905 (10) [2M
+ Na]+. Anal. Calcd for C17H30F3N5O5: C, 46.25; H, 6.85; N,
15.86. Found: C, 45.98; H, 7.03; N, 15.60.
{2-[N ′-(2-Ally lo x y c a r b o n y la m in o e t h y l)-N ′′-(2,2,2-
tr iflu or oa cetyl)gu a n id in o]eth yl}ca r ba m ic Acid ter t-Bu -
tyl Ester (4c). This compound was synthesized according to
the general procedure on a 0.19 mmol scale: yield 74 mg (92%);
(12) Boger, D. L.; Kim, S. H.; Mori, Y.; Weng, J . H., Rogel, O.; Castle,
S. L.; McAtee, J . J . J . Am. Chem. Soc. 2001, 123, 1862-1871.
9420 J . Org. Chem., Vol. 68, No. 24, 2003

Trifluoroacetyl as a Protecting Group for Guanidines
layer was concentrated and dissolved in MeOH (5 mL).
Ammonium hexafluorophosphate was added and the solution
stirred for 3 h. The solvent was removed under reduced
pressure and the residue dissolved in EtOAc (30 mL) and
washed with water (10 mL). The solution was dried over
magnesium sulfate and the solvent removed to give the crude
product, which was purified by column chromatography (5%
MeOH in CH2Cl2) to afford the pure product.
Hz, 2H), 3.54 (q, J ) 6.0 Hz, 2H), 4.58 (s, 2H), 5.77 (br s, 1H);
6.16 (br s, 2H), 6.56 (m, 2H), 7.17 (d, J ) 7.5 Hz, 2H), 7.26 (t,
J ) 7.5 Hz, 1H), 7.33 (t, J ) 7.5 Hz, 2H), 13.67 (br s, 1H); 13
C
NMR (100 MHz, CD3CN) δ 27.0, 27.3, 29.3, 34.1, 38.5, 40.4,
41.2, 52.1, 79.3, 107.7, 126.2, 127.7, 128.4, 136.0, 155.8, 173.5;
MS (ES+) m/z (rel intens) ) 486 [M]+ (100). Anal. Calcd for
C
26H40N5O4PF6‚2H2O: C, 46.77; H, 6.64; N, 10.49. Found: C,
46.69; H, 6.34; N, 10.23.
[Di(3-[(t er t -b u t oxyca r b on yl)a m in o]p r op yla m in o)-
m eth ylen e]a m m on iu m Hexa flu or op h osp h a te (5a ). This
compound was synthesized according to the general procedure
on a 42 mg (0.090 mmol) scale: yield 42 mg (90%); white foam;
Rf ) 0.19 (5% MeOH in CH2Cl2); IR (neat) νmax ) 3411 (m),
2976 (m), 2936 (m), 1658 (s), 1635 (s), 1518 (s), 1367 (s), 1277
[2-(N′-{3-[1-(4,4-Dim eth yl-2,6-d ioxocycloh exylid en e)-2-
ph en yleth ylam in o]pr opyl}gu an idin iu m h exaflu or oph os-
p h a te)eth yl]ca r ba m ic Acid ter t-Bu tyl Ester (5f). This
compound was synthesized according to the general procedure
on a 20 mg (0.034 mmol) scale: yield 17 mg (76%); white foam;
Rf ) 0.16 (5% MeOH in CH2Cl2); IR (neat) νmax ) 2970 (m),
2885 (m), 1630 (s), 1558 (s), 1517 (s), 1449 (s), 1280 (m), 1245
1
(m), 1252 (m), 1161 (s) cm-1; H NMR (400 MHz, CD3CN) δ
1
(m), 1160 (s), 1025 (m) cm-1; H NMR (400 MHz, CD3CN) δ
1.37 (s, 18H), 1.68 (quint, J ) 6 Hz, 4H), 3.13 (q, J ) 6 Hz,
4H), 3.49 (br s, 4H), 4.87 (br s, 2H), 6.08 (br s, 2H), 6.70 (br s,
2H); 13C NMR (100 MHz, CD3CN) δ 27.3, 28.8, 36.6, 38.6, 78.5,
155.5, 156.6; MS (ES+) m/z (rel intens) ) 374 [M]+ (100). Anal.
Calcd for C17H34N5O4PF6‚2H2O: C, 36.76; H, 7.26; N, 12.61.
Found: C, 36.61; H, 7.31; N, 12.40.
1.02 (s, 6H), 1.40 (s, 9H), 1.81 (quint, J ) 7.0 Hz, 2H), 2.35 (s,
4H), 3.09-3.17 (m, 6H), 3.41 (q, J ) 6.0 Hz, 2H), 3.54 (q, J )
7.0 Hz, 2H), 4.56 (s, 2H), 5.72 (br s, 1H), 6.15 (br s, 2H), 6.45-
6.55 (m, 2H), 7.16 (d, J ) 7.0 Hz, 2H), 7.23 (t, J ) 7.0 Hz,
1H), 7.29 (t, J ) 7.5 Hz, 2H), 13.64 (br s, 1H); 13C NMR (100
MHz, CD3CN) δ 27.0, 27.3, 27.6, 29.3, 34.2, 38.6, 40.0, 41.7,
52.2, 79.3, 107.3, 126.1, 127.7, 128.3, 136.1, 155.6, 173.2; MS
(ES+) m/z (rel intens) ) 500 [M]+ (100). Anal. Calcd for
[D i(3-[(t er t -b u t o x y c a r b o n y l)a m in o ]e t h y la m in o )-
m eth ylen e]a m m on iu m Hexa flu or op h osp h a te (5b). This
compound was synthesized according to the general procedure
on a 20 mg (0.045 mmol) scale: yield 20 mg (88%); white foam;
Rf ) 0.17 (5% MeOH in CH2Cl2); IR (neat) νmax ) 3409 (m),
2966 (m), 2934 (m), 1658 (s), 1635 (s), 1516 (s), 1360 (s), 1275
C
27H42N5O4PF6‚H2O: C, 48.87; H, 6.68; N, 10.55. Found: C,
48.69; H, 6.69; N, 10.30.
{2-[N ′-[2-(9H -F lu or e n -9-ylm e t h oxyca r b on yla m in o)-
e t h yl]-N ′′-(2,2,2-t r iflu or oa ce t yl)gu a n id in o]e t h yl}ca r -
ba m ic Acid ter t-Bu tyl Ester (4g). 9-Fluoroenylmethyl chlo-
roformate (184 mg, 0.7 mmol) was added to a solution of 4c
(272 mg, 0.64 mmol) in CH2Cl2 (10 mL) followed by the
addition of a palladium tetrakis(triphenylphosphine) solution
(36 mg, 32 µmol in 2 mL of CH2Cl2) and tributyltin hydride
(206 µL, 0.77 mmol).10 After the additions the solution was
stirred for 1 h at room temperature. The solvent was removed
under reduced pressure and the crude product washed with
petroleum ether. Purification by column chromatography on
silica gel (CH2Cl2/EtOAc, 50:50) afforded 4g (258 mg, 72%) as
a white solid: mp 152-154 °C; Rf ) 0.38 (CH2Cl2/EtOAc, 50:
50); 1H NMR (400 MHz, CDCl3) δ 1.34 (s, 9 H), 3.16-3.56 (br
s, 8H), 4.12 (t, J ) 7.0 Hz, 1H), 4.28-4.38 (br m, 2H), 5.00 (br
s), 5.32 (br s), 5.37 (br s), 5.92 (br s), 7.03 (br s), 7.23 (t, J )
7.5 Hz, 2H), 7.32 (t, J ) 7.5 Hz, 2H), 7.50 (d, J ) 7.5 Hz, 2H),
7.68 (d, J ) 7.5 Hz, 2H), 9.54 (br s); 13C NMR (100 MHz, CDCl3)
δ 28.7, 39.7, 40.1, 40.8, 41.2, 41.8, 42.5, 43.4, 47.6, 67.4, 81.4,
117.3 (q, J ) 285 Hz), 120.4, 125.5, 127.5, 128.1, 141.7, 144.3,
157.6, 157.8, 161.7, 167.2 (q, J ) 35.0 Hz); MS (ES+) m/z (rel
intens) ) 564 (43) [M + H]+, 586 (100) [M + Na]+.
1
(m), 1252 (m), 1161 (s) cm-1; H NMR (400 MHz, CD3CN) δ
1.43 (s, 18H), 3.20-3.22 (m, 8H), 5.70 (br s, 2H), 6.26 (br s,
2H), 6.62 (br s, 2H); 13C NMR (100 MHz, CD3CN) δ 27.3, 38.5,
41.7, 79.1, 155.9, 157.2; MS (ES+) m/z (rel intens) ) 346 [M]+
(100). Anal. Calcd for C15H32N5O4PF6‚2H2O: C, 34.16; H, 6.88;
N, 13.28. Found: C, 34.25; H, 6.60; N, 13.32.
([(2-{[(Allyloxy)car bon yl)am in o}eth yl)am in o]({2-[(ter t-
b u t o x y c a r b o n y l)a m i n o ]e t h y l }a m i n o )m e t h y le n e ]-
a m m on iu m Hexa flu or op h osp h a te (5c). This compound
was synthesized according to the general procedure on a 20
mg (0.047 mmol) scale: yield 18 mg (83%); white foam; Rf )
0.17 (5% MeOH in CH2Cl2); IR (neat) νmax ) 3414 (m), 2975
(m), 2937 (m), 1659 (s), 1634 (s), 1517 (s), 1367 (s), 1249 (m),
1
1249 (m), 1159 (s) cm-1; H NMR (400 MHz, CD3CN) δ 1.34
(s, 9H), 3.09-3.16 (m, 4H), 3.16-3.20 (m, 4H), 4.45 (d, J )
6.0 Hz), 5.12 (dd, J ) 11, 1 Hz, 1H); 5.12 (dd, J ) 17, 1 Hz,
1H), 5.59 (br s, 1H), 5.80-5.90 (m, 2H), 6.15 (br s, 2H), 6.42-
6.50 (m, 2H); 13C NMR (100 MHz, CD3CN) δ 27.3, 38.5, 38.9,
41.5, 41.6, 65.0, 79.2, 116.3, 133.0, 155.9, 157.1; MS (ES+) m/z
(rel intens) ) 330 [M]+. Anal. Calcd for C14H27N5O4PF6‚H2O:
C, 34.08; H, 6.13; N, 14.19. Found: C, 34.29; H, 6.06; N, 13.92.
{3-[N ′-(2-t er t -B u t o x y c a r b o n y la m in o e t h y l)g u a n i-
din iu m h exaflu or oph osph ate]pr opyl}car bam ic Acid Ben -
zyl Ester (5d ). This compound was synthesized according to
the general procedure on a 45 mg (0.092 mmol) scale: yield
45 mg (90%); white foam; Rf ) 0.17 (5% MeOH in CH2Cl2); IR
(neat) νmax ) 3417 (m), 2975 (m), 2938 (m), 1659 (s), 1633 (s),
1516 (s), 1453 (m), 1367 (m), 1247 (s), 1158 (s) cm-1; 1H NMR
(400 MHz, CD3CN) δ 1.33 (s, 9H), 1.64 (quint, J ) 7.0 Hz,
2H), 3.07-3.12 (m, 8H), 4.98 (s, 2H), 5.59 (br s, 1H), 5.71 (br
s, 1H), 6.07 (br s, 2H), 6.40 (br s, 2H), 7.28 (m, 5H); 13C NMR
(100 MHz, CD3CN) δ 27.3, 28.4, 37.2, 38.6, 41.7, 65.8, 79.2,
127.4, 127.7, 128.2, 137.0, 155.7, 156.8, 156.9; MS (ES+) m/z
(rel intens) ) 394 [M]+ (100). Anal. Calcd for C19H32N5O4PF6‚
H2O: C, 40.94; H, 6.15; N, 12.56. Found: C, 41.05; H, 6.01; N,
12.23.
[2-(N′-{3-[1-(4,4-Dim eth yl-2,6-d ioxocycloh exylid en e)-2-
p h en yleth yla m in o]eth yl}gu a n id in iu m h exa flu or op h os-
p h a te)eth yl]ca r ba m ic Acid ter t-Bu tyl Ester (5e). This
compound was synthesized according to the general procedure
on a 20 mg (0.034 mmol) scale: yield 16 mg (75%); white foam;
Rf ) 0.15 (5% MeOH in CH2Cl2); IR (neat) νmax ) 2969 (m),
2884 (m), 1629 (s), 1560 (s), 1517 (s), 1449 (s), 1282 (m), 1247
(m), 1158 (s) cm-1; 1H NMR (400 MHz, CD3CN) δ 1.04 (s, 6H),
1.43 (s, 9H), 2.39 (s, 4H), 3.18-3.20 (m, 4H), 3.31 (q, J ) 6.0
(4S)-4-(9H-F lu or en -9-ylm eth oxyca r bon yla m in o)-4-{2-
[N′-[2-(9H-flu or en -9-ylm eth oxyca r bon yla m in o)eth yl]-N′′-
(2,2,2-t r iflu o r o a c e t y l)g u a n id in o ]e t h y lc a r b a m o y l}-
bu tyr ic Acid ter t-Bu tyl Ester (9). 4g (204 mg, 0.36 mmol)
was stirred in a 20% solution of TFA in CH2Cl2 (12 mL) at
room temperature for 2 h. After addition of toluene (75 mL)
the solvents were removed under reduced pressure to yield
the corresponding TFA salt as an oil. A solution of N-Fmoc-
L-Glu(OtBu)-OH (183 mg, 0.43 mmol), PyBOP (224 mg, 0.43
mmol), and HOBt (66 mg, 0.43 mmol) in CH2Cl2 (20 mL) was
stirred at room temperature for 10 min and then added to a
solution of the TFA salt in CH2Cl2 (10 mL) followed by the
addition of DIPEA (0.17 mL, 0.96 mmol). After the reaction
had been stirred for 18 h, the solvent was removed under
reduced pressure to give a brown oil. Purification by column
chromatography on silica gel (CH2Cl2/EtOAc, 50:50) gave 9
rt
(250 mg, 80%) as a white solid: mp 162 °C; [R]D ) -1.7° (c )
3.0 mg/mL, l ) 2.0 dm, (CH3)2SO); Rf ) 0.16 (CH2Cl2/EtOAc,
50:50); 1H NMR (400 MHz, (CD3)2SO) δ 1.48 (s, 9H), 1.84 (m,
1H), 1.98 (m, 1H), 2.31 (m, 2H), 3.27-3.42 (m, 8H), 4.07 (m,
1H), 4.34-4.41 (m, 6H), 7.42 (t, J ) 7.5 Hz, 4H), 7.51 (t, J )
7.5 Hz, 2H), 7.52 (t, J ) 7.5 Hz, 2H), 7.60-7.78 (br m), 7.82
(d, J ) 7.0 Hz, 2H), 7.84 (d, J ) 7.0 Hz, 2H), 7.99 (d, J ) 7.7
Hz, 2H), 8.00 (d, J ) 7.7 Hz, 2H), 8.17 (br s), 8.25 (br s), 9.25
J . Org. Chem, Vol. 68, No. 24, 2003 9421

Bartoli et al.
(br s), 9.29 (br s); 13C NMR (100 MHz, (CD3)2SO) δ 28.6, 32.3,
40.4, 40.8, 41.1, 41.3, 41.5, 41.9, 42.0, 47.6, 66.6, 80.6, 121.1,
126.0, 126.3, 128.0, 128.5, 141.7, 144.7, 144.9, 156.9, 157.1,
162.0, 172.5; MS (ES+) m/z (rel intens) 871 (10) [M + H]+, 893
(50) [M + Na]+; HRMS (ES+) m/z calcd for C46H49F3N6NaO8
[M + Na]+ 893.3456, found 893.3467.
(0.17 mmol/g). A second coupling was performed with carbox-
ylic acid 10 (18.7 mg, 23 µmol), PyBOP (15 mg, 23 µmol), HOBt
(9 mg, 23 µmol), and DIPEA (9 µL, 52 µmol). The resins were
agitated on a tube rotator for 24 h at room temperature, then
washed with CH2Cl2 (3 × 5 mL), DMF (3 × 5 mL), and CH2-
Cl2 (3 × 5 mL), and dried. A second quantitative ninhydrin
test indicated the presence of free amino groups on the resin
(∼0.10 mmol/g). To cap remaining free amino functions, the
resin was agitated on a tube rotator for a further 18 h after
addition of acetic anhydride (20 µL, 0.21 mmol), DIPEA (40
µL, 0.22 mmol), and DMAP (0.4 mg, 3 µmol) in CH2Cl2 (5 mL).
Subsequent washing with CH2Cl2 (3 × 5 mL), DMF (3 × 5
mL), and CH2Cl2 (3 × 5 mL) afforded a resin, which gave a
negative ninhydrin test. Fmoc deprotection was achieved as
described above in the general procedures, and subsequent
washing with CH2Cl2 (3 × 5 mL), DMF (3 × 5 mL), and CH2-
Cl2 (3 × 5 mL) yielded a resin, which gave a positive ninhydrin
test.
Tw eezer 8. Compound 4a (220 mg, 0.47 mmol) was stirred
in a 20% solution of TFA in DCM (20 mL) for 3 h. After
addition of toluene (80 mL) the solvents were removed under
reduced pressure to give a yellow oil. The residue was
redissolved in a mixture of THF (5 mL) and DMF (5 mL) and
cooled to 0 °C. To the solution were added N-Boc-Ala-Val-Gly-
OH (436 mg, 1.25 mmol), HOBt (192 mg, 1.42 mmol), DIPEA
(400 µL, 2.3 mmol), and EDC (240 mg, 1.25 mmol), and the
mixture was stirred for 18 h (0 °C to room temperature). THF
was removed and the residue diluted with DCM (100 mL) and
washed with a 1 M solution of sodium hydrogen carbonate (50
mL), a 1 M solution of sodium hydrogen sulfate (50 mL), and
brine (50 mL). The organic layer was dried over magnesium
sulfate, and after the solvent was removed, the residue was
washed with ether, dissolved in the minimum amount of CH2-
Cl2 (1 mL), and precipitated with ether (5 mL) to yield 195
mg of 7 as a cream-colored solid (45%): 1H NMR (400 MHz,
CD3OD) δ 0.91 (d, J ) 6 Hz, 12H), 1.21 (d, J ) 7 Hz, 6H), 1.39
(s, 18H), 1.64 (m, 2H), 1.76 (m, 2H), 2.07 (m, 2H), 3.15-3.30
(m, 6H), 3.32-3.44 (m, 2H), 3.77 (m, 2H), 4.00-4.20 (m, 4H).
Tweezer 7 (25 mg, 0.027 mmol) was dissolved in a mixture of
MeOH and water (5:2, 3.5 mL), and potassium carbonate (100
mg, 0.725 mmol) was added. The mixture was stirred for 3 h
at room temperature.12 The solvents were then removed under
reduced pressure, and the residue was extracted with EtOAc
(3 × 20 mL) and chloroform (2 × 20 mL). The combined organic
layer was then concentrated to afford 8 as a white solid (23
Quantities of reagents required were calculated on the basis
of the loading of the commercially available Rink amide resin
(100 mg, 45 µmol; two amino sites per guanidine unit, 90 µmol
of free NH2). All deprotection steps were monitored by ninhy-
drin tests. A solution of Fmoc-Gly (43 mg, 144 µmol), DIC (23
µL, 144 µmol), and HOBt (22 mg, 144 µmol) in DMF (3 mL)
was stirred for 5 min and then added to the preswollen resin
in DMF (2 mL) followed by addition of DIPEA (56 µL, 324
µmol). After agitation on a tube rotator for 18 h at room
temperature, the resin was drained, washed with CH2Cl2 (3
× 5 mL), DMF (3 × 5 mL), and CH2Cl2 (3 × 5 mL), and dried.
A ninhydrin test indicated complete coupling. An Fmoc depro-
tection followed by coupling of Fmoc-L-valine (49 mg, 144 µmol)
provided the resin-bound trifluoroacetyl-protected tweezer
after a final Fmoc deprotection. Cleavage of the trifluoroacetyl
group on the guanidine was achieved by stirring the resin in
a potassium carbonate solution (0.15 M; MeOH/DMF/water,
2:2:1; 5 mL) at room temperature for 3 h. Cleavage of the
tweezer 11 from the Rink amide resin was performed in
polypropylene filtration tubes with polyethylene frits by letting
a cleavage mixture percolate slowly through the resin. A
cleavage mixture of 15% TFA, 1% TIS, 1% water, and 83%
CH2Cl2 (10 mL) was first used followed by a 30% TFA, 1%
TIS, 1% water, and 68% CH2Cl2 cleavage solution (10 mL).
After addition of toluene (4 mL), solvents were removed under
reduced pressure, and a white TFA salt of 11 precipitated after
addition of diethyl ether to the oily crude. The TFA salt was
triturated with diethyl ether and centrifuged. The precipitate
was washed once more with diethyl ether and dried to give
the TFA salt of 11 in a quantitative yield (24.1 mg) without
rt
mg, 96% yield): mp 119-120 °C; [R]D ) -15 (c ) 3.0 mg/
mL, l ) 2.0 dm, (CH3)OH); 1H NMR (400 MHz, CD3CN) δ 0.92
(d, J ) 7 Hz, 12H), 1.26 (d, J ) 7 Hz, 6H), 1.39 (s, 18H), 1.70
(m, 4H), 2.10 (m, 2H), 3.00-3.25 (m, 8H), 3.74 (d, J ) 8.0 Hz,
4H), 3.90-4.10 (m, 4H), 5.95 (br s, 2H), 6.98 (br s, 2H), 7.11
(br s, 2H), 7.23 (d, J ) 8.0 Hz, 2H); 13C NMR (100 MHz, CD3-
CN) δ 16.5, 17.2, 18.3, 27.3, 28.1, 29.6, 35.7, 38.4, 42.5, 50.4,
59.4, 79.1, 155.9, 160.8, 169.3, 171.9, 174.2; MS (ES+) m/z (rel
intens) ) 828 (100) [M]+; analytical data, HPLC 3.50 min
(S5OD, gradient 60-45% MeCN, analytical).
Gen er a l P r oced u r e for F m oc Dep r otection on a Solid
P h a se. The Fmoc-protected resin was suspended in a solution
of 20% piperidine in DMF (20 mL/g of resin) and agitated for
30-45 min. The resin was drained and washed with CH2Cl2
(3×), DMF (3×), and CH2Cl2 (3×) (10 mL of solvent/g of resin).
The procedure was repeated once and the progress of the
deprotection monitored by the ninhydrin test.
Gen er a l P r oced u r e for Boc Dep r otection on a Solid
P h a se. The Boc-protected resin was suspended in a mixture
of 45% CH2Cl2, 45% TFA, 4% EDT, 3% DMS, and 3% anisole
(20 mL/g of resin) and agitated for 60-120 min. The resin was
drained and washed with CH2Cl2 (3×), DMF (3×), a 20%
solution of DIPEA in CH2Cl2 (3×), MeOH (3×), DMF (3×), and
CH2Cl2 (3×) (10 mL of solvent/g of resin). The DIPEA solution
wash was omitted for samples requiring prolonged storage.
The progress of the deprotection was monitored by the nin-
hydrin test.
rt
any byproducts: [R]D ) 7.6° (c ) 5.0 mg/mL, l ) 2.0 dm, H2O);
1H NMR (400 MHz, D2O) δ 0.97 (d, J ) 7.0 Hz, 12H), 1.89 (m,
1H), 2.01 (m, 1H), 2.16 (m, 2H), 2.28 (t, 2H, J ) 8.0 Hz), 3.26-
3.34 (m, 8H), 3.80 (d, 2H, J ) 6.0 Hz), 3.91 (s, 2H), 3.96 (s,
2H), 4.23 (dd, J ) 9.5, 4.0 Hz, 1H); 13C NMR (100 MHz, D2O)
δ 15.9, 16.5, 26.0, 28.9, 30.1, 37.1, 39.6, 41.2, 41.3, 52.4, 57.7,
58.0, 115.4 (q, J ) 290.0 Hz) 155.1, 162.2 (q, J ) 35.0 Hz),
169.0, 169.9, 170.3, 172.8, 176.8; MS (TOF LD+) m/z (rel
intens) ) 586.6 (100) [M + H]+, 608.6 (20) [M + Na]+;
analytical data, HPLC 3.50 min (S15OD, gradient 50-60%
MeCN, analytical).
P r ep a r a tion of Tw eezer 11. The carboxylic acid 10 (46.7
mg, 57 µmol) was dissolved in a solvent mixture (DMF/CH2-
Cl2, 2:1; 3 mL) followed by addition of PyBOP (30 mg, 57 µmol)
and HOBt (8.7 mg, 57 µmol). The reaction mixture was stirred
for 5 min and then added to a preswollen and Fmoc-depro-
tected Rink amide resin (100 mg, 45 µmol of free NH2) in DMF
(2 mL) followed by addition of neat DIPEA (23 µL, 128 µmol).
After agitation on a tube rotator for 24 h at room temperature,
the resin was drained, washed with CH2Cl2 (3 × 5 mL), DMF
(3 × 5 mL), and CH2Cl2 (3 × 5 mL), and dried. A ninhydrin
test indicated the presence of free amino groups on the resin
Ack n ow led gm en t. We thank EPSRC (Grant GR/
L98916) for funding, the EC for a Marie Curie fellowship
for K.B.J ., and the Leverhulme Trust for a Research
Fellowship for J .D.K..We also thank Allison Carley for
assistance in the preparation of some intermediates.
Su p p or tin g In for m a tion Ava ila ble: General methods,
materials, and abbreviations. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O0348874
9422 J . Org. Chem., Vol. 68, No. 24, 2003