Stereoselective Modification of a Cyclopentapeptide via an α-(Ethylthio)glycine Residue

Article abstract of DOI:10.1021/jo971295a

The solid-phase synthesis of cyclo[Val-D-Gly(SEt)-Pro-Phe-D-Ala] (5) on Kaiser's oxime resin is described. Conversion of 5 to the reactive α-chloroglycine derivative 9 allowed the stereoselective addition of thiols, alcohols, amines and other nucleophiles with formation of the modified cyclopeptides 11a - n.

Full text of DOI:10.1021/jo971295a

8474
J . Org. Chem. 1997, 62, 8474-8478
Ster eoselective Mod ifica tion of a Cyclop en ta p ep tid e via a n
r-(Eth ylth io)glycin e Resid u e†
Christian Paulitz and Wolfgang Steglich*
Institut fu¨r Organische Chemie der Universita¨t Mu¨nchen Karlstrasse 23, D-80333 Mu¨nchen, Germany
Received J uly 15, 1997X
The solid-phase synthesis of cyclo[Val-D-Gly(SEt)-Pro-Phe-D-Ala] (5) on Kaiser’s oxime resin is
described. Conversion of 5 to the reactive R-chloroglycine derivative 9 allowed the stereoselective
addition of thiols, alcohols, amines and other nucleophiles with formation of the modified
cyclopeptides 11a -n .
In tr od u ction
Resu lts a n d Discu ssion
Several naturally occurring1-3 and synthetic4-11 cyclo-
peptides exhibit important biological activities. To op-
timize these properties, e.g. their binding to a recep-
tor,12,13 methods for the modification of cyclopeptides
without de novo peptide syntheses are of interest. See-
bach et al.14,15 used this approach for the synthesis of
cyclosporine A analogues by means of enolate alkylation.
We now describe the structural variation of a cyclopeptide
by use of R-(ethylthio)glycine as a stable electrophilic
glycine equivalent.16-20
Syn th esis of Cyclop ep tid e 5. cyclo[Val-D-Gly(SEt)-
Pro-Phe-D-Ala]21 (5) was chosen as a model compound.
This peptide sequence should facilitate NMR spectro-
scopic characterization and was expected to form only one
stable conformation in solution.4 Synthesis was per-
formed on Kaiser’s 4-nitrobenzophenone oxime resin22-24
starting from the D-alanine derivative 1 by application
of standard Boc methodology (Scheme 1). Due to the
known instability of N-deprotected R-heterosubstituted
glycine derivatives,25 the R-(ethylthio)glycyl residue was
introduced by coupling of resin bound H-Pro-Phe-D-Ala-
OH (2) with Boc-Val-DL-Gly(SEt)-OH (3) (vide supra)
using TBTU [2-(1H-benzotriazol-1-yl)-1,1,3,3-tetrameth-
yluronium tetrafluoroborate] as coupling reagent.26,27
During the fragment coupling the reaction time for
complete acylation had to be increased to 24 h. After
removal of the Boc protecting group from the resin bound
pentapeptide 4 only the cyclopentapeptide 5 with the
D-(R-ethylthio)glycyl residue was released28,29 whereas the
L-epimer remained fixed to the resin. This points to a
remarkable difference in the cyclization tendency of both
epimers.
The dipeptide 3 was obtained from Boc-Val-Ser-OMe
(6) in 79% overall yield. Oxidative degradation of the
serine residue in 6 with lead(IV)acetate followed by
treatment of the resulting acetoxy derivative 7 with
ethanethiol/triethylamine16-18 afforded the ester 8 which
was hydrolyzed to 3 with lithium hydroxide (Scheme 2).
St r u ct u r e of Cyclop ep t id e 5. NMR spectroscopic
evidence indicated a single preferred conformation of
cyclopeptide 5 in DMSO-d6 and CDCl3 solution at 25 °C.
The temperature gradients (-∆δ/T) for the NH-protons
of 5 in DMSO-d6 (300 MHz) were 0.3 × 10-3 for D-Ala
† This paper is dedicated to Professor Dieter Seebach on the occasion
of his 60th birthday.
X Abstract published in Advance ACS Abstracts, November 1, 1997.
(1) Auvin, C.; Baraguey, C.; Blond, A.; Lezenven, F.; Pousset, J .-L.;
Bodo, B. Tetrahedron Lett. 1997, 38, 2845-2848, and references cited
herein.
(2) Schmidt, U. Pure Appl. Chem. 1986, 58, 295-304.
(3) Schmidt, U. Nachr. Chem., Technol. Lab. 1989, 37, 1034-43.
(4) Kessler, H. Angew. Chem. 1982, 94, 509-520; Angew. Chem.,
Int. Ed. Engl. 1982, 21, 512-523.
(5) Kessler, H.; Gratias, R.; Hessler, G.; Gurrath, M.; Mu¨ller, G. Pure
Appl. Chem. 1996, 68, 1201-1205.
(6) Kessler, H.; Diefenbach, B.; Finsinger, D.; Geyer, A.; Gurrath,
M.; Goodman, S. L.; Ho¨lzemann, G.; Haubner, R.; J onczyk, A.; Mu¨ller,
G.; Graf von Roedern, E.; Wermuth, J . Lett. Pept. Sci. 1995, 2, 155-
160.
(7) Veber, D. F.; Freidinger, R. M.; Perlow, D. S.; Paleveda, W. J .;
Holly, F. W.; Strachan, R. G.; Nutt, R. F.; Arison, B. H.; Homnick, C.;
Randall, W. C.; Glitzer, M. S.; Saperstein, R.; Hirschmann, R. Nature
1981, 292, 55-58.
(8) Peishoff, C. E.; Ali, F. E.; Bean, J . W.; Calvo, R.; D′Ambrosio, C.
A.; Eggleston, D. S.; Hwang, S. M.; Kline, T. P.; Koster, P. F.; Nichols,
A.; Powers, D.; Romoff, T.; Samanen, J . M.; Stadel, J .; Vasko, J . A.;
Kopple, K. D. J . Med. Chem. 1992, 35, 3962-3969.
(9) Wermuth, J .; Goodman, S. L.; J onczyk, A.; Kessler, H. J . Am.
Chem. Soc. 1997, 119, 1328-1335, and references cited herein.
(10) Etzkorn, F. A.; Guo, T.; Lipton, M. A.; Goldberg, S. D.; Bartlett,
P. A. J . Am. Chem. Soc. 1994, 116, 10412-10425.
(11) Bach II, A. C.; Espina, J . R.; J ackson, S. A.; Stouten, P. F. W.;
Duke, J . L.; Mousa, S. A.; DeGrado, W. F. J . Am. Chem. Soc. 1996,
118, 293-294.
(21) To avoid confusion, the stereodescriptors D and L are used for
describing the stereochemistry of R-(heteroalkyl)glycine residues, the
“side chains” being considered in the usual way according to the Fischer
convention.
(12) Giannis, A.; Kolter, T. Angew. Chem. 1993, 106, 1303-1326;
Angew. Chem., Int. Ed. Engl. 1993, 32, 1244-1267.
(13) Cornish, V. W.; Mendel, D.; Schultz, P. G. Angew. Chem. 1995,
107, 677-690; Angew. Chem., Int. Ed. Engl. 1995, 34, 621-633.
(14) Seebach, D.; Beck, A. K.; Bossler, H. G.; Gerber, C.; Ko, S. Y.;
Murtiashaw, C. W.; Naef, R.; Shoda, S.-i.; Thaler, A.; Krieger, M.;
Wenger, R. Helv. Chim. Acta 1993, 76, 1564-1590.
(15) Seebach, D.; Beck, A. K.; Studer, A. In Modern Synthetic
Methods 1995; Ernst, B., Leumann, C., Eds.; Verlag Helvetica Chimica
Acta: Basel, 1995; pp 60-178.
(22) Nishino, N.; Xu, M.; Mihara, H.; Fujimoto, T. Bull. Chem. Soc.
J pn. 1992, 65, 991-994.
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1300.
(24) DeGrado, W. F.; Kaiser, E. T. J . Org. Chem. 1982, 47, 3258-
3261.
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Gilvarg, C. J . Med. Chem. 1984, 27, 1447-1451.
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4564.
(27) cf. Niddam, V.; Camplo, M.; Nguyen, D. L.; Chermann, J .-C.;
Kraus, J .-L. Bioorg. Med. Chem. Lett. 1996, 6, 609-614.
(28) Mihara, H.; Yamabe, S.; Niidome, T.; Aoyagi, H. Tetrahedron
Lett. 1995, 36, 4837-4840.
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43, 6121-6124.
(16) Apitz, G.; Steglich, W. Tetrahedron Lett. 1991, 32, 3163-3166.
(17) Apitz, G.; J a¨ger, M.; J aroch, S.; Kratzel, M.; Scha¨ffeler, L.;
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(18) Steglich, W.; J a¨ger, M.; J aroch, S.; Zistler, P. Pure Appl. Chem.
1994, 66, 2167-2170.
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S0022-3263(97)01295-4 CCC: $14.00 © 1997 American Chemical Society

Stereoselective Modification of a Cyclopentapeptide
J . Org. Chem., Vol. 62, No. 24, 1997 8475
Sch em e 1
Sch em e 2
F igu r e 1. Important NOESY correlations in cyclopeptide 5.
Sch em e 3
and 2.0 × 10-3 for Gly(SEt) in accord with two intramo-
lecular hydrogen bonds, forming a γ-turn around Val and
another turn (γ or â) on the opposite side of the cyclo-
peptide. The gradients for Phe (5.3 × 10-3) and for Val
(7.0 × 10-3) indicated exposure to the solvent.4 H,H-
COSY and NOESY spectra recorded at 600 MHz in a
mixture of pyridine-d5/DMSO-d6 5/2 allowed peak as-
signment for every single proton. Especially the diaste-
reotopic Pro δ-protons could be distinguished by deter-
mining the NOE connectivity of the protons on both sides
of the ring. Consequently, the D-configuration of the
R-(ethylthio)glycyl residue was deduced from the NOE
correlation marked with an asterisk in Figure 1, showing
the possible â/γ-turn structure of 5 together with impor-
tant NOESY correlations.
Rea ction of th e r-Ch lor oglycyl d er iva tive 9 w ith
Nu cleop h iles. On reaction with sulfuryl chloride, cy-
clopeptide 5 was smoothly converted to the R-chloroglycyl
derivative 9 (Scheme 3). Treatment of chloride 9 at -78
°C with tertiary amines generated the reactive dehydro-
glycine intermediate 10 which added nucleophiles in situ
to form the cyclopeptide derivatives 11a -m (Table 1).16-18
rigid structure of 10. The stereochemistry of the major
diastereomers of 11k -m was determined as “D”21 by
means of 2D NOESY experiments as described for
cyclopeptide 5. The H NMR data of all other adducts
were in such close agreement that the D-configuration
can also be assumed for these compounds.
1
In contrast to linear R-chloroglycyl peptides16-18 cyclo-
peptide 9 adds simple nucleophiles such as thiols, amines,
and alcohols highly stereoselective with retention of the
configuration.31 This observation can be explained by
steric shielding of the “upper” side of the conformationally
(30) Kober, R.; Papadopoulos, K.; Miltz, W.; Enders, D.; Steglich,
W.; Reuter, H.; Puff, H. Tetrahedron 1985, 41, 1693-1701.
(31) For enhanced stereoselectivity in alkylations of cyclopeptides
in comparance to linear peptides, see: Seebach, D.; Bezenc¸on, O.; J aun,
B.; Pietzonka, T.; Matthews, J . L.; Ku¨hnle, F. N. M.; Schweizer, W. B.
Helv. Chim. Acta 1996, 79, 588-608.

8476 J . Org. Chem., Vol. 62, No. 24, 1997
Paulitz and Steglich
tides yield mainly the corresponding ethers.35,36 Reaction
of 9 with trimethyl phosphite afforded the optically pure
phosphonate 11h , which failed to react with aromatic
aldehydes even under drastic Horner-Wittig reaction
conditions.16
Ta ble 1. P r od u cts 11a -n Obta in ed by th e Ad d ition of
Nu cleop h iles Nu H to r-Ch lor oglycyl Cyclop ep tid e 9
n
Treatment with Bu3SnD/AIBN converts cyclopeptide
5 directly to a 4.4:1 mixture of the corresponding mono-
deutero compounds 11m in which the R-epimer is the
major product (de 63%).17,37-39
In summary, these results demonstrate the easy
introduction of an R-(ethylthio)glycyl residue in a cyclo-
peptide by standard solid-phase methods. The exchange
of the ethylthio group against other residues allows the
synthesis of numerous peptide analogues from a single
starting material. Experiments to determine the degree
of stereoselectivity for these transformations with less
constrained cyclopeptides are in progress. The use of this
technique for combinatorial approaches, e.g. the synthesis
of libraries from libraries,40 and the optimization of
biologically active peptide ligands is under investigation.
Exp er im en ta l Section
Gen er a l Meth od s. Satisfactory elementary analysis and/
or high-resolution FAB mass spectra have been obtained for
all new compounds. CH2Cl2 was distilled from Sicapent and
THF from potassium. Phosphazene base P1-tert-butyl was
purchased from Fluka and sulfuryl chloride as a 1 M solution
in CH2Cl2 from Aldrich. All reactions were carried out under
an argon atmosphere. Kaiser’s 4-nitrobenzophenone oxime
resin was prepared according to ref 22. Standard proce-
dures26,41 were used for the synthesis of the resin-bound
tripeptide 2 in a manual solid-phase reactor.
Sch em e 4
N-(ter t-Bu tyloxyca r bon yl)-L-va lyl-L-ser in e Meth yl Es-
ter (6). To a solution of Boc-Val-OH (21.7 g, 100 mmol) and
triethylamine (15.3 mL, 110 mmol) in THF (350 mL) was
added dropwise ethyl chloroformate (10.5 mL, 110 mmol) at
-15 °C. After the mixture was stirred for 15 min at -15 °C,
a solution of L-serine methyl ester hydrochloride (15.9 g, 102
mmol) and triethylamine (15.3 mL, 110 mmol) in THF (350
mL) was added dropwise. During preparation of the ester
solution a solid was formed which was redissolved by adding
some drops of water. The reaction mixture was allowed to
warm to 25 °C over a 2 h interval, and stirring was maintained
for 3 h. The solvent was removed under reduced pressure and
the resulting oil suspended in EtOAc. The solution was
washed successively with 10% aqueous citric acid (3×), satu-
rated aqueous NaHCO3 (3×), water, and brine. The organic
layer was dried over MgSO4, filtered, and concentrated under
reduced pressure to yield 6.13 g (81%) of pure 6 as a colorless
In its reactions with nucleophiles the cyclic R-chloro-
glycyl peptide 9 not only excels its linear analogues in
respect to stereoselectivity but offers also new synthetic
possibilities.
Thus, treatment of chloride 9 with trimethylsilyl
cyanide afforded the cyanide 11i whereas linear ana-
logues form dimeric products. They arise from addition
of a second molecule of acylimine to the acidic R-CH group
of the cyano compound.32 The R-cyanoglycyl peptide 11i
can be further modified by alkylation in the presence of
a strong base (Scheme 4).33 Reaction of nitrile 11i with
benzyl bromide and sodium hydride yielded the R-(cyano)-
phenylalanyl derivative 12, and with methyl acrylate in
the presence of Schwesinger’s phosphazene base P1-tert-
butyl34 the optically pure R-(cyano)glutamyl derivative
13 was obtained. The (S)-configuration at the newly
formed stereogenic center was established by 2D NOESY
experiments. In the case of benzyl derivative 12 the
formation of small amounts of inseparable N-alkylation
products prohibited the determination of its stereochem-
istry.
solid: mp 77.5-79.5 °C; [R]20 ) -21.1 (c ) 1.03, MeOH); IR
D
(KBr) 3336 (br), 2965, 1748, 1658, 1524, 1393, 1367, 1247, 1169
cm-1 1H NMR (400 MHz, DMSO-d6) δ 8.11 (d, 1H, J ) 8.0
;
Hz), 6.62 (d, 1H, J ) 8.8 Hz), 5.00 (t, 1H, J ) 5.6 Hz), 4.34
(ddd, 1H, J ) 4.8 Hz, J ) 4.8 Hz, J ) 7.6 Hz), 3.88 (dd, 1H, J
) 6.8 Hz, J ) 8.8 Hz), 3.63-3.70 (m, 1H), 3.58-3.62 (m, 4H),
1.90-1.98 (m, 1H), 1.37 (s, 9H), 0.85 (d, 3H, J ) 6.8 Hz), 0.80
(d, 3H, J ) 6.8 Hz); MS (FAB, mNBA) m/z (%) 341 ([M + Na]+,
(35) Kober, R.; Steglich, W., unpublished results.
(36) Burgess, V. A.; Easton, C. J .; Hay, M. P.; Steel, P. J . Aust. J .
Chem. 1988, 41, 701-710.
(37) Easton, C. J .; Hutton, C. A.; Rositano, G.; Tan, E. W. J . Org.
Chem. 1991, 56, 5614-5618.
In contrast to chloride 9, which formed with water the
R-hydroxyglycyl derivative 11j, linear chloroglycyl pep-
(38) Baldwin, J . S.; Adlington, R. M.; Lowe, C.; O’Neil, I. A.; Sanders,
G. L.; Schofield, C. J .; Sweeney, J . B. J . Chem. Soc., Chem. Commun.
1988, 1030-1031.
(39) Hamon, D. P. G.; Massy-Westropp, R. A.; Razzino, P. Tetrahe-
dron 1993, 49, 6419-6428.
(32) Brem, R.; Steglich, W., unpublished results.
(33) For alkylation of R-(alkoxycarbonyl)glycyl peptides, see: Bossler,
H. G.; Waldmeier, P.; Seebach, D. Angew. Chem. 1994, 106, 455-456;
Angew. Chem., Int. Ed. Engl. 1994, 33, 439-440.
(34) Schwesinger, R.; Willaredt, J .; Schlemper, H.; Keller, M.;
Schmitt, D.; Fritz, H. Chem. Ber. 1994, 127, 2435-2454.
(40) Ostresh, J . M.; Husar, G. M.; Blondelle, S. E.; Do¨rner, B.; Weber,
P. A.; Houghten, R. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11138-
11142.
(41) Nakagawa, S. H.; Kaiser, E. T. J . Org. Chem. 1983, 48, 678-
685.

Stereoselective Modification of a Cyclopentapeptide
J . Org. Chem., Vol. 62, No. 24, 1997 8477
15), 319 ([M + H]+, 16). Anal. Calcd for C14H26N2O6: C, 52.82;
H, 8.23; N, 8.80. Found: C, 52.95; H, 8.03; N, 8.63.
of cyclopeptide 5 (49%) as a colorless solid: mp 255 °C
(sublimation); [R]20 ) +28.0 (c 0.28, DMSO); CD (c ) 1.65 ×
D
10-4 mol × L-1, MeOH) λmax nm (∆ꢀ) ) 206 (-1.9), λmin) 219
N -(t er t -Bu t yloxyca r b on yl)-L-va lyl-DL-r-a ce t oxygly-
cin e Meth yl Ester (7). To a solution of the seryl peptide 6
(5.02 g, 15.8 mmol) in EtOAc (60 mL, dried over molecular
sieve 4 Å) were added Pb(OAc)4 (10.78 g) and molecular sieve
4 Å (1.5 g). The mixture was heated to reflux for 2 h with
vigorous stirring, cooled to rt, and filtered through a pad of
Celite. After the addition of 10% aqueous citric acid (60 mL),
the mixture was stirred for 10 min. The organic layer was
separated, washed with water (3×), and dried over MgSO4.
Evaporation of the solvent afforded 5.42 g of 7 (99%) as a
colorless oil which was used for the next step without further
purification.
(-5.3), 226 (-5.1); IR (KBr) 3420 (br), 3380, 2960, 2920, 2860,
1645, 1540, 1460, 1375, 1260, 745, 700 cm-1 1H NMR (600
;
MHz, pyridine-d5/DMSO-d6 ) 5/2) δ 9.03 (d, 1H, J ) 7.8 Hz,
Gly(SEt)-NH), 8.67 (d, 1H, J ) 9.0 Hz, Val-NH), 8.22 (d, 1H,
J ) 7.2 Hz, Ala-NH), 7.94 (d, 1H, J ) 8.4 Hz, Phe-NH), 7.21-
7.43 (m, 5H, aromat H), 5.87 (d, 1H, J ) 7.8 Hz, Gly(SEt)-R-
H), 5.01-5.87 (m, 1H, Phe-R-H), 4.82 (dq, 1H, J ) 7.2 Hz, J )
6.5 Hz, Ala-R-H), 4.61 (dd, 1H, J ) 9.0 Hz (both), Val-R-H),
4.39-4.43 (m, 1H, Pro-R-H), 3.90-3.96 (m, 1H, Pro-δ′-H),
3.74-3.78 (m, 1H, Pro-δ-H), 3.58-3.63 (m, 1H, Phe-CH2),
3.04-3.10 (m, 1H, Phe-CH2), 2.74-2.88 (m, 2H, SCH2), 2.27-
2.35 (m, 1H, Val-CH(CH3)2), 2.01-2.10 (m, 1H, Pro-â-H), 1.87-
1.96 (m, 1H, Pro-γ′-H), 1.77-1.85 (m, 1H, Pro-γ-H), 1.66-
1.74 (m, 1H, Pro-â′-H), 1.37 (d, 3H, J ) 6.5 Hz, Ala-CH3),
1.26-1.29 (m, 3H, SCH2CH3), 1.09 (d, 3H, J ) 6.5 Hz, Val-
CH3), 1.07 (d, 3H, J ) 6.5 Hz, Val-CH3); 13C NMR (75.5 MHz,
DMSO-d6) δ 172.62, 171.52, 171.42, 170.91, 167.25, 138.88,
129.94, 128.79, 126.97, 62.48, 58.32, 53.86, 53.63, 49.15, 47.77,
37.50, 29.55, 28.10, 25.12, 24.58, 20.55, 19.42, 16.87, 15.64;
MS (FAB, mNBA) m/z (%) 554 ([M + Na]+, 10), 532 ([M + H]+,
45). Anal. Calcd for C26H37N5O5S: C, 58.74; H, 7.01; N, 13.17;
S, 6.03. Found: C, 58.53; H, 7.00; N, 13.08; S, 6.14.
Gen er a l P r oced u r e for th e Syn th esis of Cyclop ep tid es
11. A 0.22 mL (0.22 mmol) volume of 1 M SO2Cl2 in CH2Cl2
was added at 0 °C to a solution of 5 (106 mg, 0.2 mmol) in
CH2Cl2 (20 mL). The mixture was stirred for 1 h at 0 °C
whereby the chloropeptide 9 precipitated. The solvent and all
volatile byproducts were evaporated (cool trap), and the
resulting residue was suspended in the appropriate solvent
(THF or CH2Cl2, 20 mL). After cooling the mixture to -78
°C, the appropriate base (DIEA or DABCO, 0.42 mmol) was
added and the solution stirred for 5 min.
N-(ter t-Bu tyloxyca r bon yl)-L-va lyl-DL-r-(eth ylth io)gly-
cin e Meth yl Ester (8). To a solution of R-acetoxyglycine
peptide 7 (5.42 g, 15.6 mmol) in CH2Cl2 (150 mL) were added
at 25 °C ethanethiol (1.23 mL, 16.6 mmol) and triethylamine
(4.84 mL, 34.7 mmol). After 12 h of stirring the mixture was
diluted with tert-butyl methyl ether (150 mL) and washed
successively with 10% aqueous citric acid, saturated aqueous
NaHCO3, water, and brine. The organic layer was dried over
MgSO4, filtered, and evaporated in vacuo. The residue was
purified by column chromtography on silica gel. Elution with
petroleum ether/EtOAc 3:1 gave 8 (4.35 g, 79%) as a colorless
solid: mp 104-105 °C; IR (KBr) 3324, 2976, 2933, 2873, 1750,
1685, 1653, 1529, 1168 cm-1 1H NMR (300 MHz, CDCl3) δ
;
6.80 (d, 1H, J ) 8.5 Hz), 5.55/5.56 (each d, 1H, J ) 8.5 Hz),
4.97 (br, 1H), 3.90-4.04 (m, 1H), 3.80 (s, 3H), 2.61-2.77 (m,
2H), 2.11-2.23 (m, 1H), 1.44 (s, 9H), 1.28/1.28 (each t, 3H, J
) 7.4 Hz), 0.90-1.00 (m, 1H); MS (FAB, mNBA) m/z (%) 371
([M + Na]+, 13), 349 ([M + H]+, 17). Anal. Calcd for
C15H28N2O5S: C, 51.70; H, 8.10; N, 8.04; S, 9.20. Found: C,
51.81; H, 7.95; N, 8.12; S, 8.83.
N-(ter t-Bu tyloxyca r bon yl)-L-va lyl-DL-r-(eth ylth io)gly-
cin e (3). To a solution of ester 8 (4.35 g, 12.5 mmol) in THF
(80 mL) were added water (27 mL) and LiOH (0.68 g, 28.3
mmol) at 0 °C. After 1 h of stirring at rt, the solution was
adjusted to pH 2 with 1.1 M KHSO4 and extracted with EtOAc
(3×). The combined organic layers were washed with water
(2×) and dried over MgSO4. Evaporation of the solvent yielded
4.18 g of 3 (quant) as a colorless foam: IR (KBr) 3335 (br),
2974, 2934, 2876, 1722, 1695, 1660, 1524, 1393, 1368, 1248,
1168 cm-1; 1H NMR (400 MHz, DMSO-d6) δ 13.0 (br, 1H), 8.44/
8.53 (each d, 1H, J ) 8.8 Hz), 6.60/6.82 (each d, 1H, J ) 8.8
Hz), 5.33/5.35 (each d, 1H, J ) 8.8 Hz, J ) 10.4 Hz), 3.81-
3.92 (m, 1H), 2.54-2.68 (m, 2H), 1.86-1.94 (m, 1H), 1.36 (s,
9H), 1.15/1.16 (each t, 3H, J ) 7.4 Hz), 0.80-0.84 (m, 6H);
13C NMR (100 MHz, DMSO-d6) δ 172.53/172.61, 171.00/171.07,
156.66/156.71, 79.32/79.36, 60.69/61.02, 54.00/54.07, 31.47/
31.80, 29.43, 24.94/25.07, 20.30/20.36, 19.29/19.57, 15.67/15.72;
MS (FAB, mNBA) m/z (%) 357 ([M + Na]+, 32), 335 ([M + H]+,
5); HRMS (FAB, mNBA) calcd for C14H27N2O5S ([M + H]+)
335.1641, found 335.1638.
The nucleophile (0.21 mmol, cf. Table 1) was added, and
stirring was maintained for 12 h, allowing the solution to warm
to rt. If THF was used, the solvent was removed and the
resulting residue dissolved in EtOAc or CH2Cl2 (20 mL). The
organic layer was washed successively with 10% aqueous citric
acid, saturated aqueous NaHCO3, water, and brine, dried over
MgSO4, filtered, and concentrated in vacuo to afford the crude
cyclopeptide 11 which was purified by column chromatogra-
phy, recrystallization, or HPLC.
r-Cya n oglycyl Cyclop ep tid e 11i. As an example, the
chloro peptide 9 was suspended in THF, DIEA (72 µL, 0.42
mmol) and, after stirring for 5 min, trimethylsilyl cyanide (27
µL, 0.21 mmol) were added. Stirring was continued for 12 h,
allowing the reaction mixture to warm to rt. The solvent was
removed and the resulting residue suspended in EtOAc (20
mL). The organic layer was washed successively with 10%
aqueous citric acid, saturated aqueous NaHCO3, water, and
brine, dried over MgSO4, filtered, and concentrated in vacuo
until a crystalline solid precipitated. The precipitate was
filtered off and dried in vacuo to yield 11i (87 mg, 88%) as a
colorless solid: mp 312 °C (sublimation); IR (KBr) 3420 (br),
2950, 2910, 2840, 1650, 1515, 1450, 1370, 700 cm-1; 1H NMR
(300 MHz, DMSO-d6) δ 9.11 (d, 1H, J ) 6.0 Hz), 7.98 (d, 1H,
J ) 9.1 Hz), 7.91 (d, 1H, J ) 7.4 Hz), 7.14-7.28 (m, 6H), 5.71
(d, 1H, J ) 6.0 Hz), 4.45-4.54 (m, 1H), 4.28-4.41 (m, 1H),
4.14-4.26 (m, 2H), 3.85-3.96 (m, 1H), 3.55-3.67 (m, 1H), 3.13
(dd, 1H, J ) 7.4, 13.3 Hz), 2.73 (dd, 1H, J ) 7.4, 13.3 Hz),
1.72-2.20 (m, 4H), 1.51-1.65 (m, 1H), 1.06 (d, 3H, J ) 6.8
Hz), 0.91 (d, 3H, J ) 6.8 Hz), 0.88 (d, 3H, J ) 6.8 Hz); 13C
NMR (75.47 MHz, DMSO-d6): δ 171.56, 171.51, 170.02, 169.75,
161.43, 137.64, 128.96, 127.93, 126.11, 114.90, 61.80, 57.05,
53.55, 47.75, 47.11, 42.76, 36.69, 28.89, 28.62, 23.96, 19.30,
18.22, 15.27; MS (FAB, mNBA) m/z (%) 470 ([M-CN]+, 25).
Anal. Calcd for C25H32N6O5: C, 60.47; H, 6.50; N, 16.92.
Found: C, 60.15; H, 6.41; N, 16.80.
Cyclop ep tid e 5. A. Syn th esis of P ep tid e 4 by F r a g-
m en t Con d en sa tion : Boc-Pro-Phe-D-Ala-polymer (2) (4.9 g,
0.41 mmol/g, 2.01 mmol) was deprotected with 25% TFA in
CH2Cl2 (1 × 1 min, 1 × 30 min), washed with CH2Cl2 (2×),
i-PrOH, CH2Cl2 (2×), i-PrOH, CH2Cl2 (3×), and DMF.
A
solution of dipeptide 3 (1.01 g, 3.01 mmol), TBTU (1.45 g, 4.52
mmol), and N-hydroxybenzotriazole (HOBt, 0.46 g, 3.01 mmol)
in DMF (25 mL) was added to the resin. After 1 min of
shaking, diisopropylethylamine (DIEA, 1.38 mL, 8.04 mmol)
was added, and the shaking was maintained for 24 h. After
this period the resin was washed with DMF (1×), CH2Cl2 (3×),
CH2Cl2/EtOH ) 1/1 (2×), and CH2Cl2 (2×).
B. Cycliza tion : Boc-Val-DL-Gly(SEt)-Pro-Phe-D-Ala-poly-
mer 4 was deprotected with 25% TFA in CH2Cl2 (1 × 1 min, 1
× 30 min), washed with CH2Cl2 (2×), i-PrOH, CH2Cl2 (2×),
i-PrOH, CH2Cl2 (3×), and DMF. A solution of DIEA (0.69 mL,
4.02 mmol) and glacial acetic acid (0.23 mL, 4.02 mmol) in
DMF (75 mL) was added to the resin. After 24 h of shaking,
the resin was filtered off and washed with DMF (3×).
Evaporation of the combined filtrates in vacuo yielded a
yellowish solid which was washed with water (50 mL) and a
few milliliters of cold MeOH. Lyophylization yielded 0.53 g
r-Cya n oglu ta m yl Cyclop ep tid e 13. In a pressure tube
11i (51 mg, 0.10 mmol) was dissolved in THF (10 mL). To
the solution were added methyl acrylate (0.10 mL, 1.11 mmol)
and P1-tert-butyl base34 (53 µL, 0.21 mmol). The reaction
mixture was heated to 100 °C for 2 h and cooled to rt. After
addition of saturated aqueous NH4Cl, the mixture was ex-

8478 J . Org. Chem., Vol. 62, No. 24, 1997
Paulitz and Steglich
tracted with CHCl3 (3×), and the organic layer was washed
with water and brine. The solution was dried over MgSO4,
filtered, and concentrated in vacuo and the resulting residue
purified by column chromatography on silica gel. Elution with
CH2Cl2/EtOAc/MeOH 12:2:1 afforded 32 mg of 13 (53%) as a
colorless oil: [R]20D ) 63.8 (c 1.3, CHCl3); IR (KBr) ν 3340 (br),
2920, 1735, 1652, 1520, 1450, 1380 cm-1; 1H NMR (300 MHz,
pyridine-d5/DMSO-d6 ) 5:2) δ 10.45 (s, 1H), 9.03 (d, 1H, J )
7.7 Hz), 8.08 (d, 1H, J ) 9.6 Hz), 7.21-7.45 (m, 6H), 4.93-
5.08 (m, 1H), 4.58-4.85 (m, 3H), 4.40-4.55 (m, 1H), 4.05-
4.18 (m, 1H), 3.67 (s, 3H), 3.38-3.53 (m, 1H), 3.13-3.25 (m,
1H), 2.60-3.06 (m, 4H), 2.06-2.24 (m, 2H), 1.78-2.01 (m, 3H),
1.25 (d, 3H, J ) 6.9 Hz), 1.08 (d, 3H, J ) 6.7 Hz), 1.03 (d, 3H,
19.26, 18.60, 14.52; MS (FAB, mNBA) m/z (%) 605 ([M + Na]+,
3), 583 ([M + H]+, 86); HRMS (FAB, mNBA) calcd for
C29H39N6O7 ([M + H]+) 583.2880, found 583.2890.
Ack n ow led gm en t. Financial support of the Deut-
sche Forschungsgemeinschaft is gratefully acknowl-
edged. We thank Prof. H. Kessler for helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails for the cyclopeptides 11a -h and 11j-m , and spectral and
analytical data for these compounds (7 pages). This material
is contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J ) 6.7 Hz); 13C NMR (75.5 MHz, pyridine-d5/DMSO-d6
)
5:2): δ 174.11, 172.66, 171.99, 170.91, 170.73, 165.35, 138.24,
129.67, 128.54, 126.68, 118.61, 64.44, 57.76, 56.03, 55.13,
51.85, 49.30, 48.05, 37.90, 31.61, 31.21, 29.31, 29.05, 25.35,
J O971295A