A new silyl linker for reverse-direction solid-phase peptide synthesis

Article abstract of DOI:10.1016/S0040-4039(01)01093-0

Treatment of a free amino acid ester with CO₂ followed by exposure to a chlorosilane-containing polystyrene results in its attachment to the solid support. The newly formed silyl carbamate can be employed to build polypeptides at the carboxyl terminus. Cleavage of the (poly)peptide using aqueous HF in CH₃CN leads to its free amine form which is isolated as a Boc derivative. The polymer support can be easily recycled.

Full text of DOI:10.1016/S0040-4039(01)01093-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5629–5633
A new silyl linker for reverse-direction solid-phase
peptide synthesis
Bruce H. Lipshutz* and Young-Jun Shin
Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106, USA
Received 22 May 2001; accepted 12 June 2001
Abstract—Treatment of a free amino acid ester with CO2 followed by exposure to a chlorosilane-containing polystyrene results
in its attachment to the solid support. The newly formed silyl carbamate can be employed to build polypeptides at the carboxyl
terminus. Cleavage of the (poly)peptide using aqueous HF in CH3CN leads to its free amine form which is isolated as a Boc
derivative. The polymer support can be easily recycled. © 2001 Elsevier Science Ltd. All rights reserved.
With the advent of combinatorial chemistry, a renais-
sance in solid-phase organic synthesis (SPOS) is taking
place. Closely associated with these advances are
numerous developments in available linkers,1 which
contain various functionality and are usually modeled
on solution-based protecting groups. Many linkers,
therefore, are dependent upon silicon,2 which is sepa-
rated from a polymer backbone by a spacer group (Fig.
1). Cleavage of the desired product from a silyl linker
frequently results from acid-induced protiodesilylation
of a (substituted) aromatic ring3 or otherwise activated
silane (e.g. benzylic,4 allylic,5 etc.), or via net hydrolysis
of a silyl ether.6,7 Based on our recent report introduc-
ing Tsoc and its TBDPS analog as new protecting
groups for nitrogen,8a we envisioned the carbamate
within 1 functioning as a new linker which upon cleav-
age under the influence of fluoride ion would leave
behind a hydrogen on nitrogen.8b We now describe this
novel addition to the arsenal of SPOS, in this case as
applied to unconventional N-linked polypeptide
synthesis.9
Unlike traditional solid-phase peptide synthesis, an
approach which proceeds in the reverse direction (i.e.
N“C) leads directly to C-terminally protected/modified
products.9 Use of an N-bound silyl carbamate linker
was anticipated to offer several advantages, including:
(1) commercial availability of bromopolystyrene; (2) no
requirement for a formal spacer group; (3) mild
fluoride-based cleavage of the polypeptide from the
resin; (4) minimal loss of stereochemical integrity
throughout the sequence; and (5) ready recycling of the
polymer.
Figure 1.
* Corresponding author. Tel.: 805-893-2521; fax: 893-8265; e-mail: lipshutz@chem.ucsb.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01093-0

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B. H. Lipshutz, Y.-J. Shin / Tetrahedron Letters 42 (2001) 5629–5633
In practice, para-bromopolystyrene (1% crosslinked,
200–400 mesh, 2.6 mmol/g)10 could be lithiated and
conditions investigated (e.g. DCC, DMAP; HBTU,
HOBT, DIPEA; HOBT, DIPCDI)16 were less effective.
Subsequent couplings with a methyl or allyl ester of
phenylalanine or phenylglycine occurred smoothly.
Treatment of silyl carbamates 6 with HF/CH3CN fol-
lowed by (Boc)2O led to fully protected dipeptides 7a–c
in good overall isolated yields from 4d–f, the optical
rotation of each indicating that no loss in stereochemi-
cal integrity had occurred throughout the sequence.
11
quenched with (i-Pr)2SiCl2 to afford silyl chloride 2
(Scheme 1). Upon exposure of amines 3a–g to gaseous
8a
CO2 in CH2Cl2 followed by trapping of the intermedi-
ate salt with polymer-bound silyl chloride 2, the corre-
sponding silyl carbamates 4 were gradually formed.
Release of the amine from 4 was found to occur
smoothly using aqueous HF (3–4 equiv.) in CH3CN at
room temperature.12 Among the amines mounted via
the corresponding silyl carbamate, a fluorenylmethyl
(Fm) ester of proline (4e) was found to readily undergo
conversion to the free acid upon treatment with pyrro-
lidine in CH2Cl2 at room temperature. In general, how-
ever, Fm esters were very sensitive to base; hence,
amino acids were best introduced as their allyl ester
derivatives (e.g. 4d and 4f). These derivatives could be
unmasked to the free carboxyl using catalytic Pd(0) in
the presence of excess morpholine13 or Me2NH·BH3.14
Benzyl esters were examined, and while these under-
went clean removal using Pd(0)/Et3SiH in model com-
pounds tested in solution (as their Tsoc derivatives), the
polystyrene mounted analogs led to essentially no
reaction.
The two-step cycle (i.e. ester cleavage, then coupling)
could be applied twice to educts 4d and 4f (Scheme 3),
giving rise to polymer-bound tripeptide allyl esters 8,
10, and 12. Final release from the polymer and N-pro-
tection afforded N-Boc tripeptide esters 9, 11, and 13 in
good overall yields from the starting carbamates (4d
and 4f). The stereopurity of each product was excellent
based on comparisons with authentic materials.17 Note-
worthy is the observation that incorporation of a
phenylglycine residue was easily accommodated with-
out losses due to epimerization.
By way of comparison, the diphenylsilyl analog 14 of
diisopropylsilyl carbamate 4d was tested (Scheme 4),
using the same sequence as depicted in Scheme 3 (i.e.
acid deprotection and then coupling, done twice, then
cleavage from the resin and N-Boc protection). Overall,
the yield of tripeptide 9 was 15% lower, suggesting that
Activation of amino acids 5 was found to be most
efficient using traditional i-butyl chloroformate/N-
methylmorpholine15 (NMM; Scheme 2). Other standard
o
o
Scheme 1.

B. H. Lipshutz, Y.-J. Shin / Tetrahedron Letters 42 (2001) 5629–5633
5631
Scheme 2.
Scheme 3.
this derivative offers no obvious advantage over the
diisopropyl-substituted case (cf. overall yield for the
conversion of 4d to 9 in Scheme 3).
to the derived silyl fluoride 16, which can be efficiently
recycled upon treatment with BCl3 in CH2Cl2 at room
temperature (Scheme 5).18 Exposure of the carbamate salt
derived from phenylalanine methyl ester to regenerated
resin 2 gave the expected carbamate-derived product 17.
Release of a (poly)peptide from the resin using HF led

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B. H. Lipshutz, Y.-J. Shin / Tetrahedron Letters 42 (2001) 5629–5633
Scheme 4.
o
Scheme 5.
In summary, a novel silyl carbamate linker19 has been
developed which has been demonstrated for reverse
(N“C) direction solid-phase peptide synthesis. Overall
efficiencies are good, and the purities of the final prod-
ucts are excellent. Cleavage of the free amine moiety
from the resin occurs under mild fluoride-mediated
conditions, leaving behind only CO2 as by-product.
That the solid support can be readily recycled to the
active silyl chloride is a particularly noteworthy feature.
Other uses of this silyl linker in SPOS involving amines
are envisioned and will be reported in due course.
Ellman, J. A. J. Org. Chem. 1997, 62, 6102–6103; (d)
Hone, N. D.; Davies, S. G.; Devereux, N. J.; Taylor, S.
L.; Baxter, A. D. Tetrahedron Lett. 1998, 39, 897–900.
4. Nakamura, K.; Hanai, N.; Kanno, M.; Kobayashi, A.;
Ohnishi, Y.; Ito, Y.; Nakahara, Y. Tetrahedron Lett.
1999, 40, 515–518.
5. Schuster, M.; Lucas, N.; Blechert, S. J. Chem. Soc.,
Chem. Commun. 1997, 823–824.
6. (a) Lindsley, C.; Hodges, J.; Filzen, G.; Watson, B.;
Geyer, A. J. Comb. Chem. 2000, 2, 550–559; (b) Jes-
berger, M.; Jausems, J.; Jung, A.; Jas, G.; Schonberger,
A.; Kirschning, A. Synlett 2000, 1289–1293; (c) Doi, T.;
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Acknowledgements
7. (a) Liao, Y.; Fathi, R.; Reitman, M.; Zhang, Y.; Yang,
Z. Tetrahedron Lett. 2001, 42, 1815–1818; (b) Wang, B.;
Chen, L.; Kim, K. Tetrahedron Lett. 2001, 42, 1463–1466.
8. (a) Lipshutz, B. H.; Papa, P.; Keith, J. J. Org. Chem.
1999, 64, 3792–3793; (b) For several examples of other
carbamate linkers, see reference 1a.
Financial support provided by the NIH (GM 40287) is
warmly acknowledged. We are indebted to Dr. Craig
Lindsley (Eli Lilly) for alerting us to such applications
of the Tsoc moiety in SPOS.
9. (a) Leger, R.; Yen, R.; She, M. W.; Lee, V. J.; Hecker, S.
J. Tetrahedron Lett. 1998, 39, 4171–4174; (b) Thieriet, N.;
Guibe, F.; Albericio, F. Org. Lett. 2000, 2, 1815–1817.
10. Available from Fluka; alternatively, it can be made
according to: Frechet, M. J.; Farrall, M. J. J. Org. Chem.
1976, 41, 3877–3882.
11. Chan, T.-H.; Huang, W.-Q. J. Chem. Soc., Chem. Com-
mun. 1985, 909–911.
12. When TBAF (3 equiv., CH2Cl2, rt) was used for cleavage
of the linker, the reaction was slow (> 12 h) and afforded
products in low yields.
References
1. (a) James, I. W. Tetrahedron 1999, 55, 4855–4946; (b)
Bunin, B. A. The Combinatorial Index; San Diego: Aca-
demic Press, 1998; pp. 9–76.
2. For representative recent alternatives, see: (a) Brase, S.;
Kobberling, J.; Enders, D.; Lazny, R.; Wang, M.;
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Stieber, F.; Grether, U.; Waldmann, H. Angew. Chem.,
Int. Ed. 1999, 38, 1073–1077; (c) Zhang, H.-C.; Ye, H.;
Moretto, A. F.; Brumfield, K. K.; Maryanoff, B. E. Org.
Lett. 2000, 2, 89–92; (d) Tumelty, D.; Cao, K.; Holmes,
C. P. Org. Lett. 2001, 3, 83–86; (e) Glatthar, R.; Giese, B.
Org. Lett. 2000, 2, 2315–2317.
13. Friedrich-Bochnitschek, S.; Waldmann, H.; Kunz, H. J.
Org. Chem. 1989, 54, 751–756.
14. Gomez-Martinez, P.; Dessolin, M.; Guibe, F.; Albericio,
F. J. Chem. Soc., Perkin Trans. 1 1999, 2871–2874.
15. (a) Chen, F. M.; Lee, Y.; Steinauer, R.; Benoiton, N. L.
Can. J. Chem. 1987, 65, 613–618; (b) Chen, F. M.;
Benoiton, N. L. Can. J. Chem. 1987, 65, 619–625.
3. (a) Lee, Y.; Silverman, R. B. Org. Lett. 2000, 2, 303–306;
(b) Briehn, C. A.; Kirschbaum, T.; Bauerle, P. J. Org.
Chem. 2000, 65, 352–359; (c) Woolard, F. X.; Paetsch, J.;

B. H. Lipshutz, Y.-J. Shin / Tetrahedron Letters 42 (2001) 5629–5633
5633
16. (a) Konig, W.; Geiser, R. Chem. Ber. 1970, 103, 788; (b)
Field, G. B.; Noble, R. L. Int. J. Pept. Protein Res. 1990,
35, 161.
17. Authentic samples were prepared via solution-phase syn-
theses according to: Hess, G. P.; Sheehan, J. C. J. Am.
Chem. Soc. 1955, 77, 1067. All 1H and 13C NMR, and
HPLC data for the di- and tripeptides prepared via this
solid-phase route were compared with that obtained from
authentic samples.
mL) and CH2Cl2 (50 mL), sequentially. The resin was
dried under vacuum for 24 h, and then suspended in
anhydrous CH2Cl2 (10 mL) to which was added isobutyl
chloroformate (421 mL, 3.25 mmol) and N-methylmor-
pholine (361 mL, 3.25 mmol) at 0°C. The reaction mixture
was stirred for 2 h at this temperature and then warmed
to rt. After stirring for an additional 2 h, the resin was
filtered and washed with anhydrous CH2Cl2 (20 mL)
under Ar. The resin was suspended in anhydrous CH2Cl2
18. (a) Stranix, B. R.; Liu, H. Q.; Darling, G. D. J. Org.
Chem. 1997, 62, 6183–6186; (b) Zhang, A. J.; Russell, D.
H.; Zhu, J.; Burgess, K. Tetrahedron Lett. 1998, 39,
7439–7442.
(5 mL) and a solution of D-phenylglycine methyl ester
(536 mg, 3.25 mmol) in CH2Cl2 (1 mL) was added at 0°C.
The reaction mixture was stirred 3 h at 0°C and then
warmed to rt. After stirring for an additional 12 h at rt,
the resin was filtered and washed with CH2Cl2 (50 mL),
DMF (50 mL), 50% DMF in water (50 mL) and finally
CH2Cl2 (50 mL). The resin was dried under vacuum for
12 h and then suspended in CH3CN (5 mL) to which was
slowly added 49% aqueous HF solution (79.6 mL, 1.95
mmol). Stirring was continued for 5 h at rt after which
Et3N (453 mL, 3.35 mmol) was added at 0°C. After
raising the temperature to rt, the resin was filtered off and
washed with additional CH2Cl2 (10 mL). The combined
filtrate was concentrated in vacuo and the crude dipeptide
dissolved in CH2Cl2 (5 mL) to which was added (Boc)2O
(121 mg, 0.98 mmol) and Et3N (136 mL, 0.98 mmol) at
0°C. After stirring for 1 h, the reaction mixture was
warmed to rt with stirring for 3 h. The resin was then
filtered off and washed with CH2Cl2 (20 mL). The com-
bined filtrate was washed with H2O (20 mL), dried over
MgSO4, and concentrated under vacuum. Purification
over a silica gel (EtOA/hexanes=2/1, Rf=0.61) afforded
the desired product 7b as a white solid (216 mg, 92%);
[h]2D5=−116.7° (c 1.65, MeOH); IR (KBr) 3339, 2972,
19. General procedure for the preparation of polymer-bound,
silyl carbamate-linked amines 4: carbamate 4b: To a solu-
tion of 4-amino-1-benzyl-piperidine (0.60 g, 3.14 mmol)
in dry CH2Cl2 (5 mL) was added Et3N (0.42 mL, 3.14
mmol) at −78°C. CO2 gas was bubbled into the mixture
at −78°C for 1 h. In another flask was placed 2 (1.00 g,
1.57 mmol) suspended in dry CH2Cl2 (5 mL) and cooled
to −78°C. The reaction mixture was cannulated into the
suspension of 2 at −78°C and stirred for 2 h at this
temperature. The reaction mixture was then gradually
warmed to rt and stirred for an additional 10 h. The resin
was then filtered and washed sequentially with CH2Cl2
(100 mL), DMF (100 mL), 50% H2O/DMF (100 mL),
DMF (100 mL) and CH2Cl2 (100 mL). Drying under
vacuum (ca. 5 mmHg) for 24 h afforded 4b (1.2 g, 1.06
mmol/g) in 88% yield, based on analytical data. Anal.
calcd: N, 3.40; Si, 4.77. Found: N, 2.96; Si, 4.20. IR
(KBr) 3400, 2901, 1700, 1683, 1511, 1334, 1107, 687 cm−1
.
Representative sequence of reactions: conversion of 4f to
protected dipeptide 7b: To a suspension of 4f (0.5 g, 0.65
mmol) in anhydrous CH2Cl2 (5 mL), was added dropwise
at rt a solution of (CH3)2NH·BH3 (231 mg, 3.9 mmol) in
CH2Cl2 (2 mL). After stirring for 10 min, a solution of
Pd(PPh3)4 (74.9 mg, 0.065 mmol) in CH2Cl2 (2 mL) was
added at the same temperature. After stirring for 2 h, the
resin was filtered and washed with CH2Cl2 (50 mL),
DMF (50 mL), 50% DMF in water (50 mL), DMF (50
1747, 1649, 1532, 1390, 984, 701 cm−1 1H NMR (400
;
MHz, CDCl3): l 8.10 (s, 1H), 7.17–7.20 (m, 5H), 5.06 (m,
1H), 4.25 (m, 1H), 3.78 (s, 3H), 3.50 (m, 2H), 1.95 (m,
2H), 1.20 (s, 9H); 13C NMR (100 MHz, CDCl3): l 174.2,
172.5, 171.1, 136.4, 128.9, 128.5, 127.2, 80.5, 66.4, 56.2,
47.1, 30.1, 24.1; HRMS (FAB) calcd for C19H26N2O5
(M+Na)+ 385.1739, found 385.1736.
.