Synthesis of orthogonally protected disulfide bridge mimetics

Article abstract of DOI:10.1021/jo1018427

Concise routes to four orthogonally protected, enantiopure disulfide bridge mimetics are reported. These four dicarba analogues possess an alkyne, an (E)-alkene, a (Z)-alkene, and an alkane as substitutes for the disulfide bridge. Selective deprotection of one of these mimetics is also illustrated.

Full text of DOI:10.1021/jo1018427

pubs.acs.org/joc
Synthesis of Orthogonally Protected Disulfide
Bridge Mimetics
Andrew C. Tadd,*,† Kristian Meinander,†
‡
Kristina Luthman, and Erik A. A. Wallen
†
ꢀ
†Division of Pharmaceutical Chemistry, Faculty of Pharmacy,
PO Box 56, FI-00014, University of Helsinki, Helsinki,
Finland, and ‡Department of Chemistry-Medicinal
Chemistry, University of Gothenburg, SE-41296,
€
Goteborg, Sweden
andrew.tadd@helsinki.fi
Received September 17, 2010
FIGURE 1. Internal disulfide bridge (1) and orthogonally pro-
tected disulfide bridge mimetics (2-5).
peptide in its biologically active conformation, as demon-
strated by helical peptide stapling.4
We are primarily interested in bicyclic peptides with non-
terminal disulfide bridges. Interest in improving the in vivo
stability of these peptides has led us to require structures 2-5
possessing inexpensive, orthogonal protecting groups.5 To
our knowledge, there is no procedure for the synthesis of
orthogonally protected mimetics 2-4 in the literature. While
routes to orthogonally protected 5 have been disclosed, they
either require a cumbersome, lengthy synthesis or do not
possess the appropriate protecting groups for our study.6
Herein, we report convenient routes to orthogonally pro-
tected disulfide bridge mimetics 2-5 employing a copper-
mediated organozinc/haloalkyne coupling and a ruthenium-
catalyzed cross-metathesis reaction as key steps.
Concise routes to four orthogonally protected, enantio-
pure disulfide bridge mimetics are reported. These four
dicarba analogues possess an alkyne, an (E)-alkene, a (Z)-
alkene, and an alkane as substitutes for the disulfide bridge.
Selective deprotection of one of these mimetics is also
illustrated.
Our initial goal was the synthesis of mimetic 6. The require-
ment of an orthogonally protected, enantiopure product
severely limited the number of viable synthetic strategies. We
envisaged that employing the copper-mediated organozinc/
Cyclization stabilizes peptides and in certain cases may be
essential for biological activity. Disulfide bridge formation is
the most common mechanism for peptide cyclization. This
stabilizing effect is exemplified by a unique subclass of
biologically active peptides, known as cyclotides, which
possess multiple interlinked disulfide bridges and are re-
nowned for their resistance to thermal, chemical, and pro-
teolytic decomposition.1 Despite the stabilizing effect of
disulfide bridges on many peptides, the disulfide bond may
undergo different decomposition reactions and can also be
easily reduced in vivo to the open chain peptide.2 It has been
reported that replacing a disulfide bridge with an isosteric
carbon analogue (Figure 1, 2-5) can considerably increase
the bioavailabilty and in vivo stability of a peptide, without
significantly reducing biological activity.3 In addition, these
isosteric carbon linkers may also be used to constrain a
(4) (a) Schafmeister, C. E.; Po, J.; Verdine, G. L. J. Am. Chem. Soc. 2000,
122, 5891. (b) Walensky, L. D.; Kung, A. L.; Escher, I.; Malia, T. J.; Barbuto,
S.; Wright, R. D.; Wagner, G.; Verdine, G. L.; Korsmeyer, S. J. Science 2004,
305, 1466. (c) Kim, Y.-W.; Kutchukian, P. S.; Verdine, G. L. Org. Lett. 2010,
12, 3046.
(5) Synthesis of nonorthogonally protected disulfide bridge mimetics
(selected examples): (a) Williams, R. M.; Yuan, C. J. Org. Chem. 1992, 57,
6519. (b) Kremminger, P.; Undheim, K. Tetrahedron 1997, 53, 6925. (c) Gao,
Y.; Lane-Bell, P.; Vederas, J. C. J. Org. Chem. 1998, 63, 2133. (d) Hiebl, J.;
Blanka, M.; Guttman, A.; Kollmann, H.; Leitner, K.; Mayrhofer, G.;
Rovenszky, F.; Winkler, K. Tetrahedron 1998, 54, 2059. (e) O’Leary, D. J.;
Miller, S. J.; Grubbs, R. H. Tetrahedron Lett. 1998, 39, 1689. (f) Williams,
R. M.; Liu, J. J. Org. Chem. 1998, 63, 2130. (g) Garrard, E. A.; Borman,
E. C.; Cook, B. N.; Pike, E. J.; Alberg, D. G. Org. Lett. 2000, 2, 3639. (h)
Lygo, B.; Crosby, J.; Peterson, J. A. Tetrahedron 2001, 57, 6447. (i) IJsselstijn,
M.; Kaiser, J.; van Delft, F. L.; Schoemaker, H. E.; Rutjes, F. P. J. T. Amino
Acids 2003, 24, 263. (j) Robinson, A. J.; Elaridi, J.; Patel, J.; Jackson, W. R.
Chem. Commun. 2005, 5544. (k) Schmidtmann, F. W.; Benedum, T. E.;
McGarvey, G. J. Tetrahedron Lett. 2005, 46, 4677. (l) Elaridi, J.; Patel, J.;
Jackson, W. R.; Robinson, A. J. J. Org. Chem. 2006, 71, 7538.
(1) (a) Colgrave, M. L.; Craik, D. J. Biochemistry 2004, 43, 5975. (b)
(6) Synthesis of orthogonally protected mimetic 5: (a) Nutt, R. F.;
Strachan, R. G.; Veber, D. F.; Holly, F. W. J. Org. Chem. 1980, 45, 3078.
(b) Hiebl, J.; Kollmann, H.; Rovenszky, F.; Winkler, K. Bioorg. Med. Chem.
Lett. 1997, 7, 2963. (c) Lange, M.; Fischer, P. M. Helv. Chim. Acta 1998, 81,
2053. (d) Aguilera, B.; Wolf, L. B.; Nieczypor, P.; Rutjes, F. P. J. T.;
Overkleeft, H. S.; van Hest, J. C. M.; Schoemaker, H. E.; Wang, B.; Mol,
ꢁ
ꢁ
Cemazar, M.; Craik, D. J. Int. J. Pept. Res. Ther. 2006, 12, 253.
(2) Manning, C. M.; Chou, D. K.; Murphy, B. M.; Payne, R. W.;
Katayama, D. S. Pharm. Res. 2010, 27, 544.
(3) (a) Veber, D. F.; Strachan, R. G.; Bergstrand, S. J.; Holly, F. W.;
Homnick, C. F.; Hirschmann, R. J. Am. Chem. Soc. 1976, 98, 2367. (b)
Stymiest, J. L.; Mitchell, B. F.; Wong, S.; Vederas, J. C. J. Org. Chem. 2005,
70, 7799 and references cited therein. (c) Derksen, D. J.; Stymiest, J. L.;
Vederas, J. C. J. Am. Chem. Soc. 2006, 128, 14252.
€
J. C.; Furstner, A.; Overhand, M.; van der Marel, G. A.; van Boom, J. H.
J. Org. Chem. 2001, 66, 3584. (e) Hernandez, N.; Martı
ꢀ
´
n, V. S. J. Org. Chem.
2001, 66, 4934.
DOI: 10.1021/jo1018427
r
Published on Web 12/22/2010
J. Org. Chem. 2011, 76, 673–675 673
2010 American Chemical Society

Tadd et al.
JOCNote
SCHEME 1. Synthesis of Mimetic 6
smoothly using hydrochloric acid to remove the Boc group
and diethylamine to remove the Fmoc moiety. Amines 11 and
12 were isolated in 93% and 98% yield respectively. Deprotec-
tion of the methyl ester to deliver carboxylic acid 13 initially
proved challenging; treatment of 6 with LiOH resulted in
partial cleavage of the Fmoc group and an unsatisfactory yield
of the desired product. However, employing the conditions
reported by Nicolaou and co-workers11 resulted in a smooth
transformation; subjecting 6 to Me3SnOH at 70 °C allowed 13
to be cleanly isolated in 78% yield. Finally, carboxylic acid 14
was synthesized in two steps from 6 by using a double
deprotection-reprotection strategy.
haloalkyne chemistry pioneered by Yeh and Knochel would
provide an efficient route (Scheme 1).7 Bromoacetylene 7
was prepared in two steps from protected propargyl
glycine 88 and serine iodide 9 was synthesized from the
commercially available, protected serine 10.9 The key step,
employing Zn/I2 followed by CuCN with LiCl in DMF,
provided mimetic 6 in 63% yield. Altering the copper source
SCHEME 3. Partial and Full Reduction of Disulfide Bridge
Analogue 6
to CuBr DMS (32% yield10) or changing the solvent to THF
3
(<10% yield) resulted in less efficient reactions.
SCHEME 2. Selective Deprotection of Mimetic 6
With isostere 6 in hand, it was predicted that partial and full
hydrogenation of the alkyne would provide analogues 3 and 5,
respectively. Initial conditions to construct linker 15 by using
Lindlar’s catalyst gave poor results but, fortunately, switching the
catalyst to Pd/BaSO4 (Rosenmund’s catalyst)12 delivered an
88% yield of the unsaturated mimetic. Similarly, the fully satu-
rated isostere 16 was synthesized in quantitative yield employing
a Pd/C catalyst under a hydrogen atmosphere (Scheme 3).
To complete the set of disulfide bridge analogues, we also
required a route to orthogonally protected isostere 4, con-
taining an (E)-alkene. As the selective reduction of functio-
nalized alkynes to (E)-alkenes is still a synthetic challenge,13
another approach was adopted. We predicted a ruthenium-
catalyzed cross-metathesis reaction would provide access to
the final mimetic from two differently protected allyl glycine
units (Scheme 4, 17 þ 18 f 19).14,15 Standard literature
(11) Nicolaou, K. C.; Estrada, A. A.; Zak, M.; Lee, S. H.; Safina, B. S.
Angew. Chem., Int. Ed. 2005, 44, 1378.
(12) Procedure adapted from: Hunt, T. A.; Parsons, A. F.; Pratt, R.
J. Org. Chem. 2006, 71, 3656.
€
(13) Trost, B. M.; Zachary, B. T.; Joge, T. J. Am. Chem. Soc. 2002, 124,
7922 and references cited therein.
(14) Key olefin metathesis articles/reviews (selected examples): (a)
Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem.
Soc. 2003, 125, 11360. (b) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45,
3760. (c) Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243. (d) Morris,
T.; Sandham, D.; Caddick, S. Org. Biomol. Chem. 2007, 5, 1025. (e) Brik, A.
Adv. Synth. Catal. 2008, 350, 1661.
(15) Examples of disulfide bridge mimetic formation by ruthenium-
catalyzed ring closing metathesis (for synthesis of disulfide bridge mimetics
using cross-metathesis see refs 5e, 5j, 5k, and 14d): (a) Miller, S. J.; Blackwell,
H. E.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 9606. (b) Creighton, C. J.;
Reitz, A. B. Org. Lett. 2001, 3, 893. (c) Stymiest, J. L.; Mitchell, B. F.; Wong,
S.; Vederas, J. C. Org. Lett. 2003, 5, 47. (d) Ghalit, N.; Riijkers, D. T. S.;
Kemmink, J.; Versluis, C.; Liskamp, R. M. J. Chem. Commun. 2005, 192. (e)
Derksen, D. J.; Stymiest, J. L.; Vederas, J. C. J. Am. Chem. Soc. 2006, 128,
14252. (f) Addona, D. D.; Carotenuto, A.; Novellino, E.; Piccand, V.; Reubi,
J. C.; Cianni, A. D.; Gori, F.; Papini, A. M.; Ginanneschi, M. J. Med. Chem.
2008, 51, 512.
To demonstrate the full orthogonality of the chosen
protecting groups, selective deprotection of 6 was investi-
gated. Pleasingly, four different monodeprotected analogues
can be easily accessed under standard conditions (Scheme 2).
Cleavage of the two amine protecting groups proceeded
(7) Yeh, M. C. P.; Knochel, P. Tetrahedron Lett. 1989, 30, 4799.
(8) Alkyne bromination adapted from: Hofmeister, H.; Annen, K.;
Laurent, H.; Wiechert, R. Angew. Chem., Int. Ed. 1984, 23, 727.
(9) Huber, T.; Manzenrieder, F.; Kuttruff, C. A.; Dorner-Ciossek, C.;
Kessler, H. Bioorg. Med. Chem. Lett. 2009, 19, 4427.
(10) Procedure adapted from: Rodrıguez, A.; Miller, D. D.; Jackson,
´
R. F. W. Org. Biomol. Chem. 2003, 1, 973.
674 J. Org. Chem. Vol. 76, No. 2, 2011

Tadd et al.
JOCNote
SCHEME 4. Synthesis of Mimetic 19
to give the corresponding organozinc intermediate (TLC analysis
was used to confirm the complete consumption of the starting
material). In a separate flask, CuCN (118 mg, 1.32 mmol) and
LiCl (112 mg, 2.63 mmol) were heated to 150 °C for 2 h under
argon and then cooled to room temperature. DMF (2.2 mL) was
added and the solution was stirred for 5 min to form the soluble
CuCN 2LiCl complex. The copper complex was then cooled
3
to -15 °C. Once the zinc insertion process was judged to have
reached completion, stirring was ceased to allow the zinc powder
to settle to the bottom of the flask. The supernatant was removed
via syringe under argon (with care being taken to avoid the
transfer of zinc) and added dropwise to the copper complex
at -15 °C. Compound 7 (476 mg, 1.01 mmol) was then dissolved
in DMF (1.5 mL) and also added dropwise to the copper complex
at -15 °C. The cooling bath was removed and the reaction
mixture was stirred at room temperature for 16 h under argon.
Water (ca. 30 mL) was added and the suspension was extracted
with diethyl ether (3 ꢀ 100 mL), washed with brine (ca. 60 mL),
dried over anhydrous sodium sulfate, and concentrated in vacuo.
The crude product was purified via flash column chromatography
(5:1 hexane:ethyl acetate) to yield alkyne 6 (376 mg, 63%) as an
amorphous white solid. νmax (KBr)/cm-1 3376, 3358, 3064, 3038,
2977, 2934, 1746, 1737, 1704, 1516, 1439, 1389, 1362, 1219, 1168,
1058, 1016, 845, 759, 743; δH (300 MHz; CDCl3) 7.82-7.65 (4H,
m), 7.45-7.26 (4H, m), 6.23 (1H, d, J=8.5 Hz), 6.03 (1H, d, J=8.7
Hz), 4.65-4.15 (5 H, m), 3.71 (3H, s), 2.78-2.52 (4H, m), 1.52
(9H, s), 1.46 (9H, s); δC (75 MHz; CDCl3) 172.2, 170.4, 156.1,
155.5, 144.1, 141.5, 127.9, 127.4, 127.3, 125.6, 125.5, 120.2, 120.1,
82.9, 80.1, 78.5, 78.2, 67.3, 53.01, 52.95, 52.3, 47.4, 28.6, 28.3, 24.2,
24.1; m/z LRMS (EI) 592 (Mþ, 20), 492 (25), 391 (50), 269 (45),
241 (55), 178 (100). Anal. Calcd for C33H40N2O8: C, 66.87; H,
6.80; N, 4.73. Found: C, 66.76; H, 6.78; N, 4.70.
conditions were tested (10 mol % 20, DCM, reflux), which
gave a 57% yield of analogue 19. Switching from thermal
heating to microwave irradiation (36% yield16) or altering the
catalyst loading (5 mol % 20, 46% yield) was not advantageous.
In summary, we have developed efficient routes to four
enantiopure, orthogonally protected disulfide bridge mi-
metics employing a copper-mediated organozinc/haloalkyne
coupling and a ruthenium-catalyzed cross-metathesis reac-
tion as key steps. This chemistry now enables these central
peptidomimetic building blocks to be easily synthesized,
selectively deprotected, and incorporated into peptides.
Experimental Section
Procedure for the Synthesis of Mimetic 6. Zinc dust (331 mg,
5.05 mmol) was weighed into a round-bottomed flask. Iodine
(12.8 mg, 0.05 mmol) was added and the flask was heated with a
heat gun, under vacuum, for 10 min and then flushed with argon.
The flask was evacuated and flushed with argon a further 3 times
and then cooled to 0 °C. Compound 9 (500 mg, 1.52 mmol) was
dissolved in anhydrous DMF (1.5 mL) and added dropwise, via
syringe, to the activated zinc at 0 °C. The reaction mixture was
then allowed to warm to room temperature and stirred for 90 min
Acknowledgment. Financial support was obtained from
the Academy of Finland (grant no. 126969), the research funds
of the University of Helsinki, and the Magnus Ehrnrooth
foundation. We thank Timo Mauriala, Janne Weisell, and
Jorma Matikainen for mass analysis.
Supporting Information Available: Detailed experimental
procedures and characterization data for new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(16) Reaction conditions: 17 (1.0 equiv, 0.42 mmol), 18 (1.1 equiv, 0.46
mmol), 20 (10 mol %), DCM (4 mL), 100 °C (microwave irradiation), 10 min.
J. Org. Chem. Vol. 76, No. 2, 2011 675