High-yielding macrocyclization conditions used in the synthesis of novel Sansalvamide A derivatives

Article abstract of DOI:10.1016/j.tetlet.2005.11.056

Described are the syntheses of nine Sansalvamide A derivatives using new, high-yielding, in situ deprotection-cyclization conditions that are general for this series of macrocycles, 55% average for both steps. This is 10-fold greater than previously repor

Full text of DOI:10.1016/j.tetlet.2005.11.056

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 515–517
High-yielding macrocyclization conditions used in the synthesis
of novel Sansalvamide A derivatives
Thomas J. Styers, Rodrigo Rodriguez, Po-Shen Pan and Shelli R. McAlpine*
Department of Chemistry and Biochemistry, 5500 Campanile Dr., San Diego State University, San Diego, CA 92182-1030, USA
Received 21 October 2005; revised 9 November 2005; accepted 10 November 2005
Available online 28 November 2005
Abstract—Described are the syntheses of nine Sansalvamide A derivatives using new, high-yielding, in situ deprotection–cyclization
conditions that are general for this series of macrocycles, 55% average for both steps. This is 10-fold greater than previously reported
yields.
Ó 2005 Elsevier Ltd. All rights reserved.
Table 1. Comparison of macrocyclization conditions
Sansalvamide A is composed of four hydrophobic ami-
Original yield (%)a
New yield (%)b
no acids and one hydrophobic hydroxy acid. Herein, we
describe the synthesis of nine novel Sansalvamide A
derivatives using conditions that give high yields for
the in situ deprotections–cyclizations. Cyclizing large
macrocycles is usually very challenging, typically the
yields are low, and they often take long reaction times
(2–4 days).1 Peptide cyclization yields depend on the
coupling agent and the linear precursor conformation.2
Most cyclizations are reported as single examples, where
the conditions were optimized for coupling agents, con-
centration conditions, and solvents. What is unique
about the work reported here is that not only do we
cyclize in relatively high yields, we do so for nine different
substrates. Thus, despite structural and likely conforma-
tional differences, these new conditions produced nine
macrocycles in reasonable yields. Furthermore, these
conditions give approximately 10-fold greater yields
when compared to our previous conditions. This work
opens the possibility for the synthesis of focused
libraries in practical quantities of material, which is criti-
cal for biological assays.
Compound #
1
2
3
4
5
6
7
8
9
5
2
5c
5c
9
0.7
5c
5c
5c
65
39
85
65
53
37
81
40
35
a Old cyclization conditions, referred to as conditions I.
b New cyclization conditions, referred to as conditions II.
c The cyclization was only run under the new conditions. Reported is
the average yield of all Sansalvamide A derivatives synthesized in our
lab to date using the old conditions.7
duced insoluble salts with the acid of the linear penta-
peptide. Investigation into more efficient in situ
deprotection and cyclization conditions led to the dis-
covery that HCl deprotected both the amine and the
acid without forming insoluble salts, thus furnishing
cleaner cyclization products in high yields (Fig. 2). The
average yield for the new macrocyclization conditions
(conditions II) is approximately 10-fold higher than
the original cyclization conditions (conditions I). In
addition, the new cyclization conditions are general for
nine substrates, which is unusual for cyclizations of large
macrocycles.
Initial in situ deprotection–cyclization conditions for the
first- and second-generation Sansalvamide A derivatives
(reaction conditions I, Table 1) involved deprotection of
the amine on the pentapeptide using trifluoroacetic acid
(TFA), which gave insoluble TFA salts. Subsequent
deprotection of the acid using lithium hydroxide pro-
The synthesis of nine derivatives utilized the amino acids
shown in Figure 1, where the amino acids were chosen
*
Corresponding author. Tel.: +1 619 594 5580; fax: +1 619 594
4634; e-mail: mcalpine@chemistry.sdsu.edu
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.11.056

516
T. J. Styers et al. / Tetrahedron Letters 47 (2006) 515–517
Amino Acid 4
Amino Acid 3
3
O
O
OMe
NH2
O
4
O
O
N
R
NHBoc
HO
HO
NHBoc
a
RN
NH
O
O
OMe
NH2
a
5
b
NH HN
O
O
O
2
b
NBoc
HO
O
OMe
NH
1
c
c
Amino Acid 2
Amino Acid 1
MeO
OH
OH
Amino Acid 5
NHBoc
NHBoc
O
OH
O
O
b
O
a
a
NHBoc
OH
NH2
MeO
a
OH
O
O
NBoc
O
b
NHBoc
c
NH2
b
Figure 1. Amino acids used in synthesis of nine derivatives.
based on the structure–activity relationship (SAR) of
the first generation.3
Table 1 shows the macrocyclization yields using our new
method (conditions II). Although there is some varia-
tion in these yields, the average yield is 55%. By compar-
ison, the average yield using our previous cyclization
conditions is 5% (this includes both first and second-
generation data). The cyclization yields for compounds
3, 4, 7, 8, and 9 were not available using the old
approach (I) because these compounds were synthesized
after realization that the new method substantially
improved yields.6 LCMS data of crude cyclization reac-
tions show only one product peak, which is around
6.1–7 min.5,8 HPLC purification gave yields in Table 1
(Ônew yieldsÕ). The nine macrocycles are shown in Figure
3.
Our synthesis utilized a convergent approach and stan-
dard conditions to reach the linear pentapeptide.3 The
high yielding macrocyclization conditions (II) involved
dissolving the linear pentapeptide in THF (0.05 M,
Fig. 2), addition of two equivalents of anisole, and add-
ing approximately eight drops concentrated HCl per
0.3 mmol of linear pentapeptide. The reaction mixture
was stirred at room temperature for 24 h, whereupon
LCMS usually indicated that the acid and amine were
partially deprotected. At this point, four drops of HCl
per 0.3 mmol of peptide were added, and reactions were
monitored by LCMS. Typically deprotection reactions
were completed within four days.4 Upon completion,
the reaction was concentrated in vacuo, and dried on
the high-vac. The dried, crude linear pentapeptide was
dissolved in a 2:2:1 ratio of THF/CH3CN/CH2Cl2
(0.007 M), addition of DIPEA (6 equiv), and three cou-
pling agents (HATU, DEPBT, and TBTU 0.7 equiv ea)
to reaction gave a clear solution. Reactions were usually
completed in 1–2 h, which is significantly faster than
most published cyclization conditions.1,5
In summary, new cyclizations conditions have provided
nine novel, Sansalvamide A derivatives in high yields.
These yields are ꢀ10-fold greater than previously
reported. The LCMS data of the crude reactions show
the product is the primary peak in all examples.8 Most
importantly, these cyclization conditions appear to be
general for this series of macrocycles. Recent assay data
of these nine compounds9 have provided insight into
their SAR, and these results will be reported in due
course.
O
O
O
Acknowledgements
O
O
N
H
N
1. (a)
2. (b)
RN
H
O
NR
MeO
O
RN
NR
O
We thank Pfizer La Jolla for their fellowship to T.J.S.
(summer 2005). S.R.M. was supported by NIH
(AI058241-01).
HN
NH HN
O
NH
Boc
O
R = H or Me
Supplementary data
Figure 2. New cyclization conditions (conditions II): (a) HCl in THF
(0.05 M), anisole (2 equiv); (b) HATU (0.7 equiv), DEPBT (0.7 equiv),
TBTU (0.7 equiv), DIPEA (6 equiv), THF/CH3CN/DCM (2:2:1)
0.007 M.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.11.056.

T. J. Styers et al. / Tetrahedron Letters 47 (2006) 515–517
517
4a
3a
O
3a
O
3a
4a
O
4a
1a
O
N
O
O
O
N
H
H
N
H
HN
HN
NH
2b
HN
NH
2a
O
HN
NH
O
O
5a
5a
NH
O
O
2a
NH HN
O
NH HN
O
O
5a
1a
1b
Compound 1
Compound 2
Compound 3
O
3b
3a
O
4a
O
3a
4b
4a
O
O
N
H
N
H
O
N
H
HN
HN
NH
2a
2a
NH
HN
O
HN
HN
O
2b
NH
O
5a
5a
NH
O
5a
NH HN
O
O
NH
O
O
O
1a
1a
1b
Compound 4
Compound 5
3a
Compound 6
3a
O
O
4c
4a
3a
O
O
O
4b
N
H
O
N
H
N
H
NH
N
2c
2a
2a
O
N
HN
HN
NH
HN
O
O
NH HN
O
NH
O
NH HN
O
O
5a
O
1a
1a
O
5b
5a
1a
Compound 7
Compound 8
Compound 9
Figure 3. Compounds synthesized via new conditions.
5. It was straightforward to follow the reactions via LCMS as
the starting material double deprotected linear precursor
would appear at 5.0–5.5 min, the cyclized product would
appear between 6.1 and 7.0 min.
6. Because of the high yields achieved with the cyclizations
using the new conditions (B), we had enough material
for bioassays. Thus, it was not necessary to resynthesize
these compounds using a low yielding conditions (the old
approach A) in order to recognize the value of this
cyclization reaction.
7. Yields previously reported for cyclizations in Org. Lett. 2005,
7, 3481 were yields after silica gel flash chromatography.
8. The nine macrocyclic peptides have LCMS spectra and
yields given in the supplementary data. In addition, several
representative NMRs are shown.
References and notes
1. (a) Downing, S. V.; Aguilar, E.; Meyers, A. I. J. Org.
Chem. 1999, 64, 826–831; (b) Samy, R.; Kim, H. Y.;
Brady, M.; Toogood, P. L. J. Org. Chem. 1999, 64, 2711–
2728; (c) Wipf, P.; Uto, Y. J. Org. Chem. 2000, 65, 1037–
1049.
2. (a) Li, P.; Roller, P. P. Curr. Topics Med. Chem. 2002, 2,
325–341; (b) Lambert, J. N.; Mitchell, J. P.; Roberts, K. D.
J. Chem. Soc., Perkin Trans. 1 2001, 471–484.
3. Carroll, C. L.; Johnston, J. V. C.; Kekec, A.; Brown, J. D.;
Parry, E.; Cajica, J.; Medina, I.; Cook, K. M.; Corral, R.;
Pan, P.-S.; McAlpine, S. R. Org. Lett. 2005, 7, 3481–
3484.
4. For details on the reaction conditions see supplementary
data.
9. Unpublished results.