Preparation and reactions of N-thioformyl peptides from amino thioacids and isonitriles

Article abstract of DOI:10.1016/j.tetlet.2009.02.205

The preparation of N-thioformyl peptides from amino thioacids and isonitriles at room temperature is described.

Full text of DOI:10.1016/j.tetlet.2009.02.205

Tetrahedron Letters 50 (2009) 2329–2333
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Tetrahedron Letters
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Preparation and reactions of N-thioformyl peptides from amino thioacids
and isonitriles
Yu Yuan a, Jianglong Zhu a, Xuechen Li a, Xiangyang Wu a, Samuel J. Danishefsky a,b,
*
a Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY 10065, USA
b Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of N-thioformyl peptides from amino thioacids and isonitriles at room temperature is
Received 8 February 2009
Revised 23 February 2009
Accepted 26 February 2009
Available online 4 March 2009
described.
Ó 2009 Elsevier Ltd. All rights reserved.
Since their discovery in the mid 1800s,1,2 isonitriles have been
employed in many synthetically useful transformations. Included
among these are the Ugi multicomponent couplings3 and the
Passerini reaction.4,5 Recently, we had occasion to explore some
new possibilities for amide bond construction. Toward this end,
we examined the feasibility of a very simple idea which, remark-
ably, had thus far been overlooked.6 It was anticipated that another
venerable functional type in organic chemistry, that is, a carboxyl
group, would react with an isonitrile to produce a formimidate car-
boxy mixed anhydride (cf. systems 3a and 3s which we abbreviate
as FCMAs). While, in practice, we have not isolated or character-
ized either FCMA from the reaction of 1 and 2, we do obtain the
N-formylamide 4 when the reaction is conducted at ca 130 °C un-
der microwave thermolysis.7 Clearly, the nitrogen in the context of
its two carbonyl attachments corresponds to a mixed imide
ensemble. Not surprisingly, nucleophile induced deacylation oc-
curs specifically at the N-formyl bond to afford 5. Other productive
options for product 4 were studied and reported.8
While we do not detect any ‘in hand product’ upon mixing of 1
and 2, strong inferential evidence was brought to bear that, in fact,
FCMA system 3 is being produced. Thus, when 1 and 2 are heated
in the presence of amines (see 6), amides 7 (the apparent products
of interdiction of 3) are produced in modest yield, though the acyl
transfer product 5 is not detected.7 These findings invite the inter-
pretation that the FCMA structures are, in fact, slowly generated at
room temperature but that the presumed 1,3 O?N-acyl transfer
requires significant thermal activation6,9 (see Scheme 1).
we term ‘2 component coupling strategy’ (2CC), the carboxyl is
activated in the natural context of its progression to amides.
Of course, we were not unmindful of the potential limitations
that ensue from the requirement of thermal activation, particularly
as we approach challenging fragment mergers. In this Letter, we
report on the use of thioacids, in place of normal carboxylic acids,
hoping to realize N-thioformyl imide formation under mild condi-
tions. To the best of our knowledge, there had been only one liter-
ature precedent for the formation of N-thioformyl imide, prior to
the work documented here, from the reaction of thiocarboxylic
acids and isonitriles. It was reported by Chupp and Leschinsky
but only in the context of very simple thioacids (acetic, benzoic
thioacids and their derivatives).10 In the study reported below,
we sought to evaluate how the substitution of a thioacid in place
of 1 would alter the chemistry described above in more complex
settings.
Our initial studies focused on the coupling between Fmoc-Ala-
SH 911 and commercially available cyclohexyl isonitrile 10. Under
mild thermolysis, N-thioformyl imide 11 was indeed obtained in
21% yield, along with a significant amount of cyclohexyl thioforma-
mide 12 (35%). When the reactions were conducted in diethyl
ether or in chloroform, transformation to product 11 occurred in
improved yield with correspondingly diminished formation of
small amounts of side product, to the level of a few percent. At this
stage, we could not rigorously establish whether 12 arises via
adventitious cleavage of 11 or, more likely, by nucleophile induced
cleavage of the presumed thio FCMA intermediate 13. It is conceiv-
able that in entries 1 and 2, DMF may serve in the role of NuH (see
Table 1).
We feel that this line of chemistry is quite promising because
the versatile mixed imide products 4 are generated from acids
without the need for either separate external activation of the car-
boxyl group to a higher acyl donor level, or coupling agents which
function by in situ acyl donor activation. In our chemistry, which
The generality of the reaction was examined by recourse to dif-
ferent amino acid derivatives reacting with amino acid isonitriles.
As demonstrated in Table 2, both sterically hindered thioacids (va-
line derivative, entries 1–3) and less hindered thioacids (glutamic
acid derivative, entries 5 and 6) reacted with isonitriles to give
dipeptides in reasonable yields. Reactions of thioacids with aro-
matic functional groups (tyrosine derivative, entries 7 and 8) pro-
* Corresponding author. Tel.: +1 212 639 5501; fax: +1 212 772 8691.
E-mail address: s-danishefsky@ski.mskcc.org (S.J. Danishefsky).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.205

2330
Y. Yuan et al. / Tetrahedron Letters 50 (2009) 2329–2333
O
R2 NC
+
R1
OH
R1
1
2
O
H
O
O
1,3 O→N
acyl transfer
O
O
N
O
1
R2
H
R2
R1
O
N
R1
N
R1
O
R1
H
O
R2
8
3a
3s
4
R3 NH2
6
O
O
R2
R3
R1
N
R1
N
H
H
7
5
Scheme 1.
ceeded equally well to afford the desired N-thioformyl imides in
good yields. In the reaction of valine thioacid, we observed that
the yields of the N-thioformyl imides were increased with ascend-
ing isonitrile steric hindrance. We surmise that perhaps the rate of
cleavage of the thio FCMA is suppressed in hindered cases. Alterna-
tively, and in the same direction, the rate of syn?anti isomeriza-
tion of intermediate FCMA 23 is increased in the most hindered
substrates (Scheme 2).12 Indeed, in the least hindered glycine iso-
nitrile case (entry 4), the desired imide was isolated in 14% yield
along with 68% of thioformyl amide and 67% of the symmetric thi-
oanhydride. Efforts to minimize this unproductive but competitive
pathway have thus far proven to be unsuccessful.
Unfortunately, we have not been able to extend this chemistry
effectively to the coupling of a dipeptide with a single amino acid.
We attempted to accomplish this in each of the two possible direc-
tions. In the case of the reaction of the C-terminal dipeptide thio-
acid with the phenylalanine derived isonitrile 25, only the
phenylalanine-derived thioformamide 27 was obtained. A reason-
able interpretation of this breakdown presupposes oxazalone 28
formation from the activated thio FCMA 26. When conducted in
the opposite sense, that is, dipeptide isonitrile 25 and valine-de-
rived thioacid 22, albeit in only 18%, the tripeptide 29 and also
thioformamide 27 were obtained. We note that in this case, inter-
nal cleavage of the thio FCMA via oxazalone formation could not
occur, hence it cannot be the sole course of the problem (see
Scheme 3).
The viability of methodology for native peptide bond formation
required dethioformylation of 21. An initial attempt at dethio-
formylation was performed according to the procedure reported
by Chupp and co-workers.10 However, treatment of 21 with aniline
hydrochloride in toluene for 20 h under reflux afforded thioforma-
mide 30 only. The same regioselectivity was observed under basic
conditions as well. This surprising result indicates how a subtle struc-
tural change (from formyl to thioformyl) can control fundamental
chemical properties (see Scheme 4).
Fortunately dethioformylation could be accomplished by a two-
step protocol (Scheme 5). Treatment of 21 with mCPBA in dichlo-
romethane at low temperature afforded the corresponding N-for-
Table 1
S
NuH
O
H
H
N
N
H
Fmoc
SH
12
NuH
O
Me
+
9
conditions
N
amide bond
cleavage
H
N
CN
Fmoc
S
H
Me
13
Fmoc
HN
O
N
10
1,3 acyl
migration
Me
H
S
11
Entry
Conditionsa
DMF, 40 °C, 2 h
DMF, microwave, 60 °C, 25 min
Ether, rt, 6 h
Yieldb (%)
1
2
3
4
21c
22
50
CHCl3, rt, 6 h
53d
a
1.0 equiv of thioacid and 2.0 equiv of isonitrile were used for all reactions.
Isolated yields for 11.
35% of the cyclohexyl thioformamide was isolated.
71% based on the recovery of thioacid.
b
c
d

Y. Yuan et al. / Tetrahedron Letters 50 (2009) 2329–2333
2331
Table 2
S
R1
O
O
R1
O
CHCl3
H
SH
CN
H
N
+
N
+
HN
Fmoc
OR
O
r.t.
12 h
OR
HN
Fmoc
OR
O
R2
S
R2
O
R2
Entry
Producta
Yieldb (%)
½ ꢀ
a 2D5
Me
FmocHN
14
Me
S
N
1
60 (82)
ꢁ12.0
ꢁ11.6
ꢁ17.0
+34.4
+14.8
ꢁ29.7
ꢁ7.2
OBn
O
Me
Me
Me
Me
S
O
N
2
3
4
5
6
7
50
FmocHN
OMe
O
Ph
15
Me
Me
S
O
N
28
FmocHN
OBn
OMe
O
Me
16
Me
Me
S
O
14
N
FmocHN
O
17
O
O
S
O
49 (61)
N
N
FmocHN
O
OMe
O
18
Ph
O
S
O
59
FmocHN
OBn
O
Me
19
tBuO
S
O
60
N
FmocHN
20
OBn
OBn
O
Me
tBuO
S
O
8
63
ꢁ71.1
N
FmocHN
21
O
Me
Me
a
1.0 equiv of thioacid and 1.0 equiv of isonitrile were used for all reactions.
Yields refer to isolated yields. The yields in parentheses are based on recovery of the starting isonitriles.
b

2332
Y. Yuan et al. / Tetrahedron Letters 50 (2009) 2329–2333
Me
O
Me
O
H
R
Me
SH
Me
O
H
R
FmocHN
Me
S
N
O
22
+
Me
FmocHN
S
N
FmocHN
O
OR'
R
23a
23s
OR'
CN
CO2R'
Me
O
Me
1,3 acyl
migration
OR'
Me
N
FmocHN
O
S
Scheme 2.
O
O
Me
Bn
O
H
N
CHCl3
r.t.
SH
O
N
+
CN
OEt
25
Bn
H
FmocHN
O
O
H
Bn
24
O
Me
S
O
H
R'
FmocHN
S
N
N
N
H
R
N
H
OEt
100%
O
H
O
Bn
26
27
Bn
O
Me
O
Bn
S
O
Me
O
H
H
N
CHCl3, rt
18%
N
CN
SH +
OEt
Me
FmocHN
N
OEt
Me
O
Bn
25
O
Bn
FmocHN
H
22
29
O
O
R'
FmocHN
Me
28
N
Scheme 3.
myl imide, which was smoothly converted to the desired amide 31
in 80% yield whose spectroscopic properties are identical to those
of authentic sample prepared by other standard methods.13 To our
knowledge, the work described here is the first recognition and
solution of this problem.
In summary, we have shown that the reaction of thioacids with
isonitriles can be used to generate N-thioformylamides at rt in con-
trast to the requirement for thermolysis in the carboxy counter-
part. The reaction is applicable to the synthesis of dipeptides. It
is, however, complicated by the high vulnerability of the thio FCMA
intermediate to interdiction, competitive with the key S?N acyl
transfer. This problem is, at this writing, unmanageable in at-
tempted 2 + 1 couplings in peptides. Finally effective methodology
to accomplish the transformylation of N-thioformyl amides is
described.
Applications of these general ideas to some synthetic targets
which are otherwise difficult to access will be described in due
course.
tBuO
S
aniline•HCl
toluene, reflux
O
O
H
N
H
N
OBn
Me
FmocHN
OBn
Me
or
S
K2CO3, MeOH
0 ºC, 0.5 h
O
Me
21
Me
30
55-65%
Scheme 4.

Y. Yuan et al. / Tetrahedron Letters 50 (2009) 2329–2333
2333
tBuO
tBuO
S
S
1.mCPBA, Š30 °C
CH2Cl2, 2 h
O
O
H
N
N
FmocHN
OBn
FmocHN
31
OBn
2.NaHCO3, 0 °C
MeOH, 3 h
O
O
Me
21
Me
Me
Me
80% for 2 steps
Me
FmocHN
14
Me
Me
Me
1.mCPBA, Š30 °C
CH2Cl2, 2 h
O
O
H
N
N
OBn
Me
FmocHN
OBn
Na
2.NaHCO3, 0 °C
MeOH, 3 h
O
O
32
Me
Me
Me
76% for 2 steps
Scheme 5.
3. Ugi, I.; Lohberger, S.; Karl, R. The Passerini and Ugi Reactions. Comprehensive
Organic Synthesis; Pergamon Press: 1991; Vol. 2, Chapter 4.6, p 1083.
4. Passerini, M.; Simone, L. Gazz. Chim. Ital. 1921, 51, 126.
5. Passerini, M.; Ragni, G. Gazz. Chim. Ital. 1931, 61, 964.
6. In this regard, some initiatives, which invoked incorrect structural
assignments, have been reported in the literature. For details, see: Li, X.;
Yuan, Y.; Berkowitz, W. F.; Todaro, L. J.; Danishefsky, S. J. J. Am. Chem. Soc. 2008,
130, 13222.
Acknowledgment
This work was supported by Grant CA103823.
Supplementary data
7. Li, X.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130, 5446.
8. Corriu, R. J. P.; Lanneau, G. F.; Perrot-Petta, M.; Mehta, V. D. Tetrahedron Lett.
1990, 31, 2585.
9. Li, X.; Yuan, Y.; Kan, C.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130, 13225.
10. Chupp, J. P.; Leschinsky, K. L. J. Org. Chem. 1975, 40, 66.
11. Fu, X.; Jiang, S.; Li, C.; Xin, J.; Yang, Y.; Ji, R. Bioorg. Med. Chem. Lett. 2007, 17,
465.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.02.205.
References and notes
12. Marcelli, T.; Himo, F. Eur. J. Org. Chem. 2008, 4751.
13. Significant level of epimerization was observed, if the reaction was performed
at higher temperature.
1. Gautier, A. Liebigs Ann. Chem. 1867, 142, 289.
2. Hofmann, A. W. Liebigs Ann. Chem. 1867, 144, 114.