Mild deprotection of primary N-(p-toluenesulfonyl) amides with SmI 2 following trifluoroacetylation

Article abstract of DOI:10.1055/s-2006-951530

A mild deprotection method for notoriously difficult to unmask primary N-(p-toluenesulfonyl) amides was developed during our total synthesis studies toward the marine toxin, gymnodimine. The deprotection occurs at low temperature (-78 °C) under mild conditions by initial activation of the nitrogen with a trifluoroacetyl group, followed by reductive cleavage of the p-toluenesulfonyl group with samarium diiodide. The substrate scope and functional group tolerance of this useful N-S cleavage process, which builds on related cleavage processes of other nitrogen-heteroatom bonds, is explored. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-951530

3294
LETTER
Mild Deprotection of Primary N-(p-Toluenesulfonyl) Amides with SmI2
Following Trifluoroacetylation
M
ildDeprotecti
i
onof
P
a
r
imary
N
-
(
d
p
-Toluenesulfonyl)
MAmides oussa, Daniel Romo*
Department of Chemistry, Texas A&M University, P. O. Box 30012, College Station, TX 77842-3012, USA
Fax +1(979)8624880; E-mail: romo@mail.chem.tamu.edu
Received 13 June 2006
O
S
H
R
H
R
Abstract: A mild deprotection method for notoriously difficult to
unmask primary N-(p-toluenesulfonyl) amides was developed dur-
ing our total synthesis studies toward the marine toxin, gymnodi-
mine. The deprotection occurs at low temperature (–78 °C) under
mild conditions by initial activation of the nitrogen with a trifluoro-
acetyl group, followed by reductive cleavage of the p-toluenesulfo-
nyl group with samarium diiodide. The substrate scope and
functional group tolerance of this useful N–S cleavage process,
which builds on related cleavage processes of other nitrogen–het-
eroatom bonds, is explored.
R = alkyl, aryl
N
H
N
O
Scheme 1 Previous methods employed for deprotection of the pri-
mary N-tosyl group. Reagents and Conditions: Na–naphthalenide;
HBr–phenol; Na–Hg, Na2HPO4; Na–NH3; SmI2–HMPA or DMPU;
Bu3SnH–AIBN; Mg–MeOH–sonication; electrolysis
The N-tosyl moiety was critical for achieving successful
nucleophilic ring opening of this hindered lactam and it
also provided the functional group compatibility required
for our endgame toward the natural product. Unfortunate-
ly, despite numerous methods available for the cleavage
of the tosyl group, these proved ineffective, returning ei-
ther starting material or complex mixtures of products due
to functional group incompatibility. While secondary
(N,N-disubstituted) sulfonamides may undergo facile
cleavage in many cases, the analogous primary (N-mono-
substituted) sulfonamides are notoriously difficult to un-
mask. Thus, we became interested in developing a mild
method for removal of the tosyl group from primary sul-
fonamides.
Key words: sulfonamide, samarium diiodide, reductive cleavage,
trifluoroacetylation
Arylsulfonyl substituents have found widespread utility in
the activation and protection of oxygen and amino groups,
respectively.1 In particular, the N-p-toluenesulfonyl (to-
syl, Ts) group has served as a highly effective protecting
group for nitrogen because it is readily introduced and sig-
nificantly lowers the basicity of nitrogen. In addition, the
resulting sulfonamides are generally crystalline, strong
chromophores, stable to a variety of reaction conditions,
and more resistant to nucleophilic attack than carbamates.
However, the tosyl group ranks amongst the most stable
of the amino protecting groups, thus requiring harsh con-
ditions for subsequent reductive cleavage. Several early
deprotection protocols for the tosyl group involved the use
of rather harsh conditions.2–7 These drastic conditions
present functional group incompatiblity and limit the
utility of the tosyl group and related protecting groups.
The need for milder and neutral reaction conditions
prompted recent interest in the development of alternative
desulfonylation methods which have included reagents
such as refluxing SmI2 in THF–DMPU,8 Bu3SnH–AIBN,9
magnesium in methanol under ultrasonication con-
ditions10 and electrolysis (Scheme 1).11 Important recent
developments in this area also include more labile aryl-
sulfonyl protecting groups12 including the now widely
employed o- and p-nitrosulfonyl (nosyl) group pioneered
by the Fukuyama group, which has found widespread
use.13
O
O
TsHN
H2N
OTBS
OTBS
1
2
Scheme 2 Required N-tosyl deprotection in a model sulfonamide 1
The application of samarium diiodide (SmI2) as a one-
electron-transfer reagent has been extensively studied and
its functional group compatibility has been well docu-
mented.15 While the SmI2–DMPU system was previously
reported by Vedejs to cleave arenesulfonamides, it was
limited to only a few substrates and worked best with the
N-(phenylsulfonyl) group and N,N-disubstituted sulfon-
amides.8 Building on this work, Parsons recently reported
the desulfonylation of N-sulfonyl amides without an addi-
tive using samarium diiodide.16 This latter procedure re-
quired isolation of the sulfonamides, the use of the rather
robust benzoyl group as an activating group, and the re-
quirement of warming to room temperature to achieve
desulfonylation. Prior to Parsons’s work, Keck and subse-
quently Marco–Contelles reported an efficient process for
the reductive cleavage of N–O bonds of acylated hydrox-
ylamines and hydroxamic acid derivatives using SmI2.17
Recently, in our efforts toward the total synthesis of gym-
nodimine, we were faced with the deprotection of a prima-
ry sulfonamide 1 to regenerate the amine 2 (Scheme 2).14
SYNLETT 2006, No. 19, pp 3294–3298
0
1
.1
2
.2
0
0
6
Advanced online publication: 23.11.2006
DOI: 10.1055/s-2006-951530; Art ID: S12906ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Mild Deprotection of Primary N-(p-Toluenesulfonyl) Amides
3295
TFAA (4 equiv), Et3N (4 equiv)
NHTs
NTs
CF3
THF, 23 °C, 10 min
O
4
3
O
SmI2
(7.4 equiv)
NH
+
N
CF3
CF3
+
4 (6%)
+
3 (9%)
–78 °C
O
CF3
O
5 (26%)
6 (59%)
Scheme 3 One-pot reductive cleavage of a sulfonamide
The acyl function was originally utilized to obviate isola- chemical shift of the methine proton of amide 4 (q, d =
tion difficulties associated with low molecular weight pri- 5.52 ppm, J = 7.0 Hz) exhibited a Dd of 1.0 relative to sul-
mary amines and their inherently polar nature.17a fonamide 3 (app. quintet, d = 4.48 ppm, J = 7.0 Hz).
However, recent studies have suggested that the acyl sub-
Surprisingly, when the N-trifluoroacetylsulfonamide in-
termediate 4 was treated with excess SmI2 (7.4 equiv) at
stitutent allows for reductive cleavage at lower tempera-
tures and leads to greatly improved yields.17b
–78 °C and the crude mixture was analyzed by 1H NMR,
Furthermore, a related, mild N–O cleavage with SmI2 was
only ca. 26% of the desired trifluoroacetamide 5 was
employed by us in our pateamine A synthesis.18 Based on
present relative to other products (Scheme 3). The major
these precedents and in conjunction with our studies to-
component of the reaction mixture was the bistrifluoro-
acetamide derivative 6 (59%) presumably resulting from
ward gymnodimine,14 we proceeded to investigate the
cleavage of N–S bonds of acylated N-monosubstituted
subsequent trifluoroacetylation following reductive
sulfonamides with SmI2. The trifluroacetyl group was of
cleavage.20 In addition, unreacted N-trifluoroacetylsul-
fonamide 4 and sulfonamide 3 comprised 6% and 9% of
the mixture, respectively. Clearly, the presence of excess
particular interest and represented an attractive activating
group due to its ease of installation, expected activation
leading to low temperature N–S bond cleavage,and mild
amounts of TFAA and triethylamine is not desirable.
removal (e.g. NH3–MeOH). The use of the trifluoroacetyl
Thus, in order to suppress the subsequent acylation of the
product we initially attempted to isolate and purify (SiO2)
the intermediate N-trifluoracetylsulfonamide prior to
treatment with SmI2 and remove excess reagents by chro-
group contrasts to related methods employing a tert-but-
oxycarbonyl (Boc) activating group which require room
temperature cleavage with Mg and sonication which
could be useful if a protecting group switch is required
matography. Unfortunately, the acetylated intermediates
and the Boc group is to be carried through additional
generally proved very labile undergoing cleavage of the
steps.10 We now report the development of a mild and
trifluoroacetamide group during workup and purification.
convenient method for the deprotection of N-monosubsti-
After extensive investigation of solvents and bases, we
tuted aryl- and alkylsulfonamidesinvolving initial N-acy-
found that a significant reduction in the amount of re-
lation/activation with trifluoroacetic anhydride (TFAA)
agents required for complete acetylation could be
followed by direct reductive cleavage with SmI2 at –78
achieved with dichloromethane as solvent. In this case,
two equivalents of TFAA and triethylamine resulted in
°C.19
We were initially interested in developing a one-pot pro- complete trifluoroacetylation at 23 °C within ten minutes.
cedure involving in situ acylation of N-monosubstituted Since dichloromethane is not an ideal solvent for SmI2 re-
sulfonamides with TFAA, followed by treatment with actions and unreacted TFAA remains, excess TFAA (bp
SmI2. (S)-N-(1-Phenylethyl) toluenesulfonamide (3) was 39–40 °C) and triethylamine (bp 88 °C) were removed by
employed as a model substrate since it is readily prepared concentration in vacuo followed by addition of tetrahy-
and presents a more sterically encumbered a-substituted drofuran. Under these conditions, the desired trifluoro-
amine. Employing tetrahydrofuran as solvent for both op- acetamide 5 was isolated in 85% yield (99%, based on
erations, we screened several bases (NaH, DBN, DBU, 2- recovered sulfonamide 3), following treatment of the in-
tert-butyl-1,1,3,3-tetramethylguanidine and Et3N) and de- termediate N-trifluoroacetylsulfonamide with 4.9 equiva-
termined that efficient acylation of amine 3 required the lents of SmI2 (Scheme 4). The spectral and physical data
use of four equivalents of both triethylamine and TFAA
(Scheme 3). The extent of conversion to amide 4 was
readily monitored by aliquot NMR and complete acyla-
TFAA (2 equiv), Et3N (2 equiv)
NHTs
NH
CH2Cl2, 23 °C, 10 min; then,
tion was observed within ten minutes as indicated by the
complete disappearance of the N–H proton and other di-
agnostic signals of amine 3 and the appearance of new sig-
nals that had the characteristic downfield shift expected
upon N-acylation (d = 0.80–1.04 ppm). For example, the
O
CF3
SmI2 (4.9 equiv), THF, –78 °C
5 (85%)
Scheme 4 Optimized conditions for reductive cleavage of a sulfon-
amide
Synlett 2006, No. 19, 3294–3298 © Thieme Stuttgart · New York

3296
Z. Moussa, D. Romo
LETTER
O
O
H
a
F3C
N
TsHN
O
OTBS
OTBS
24
1
N
b
OTBS
25
Scheme 5 Reagents and conditions: (a) (CF3CO)2O, Et3N, THF, then SmI2, 63%; (b) NH3·H2O, MeOH, 80 ºC, 89%.
for amide 5 were in accord with spectral and physical data 24 was converted in one step into the desired cyclic imine
reported in the literature.21
under basic conditions.
In order to investigate the scope and limitations of this In summary, we have developed an improved and more
procedure, we examined the trifluoroacetylation and re- general method for deprotection of primary N-toluene-
ductive cleavage of several primary N-tosyl sulfonamides sulfonamides at low temperature building on previous re-
(Table 1).22 The aryl- and alkylsulfonamides used as start- lated reductions of N–S and N–O bonds. This one-pot
ing material in this work were prepared from commercial method should find utility in synthesis due to the mild re-
primary amines by standard conditions.23 With the excep- action conditions employed, excellent chemoselectivity,
tion of substrate 8 (entry 3), all the sulfonamides shown in and high yields obtained with a variety of alkyl and aryl
Table 1 underwent clean and complete trifluoroacetyla- N-sulfonamides. In addition, this deprotection protocol
tion according to the established conditions.24 Subsequent should prove successful with other types of sulfonamides.
treatment with SmI2 at –78 °C typically afforded good to Finally, unmasking of the trifluoroacetamides to the cor-
excellent yields of the desired trifluoroacetamides, (60– responding aliphatic and aromatic amines is facile and
95%).22 As can be seen in Table 1, the deprotection meth- readily accomplished using several published methods
od was useful for a variety of substrates and compatible thus providing access to the parent amines.26
with a wide range of functional groups including carbam-
ates, bromides, acetals, nitriles, ketones, lactones, and lac-
tams. Detosylation of the relatively hindered a-substituted
Acknowledgment
The authors thank the NIH (GM52964), the Robert A. Welch Foun-
dation (A-1280), and the Life Sciences Task Force (TAMU) for ge-
nerous financial support. The NSF (CHE-0077917) provided funds
for the purchase of NMR instrumentation.
sulfonamide 3 (entry 1) and N-Boc-piperidine sulfon-
amide 7 (entry 2) produced the corresponding acetamides
in high yields without cleavage of the carbamate group in
the latter case. When the reaction of the bromosulfon-
amide 8 (entry 3) was conducted at 23 °C, the isolated
yield was low (24%), presumably due to reduction of the References and Notes
primary bromide. However at –78 °C, the reduction was
(1) (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
highly chemoselective providing acetamide 17 in excel-
lent yield (88%).25 Similarly, N-piperonyl-p-toluene-
sulfonamide 9 and its cyanobenzyl derivative 10
underwent smooth cleavage (entries 4 and 5). We also
examined the detosylation of arylsulfonamides 14 and 15
(entries 9 and 10) and these deprotections proceeded
readily, albeit in lower yields with respect to alkylsulfon-
amides. In the former case, the low yield (71%) was due
to incomplete detosylation and recovery of sulfonamide
14 (yield of acetamide 23 was 77% based on recovered
sulfonamide 14).
Organic Synthesis, 3rd Ed.; Wiley-Interscience: New York,
1999. (b) Kocienski, P. J. Protecting Groups; Thieme: New
York, 1994.
(2) du Vigneaud, V.; Behrens, O. K. J. Biol. Chem. 1937, 117,
27.
(3) Kharasch, M. S.; Priestley, H. M. J. Am. Chem. Soc. 1939,
61, 3425.
(4) Snyder, H. R.; Heckert, R. E. J. Am. Chem. Soc. 1952, 74,
2006.
(5) Li, S.; Gortler, L. B.; Waring, A.; Battisti, A.; Bank, S.;
Closson, W. D.; Wriede, P. J. Am. Chem. Soc. 1967, 89,
5311.
(6) Gold, E. H.; Babad, E. J. Org. Chem. 1972, 37, 2208.
(7) (a) Sundberg, R. J.; Laurino, J. P. J. Org. Chem. 1984, 49,
249. (b) Anderson, H. J.; Loader, C. E.; Xu, R. X.; Le, N. L.;
Gogan, N. J.; McDonald, R.; Edwards, L. G. Can. J. Chem.
1985, 63, 896.
(8) (a) Vedejs, E.; Lin, S. J. Org. Chem. 1994, 59, 1602.
(b) Goulaouic-Dubois, C.; Guggisberg, A.; Hesse, M. J.
Org. Chem. 1995, 60, 5969. (c) Goulaouic-Dubois, C.;
Guggisberg, A.; Hesse, M. Tetrahedron 1995, 51, 12573.
Finally, we previously demonstrated the applicability of
this approach to the gymnodimine intermediate, sulfon-
amide 1, possessing a labile silyl enol ether. Importantly,
we were able to perform a one-pot activation–deprotec-
tion without the need of removing excess reagents and
switching solvents (Scheme 5).14 This demonstrates the
utility of this approach and the feasibility of performing
detosylation in a single operation. The resulting acetamide
Synlett 2006, No. 19, 3294–3298 © Thieme Stuttgart · New York

LETTER
Mild Deprotection of Primary N-(p-Toluenesulfonyl) Amides
3297
Table 1 Deprotection of Primary Sulfonamides via Trifluoroacetylation and Reductive Cleavage with SmI2
TFAA (2 equiv), Et3N, (2 equiv)
CH2Cl2, 23 °C, 10 min, then,
R-NHC(O)CF3
R-NHTs
Sml2 (4.9 equiv), THF, –78 °C
Entry
1
Substrate
Product
Yield (%)
85
O
NHTs
N
CF3
H
3
5
O
CF3
2
Boc
N
N
H
95
88
Boc
N
NHTs
7
16
O
Ts
3a
N
N
H
Br
CF3
O
Br
H
8
17
O
O
N
CF3
NHTs
4
5
84
85
H
O
O
9
18
O
NHTs
N
CF3
H
CN
CN
10
11
19
20
O
H
O
NHTs
O
N
O
CF3
6
82
[ ]3
[ ]3
O
O
7
8
95
77
N
N
NHTs
N
H
CF3
12
21
O
O
H
N
CF3
O
NHTs
O
O
13
22
O
O
H
N
CF3
NHTs
9
71
60
O
14
23
24
N
10
HN
CF3
N
NHTs
O
15
a In this case, Et3N (3 equiv) and TFAA (3 equiv) were used.
Synlett 2006, No. 19, 3294–3298 © Thieme Stuttgart · New York

3298
Z. Moussa, D. Romo
LETTER
(9) Parsons, A. F.; Pettifer, R. M. Tetrahedron Lett. 1996, 37,
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Molinoff, P. B.; Childers, S. R.; Ehrenkaufer, R. L. J. Med.
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Brierley, M. J.; Andersen, K.; Sløek, F. A.; Mellor, I. R.;
Usherwood, P. N. R.; Krogsgaard-Larsen, P.; Jaroszewski, J.
W. J. Med. Chem. 1999, 42, 5224. (g) Acetamide 18: Keck,
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R.; Kaur, S.; Singh, P. P. Bioorg. Med. Chem. 2004, 12,
2501. (i) General Procedure for the Reductive Cleavage
of the Tosyl Group of Monosubstituted N-(p-Toluene-
sulfonyl) Amides as Described for Preparation of Amide
5: To a stirred solution of sulfonamide 3 (55.1 mg, 0.20
mmol) and Et3N (60 mL, 0.40 mmol) in CH2Cl2 (3.0 mL) was
added TFAA (57 mL, 0.40 mmol) dropwise via syringe.
After aliquot NMR analysis indicated complete reaction (ca.
10 min), the solvent was concentrated in vacuo and the
residue was diluted with THF (1.0 mL), cooled to –78 °C and
subsequently treated dropwise with SmI219 in THF (10.0 mL,
0.098 M, 0.98 mmol). The resulting mixture was stirred for
30 min and then quenched by filtering through a compressed
pad of silica gel. The filter pad was washed with EtOAc and
the solvent was removed in vacuo. The residue was
chromatographed (elution with 10% EtOAc–hexanes) to
afford acetamide 5 (37 mg, 85%) as a white solid, followed
by the starting material 3 (7.5 mg). The spectral data for the
isolated material was in accord with published spectral
data.21
(11) (a) Nishimura, O.; Fujino, M. Chem. Pharm. Bull. 1976, 24,
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(19) SmI2 was prepared according to the published procedure,
see: Imamoto, T.; Ono, M. Chem. Lett. 1987, 501.
(20) We were unable to isolate a clean sample of
bistrifluoroacetamide 6 as it underwent partial deacetylation
to acetamide 5 during chromatography.
(23) Et3N was not suitable for the tosylation of 3¢-aminoaceto-
phenone (Table 1, sulfonamide 14) as it resulted in an
inseparable mixture of 14 and its bistosylated derivative.
However, employing pyridine as a base led to exclusive
monotosylation.
(24) Substrate 8 (Table 1, entry 3) required TFAA (3 equiv) and
Et3N for complete acetylation.
(25) Reductive cleavage of the tosyl group at 23 °C led to
significantly reduced yields of trifluoroacetamide.
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(22) The following compounds are known and spectroscopic data
have been previously reported in the literature:
(a) Sulfonamide 3: Soeta, T.; Nagai, K.; Fujihara, H.;
Kuriyama, M.; Tomioka, K. J. Org. Chem. 2003, 68, 9723.
(b) Sulfonamide 8: Di Martino, A.; Galli, C.; Gargano, P.;
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Org. Lett. 2005, 7, 1031. (d) Sulfonamide 10: Bookser, B.
C.; Bruice, T. C. J. Am. Chem. Soc. 1991, 113, 4208.
(e) Acetamide 16: Mach, R. H.; Luedtke, R. R.; Unsworth,
C. D.; Boundy, V. A.; Nowak, P. A.; Scripko, J. G.; Elder,
Synlett 2006, No. 19, 3294–3298 © Thieme Stuttgart · New York