Activation Method to Prepare a Highly Reactive Acylsulfonamide "Safety-Catch" Linker for Solid-Phase Synthesis

Full text of DOI:10.1021/ja9535165

J. Am. Chem. Soc. 1996, 118, 3055-3056
3055
Herein, we report on the potential applications of an activation
method to prepare a highly reactive acylsulfonamide linkage.
Aminomethylated macroreticular resin is treated with 4-car-
boxybenzenesulfonamide, N,N-diisopropylcarbodiimide (DICI),
and 1-hydroxybenzotriazole (HOBt) to provide the sulfonamide-
derivatized resin 1 (Scheme 1). Acylsulfonamide 2 is then
prepared by treating sulfonamide resin 1 with i-Pr2EtN, catalytic
DMAP, and the symmetrical anhydride of a carboxylic acid
prepared in situ.9 At the end of a given synthesis sequence,
we had previously activated the acylsulfonamide 2 for nucleo-
philic cleavage by treatment with CH2N2 according to the
procedure of Kenner to provide the N-methyl acylsulfonamide
3a. We hypothesized that alkylation to introduce an electron-
withdrawing N-alkyl group would provide enhanced reactivity
toward nucleophilic displacement. Several alkyl groups were
evaluated for activation of acylsulfonamide 2, with the cya-
nomethyl group proving to be optimal. Treatment of 2 with
bromoacetonitrile or iodoacetonitrile and i-Pr2EtN in DMSO
or 1-methyl-2-pyrrolidinone (NMP)10 provides the N-cyanom-
ethyl acylsulfonamide 3b. The cyanomethyl derivative 3b (R1
) (CH2)2Ph-3,4,5-tri-OMe) is highly labile to nucleophilic
displacement, with a t1/2 of <5 min for displacement with 0.007
M benzylamine in DMSO. In comparison, the t1/2 for the
corresponding N-methyl derivative 3a under the same conditions
is approximately 790 min. The acylsulfonamide 3b (R1 )
(CH2)2Ph-3,4,5-tri-OMe) is rapidly cleaved with a number of
amines at room temperature to give the corresponding amide
products 4a-h in high yield based upon the initial aminomethyl
substitution of the resin (Table 1). This includes both sterically
hindered amines and nonbasic amines, as shown by the high
yields for cleavage with tert-butylamine and aniline, respectively
(entries 4f and 4g, Table 1). The latter result is noteworthy
since no cleavage of the analogous N-methyl acylsulfonamide
is observed upon treatment with aniline, even under forcing
conditions.
Due to the high reactivity of 3b, treatment with limiting
amounts of an amine nucleophile results in complete consump-
tion of the amine to provide the pure amide product 4,
uncontaminated with excess amine. For example, acylsulfona-
mide 3b (R1 ) (CH2)2Ph-3,4,5-tri-OMe) was treated with a
limiting amount of benzylamine at room temperature to provide
pure N-benzyl amide 4c in 98% yield based upon the benzyl-
amine reagent. The high efficiency of this process provides
the opportunity to apply novel pooling strategies, whereby
equimolar quantities of highly pure amide products are obtained
by treating support-bound 3b with a limiting amount of an
equimolar mixture of several amines. When support-bound 3b
(R1 ) (CH2)2Ph-3,4,5-tri-OMe) is treated with a limiting amount
(0.5 equiv total amine) of an equimolar mixture of the five
amines (4-(3-aminopropyl)morpholine, morpholine, benzyl-
amine, piperidine, cyclohexylamine), a pool of the five amide
products 4a-e is obtained (Figure 1). Equal amounts of the
five products ((3%) are observed by HPLC analysis, and the
free acid (<0.5% of combined amide products) resulting from
acylsulfonamide hydrolysis is the only side product observed.
When less nucleophilic amines (aniline, tert-butylamine) are
included in pooling experiments, heating is required and
significantly more competitive hydrolysis occurs.11
Activation Method to Prepare a Highly Reactive
Acylsulfonamide “Safety-Catch” Linker for
Solid-Phase Synthesis1
Bradley J. Backes, Alex A. Virgilio, and
Jonathan A. Ellman*
Department of Chemistry, UniVersity of California
Berkeley, California 94720
ReceiVed October 19, 1995
Solid-phase synthesis methods are commonly employed for
the preparation of oligonucleotides and peptides and are
becoming increasingly important for the preparation of small
organic molecules, in particular for the preparation of compound
libraries for drug development programs.2 In almost all solid-
phase peptide synthesis efforts,3 and in a majority of small
molecule solid-phase synthesis approaches, the compound is
attached to the support through a carboxylic acid functionality.
Linkage to support is most often accomplished with amide-based
linkage elements that provide primary amide products upon
cleavage, or with ester-based linkage elements that provide
carboxylic acid products upon cleavage. However, for many
synthesis efforts it is desirable to cleave the compound from
the support by nucleophilic displacement with amines or
alcohols to provide the corresponding amide or ester products.4,5
To achieve this goal, researchers have worked to develop linkers
that are stable through a given synthesis sequence, yet can be
activated for nucleophilic cleavage upon synthesis completion.6
Of these, only Kenner’s acylsulfonamide safety-catch linker7
is completely stable to basic or strongly nucleophilic conditions.
Activation is accomplished by treatment with diazomethane to
provide the N-methyl acylsulfonamide, which can then be
cleaved with hydroxide or with nucleophilic amines. Kenner
initially developed this linker for peptide synthesis and dem-
onstrated the preparation of acid, primary amide, and hydrazide
products. We have used an adaptation of the linker for the solid-
phase synthesis of the arylacetic acid class of cyclooxygenase
inhibitors where basic reaction conditions were employed
including acylsulfonamide enolate alkylation reactions.8 For
both peptide and small molecule synthesis, however, the
reactivity of the N-methyl acylsulfonamide is poor. Non-
nucleophilic amines do not react with the N-methylated acyl-
sulfonamide, and even for nucleophilic amines, excess reagent
is usually employed which can complicate product isolation.
(1) A preliminary account was presented at the 209th American Chemical
Society National Meeting, Anaheim, CA, 1995; Abstract ORGN 261.
(2) Reviewed in the following: (a) Gallop, M. A.; Barrett, R. W.; Dower,
W. J.; Fodor, S. P. A.; Gordon, E. M. J. Med. Chem. 1994, 37, 1233-
1251. (b) Gallop, M. A.; Barrett, R. W.; Dower, W. J.; Fodor, S. P. A.;
Gordon, E. M. J. Med. Chem. 1994, 37, 1385-1401. (c) Terrett, N. K.;
Gardner, M.; Gordon, D. W.; Kobylecki, R. J.; Steele, J. Tetrahedron 1995,
51, 8135-8173. (d) Thompson, L. A.; Ellman, J. A. Chem. ReV. 1996, 96,
555-600.
(3) (a) Atherton, E.; Sheppard, R. C. Solid Phase Peptide Synthesis: A
Practical Approach; IRL Press: Oxford, England, 1989. (b) Fields, G. B.;
Noble, R. L. Int. J. Pept. Protein Res. 1990, 161-214.
(4) Oxime resin has been used with success for the preparation of N-alkyl
amides, peptide cyclization, and segment condensation. Selected examples
follow: (a) Degrado, W. F.; Kaiser, E. T. J. Org. Chem. 1980, 45, 1295-
1300; 1982, 47, 3258-3261. (b) O¨ sapay, G.; Taylor, J. W. J. Am. Chem.
Soc. 1990, 112, 6046-6124. (c) Kaiser, E. T.; Mihara, H.; Laforet, G. A.;
Kelly, J. W.; Walters, L.; Findeis, M. A.; Sasaki, T. Science 1989, 243,
187-192.
Extension of this activation protocol to carboxylic acids that
possess R-electronegative substituents initially proved to be
problematic. Acylation of support-bound sulfonamide 1 with
(5) For solid-phase strategies to prepare peptide esters, see: Seebach,
D.; Thaler, A.; Blaser, D.; Ko, S. Y. HelV. Chim. Acta 1991, 74, 1102-
1119 and references cited therein.
(6) (a) Marshall D. L.; Liener, I. E. J. Org. Chem. 1970, 35, 867-868.
(b) Flanigan, E.; Marshall, G. R. Tetrahedron Lett. 1970, 27, 2403-2406.
(c) Wieland, T.; Lewalter, J.; Burr, C. Justes Liebigs Ann. Chem. 1970,
740, 31-47.
(9) ) This method has proven to be successful with a number of carboxylic
acids and represents an improvement over the method reported previously
employing pentafluorophenyl esters.
(10) NMP is employed rather than DMSO when using gel-form resin to
better solvate the resin.
(7) Kenner, G. W.; McDermott, J. R.; Sheppard, R. C. J. Chem. Soc.,
Chem. Commun. 1971, 636-637.
(11) In initial amine pooling experiments where aniline was employed,
only a 70-80% yield of the anilide product was observed due to competitive
hydrolysis or slow reaction rates.
(8) Backes, B. J.; Ellman, J. A. J. Am. Chem. Soc. 1994, 116, 11171-
11172.
0002-7863/96/1518-3055$12.00/0 © 1996 American Chemical Society

3056 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Communications to the Editor
Scheme 1a
Scheme 2a
a (a) Boc-L-Phe-OH, PyBOP, i-Pr2EtN (2x); (b) ICH2CN, i-Pr2EtN;
(c) Leu-OMe.
a (a) (R1CO)2O, catalytic DMAP, i-Pr2EtN; (b) CH2N2; (c) XCH2CN,
i-Pr2EtN; (d) R3R4NH.
of acylsulfonamide 2 (R1 ) Ph) in the cyanomethylation step.12
However, efficient activation and displacement with both of
these substrates are accomplished by employing an aliphatic
sulfonamide linker rather than the aryl sulfonamide linker 1
(Scheme 2), apparently due to the increased basicity and
resulting enhanced nucleophilicity of an aliphatic acylsulfona-
mide anion.13 Accordingly, 3-carboxypropanesulfonamide14 is
coupled to aminomethyl Merrifield resin employing DICI and
HOBt in DMF. The support-bound aliphatic sulfonamide 5 is
then submitted twice to coupling conditions employing Boc-L-
phenylalanine, (1H-benzotriazol-1-yloxy)tripyrrolidinophospho-
nium hexafluorophosphate (PyBOP), and i-Pr2EtN in DMF to
provide acylsulfonamide 6.15 Alkylation with iodoacetonitrile
and i-Pr2EtN in NMP at ambient temperature to give 7, followed
by cleavage with L-leucine methyl ester, provides the dipeptide
product 8a in 77% yield with 1.2-1.5% of the L,D-epimer being
observed.16 Displacement of 7 with D-leucine methyl ester gives
8b in 81% yield (1.3% D,D-epimer).17,18
Table 1. Synthesis of Amide Products 4 (Scheme 1)
%
cmpd
R1
amine
yielda
4a (CH2)2Ph-3,4,5-tri-OMe 4-(3-NH2(CH2)3)morpholine 98
4b (CH2)2Ph-3,4,5-tri-OMe morpholine
4c (CH2)2Ph-3,4,5-tri-OMe benzylamine
4d (CH2)2Ph-3,4,5-tri-OMe piperidine
4e (CH2)2Ph-3,4,5-tri-OMe cyclohexylamine
98
98
98
98
92
96b
97c
4f
(CH2)2Ph-3,4,5-tri-OMe tert-butylamine
4g (CH2)2Ph-3,4,5-tri-OMe aniline
4h (CH2)2Ph-3,4,5-tri-OMe benzylamine
a Yields of analytically pure material based upon the initial ami-
nomethyl substitution level of the resin. b Elevated temperature (65 °C)
and 1 M aniline were employed. c Yield for the four-step synthesis
sequence including the aminolysis step, as based upon the original
loading level of 4-bromophenylacetic acid (see ref 8).
We are currently optimizing the acylation of sulfonamide 5
with protected amino acids to eliminate epimerization for peptide
segment condensation and cyclization strategies. In addition,
using the N-cyanomethyl activation method, the acylsulfonamide
linker is being utilized in the synthesis of other compound
classes.
Acknowledgment. We gratefully acknowledge NSF, the Beckman
Foundation, the Burroughs Wellcome Fund, the A.P. Sloan Foundation,
Affymax, Tularik, and Hoffman-La Roche for support of this work.
A.A.V. also thanks Berlex for a Graduate Fellowship.
Supporting Information Available: Experimental details, including
analytical data for all compounds described in the article (6 pages).
This material is contained in many libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can be
ordered from the ACS, and can be downloaded from the Internet; see
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instructions.
Figure 1. (a) HPLC trace of addition of limiting amounts of five
amines to acylsulfonamide resin 3b (R1 ) (CH2)2Ph-3,4,5-tri-OMe)
resulted in equimolar amounts ((3%) of the five amide products listed
in order of elution (relative peak area): 4a 4-(3-aminopropyl)morpholine
(0.97), 4b morpholine (1.00), 4c benzylamine (1.00), 4d piperidine
(1.01), 4e cyclohexylamine (1.01). (b) HPLC trace of standard solution
containing an equimolar mixture of the five amide products.
JA9535165
(12) Acylsulfonamide 2 (R1 ) Ph) is likely to be more acidic than
acylsulfonamide 2 (R1 ) alkyl) since the pKa values of benzamide and
acetamide in DMSO solution are 23.35 and 25.5, respectively: Bordwell,
F. G. Acc. Chem. Res. 1988, 21, 456-463.
(13) The pKa values for methanesulfonamide and benzenesulfonamide
in DMSO solution are 17.5 and 16.1, respectively: Bordwell, F. G. Acc.
Chem. Res. 1988, 21, 456-463.
(14) 3-Carboxypropanesulfonamide was prepared from commercially
avaliable 4,4′-dithiobutyric acid. See supporting information for details.
(15) Treatment of 5 with the symmetrical anhydride of Boc-L-phenyl-
alanine, using catalytic DMAP, and i-Pr2EtN followed by cyanomethylation
and cleavage with L-leucine methyl ester resulted in good yields of 8 (80-
86%) but high levels of racemization (6-7% L,D-epimer).
(16) Although a number of coupling reagents and conditions were
surveyed, PyBoP with i-Pr2EtN in DMF provided the best combination of
efficient loading with reduced epimerization.
(17) Increased levels of epimerization are not observed when submitting
7 to the alkylation conditions a second time before cleavage with L-leucine
methyl ester, indicating that racemization does not occur in the activation
step.
the symmetrical anhydride of Boc-L-phenylalanine employing
catalytic DMAP and i-Pr2EtN, followed by alkylation with
iodoacetonitrile and i-Pr2EtN in NMP, and displacement with
excess benzylamine resulted in only a 29% yield of the N-benzyl
amide product. In contrast, alkylation of the Boc-L-phenyl-
alanine acylsulfonamide resin with CH2N2 followed by ben-
zylamine treatment resulted in a 94% yield of the N-benzyl
amide product, suggesting that incomplete cyanomethylation of
the acylsulfonamide had occurred. Presumably, the electrone-
gative R-protected amine attenuates the nucleophilicity of the
acylsulfonamide anion. Similarly, low yields were also observed
for cyanomethylation followed by benzylamine cleavage of
acylsulfonamide 1 (R1 ) Ph) that was prepared by acylation of
1 with benzoic acid. Again, this is due to incomplete alkylation
(18) Complete cyanomethylation and benzylamine cleavage is also ob-
served with the acylsulfonamide prepared from sulfonamide 5 and benzoic
acid.