Solid-phase synthesis of N(α)-benzyl-N(α)-cinnamyl lysine and glutamic acid derivatives

Article abstract of DOI:10.1016/S0040-4039(00)00834-0

Several variations of a solid-phase strategy for the synthesis of N(α)- benzyl-N(α)-cinnamyl lysine and glutamic acid derivatives are presented. Starting from the corresponding N(α)-Fmoc amino acids on Wang resin, reductive alkylation using nitrocinnamaldehyde or a substituted benzaldehyde was followed by nucleophilic displacement of a substituted benzyl halide or nitrocinnamyl bromide to provide resin-bound intermediates. Diversity was added by reduction of the nitro group and derivatization of the resulting aminocinnamyl moiety with a variety of acylating or sulfonylating reagents. Using an orthogonal protecting group strategy, N(ε)-Dde-protected lysine derivatives were further functionalized at the side-chain amino group prior to cleavage from resin. This method allows for the preparation of analog libraries having up to four points of diversity. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00834-0

Tetrahedron Letters 41 (2000) 5187±5191
Solid-phase synthesis of Na-benzyl-Na-cinnamyl lysine and
glutamic acid derivatives
Peter J. Connolly,* Kimberly N. Beers, Steven K. Wetter and William V. Murray
The R. W. Johnson Pharmaceutical Research Institute, 1000 Route 202, Raritan, NJ 08869, USA
Received 18 April 2000; revised 18 May 2000; accepted 22 May 2000
Abstract
Several variations of a solid-phase strategy for the synthesis of Na-benzyl-Na-cinnamyl lysine and glutamic
acid derivatives are presented. Starting from the corresponding Na-Fmoc amino acids on Wang resin,
reductive alkylation using nitrocinnamaldehyde or a substituted benzaldehyde was followed by nucleo-
philic displacement of a substituted benzyl halide or nitrocinnamyl bromide to provide resin-bound inter-
mediates. Diversity was added by reduction of the nitro group and derivatization of the resulting
aminocinnamyl moiety with a variety of acylating or sulfonylating reagents. Using an orthogonal protect-
ing group strategy, Ne-Dde-protected lysine derivatives were further functionalized at the side-chain amino
group prior to cleavage from resin. This method allows for the preparation of analog libraries having up to
four points of diversity. # 2000 Elsevier Science Ltd. All rights reserved.
Over the past decade, solid-phase organic chemistry has become a valuable tool for the rapid
preparation of small molecules.1,2 In the pharmaceutical industry, solid phase synthesis provides
the medicinal chemist with the means to quickly generate analogs for structure±activity relation-
ship (SAR) studies and more eciently optimize biological activity. Using both combinatorial
and parallel techniques, many groups have constructed large diverse libraries or small- to medium-
sized targeted libraries for evaluation in biological assays. One convenient source of solid-supported
starting materials for the construction of libraries is the collection of commercially available
resin-bound protected amino acids. While this `chiral pool' has been used in peptide synthesis for
more than 35 years,3 it has more recently been employed in the construction of heterocycles and
other non-peptidic molecules of biological interest.4,5 In our laboratory, work directed toward the
discovery of small molecule mimics of protein±protein interactions led to a series of N,N-disub-
stituted amino acids having micromolar anity for the erythropoietin receptor, which we were
interested in expanding via solid phase parallel synthesis techniques.6 A common characteristic of
the highest anity analogs was the presence of phenoxy or benzyloxy substituents on the two
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00834-0

5188
phenyl rings of core structure 1 (Fig. 1). Although synthesis of phenyl ethers on solid support
using the Mitsunobu reaction has been reported, we decided to approach the problem di€erently
by preparing amide analogs of the original benzyl ether leads.7 9 In this way, we could take
advantage of readily available carboxylic and sulfonic acid derivatives for use as coupling partners
with a primary amino group at R3 or R4 of the core structure.
Figure 1.
The synthesis of Na-benzyl-Na-cinnamyl lysine derivatives was carried out according to the
following procedure (Scheme 1). Commercially available Wang resin-bound Na-Fmoc-Ne-Boc
lysine (2, R1=Boc) was deprotected with 40% piperidine/DMF and subjected to reductive alkyl-
ation with a substituted benzaldehyde using (MeO)3CH (overnight imine formation) followed
by NaBH(OAc)3 in CH2Cl2.10,11 The resulting secondary amine (3, R1=Boc) was reacted with
3-nitrocinnamyl bromide and diisopropylethylamine (DIEA) in DMF; subsequent nitro reduction
using SnCl2 in DMF a€orded the corresponding 3-aminocinnamyl derivative.12,13 Treatment
with various sulfonyl chlorides, carboxylic acids, or carboxylic anhydrides in pyridine/CH2Cl2
Scheme 1. (a) 40% piperidine/DMF; (b) R2C6H4CHO, (MeO)3CH; (c) NaBH(OAc)3, CH2Cl2; (d) 3-
3
.
NO2C6H4CHˆCHCH2Br, DIEA, DMF; (e) SnCl2 2H2O, DMF; (f) R -X, pyridine; (g) 50% TFA/CH2Cl2; (h) 2%
N2H4/DMF; (i) succinic anhydride, pyridine, DMF

5189
provided protected targets 4 (R1=Boc) which were cleaved from resin (50% TFA/CH2Cl2) to
give Ne-unsubstituted lysine derivatives 5. In order to prepare analogs having functionality on the
e-amino group, Ne-Dde-protected lysine (Dde=1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl)
was used in place of Ne-Boc-lysine.14 To begin, commercially available Fmoc-Lys(Dde)OH was
coupled to Wang resin using the standard Sieber technique.15 The resulting resin (2, R1=Dde)
was treated in the same manner as the Ne-Boc-lysine resin until just prior to the cleavage step. At
this point, the Ne-protected derivative 4 (R1=Dde) was shaken with 2% hydrazine in DMF to
remove the Dde group. The resin-bound free amine 4 (R1=H) was acylated with succinic anhydride
and cleaved from resin with TFA/CH2Cl2 to give the target N-benzyl-N-cinnamyl lysine derivatives
6.16 Using this protocol, we generated a 17-member library incorporating four points of diversity
in 35±85% yield.17
A variation of the lysine route was used to synthesize N-benzyl-N-cinnamyl glutamic acid
analogs bearing substitution at the 4-position of the cinnamyl phenyl ring. Interestingly, when we
®rst applied the 3-nitrocinnamyl methodology to a 4-nitrocinnamyl system, we observed compli-
cations in the nitro reduction step (Scheme 2). Speci®cally, attempted reduction of amine 7
(R=NO2) with SnCl2 in MeOH did not provide the expected 4-aminocinnamyl derivative,
resulting instead in loss of the 4-nitrocinnamyl group and recovery of secondary amine 8. This
unexpected result was accompanied by the appearance of an intense maroon color in the reaction
mixture, consistent with the presence of 1-imino-4-methylene-2,5-cyclohexadiene 9, possibly
formed by decomposition of 4-aminocinnamyl product 7 (R=NH2).18 Similar behavior has been
observed in the SnIV-mediated cleavage of p-nitrobenzyl ethers, esters, and carbamates.19 We
postulated that protonation of the alpha nitrogen of 7 (R=NH2) under the acidic SnCl2 reduction
conditions favors solvolysis of the 4-aminocinnamyl moiety, and that elimination of the positive
charge on Na (by acylation, for example) would minimize the cleavage reaction. Therefore, we
redesigned the solid-phase reaction scheme to include Fmoc reprotection at Na (Scheme 3). Toward
that end, commercially available Wang resin-bound N-Fmoc glutamic acid (10) was deprotected
and reductively alkylated with 4-nitrocinnamaldehyde using the conditions described above,
giving the expected resin-bound secondary amine. After N-protection using Fmoc-Cl/pyridine,
SnCl2 reduction of N-Fmoc intermediate 11 proceeded cleanly without N-dealkylation to provide
the desired 4-aminocinnamyl compound. Next, the aniline nitrogen was derivatized with various
carboxylic anhydrides, acid chlorides, or sulfonyl chlorides. After removal of the Fmoc protecting
group with 40% piperidine/DMF, Na-alkylation of the resulting secondary amine 12 was carried
out using 4-benzyloxybenzyl chloride, NaI, and DIEA. Finally, cleavage from resin using 50%
TFA/CH2Cl2 provided the target N-benzyl-N-cinnamyl glutamic acids 13 in yields ranging from
30±85%.
Scheme 2. (a) 4-NO2C6H4CHˆCHCHO, (MeO)3CH; (b) NaBH(OAc)3, CH2Cl2; (c) 4-BnOC6H4CH2Cl, NaI, DIEA,
.
DMF; (d) SnCl2 2H2O, DMF

5190
Scheme 3. (a) 40% piperidine/DMF; (b) 4-NO2C6H4CHˆCHCHO, (MeO)3CH; (c) NaBH(OAc)3, CH2Cl2; (d) Fmoc-
.
Cl, pyridine; (e) SnCl2 2H2O, DMF; (f) R-X, pyridine; (g) 4-BnO-C6H4CH2Cl, NaI, DIEA, DMF; (h) 50% TFA/
CH2Cl2
In conclusion, solid phase techniques have been employed in a straightforward and convenient
synthesis of a series of biologically interesting Na-benzyl-Na-cinnamyl amino acids. Using a
sequence of reductive alkylation followed by nucleophilic displacement, core structure 1
(R3=NO2) was easily prepared. Subsequent elaboration of the nitrocinnamyl group and, in the
case of Ne-Dde-protected lysine, the side chain amino group provided two small libraries (5, 6,
and 13) incorporating up to four points of diversity.
References
1. Fruechtel, J. S.; Jung, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 17±42.
2. Hermkens, P. H. H.; Ottenheijm, H. C. J.; Rees, D. Tetrahedron 1996, 52, 4527±4554.
3. Merri®eld, R. B. Science 1965, 151, 178±185.
4. Bicknell, A. J.; Hird, N. W. Bioorg. Med. Chem. Lett. 1996, 6, 2411±2444.
5. Hulme, C.; Ma, L.; Kumar, N. V.; Krolikowski, P. H.; Allen, A. C.; Labaudiniere, R. Tetrahedron Lett. 2000, 41,
1509±1514.
6. Connolly, P. J.; Wetter, S. K.; Murray, W. V.; Johnson, D. L.; McMahon, F. J.; Farrell, F. X.; Tullai, J.; Jolli€e,
L. K. Bioorg. Med. Chem. Lett., submitted.
7. Richter, L. S.; Gadek, T. R. Tetrahedron Lett. 1994, 35, 4705±4706.
8. Rano, T. A.; Chapman, K. T. Tetrahedron Lett. 1995, 36, 3789±3792.
9. Krchnak, V.; Weichsel, A. S.; Issakova, O.; Lam, K. S.; Lebl, M. Mol. Diversity 1996, 1, 177±182.
10. Murphy, M. M.; Schullek, J. R.; Gordon, E. M.; Gallop, M. A. J. Am. Chem. Soc. 1995, 117, 7029±7030.
11. Gordon, D. W.; Steele, J. Bioorg. Med. Chem. Lett. 1995, 5, 47±50.
12. Kiselyov, A. S.; Smith II, L.; Armstrong, R. W. Tetrahedron 1998, 54, 5089±5096.
13. Lee, J.; Gauthier, D.; Rivero, R. A. Tetrahedron Lett. 1998, 39, 201±204.
14. Bycroft, B. W.; Chan, W. C.; Chharbra, S. R.; Hone, N. D. J. Chem. Soc., Chem. Commun. 1993, 778±779.
15. Sieber, P. Tetrahedron Lett. 1987, 28, 6147±6150.

5191
16. Functionalization of lysine side chains on solid support is known: (a) Lanter, C. L.; Guiles, J. W.; Rivero, R. A.
Mol. Diversity 1999, 4, 149±153; (b) Roberts, K. D.; Lambert, J. N.; Ede, N. J.; Bray, A. M. Tetrahedron Lett.
1998, 39, 8357±8360; (c) Yong, Y. F.; Kowalski, J. A.; Thoen, J. C.; Lipton, M. A. Tetrahedron Lett. 1999, 40, 53±
56.
1
17. All compounds were characterized by H NMR (>85% purity) and mass spectroscopy (ES+).
18. Reduction of 4-nitrocinnamylamines using SmI2 or Fe/HCl, respectively, has been reported: (a) Baldwin, J. E.;
Turner, S. C. M.; Moloney, M. G. Tetrahedron 1994, 50, 9411±9424; (b) Dodic, N.; Dumaitre, B.; Dougan, A.;
Pianetti, P. J. Med. Chem. 1995, 38, 2418±2426.
19. Hocker, M. D.; Caldwell, C. G.; Macsata, R. W.; Lyttle, M. H. Pept. Res. 1995, 8, 310±315.