Synthesis of the peptide moiety of the jamaicamides

Article abstract of DOI:10.1016/j.tetlet.2011.07.078

The jamaicamides, isolated from cyanobacterium Lyngbya majuscula in Jamaica, are unique mixed polyketide-peptides that are reported to be blockers of the sodium channels. The peptide moiety contains a pyrrolinone ring and a β-methoxy enone functionality. Herein, we report the stereoselective synthesis of the N-(Boc)₂-protected peptide moiety of the jamaicamides by utilizing Meldrum's acid starting from l-alanine and N-Boc-β-alanine.

Full text of DOI:10.1016/j.tetlet.2011.07.078

Tetrahedron Letters 52 (2011) 5036–5038
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Tetrahedron Letters
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Synthesis of the peptide moiety of the jamaicamides
⇑
Ayano Tanaka, Toyonobu Usuki
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The jamaicamides, isolated from cyanobacterium Lyngbya majuscula in Jamaica, are unique mixed poly-
ketide-peptides that are reported to be blockers of the sodium channels. The peptide moiety contains a
pyrrolinone ring and a b-methoxy enone functionality. Herein, we report the stereoselective synthesis of
Received 28 June 2011
Revised 14 July 2011
Accepted 19 July 2011
Available online 24 July 2011
the N-(Boc)2-protected peptide moiety of the jamaicamides by utilizing Meldrum’s acid starting from
alanine and N-Boc-b-alanine.
L-
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Marine natural products
Jamaicamides
Peptide
Total synthesis
Amino acids
Meldrum’s acid
The jamaicamides A, B, and C (Fig. 1) were isolated from a dark
green strain of cyanobacterium Lyngbya majuscula in Hector’s Bay
in Jamaica by Gerwick and co-workers in 2004.1 The marine natu-
ral products are unique mixed polyketide-peptide neurotoxins
containing a trisubstituted E chloroolefin, an undetermined methyl
stereocenter (C-9), a rare alkynyl bromide (only in jamaicamide A),
a pyrrolinone ring, and a b-methoxy enone. The jamaicamides are
sodium channel blockers and exhibit fish toxicity as well as cyto-
toxicity to both H-460 human lung and Neuro-2a mouse neuro-
blastoma cell lines. Interestingly, only jamaicamide B displays
anti-malarial activity and cytotoxicity to Vero cells.2
The attractive biological aspects and biosynthesis of these sec-
ondary metabolites,3 as well as the opportunity to decipher the
structure–activity relationships of the jamaicamides at atomic le-
vel prompted us to conduct a study on the synthesis of the neuro-
toxins. Synthesis of jamaicamide C carboxylic acid by employing
Johnson–Claisen rearrangement and Negishi coupling strategy
was reported by Paige and co-workers in 2009.4 In the present
study, we report the stereoselective synthesis of the peptide moi-
ety of the jamaicamides, of which the amine group was protected
by di-t-butoxycarbonyl ((Boc)2). The synthesis utilizes Meldrum’s
an amide bond between 2 and 3 would be accomplished. Both seg-
ments 2 and 3 can be then synthesized starting from the commer-
cially available L-alanine and N-Boc-b-alanine, respectively, by
treatment with Meldrum’s acid as the key reaction.5
Preparation of the pyrrolidone ring segment 2 began with
L
-alanine (Scheme 2).6 Protection of the amino group in
L
-alanine
-alanine 4 in 96% yield.7 Compound
was then treated with recrystallized Meldrum’s acid8 in
with (Boc)2O provided N-Boc-
4
L
the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDCꢀHCl) and 4-dimethylaminopyridine (DMAP)
in CH2Cl2 to give the intermediate 5.9 Reflux of 5 in ethyl acetate
for 30 min gave tetramic acid 6 quantitatively. The regioselective
reduction of 6 was accomplished by sodium borohydride (NaBH4)
in CH2Cl2-acetic acid (10:1) to afford alcohol 7, followed by the for-
mation of mesylate which was spontaneously eliminated in the
presence of triethylamine.10,11 Thus, N-Boc-5-methyl pyrrolinone
8 was obtained from N-Boc-L-alanine 4 in 68% yield over four
steps. When the reaction was performed in the presence of N,
acid starting from L-alanine and N-Boc-b-alanine.
As illustrated in Scheme 1, N-(Boc)2-protected peptide moiety 1
of the jamaicamides could be retrosynthetically divided into two
parts through the amide bond: the pyrrolidone ring segment 2
and the N-Boc-b-methoxy enone carboxylic acid 3. Before con-
struction of a relatively labile pyrrolinone system, formation of
⇑
Corresponding author. Tel.: +81 3 3238 3446; fax: +81 3 3238 3361.
E-mail address: t-usuki@sophia.ac.jp (T. Usuki).
Figure 1. Structures of the jamaicamides, mixed polyketide-peptide neurotoxins.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.078

A. Tanaka, T. Usuki / Tetrahedron Letters 52 (2011) 5036–5038
5037
As an alternative route, N-Boc-pyrrolidone 10 was also provided
from 4 in 78% yield in three steps (Scheme 2).12 Condensation of
carboxylic acid 4 with Meldrum’s acid and regioselective reduction
of 5 gave 11. The ring closure of 11–10 did not proceed under the
conditions of reflux in ethyl acetate as in the case of 5–6. However,
reflux of 11 in toluene at 110 °C for 3 h successfully gave 10. Thus,
segment 2 was obtained from
five steps in this route, which is an improvement over the former
route (50% yield in seven steps from -alanine).
L-alanine with an overall 67% yield in
L
The preparation of N-Boc-b-methoxy enone carboxylic acid 3
began with N-Boc-b-alanine (Scheme 3). Condensation of N-
Boc-b-alanine with Meldrum’s acid, followed by thermolysis of
intermediate 12 in ethyl acetate produced N-Boc-protected pipe-
ridinedione 13 in 74% yield in two steps.13 Compound 13 was
regioselectively methylated with dimethyl sulfate and K2CO3 in
acetone to give 14 in 91% yield.14 However, treatment of 14 with
lithium hydroperoxide did not result in cleavage of the ring to form
carboxylic acid 3. As an alternative route, N-Boc-b-alanine was
treated with Meldrum’s acid, followed by refluxing intermediate
12 in anhydrous methanol to give the b-keto ester 15 in 89% yield
in two steps.15,16 Formation of E methyl enol ether 16 was accom-
plished from 15 in 22% yield by treatment with trimethyl orthofor-
mate under acidic conditions.17–19 Treatment of 15 with dimethyl
sulfate and K2CO3 under basic conditions gave the desired 16 ste-
reoselectively in 57% yield as well as the byproduct 16’ in 24%
yield.18 Nuclear Overhauser effect (NOE) was observed between
H18 and the methyl protons of OMe. Hydrolysis of ester 16 with
lithium hydroxide in THF/H2O at 50 °C gave 3 in 87% yield.17 Thus,
the desired carboxylic acid 3 was successfully prepared in 44%
yield over four steps.
Scheme 1. Retrosynthetic analysis of N-(Boc)2 protected peptide moiety 1 of the
jamaicamides.
With pyrrolidone lactam 2 and acid 3 in hand, we focused our
efforts on the formation of the peptide moiety of the jamaicamides
(Scheme 4). First, activation of the carboxylic functionality of 3 was
achieved by conversion to its pentafluorophenol (Pfp) ester 17 with
EDCꢀHCl in the presence of an equivalent amount of DMAP in 83%
yield. While treatment with EDCꢀHCl without DMAP afforded 17 in
68% yield, treatment with DCC afforded 17 in 49% yield as well as a
cyclization by-product.20 Next, treatment of amine 2 in THF with
lithium hexamethyldisilazide (LiHMDS) as a base at ꢂ55 °C for
30 min provided the lithiated anion, which was then reacted with
the activated ester 17 at ꢂ55 °C to 0 °C for 9 h to afford the amide
Scheme 2. Synthesis of pyrrolidone ring segment 2. Reagents and conditions: (a)
(Boc)2O, NaOH, dioxane, 0 °C to rt, 6 h, 96%; (b) Meldrum’s acid, EDCꢀHCl, DMAP,
CH2Cl2, 0 °C to rt, 2 h; (c) EtOAc, reflux, 0.5 h; (d) NaBH4, AcOH/CH2Cl2, 0 °C, 8.5 h;
(e) MsCl, Et3N, CH2Cl2, rt, 1 h, 68% (4 steps from 4); (f) TFA, CH2Cl2, rt, 15 min, 0%; (g)
H2, Pd/C, THF, rt, 3 d, 84%; (h) NaBH4, AcOH/CH2Cl2, 0 °C, 18 h; (i) toluene, reflux,
3 h, 78% (3 steps from 4); (j) TFA, CH2Cl2, rt, 20 min, 90%.
N0-dicyclohexylcarbodiimide (DCC) and DMAP,10 the desired 8 was
obtained from 4 in 56% yield. However, removal of the N-Boc pro-
tecting group in 8 with trifluoroacetic acid (TFA) did not proceed to
give 5-methyl pyrrolinone 9, probably due to the instability of the
product. N-Boc-5-methyl pyrrolinone 8 was thereby converted into
5-methyl pyrrolidone 10 by hydrogenation in 85% yield. Removal
of the N-Boc protecting group in 10 was accomplished with TFA
in 90% yield to give the desired non-racemic pyrrolidone ring seg-
Scheme 3. Synthesis of N-Boc-b-methoxy enone carboxylic acid 3. Reagents and
conditions: (a) Meldrum’s acid, EDCꢀHCl, DMAP, CH2Cl2, 0 °C to rt, 3.5 h; (b) EtOAc,
reflux, 3 h, 74% (2 steps); (c) Me2SO4, K2CO3, acetone, rt, 12 h, 91%; (d) LiOH, H2O2,
H2O/THF, rt, 24 h, 0%; (e) MeOH, reflux, 5 h, 89% (2 steps from Boc-b-alanine); (f)
Me2SO4, K2CO3, acetone, rt, 24 h, 57% (16), 24% (160); (g) LiOH, H2O/THF, 50 °C, 24 h,
87%.
ment 2 (½a 2D2
= ꢂ50.5 (c 0.84, MeOH)).
ꢁ

5038
A. Tanaka, T. Usuki / Tetrahedron Letters 52 (2011) 5036–5038
Supplementary data
Supplementary data (experimental procedures and character-
ization data) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2011.07.078.
References and notes
Scheme 4. Synthesis of 1. Reagents and conditions: (a) PfpOH, EDCꢀHCl, DMAP,
EtOAc, rt, 25 h, 83%; (b) (Boc)2O, DMAP, MeCN, rt, 7 h, 87%; (c) LDA, PhSeBr, THF,
ꢂ78 °C, then, H2O2, THF, rt, 1 h, 41% (2 steps from 20).
1. Edwards, D. J.; Marquez, B. L.; Nogle, L. M.; McPhail, K.; Goeger, D. E.; Roberts,
M. A.; Gerwick, W. H. Chem. Biol. 2004, 11, 817–833.
2. McPhail, K. L.; Correa, J.; Linington, R. G.; González, J.; Ortega-Barría, E.; Capson,
T. L.; Gerwick, W. H. J. Nat. Prod. 2007, 70, 984–988.
3. Gu, L.; Wang, B.; Kulkarni, A.; Geders, T. W.; Grindberg, R. V.; Gerwick, L.;
Håkansson, K.; Wipf, P.; Smith, J. L.; Gerwick, W. H.; Sherman, D. H. Nature
2009, 459, 731–735.
Table 1
Amide formation between 2 and 17 or 19 to produce 18 or 20 in THF at ꢂ55 °C to
room temperature
4. Graf, K. M.; Tabor, M. G.; Brown, M. L.; Paige, M. Org. Lett. 2009, 11, 5382–5385.
5. Jouin, P.; Castro, B.; Nisato, D. J. Chem. Soc., Perkin Trans. 1 1987, 1177–1182.
6. Cuiper, A. D.; Brzostowska, M.; Gawronski, J. K.; Smeets, W. J. J.; Spek, A. L.;
Hiemstra, H.; Kellogg, R. M.; Feringa, B. L. J. Org. Chem. 1999, 64, 2567–2570.
7. Ray, S.; Das, A. K.; Banerjee, A. Chem. Commun. 2006, 2816–2818.
8. Davidson, D.; Bernhard, S. A. J. Am. Chem. Soc. 1948, 70, 3426–3428.
9. Hosseini, M.; Kringelum, H.; Murray, A.; Tønder, J. E. Org. Lett. 2006, 8, 2103–
2106.
10. Ma, D.; Ma, J.; Ding, W.; Dai, L. Tetrahedron: Asymmetry 1996, 7, 2365–2370.
11. Decicco, C. P.; Grover, P. J. Org. Chem. 1996, 61, 3534–3541.
12. Smrcina, M.; Majer, P.; Majerová, E.; Guerassina, T. A.; Eissenstat, M. A.
Tetrahedron 1997, 53, 12867–12874.
Entry
17 or 19a
2 (equiv)
LiHMDS (equiv)
Time (h)
Yield (%)
1
2
3
4
17
19
19
19
1.5
2
2
1.5
1.5
2
9
5
5
4
34 (18)
46 (20)
54 (20)
70 (20)
3
3
a
1 equiv of 17 or 19 was used.
18 in 34% yield (Table 1, entry 1).9,20 Although the desired 18 could
be obtained as the major product, it was difficult to separate from
its byproduct due to similar polarity. In order to prevent the appar-
ent side reactions, the amino group in 17 was converted into the
bis-Boc product 19 with (Boc)2O in the presence of DMAP in 87%
yield. When amide formation between 2 and 19 was carried out
(Table 1, entries 2–4), three equivalents of amine 2 and one equiv-
alent of 19 by three equivalents of LiHMDS were found to give the
amide 20 in 70% yield (Table 1, entry 4). Formation of the enamide
was achieved by treatment with lithium diisopropylamide (LDA)
and phenylselenenyl bromide at ꢂ78 °C, followed by syn elimina-
tion of the selenoxide by treatment with hydrogen peroxide to af-
ford compound 1 stereoselectively in 41% yield over two steps.21–23
13. Vanotti, E.; Amici, R.; Bargiotti, A.; Berthelsen, J.; Bosotti, R.; Ciavolella, A.; Cirla,
A.; Cristiani, C.; D’Alessio, R.; Forte, B.; Isacchi, A.; Martina, K.; Menichincheri,
M.; Molinari, A.; Montagnoli, A.; Orsini, P.; Pillan, A.; Roletto, F.; Scolaro, A.;
Tibolla, M.; Valsasina, B.; Varasi, M.; Volpi, D.; Santocanale, C. J. Med. Chem.
2008, 51, 487–501.
14. Paik, S.; Carmeli, S.; Cullingham, J.; Moore, R. E.; Patterson, G. M. L.; Tius, M. A. J.
Am. Chem. Soc. 1994, 116, 8116–8125.
15. Kralj, D.; Friedrich, M.; Grošelj, U.; Kiraly-Potpara, S.; Meden, A.; Wagger, J.;
Dahmann, G.; Stanovnik, B.; Svete, J. Tetrahedron 2009, 65, 7151–7162.
16. Baoqing, L.; Franck, R. W. Bioorg. Med. Chem. Lett. 1999, 9, 2629–2634.
17. Krebs, O.; Taylor, R. J. K. Org. Lett. 2005, 7, 1063–1066.
18. Chen, J.; Fu, X.-G.; Zhou, L.; Zhang, J.-T.; Qi, X.-L.; Cao, X.-P. J. Org. Chem. 2009,
74, 4149–4157.
19. Chen, J.; Huang, P.-Q.; Queneau, Y. J. Org. Chem. 2009, 74, 7457–7463.
20. Jin, Y.; Liu, Y.; Wang, Z.; Kwong, S.; Xu, Z.; Ye, T. Org. Lett. 2010, 12, 1100–1103.
21. Andrus, M. B.; Li, W.; Keyes, R. F. J. Org. Chem. 1997, 62, 5542–5549.
22. Reich, H. J.; Renga, J. M.; Reich, I. L. J. Am. Chem. Soc. 1975, 5434–5447.
23. Data of 1: colorless oil; ½a D20
ꢁ
+86.1 (c 0.48, MeOH); IR (cmꢂ1) 3120, 2979, 2935,
2660, 1787, 1720, 1604, 1434, 1395, 1367, 1200, 1175, 1140, 1113, 1050, 1004,
825, 756, 704, 667, 607, 413; 1H NMR (500 MHz, CDCl3) d 7.18 (1H, dd, J = 6.2,
2.0 Hz, H22), 6.74 (1H, s, H18), 6.04 (1H, dd, J = 6.2, 1.5 Hz, H21), 4.85 (2H, ddq,
J = 6.8, 1.7, 1.7 Hz, H23), 3.89 (2H, m, H15), 3.72 (3H, s, H25), 3.13 (1H, m, H16),
3.05 (1H, m, H16), 1.51 (18H, s, t-Bu ꢃ 2), 1.44 (3H, d, J = 6.9 Hz, H24); 13C NMR
(125 MHz, CDCl3) d 175.8 (C17), 170.3 (C20), 164.9 (C19), 153.1 (C22), 152.5
(Boc), 126.1 (C21), 94.2 (C18), 82.2 (Boc), 58.3 (C23), 56.1 (C25), 44.3 (C15),
32.9 (C16), 28.2 (Boc), 18.1 (C24); ESI-HRMS (m/z) Calcd for C21H32N2O7Na
[M+Na]+ 447.2107, found 447.2108.
Conclusion
In summary, we achieved the stereoselective synthesis of the
peptide moiety 1 of the jamaicamides starting from natural amino
acids and utilizing Meldrum’s acid. The present study could also be
applied to the synthesis of palmyrrolinone, recently isolated from
cyanobacteria in Palmyra Atoll.24 Studies toward the first total syn-
thesis and stereochemistry of the C-9 methyl group of the jamaica-
mides are currently underway in our laboratory.
24. Pereira, A. R.; Etzbach, L.; Engene, N.; Müller, R.; Gerwick, W. H. J. Nat. Prod.
2011, 74, 1175–1181.
Acknowledgments
This work was supported in part by a Grant-in-Aid for Young
Scientists (B) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT) of Japan.