Enantioselective synthesis of the R-enantiomer of the feeding deterrent (S)-ypaoamide

Article abstract of DOI:10.1021/jo901557h

(Chemical Equation Presented) The enantioselective synthesis of the R-enantiomer of the marine natural product (S)-ypaoamide (5) is reported. The synthesis features both a flexible racemization-free approach to the 5-substituted 3-pyrrolin-2-one segment,

Full text of DOI:10.1021/jo901557h

pubs.acs.org/joc
Enantioselective Synthesis of the R-Enantiomer of
the Feeding Deterrent (S)-Ypaoamide
Jie Chen,†,‡ Pei-Qiang Huang,*,†,§ and Yves Queneau*,‡
†Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, People’s Republic of
‡
ꢀ
ꢀ
ꢀ
China, Institut de Chimie et Biochimie Moleculaires et Supramoleculaires, UMR 5246, CNRS, Universite de
Lyon 1, INSA-Lyon, CPE-Lyon, Laboratoire de Chimie Organique, INSA Lyon, Ba^timent J. Verne,
20 av A. Einstein, F-69621 Villeurbanne, France, and §State Key Laboratory of Bioorganic and Natural
Products Chemistry, 354 Fenglin Lu, Shanghai 200032, People’s Republic of China
pqhuang@xmu.edu.cn; yves.queneau@insa-lyon.fr
Received July 18, 2009
The enantioselective synthesis of the R-enantiomer of the marine natural product (S)-ypaoamide (5)
is reported. The synthesis features both a flexible racemization-free approach to the 5-substituted
3-pyrrolin-2-one segment, and a lipase (CCL)-promoted deacetylation reaction to reach the ortho-
gonal deprotection. Through this work the absolute configuration of the natural ypaoamide was
determined as S.
Introduction
and antiproliferative properties (Figure 1).5 Jamaicamides
A-C (e.g., 3 and 4) are a series of potent neurotoxins isolated
from a chemically rich Jamaican strain of L. majuscula.6
Given that immunosuppressive agents are indispensable for
the success of transplantation, and microcolins A (1) and B
(2) are more potent inhibitors of the human two-way mixed
lymphocyte response (MLR) than FK506 and cyclosporin
A, two drugs currently in clinical use in transplantation, they
have attracted multidisciplinary attention.4,5,7-9 It has been
Many marine natural products possess diverse and im-
portant bioactivities1,2 and/or have ecological significance.3
The marine cyanobacteria (blue-green algae) have been
proven to be a prolific source of novel bioactive natural
products. Over 300 nitrogen-containing natural products
have been identified from marine cyanobacteria.2 For ex-
ample, microcolins A (1) and B (2) were lipopeptides isolated
from a Venezuelan sample of the blue-green alga Lyngbya
majuscula,4 which exhibited very potent immunosuppressive
(5) (a) Zhang, L.-H.; Langley, R. E.; Koehn, F. E. Life Sci. 1997, 60, 751–
762. (b) Zhang, L.-H.; Langley, R. E.; Koehn, F. E. Life Sci. 1999, 64, 1013–
1028. (c) Takamatsu, S.; Nagle, D. G.; Gerwick, W. H. Planta Med. 2004, 70,
127–131.
(6) 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.
(7) Koehn, F. E.; McConnell, O. J.; Longley, R. E.; Sennett, S. S.; Reed, J.
K. J. J. Med. Chem. 1994, 37, 3181–3186.
(1) For reviews, see: (a) McConnell, O. J.; Longley, R. E.; Koehn, F. E. The
Discovery of Marine Natural Products with Therapeutic Potential. In The Dis-
covery of Natural Products with Therapeutic Potential: Gullo, V., Ed.: Springer-
Verlag: New York, 1994; pp 109-174. (b) Blunt, J. W.; Copp, B. R.; Hu, W. P.;
Munro, M. H. G.; Northcote, P. T.; Prinsep, M. R. Nat. Prod. Rep. 2009, 26, 170–
244.
(2) (a) Nagle, D. G.; Camacho, F. T.; Paul, V. J. Mar. Biol. 1998, 132,
267–273. (b) Burja, A. M.; Banaigs, B.; Abou-Mansour, E.; Burgess, G.;
Wright, P. C. Tetrahedron 2001, 57, 9347–9377. (c) Paul, V. J.; Puglisi, M. P.
Nat. Prod. Rep. 2006, 23, 153–180. (d) Paul, V. J.; Ritson-Williams, R. Nat.
Prod. Rep. 2008, 25, 662–695. (e) Tan, L. T. Phytochemistry 2007, 68, 954–
968.
(8) Mandal, A. K.; Hines, J.; Kuramochi, K.; Crews, C. M. Bioorg. Med.
Chem. Lett. 2005, 15, 4043–4047.
(9) For the total synthesis of microcolin A, see: (a) Decicco, C. P.; Grover,
P. J. Org. Chem. 1996, 61, 3534–3541. For the total synthesis of microcolin B,
see: (b) Andrus, M. B.; Li, W.-K.; Keyes, R. E. J. Org. Chem. 1997, 62,
5542-5549. For synthetic studies on microcolin B, see: (c) Mattern, R. H.;
Gunasekera, S.; McConnell, O. Tetrahedron 1996, 52, 425–434.
(3) Meinwald, J.; Eisner, T. Helv. Chim. Acta 2003, 86, 3633–3637.
(4) Koehn, F. E.; Longley, R.; Reed, J. K. J. Nat. Prod. 1992, 55, 613–619.
DOI: 10.1021/jo901557h
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Chen et al.
JOCArticle
outlined in Scheme 1, which suggested a convergent synthesis
involving the coupling of the lipid moiety with the 5-alkyl-
3-pyrrolin-2-one portion. The tert-butyl-containing lipid side
chain was envisioned to be synthesized from glutarimide or
pivaldehyde (Scheme 1). As regarding the 5-alkyl-3-pyrrolin-
2-one moiety, although there are several reports on their
synthesis,9,15,16 most of them use R-amino acids as the starting
materials,9,15 which may suffer from partial racemization,15b,d
and lack generality. In view of the presence of 5-alkyl-
3-pyrrolin-2-one subunits in several marine cyanobacteria-
origin natural products and the possibility to use them as leads
for drug development, a nonamino acid-based approach would
offer a greater flexibility for the synthesis of synthetic analogues
bearing different C-5 substituents. On the basis of these
considerations, we envisioned the use of lactams such as 12
and 6 as latent forms of the 5-alkyl-3-pyrrolin-2-one portion,
which, according to our previous results, can be derived from
(S)-malimide 13 by Grignard addition.17 There are two ad-
vantages in this approach. First, as we have demonstrated
earlier,14 the installation of different C-5 alkyl groups is easy by
just changing the Grignard reagent, and second, the coupling of
lactams (e.g., 12) with the lipid chain 7 can be performed under
racemization-free conditions.
FIGURE 1. Structures of some 5-alkyl-3-pyrrolin-2-one substructure-
containing marine natural products.
revealed that the 3-pyrrolin-2-one moiety is mandatory for
the immunosuppressive activity.7
In May 1994 at Ypao Beach on Guam (a Micronesia
island), a simultaneous blue-green algal bloom and a massive
die off of pelagic larval rabbitfish occurred, which led to
a temporary closure of the beach. From the extract of a
mixed cyanobacteria assemblage, which was composed of
Schizothrix calcicola and Lyngbya majuscule, ypaoamide (5)
was isolated. It was shown that this compound can also be
produced from the isolated cells of the L. majuscula in
laboratory culture.10 Ypaoamide was shown to be a new
broadly acting feeding deterrent, and its ecological signifi-
cance has been studied.11 The structure of ypaoamide (5) was
determined by 2D NMR and high-resolution FAB mass
spectroscopy; however, its absolute configuration was not
established. While ypaoamide (5) presents partial structural
similarity with microcolins A and B (1 and 2),4 jamaicamides
A and B (3 and 4),6 majusculamide D,12 and malynga-
mides,13 its unusual tert-butyl lipid side chain has little
biosynthetic precedent. As a continuation of a program
aimed at the development of the malimide-based synthetic
methodology for the asymmetric synthesis of bioactive ni-
trogen-containing natural products,14 we now report the first
total synthesis of (R)-ypaoamide (5).
The synthesis started with N-(p-methoxybenzyl)glutarimide
10, which was derived from glutaric anhydride 14 (Scheme 2).
Disappointingly, addition of tert-butyl lithium led to the desired
keto-ester 15 in only 17% yield, along with 38% of the recovered
starting material. A similar low yield (16%) was reported in the
reaction of tert-butyl lithium with a glutarimide derivative.18
The low yield may be attributed to the strong basicity of tert-
butyllithium, which abstracts an acidic R-proton. The deoxy-
genation of 15 was achieved by Maryanoff’s method,19 namely,
reduction of the p-tosylhydrazone derivative with NaBH3CN,
which gave the desired side chain 9 in 55% yield.
Although amide 9 can be obtained in only three steps from
glutaric anhydride 14, the low yield in the formation of keto
amide 15 led us to consider an alternative approach starting
from ethyl 4-bromobutyrate (16) as shown in Scheme 3. The
Wittig reaction of the ylide, generated by deprotonation of
the phosphonium bromide salt 11 with sodium hexamethyl-
disilylamide (NHMDS), with pivaldehyde gave volatile olefin
Z-17 as a single product in 74% yield.20 The ester 17 was
(15) For amino acid-based synthetic approaches to 5-alkyl-3-pyrrolin-2-
ones, see: (a) Schmidt, U.; Riedl, B.; Haas, G.; Griesser, H.; Vetter, A.;
Weinbrenner, S. Synthesis 1993, 216–220. (b) Mattern, R. H. Tetrahedron
Lett. 1996, 37, 291–294. (c) Reddy, G. V.; Rao, G. V.; Iyengar, D. S.
Tetrahedron Lett. 1999, 40, 775–776. (d) 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. (e) Courcambeck, J.; Bihel,
F.; Michelis, C. D.; Quelever, G.; Kraus, J. L. J. Chem. Soc., Perkin Trans. 1
2001, 1421–1430. For a versatile approach, see: (f) Bower, J. F.; Chakthong,
S.; Svenda, J.; Williams, A. J.; Lawrence, R. M.; Szeto, P.; Gallagher, T. Org.
Biomol. Chem. 2006, 4, 1868–1877.
(16) For nonamino acid-based synthetic approaches to 5-alkyl-3-pyrro-
lin-2-ones, see: (a) Merino, P.; Castillo, E.; Franco, S.; Merchan, F. L.;
Tejero, T. Tetrahedron: Asymmetry 1998, 9, 1759–1769. (b) Sanyal, A.;
Yuan, Q.; Snyder, J. K. Tetrahedron Lett. 2005, 46, 2475–2478. (c) Spiess,
S.; Berthold, C.; Weihofen, R.; Helmchen, G. Org. Biomol. Chem. 2007, 5,
2357–2360. (d) Burgess, K. L.; Lajkiewicz, N. J.; Sanyal, A.; Yan, W. L.;
Snyder, J. K. Org. Lett. 2005, 7, 31–34.
Results and Discussion
The enantioselective synthesis of ypaoamide (5) presents
several challenges. The most difficult one is the incorpora-
tion of two acid and/or base sensitive functional groups (the
5-alkyl-3-pyrrolin-2-one subgroup and the E-β-methoxy-
R,β-unsaturated imide moiety) together with a phenolic
moiety. Our retrosynthetic analysis of (R)-ypaoamide (5) is
(10) Nagle, D. G.; Paul, V. J.; Roberts, M. A. Tetrahedron Lett. 1996, 37,
6263–6266.
(11) Nagle, D. G.; Paul, V. J. J. Exp. Mar. Biol. Ecol. 1998, 225, 29–38.
(12) Moore, R. E.; Entzeroth, M. Phytochemistry 1988, 27, 3101–3103.
(17) Huang, P.-Q.; Chen, Q.-F.; Chen, C.-L.; Zhang, H.-K. Tetrahedron:
Asymmetry 1999, 10, 3827–3832.
(18) Ardeo, A.; Garcia, E.; Arrasate, S.; Lete, E.; Sotomayor, N. Tetra-
hedron Lett. 2003, 44, 8445–8448.
ꢀ
(13) Milligan, K. E.; Marquez, B.; Williamson, R. T.; Davies-Coleman,
M.; Gerwick, W. H. J. Nat. Prod. 2000, 63, 965–968.
(14) (a) Huang, P.-Q. Synlett 2006, 1133–1149. (b) Huang, P.-Q.; Wang,
S.-L.; Ye, J.-L.; Ruan, Y.-P.; Huang, Y.-Q.; Zheng, H.; Gao, J.-X. Tetra-
hedron 1998, 54, 12547–12560.
(19) Hutchins, R. O.; Milewski, C. A.; Maryanoff, B. E. J. Am. Chem.
Soc. 1973, 95, 3662–3668.
(20) Schlosser, M.; Christmann, K. F. Angew. Chem., Int. Ed. Engl. 1966,
5, 126–126.
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SCHEME 1
SCHEME 2
was dissolved in CH2Cl2, neutralized with sodium bicarbonate
at -20 °C, extracted with CH2Cl2, and chromatographed on
SiO2 to give 20 in 86% yield.
The cross-Claisen condensation reaction was carried out
by deprotonation of EtOAc with LDA, allowing the result-
ing enolate to react with ester 20, which gave keto ester 8a in
50% yield (Scheme 4). To convert 8a into 22a, Pinnick
method24 was adopted. Thus, in the presence of a catalytic
amount of concentrated H2SO4, a mixture of 8a and trimethyl
orthoester was heated at reflux to yield a mixture of 21a and
22a, which was heated at 80 °C under reduced pressure for 5 h.
In such a manner, 21a was further converted to 22a, and the
latter was isolated in 71% yield. Thefinalstep for the synthesis
of the fragment A was the saponification of 22a. Disappoint-
ingly, attempted saponification under a number of conditions
(e.g., LiOH-MeOH-H2O; 1, 4, or 6 N aq NaOH) failed, and
keto ester 8a was obtained after acidic workup. At this point,
we decided to synthesize benzyl ester 22b in the hope that
it would be cleavable under hydrogenolytic debenzylation
conditions. The cross-condensation of benzyl acetate with
20 gave 8b in 62% yield. The latter was converted to 22b in
75% yield by following the procedure described for 8a. The
E-stereochemistry of 22b was assigned on the basis of the
observed NOEs between the methoxyl (δ 3.62) and olefinic
(δ 5.14) protons. Under the catalytic transfer hydrogenolytic
conditions (10% Pd/C, HCO2H/EtOH = 1:9, v/v, rt, 3 h), 22b
was smoothly transformed into the desired acid 7 in 92% yield.
After achieving the synthesis of 7, we turned to the
synthesis of the 5-(p-hydroxyphenylmethyl)-3-pyrrolin-
2-one subunit (12). Stepwise reductive p-methoxybenzyla-
tion of (S)-malimide 13a afforded 23a as the only observable
regio- and diastereomer in 83% overall yield (Scheme 5). For
the cleavage of the p-methoxybenzyl (PMB) group in 23a, the
CAN-mediated oxidative deprotection14 gave a complex
mixture, from which the desired product could not be
isolated. This failure may be due to both over reduction
converted to amide 18 in 94% yield by the method previously
developed in these laboratories.21 Catalytic hydrogenation of
olefin 18 gave amide 9 in 95% yield. For the aza-Michael
addition of the secondary amide 9 with ethyl acrylate, a number
of conditions were tested. When 0.2 mol equiv of potassium
tert-butoxide was used, the desired product 19 was formed in
55% yield, along with 43% of the recovered starting material.
A better result was obtained (19: 68%; 9: 30%) when the
TBSOTf/ NEt3 reagent system22 was used as the promoter. To
undertake the chain elongation of 19 by a base-mediated cross-
Claisen reaction, a prior N-deprotection was necessary to avoid
the possible retro-Michael reaction of 19 under basic condi-
tions. Attempts to cleave the N-PMB group under oxidative
conditions (CAN-MeCN-H2O)14b produced 20 in only 20%
yield. An alternative method by heating 19 in trifluoroacetic
acid23 at reflux for 5 h was tried. However, due to both the poor
visibility of the desired product on TLC detection and the
formation of a large number of colored side products, isolation
of the desired product was difficult even by following the
Brooke’s workup procedure,23 which led to amide 20 in only
26% yield. This difficulty was overcome by modifying the
Brooke’s workup procedure: after the reaction, most of the
TFA was evaporated under reduced pressure, and the residue
(21) Huang, P.-Q.; Zheng, X.; Deng, X.-M. Tetrahedron Lett. 2001, 42,
9039–9041.
(22) Mekouar, K.; Greene, A. E. J. Org. Chem. 2000, 65, 5212–5215.
(23) Brooke, G. M.; Mohammed, S.; Whiting, M. C. Chem. Commun.
1997, 1511–1512.
(24) (a) Kochhar, K. S.; Pinnick, H. W. J. Org. Chem. 1984, 49, 3222–3224.
(b) Duc, L.; McGarrity, J. F.; Meul, T.; Warm, A. Synthesis 1992, 391–394.
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SCHEME 3
SCHEME 4
SCHEME 5
Now the stage was set for the coupling of segment A (7)
with segment B (12b). Although several methods have been
reported,27,28 we elected to use the Andrus’s pentafluoro-
phenyl ester method.9b,28 Thus, using DCC as a coupling
agent, the reaction of the acid 7 with pentafluorophenol
afforded the activated ester 24 in 94% yield. Reaction of ester
24 (-78 °C; then warm up to -5 °C) with the anion,
generated by deprotonation of the lactam 12b with 1.0 mol
equiv of n-butyllithium at -78 °C, furnished the coupling
product 25 in 63% yield (Scheme 6). To form the pyrrolinone
moiety in a racemization-free manner, a switch of protec-
ting groups for the C-4 hydroxyl group was necessary.
Compound 25 was subjected to catalytic hydrogenolysis
(10% Pd/C, HCO2H, EtOH), which produced the bis-
debenzylated product 6 in 86% yield. In light of Mattern’s
report,15b the 4-O-Boc-derivative of 6 was envisioned as a ready
precursor of 5. Treatment of compound 6 with 2.0 mol equiv
of di-tert-butyl dicarbonate (Boc2O, DMAP, THF) at room
temperature for 5 h led to the desired elimination in the
pyrrolidinone subunit and concomitant protection of the phenol
group, yielding compound 26, the Boc-protected analogue of
and concomitant oxidation of the methoxybenzyl group at
C-5. At this point, the protecting group strategy was taken
into consideration, and we decided to synthesize N-allyl
protected25 lactam 23b as the precursor of the pyrrolinone
segment. Thus, the known N-allyl malimide 13b26 was used
as the starting material, which has been converted to lactam
12b in our recent synthesis of melleumin A.
(25) (a) Koch, T.; Hesse, M. Synthesis 1992, 931–932. (b) Kanno, O.;
Miyauchi, M.; Kawamoto, I. Heterocycles 2000, 53, 173–181. (c) Kitov, P. I.;
Bundle, D. R. Org. Lett. 2001, 3, 2835–2838. (d) Zacuto, M. J.; Xu, F. J. Org.
Chem. 2007, 72, 6298–6300.
(26) Luo, J.-M.; Dai, C.-F.; Lin, S.-Y.; Huang, P.-Q. Chem. Asian J. 2009,
4, 328–335.
(27) (a) Mattern, R. H.; Gunasekera, S. P.; McConnell, O. J. Tetrahedron
Lett. 1997, 38, 2197–2200. (b) Yamada, S.; Yaguchi, S.; Matsuda, K.
Tetrahedron Lett. 2002, 43, 647–651.
(28) (a) Andrus, M. B.; Li, W.; Keyes, R. F. Tetrahedron Lett. 1998, 39,
5465–5468. (b) Hosseini, M.; Kringrlum, H.; Murray, A.; Tonder, J. E. Org.
Lett. 2006, 8, 2103–2106.
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SCHEME 6
SCHEME 7
natural product.10 Unfortunately, no optical rotation
was observed, which implied that a complete racemization
occurred.
TABLE 1. Acetylation of Compound 6
Ac2O
(equiv)
pyridine
(equiv)
temp
(°C)
time
(h)
ratio
28:29
yield
(%)
entry
We next turned our attention to the monoacetylated
product 28. Compound 28 was converted to the known
compound 3010 by reacting with Boc2O in the presence of
0.1 mol equiv of DMAP. Treatment of a methanolic solution
of compound 30 with 1.0 mol equiv of K2CO3 at room
temperature for 1.5 h produced ypaoamide (5) in 55% yield.
The enantiomeric excess of the product 5 was 10%, indicat-
ing that substantial racemization occurred.
1
2
3
1.5
2.0
2.0
1.5
2.0
2.0
0
20
20
2
0.5
8
100:0
52:48
0:100
92
90
94
ypoamide (5). However, due to the fragility of both molecules 26
and 5, the last deprotection step (TFA, CH2Cl2) appeared
problematic, due to a concomitant hydrolysis of the enol ether
function (at either 0 or -20 °C), leading to compound 27.
At this stage, an acetyl protecting group was selected to
replace the Boc group. As can be seen from Scheme 7 and
Table 1, no desired C-4 O-monoacetylated product could be
obtained. Instead, either phenol monoacetylated product 28
or bis-acetylated product 29 was obtained in high yield. In an
attempt to eliminate the acetic acid, bis-acetate 29 was
treated with DBN in CH2Cl2. After reaction at rt for 1.5 h,
the desired product 5 was obtained in 52% yield, whose
spectral data were in agreement with those reported for the
At this stage, enzymatic deacetylation29 of compound 30 was
envisioned. Initial attempts with use of porcine pancreatic lipase
(PPL) were unsuccessful. To our satisfaction, when com-
pound 30 was treated with CCL (Candida cylindracea lipase)
in a buffer solution30 at pH 7.2, and at rt for 2 days, the desired
(29) For a review on the enzymatic catalyzed reaction, see: Ghanem, A.
Tetrahedron 2007, 63, 1721–1754.
(30) Parmar, V. S.; Kumar, A.; Bisht, K. S.; Mukherjee, S.; Prasad, A. K.;
Sharma, S. K.; Wengel, J.; Olsen, C. E. Tetrahedron 1997, 53, 2163–2176.
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ypaoamide (5) was obtained in 88% yield based on the recov-
ered starting material 30 (50%). The physical and spectroscopic
data of the synthetic compound were in agreement with those
reported for the natural product.10 Comparing the optical
dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified by flash column chro-
matography on silica gel eluting with EtOAc-PE (1:3) to give
compound 8b (309 mg, 0.85 mmol, 62%) as a pale yellow oil. IR
(film) νmax 3303, 2952, 1743, 1716, 1545, 1262, 1215 cm-1;1HNMR
(400 MHz, CDCl3) δ 0.85 (s, 9H), 1.12-1.19 (m, 2H), 1.20-1.30
(m, 2H), 1.50-1.60 (m, 2H), 2.11 (t, J = 7.7 Hz, 2H), 2.77 (t, J =
5.6 Hz, 2H), 3.44-3.50 (m, 2H), 3.49 (s, 2H), 5.18 (s, 2H), 5.90 (br s,
1H), 7.30-7.40 (m, 5H); 13C NMR (100 MHz, CDCl3) δ24.2, 26.5,
29.3 (3C), 30.2, 33.7, 36.7, 42.6, 43.8, 49.1, 67.2, 128.3 (2C), 128.5
(2C), 128.6, 135.0, 166.6, 173.1, 202.5; MS (ESI) m/z 362 (M þ Hþ,
100); ESI-HRMS calcd for [C21H31NO4 þ Naþ] 384.2151, found
384.2148.
rotations {synthetic 5: [R]20 -194 (c 0.8, CHCl3); natural
D
product: [R]20 þ197 (c 1.0, CHCl3)} allowed the conclusion
D
that the absolute configuration of the natural ypaoamide (5) is
S, and our synthetic product is the enantiomer of the natural
product. The recovered starting material showed the same
optical rotation as that of the starting material 30, implying
that the high enantioselectivity of compound 5 was a result of
the mild racemization-free conditions of the enzyme-catalyzed
reaction, not of a kinetic resolution.
Benzyl (E)-5-(6,6-Dimethylheptanamido)-3-methoxypent-2-
enoate (22b). To a mixture of compound 8b (892 mg, 2.47
mmol) and HC(OMe)3 (1.0 mL, 9.1 mmol) was added 5 drops
of concentrated H2SO4. The mixture was stirred at 40 °C for
4 days then concentrated under reduced pressure. The oily
residue was purified by chromatographic purification on silica
gel eluting with EtOAc-PE (1:3) to give an oily residue, along
with the recovered starting material (110 mg). The oily residue
was heated at 80 °C under reduced pressure for 3 h. The residue
was purified by flash column chromatography on silica gel
(EtOAc/PE = 1/2) to give compound 22b (680 mg, 1.81 mmol,
75%) as a colorless oil. IR (film) νmax 3302, 2951, 1712, 1632, 1620,
1133 cm-1; 1H NMR (400 MHz, CDCl3) δ 0.85 (s, 9H), 1.12-1.18
(m, 2H), 1.18-1.29 (m, 2H), 1.49-1.58 (m, 2H), 2.08 (t, J =
7.6 Hz, 2H), 2.93 (t, J = 6.3 Hz, 2H), 3.46-3.52 (m, 2H), 3.62 (s,
3H), 5.14 (s, 2H), 5.16 (s, 1H), 6.22 (br s, 1H), 7.28-7.40 (m, 5H);
13C NMR (100 MHz, CDCl3) δ 24.3, 26.5, 29.3 (3C), 30.2, 31.4,
36.8, 37.9, 43.9, 55.8, 65.8, 92.0, 128.1 (3C), 128.5 (2C), 136.2,
168.3, 173.2, 174.2; MS (ESI) m/z 398 (M þ Naþ, 100); ESI-
HRMS calcd for [C22H33NO4 þ Hþ] 376.2476, found 376.2478.
N-[(E)-5-((2R,3S)-2-(4-(Benzyloxy)benzyl)-3-(benzyloxy)-5-oxo-
pyrrolidin-1-yl)-3-methoxy-5-oxopent-3-enyl]-6,6-dimethylhep-
tanamide (25). To a solution of (4S,5R)-5-(4-(benzyloxy)-
benzyl)-4-(benzyloxy)pyrrolidin-2-one 12b (70 mg, 0.18 mmol)
in THF (1.0 mL) was added dropwise n-BuLi (2.5 M in hexane)
(0.9 mL, 0.18 mmol) at -80 °C. The mixture was stirred for
15 min. To the resulting mixture was added a THF solution
(2.0 mL) of ester 24 (96 mg, 0.21 mmol). After the solution was
stirred for 3 h at -80 °C, the reaction temperature was allowed
to rise to -5 °C. The mixture was quenched with a saturated
aqueous solution of NH4Cl (2.0 mL). The resulting mixture
was extracted with EtOAc (3 ꢀ 4 mL). The combined organic
phases were washed with brine, dried over anhydrous Na2SO4,
filtered, and concentrated under reduced pressure. The residue
was purified by flash column chromatography on silica gel
eluting with EtOAc-PE (1:2) to give compound 25 (74 mg, 0.11
mmol, 63%) as a pale yellow oil. [R]20D -28 (c 0.6, CHCl3); IR
Conclusion
In summary, the first enantioselective synthesis of the
unnatural enantiomer of ypaoamide (5) was achieved in a
convergent manner in 19 steps with 1.0% overall yield
starting from (S)-N-allylmalimide 13b and ethyl 4-bromo-
butyrate (16). This work established the absolute configura-
tion of the natural ypaoamide (5) as S. In this synthesis, we
were able to demonstrate that the difficult problem asso-
ciated with the need for an orthogonal deprotection can be
tackled by mild enzymatic deacetylation. Our approach to
the 5-substituted 3-pyrrolin-2-one segment is both racemiza-
tion-free and flexible. By using other Grignard reagents, this
method should allow access to other 5-substituted 3-pyrro-
lin-2-ones, which are found as key structural features in
many bioactive natural products. This work is currently in
progress in these laboratories, and will be reported in due
course.
Experimental Section
Ethyl 3-(6,6-Dimethylheptanamido)propanoate (20). A mix-
ture of compound 19 (1.35 g, 3.57 mmol) and TFA (17.8 mL)
was refluxed for 6 h and then cooled to room temperature. The
mixture was concentrated under reduced pressure, then dis-
solved in CH2Cl2 (10 mL). The solution was neutralized with a
saturated aqueous solution of NaHCO3 at -20 °C and extracted
with CH2Cl2 (3 ꢀ 20 mL). The combined extracts were washed
with brine, dried over anhydrous Na2SO4, filtered, and concen-
trated under reduced pressure. The residue was purified by flash
column chromatography on silica gel eluting with EtOAc-PE
(1: 3) to give compound 20 (763 mg, 2.98 mmol, 83%) as a pale
yellow oil. IR (film) νmax 3419, 2953, 1736, 1650, 1548, 1364,
1184 cm-1; 1H NMR (400 MHz, CDCl3) 0.85 (s, 9H), 1.09-1.16
(m, 2H), 1.17-1.26 (m, 2H), 1.22 (t, J = 7.1 Hz, 3H), 1.49-1.58
(m, 2H), 2.12 (t, J = 7.6 Hz, 2H), 2.48 (t, J = 6.0 Hz, 2H),
3.43-3.50 (dd, J = 12.0, 6.0 Hz, 2H), 4.10 (q, J = 7.1 Hz, 2H),
6.15 (br s, 1H); 13C NMR (100 MHz, CDCl3) δ 14.0, 24.1, 26.4,
29.2 (3C), 30.1, 33.9, 34.6, 36.7, 43.7, 60.5, 172.6, 173.1; MS
(ESI) m/z 280 (M þ Naþ, 100); ESI-HRMS calcd for
[C14H27NO3 þ Hþ] 257.1991, found 257.1994.
(film) νmax 3318, 2937, 1731, 1664, 1599, 1510, 1196, 1167 cm-1
;
1H NMR (400 MHz, CDCl3) δ 0.80 (s, 9H), 1.10-1.17 (m, 2H),
1.18-1.28 (m, 2H), 1.57 (p, J = 7.6 Hz, 2H), 2.16 (t, J = 7.7 Hz,
2H), 2.53 (dd, J = 13.6, 4.5 Hz, 1H), 2.53-2.64 (m, 2H),
2.87-2.97 (m, 1H), 2.93-3.03 (m, 1H), 3.16 (dd, J = 13.6, 3.2
Hz, 1H), 3.50-3.55 (m, 1H), 3.55-3.59 (m, 1H), 3.74 (s, 3H),
3.80-3.83 (m, 1H), 4.14 (d, J = 12.1 Hz, 1H), 4.34 (d, J =
12.1 Hz, 1H), 4.66-4.72 (dm, J = 9.1 Hz, 1H), 5.05 (s, 2H), 6.69
(br s, 1H), 6.74 (s, 1H), 6.90-7.50 (m, 14H); 13C NMR (100
MHz, CDCl3) δ 24.2, 26.5, 29.2 (3C), 30.1, 32.3, 36.5, 36.8,
38.1, 39.5, 41.9, 43.7, 56.0, 64.2, 70.0, 71.9, 94.9, 115.1 (2C),
127.3 (2C), 127.6 (2C), 127.7, 127.9, 128.3 (2C), 128.5 (2C),
128.7, 130.2 (2C), 136.8, 136.9, 157.8, 166.9, 173.2, 173.6, 175.5;
MS (ESI) m/z 677 (M þ Naþ, 100); ESI-HRMS calcd for
[C40H50N2O6 þ Hþ] 655.3747, found 655.3737.
Benzyl 5-(6,6-Dimethylheptanamido)-3-oxopentanoate (8b).
To a solution of LDA (0.65 M solution in THF, 9.5 mL, 6.2 mmol)
was added dropwise benzyl acetate (0.9 mL, 6.2 mmol) at -78 °C
under a nitrogen atmosphere over 15 min. After 15 min a solution of
compound 20 (355 mg, 1.39 mmol) in THF (8.0 mL) was added
over 15 min. The reaction mixture was stirred overnight while the
temperature was allowed to warm to room temperature. The
reaction was quenched with a saturated aqueous solution of NH4Cl
(10.0 mL). The resulting mixture was extracted with EtOAc (3 ꢀ
10 mL). The combined organic phases were washed with brine,
N-[(E)-5-((2R,3S)-2-(4-Hydroxybenzyl)-3-hydroxy-5-oxopyr-
rolidin-1-yl)-3-methoxy-5-oxopent-3-enyl]-6,6-dimethylheptan-
amide (6). To 10% Pd/C (50 mg) was added a solution of
7462 J. Org. Chem. Vol. 74, No. 19, 2009

Chen et al.
JOCArticle
compound 25 (64 mg, 0.10 mmol) in anhydrous EtOH (1.6 mL)
and formic acid (0.2 mL). The mixture was stirred for 5 h at room
temperature under a nitrogen atmosphere. The reaction mixture
was filtered and concentrated under reduced pressure. The oily
residue was purified by flash column chromatography on silica gel
eluting with EtOAc-PE (2:1) to give compound 6 (40 mg, 0.08
mmol, 86%) as a white solid. Mp 140-141 °C (EtOAc-PE);
[R]20D -36 (c 0.8, CHCl3); IR (film) νmax 3325, 2951, 1731, 1722,
1650, 1595, 1197, 1165 cm-1; 1H NMR (400 MHz, CDCl3) δ 0.80
(s, 9H), 1.10-1.17 (m, 2H), 1.18-1.28 (m, 2H), 1.57 (p, J =
7.6 Hz, 2H), 2.16 (t, J = 7.6 Hz, 2H), 2.32-2.36 (m, 1H),
2.50-2.61 (m, 2H), 2.84-2.96 (m, 2H), 2.99-3.07 (m, 1H),
3.40-3.50 (m, 2H), 3.59 (s, 3H), 3.90 (s, 1H), 4.14-4.24 (m,
1H), 4.47-4.54 (m, 1H), 6.70 (s, 1H), 6.77 (d, J = 8.4 Hz, 2H),
6.85 (br s, 1H), 6.99 (d, J = 8.4 Hz, 2H), 8.05 (s, 1H); 13C NMR
(100 MHz, CDCl3) δ 24.2, 26.5, 29.3 (3C), 30.1, 32.4, 36.3, 36.7,
38.1, 41.9, 43.7, 56.1, 66.0, 67.6, 94.9, 115.8 (2C), 127.5, 130.3 (2C),
155.6, 166.9, 174.3, 174.4, 175.4; MS (ESI) m/z 497 (M þ Naþ,
100). Anal. Calcd for C26H38N2O6: C, 65.80; H, 8.07; N, 5.90; O,
20.23. Found: C, 65.84; H, 8.10; N, 5.91; O, 20.26.
column chromatography on silica gel eluting with EtOAc-PE
(1:1) to give the known compound10 30 (35 mg, 0.07 mmol, 73%)
as a pale yellow oil. [R]20 -110 (c 1.0, CHCl3); IR (film)
D
1
νmax 3310, 2938, 1720, 1664, 1598, 1196, 1167 cm-1; H NMR
(400 MHz, CDCl3) δ 0.80 (s, 9H), 1.10-1.18 (m, 2H), 1.19-1.30
(m, 2H), 1.57 (p, J = 7.7 Hz, 2H), 2.18 (t, J = 7.7 Hz, 2H), 2.28 (s,
3H), 2.75 (dd, J = 13.4, 9.1 Hz, 1H), 2.94 (ddd, J = 13.3, 7.1,
4.9 Hz, 1H), 3.04 (ddd, J = 13.3, 8.3, 4.9 Hz 1H), 3.50-3.60 (m,
2H), 3.60 (dd, J = 13.4, 3.7 Hz, 1H), 3.75 (s, 3H), 4.96-5.08 (dm,
J = 9.1 Hz, 1H), 6.05 (dd, J = 6.0, 1.6 Hz, 1H), 6.71 (s, 1H), 6.74
(br s, 1H), 7.05 (dt, J = 8.6, 2.0 Hz, 2H), 7.15-7.17 (m, 1H), 7.18
(dt, J = 8.6, 2.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 21.1,
24.3, 26.6, 29.3 (3C), 30.2, 32.4, 36.9, 37.4, 38.2, 43.8, 56.2, 62.8,
94.8, 121.7 (2C), 126.7, 130.3 (2C), 133.3, 149.7, 151.0, 166.2,
169.3, 170.0, 173.2, 175.7; MS (ESI) m/z 521 (M þ Naþ, 100);
ESI-HRMS calcd for [C28H38N2O6 þ Hþ] 499.2808, found
499.2823.
(R)-Ypaoamide (R-5). To a solution of compound 30 {20 mg,
0.04 mmol, [R]20 -110 (c 1.0, CHCl3)} in MeCN (2 mL) and
D
pH 7.2 phosphate buffer (2 mL) was added CCL (25 mg) at room
temperature. The mixture was stirred for 2 days. The enzyme
was filtered off and the solvent was removed under reduced
pressure. The residue was purified by flash column chromato-
graphy on silica gel eluting with EtOAc-PE (1:1) to give
(R)-ypaoamide (R-5) (8.0 mg, 0.02 mmol, 44%) as a pale yellow
oil, and the recovered starting material {30, 10 mg, 0.02 mmol,
[R]20D -106 (c 0.7, CHCl3)}. (R)-5: [R]20D -194 (c 0.8, CHCl3)
4-[((2R,3S)-1-((E)-5-(6,6-dimethylheptanamido)-3-methoxypent-
2-enoyl)-3-hydroxy-5-oxopyrrolidin-2-yl)methyl]phenyl Acetate (28).
To a solution of compound 6 (132 mg, 0.28 mmol) in anhydrous
CH2Cl2 (1.8 mL) was added Ac2O (0.04 mL, 0.42 mmol) and
pyridine (0.03 mL, 0.42 mmol) at 0 °C. The mixture was stirred for
2 h. The reaction was quenched with a saturated aqueous solution of
NH4Cl (2 mL). The resulting mixture was extracted with CH2Cl2
(3ꢀ 1.0 mL). The combined organic phases were washed with brine,
dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified by flash column chro-
matography on silica gel eluting with EtOAc-PE (1:1) to give
{lit.10 [R]20 þ197 (c 1.0, CHCl3)}; IR (film) νmax 3310, 2952,
D
1720, 1712, 1648, 1596, 1199, 1170 cm-1; 1H NMR (400 MHz,
CDCl3) δ 0.79 (s, 9H), 1.04-1.16 (m, 2H), 1.18-1.28 (m, 2H),
1.57 (p, J = 7.7 Hz, 2H), 2.18 (t, J = 7.7 Hz, 2H), 2.72 (dd, J =
13.4, 9.1 Hz, 1H), 2.88 (ddd, J = 13.3, 7.1, 5.0 Hz, 1H), 3.04
(ddd, J = 13.3, 8.3, 5.0 Hz, 1H), 3.47 (dd, J = 13.4, 3.7 Hz, 1H),
3.50-3.58 (m, 1H), 3.54-3.64 (m, 1H), 3.74 (s, 3H), 4.92-5.02
(dm, J = 9.1 Hz, 1H), 6.02 (dd, J = 6.0, 1.6 Hz, 1H), 6.15 (br s,
1H), 6.71 (s, 1H), 6.78 (dt, J = 8.6, 2.0 Hz, 2H), 6.92 (br s, 1H),
compound 28 (132 mg, 0.25 mmol, 92%) as a pale yellow oil. [R]20
D
-21 (c 1.0, CHCl31); IR (film) νmax 3322, 2951, 1731, 1649, 1599,
1195, 1165 cm-1; H NMR (400 MHz, CDCl3) δ 0.80 (s, 9H),
1.10-1.19 (m, 2H), 1.19-1.30 (m, 2H), 1.57 (p, J = 7.7 Hz, 2H),
2.12 (t, J = 7.7 Hz, 2H), 2.30 (s, 3H), 2.42-2.46 (m, 1H), 2.54-2.70
(m, 2H), 2.78-2.95 (m, 2H), 3.20 (dd, J = 14.0, 3.5 Hz, 1H),
3.42-3.58 (m, 2H), 3.72 (s, 3H), 3.90 (s, 1H), 4.12-4.20 (m, 1H),
4.50-4.60 (dm, J = 9.1 Hz, 1H), 6.70 (br s, 1H), 6.71 (s, 1H), 7.05
(dt, J = 8.6, 2.0 Hz, 2H), 7.23 (dt, J = 8.6, 2.0 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ 21.0, 24.2, 26.5, 29.3 (3C), 30.1, 32.3, 36.6,
36.8, 38.0, 41.8, 43.8, 56.1, 65.8, 67.4, 94.9, 121.9 (2C), 130.2 (2C),
134.4, 149.6, 166.9, 169.5, 173.5, 173.8, 175.5; MS (ESI) m/z 539
(M þ Naþ, 100); ESI-HRMS calcd for [C28H40N2O7 þ Hþ]
517.2913, found 517.2903.
6.98 (dt, J = 8.6, 2.0 Hz, 2H), 7.16 (dd, J = 6.0, 2.0 Hz, 1H); 13
C
NMR (100 MHz, CDCl3) δ 24.3, 26.6, 29.3 (3C), 30.2, 32.3, 37.0,
37.1, 38.3, 43.8, 56.2, 63.2, 95.1, 115.5 (2C), 126.4, 126.9, 130.5
(2C), 151.6, 155.5, 166.3, 170.3, 173.9, 175.5; MS (ESI) m/z 479
(M þ Naþ, 100); ESI-HRMS calcd for [C26H36N2O5 þ Hþ]
457.2702, found 457.2707.
Acknowledgment. The authors are grateful to the NSF
of China (20832005), China Scholarship Council, and the
National Basic Research Program (973 Program) of China
(Grant No. 2010CB833206). Financial support from CNRS
and MESR is also gratefully acknowledged.
(R,E)-4-[(1-(5-(6,6-Dimethylheptanamido)-3-methoxypent-
2-enoyl)-5-oxo-2,5-dihydro-1H-pyrrol-2-yl)methyl]phenyl Acetate
(30). To a solution of compound 28 (50 mg, 0.10 mmol) and
DMAP (1.5 mg, 0.01 mmol) in anhydrous CH2Cl2 (1.0 mL) was
added (Boc)2O (0.05 mL, 0.19 mmol) at 0 °C. The mixture was
allowed to rise to room temperature and stirred for 4 h. The
reaction was quenched with a saturated aqueous solution of
NH4Cl (1.0 mL). The resulting mixture was extracted with
CH2Cl2 (3 ꢀ 1.0 mL). The combined organic phases were washed
with brine, dried over anhydrous Na2SO4, filtered, and concen-
trated under reduced pressure. The residue was purified by flash
Supporting Information Available: Experimental proce-
dures and spectral data for compounds 10, 15, 9, 17, 18, 19,
8a, 22a, 7, 24, 27, and 29, 1H NMR and 13C NMR spectra for the
intermediates and final products, and chiral HPLC chromato-
grams of ypaoamide (5). This material is available free of charge
via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 19, 2009 7463