Convenient synthesis of pulchellalactam, a CD45 protein tyrosine phosphatase inhibitor from the marine fungus Corollospora pulchella, and its related compounds

Article abstract of DOI:10.3987/com-04-10030

Convenient synthesis of pulchellalactam (1a) and its related compounds was accomplished. Total yields of (Z)-pulchellalactam and (E)-pulchellalactam were 55% for 5 steps and 43% for 8 steps from commercially available and inexpensive citraconic anhydride, respectively.

Full text of DOI:10.3987/com-04-10030

HETEROCYCLES, Vol. 63, No. 5, 2004, pp. 1013 - 1016
Received, 4th February, 2004, Accepted, 5th March, 2004, Published online, 12th March, 2004
CONVENIENT SYNTHESIS OF PULCHELLALACTAM, A CD45
PROTEIN TYROSINE PHOSPHATASE INHIBITOR FROM THE
MARINE FUNGUS COROLLOSPORA PULCHELLA, AND ITS RELATED
COMPOUNDS
Jun-ichiro Bessho, Yasuhiko Shimotsu, Sachiko Mizumoto, Nobuyuki Mase,
Hidemi Yoda, and Kunihiko Takabe*
Department of Molecular Science, Faculty of Engineering, Shizuoka University,
3-5-1 Johoku, Hamamatsu 432-8561, Japan. e-mail: tcktaka@ipc.shizuoka.ac.jp
Abstract – Convenient synthesis of pulchellalactam (1a) and its related
compounds was accomplished. Total yields of (Z)-pulchellalactam and
(E)-pulchellalactam were 55% for 5 steps and 43% for 8 steps from commercially
available and inexpensive citraconic anhydride, respectively.
Pulchellalactam (1a), (5Z)-1,5-dihydro-4-methyl-5-(2-methylpropylidene)pyrrol-2(2H)-one, was isolated
from the marine fungus Corollospora pulchella in 1998 by Alvi et al.1 Its bioactivity is a dose-dependent
inhibitor of CD45 protein tyrosine phosphatase with an IC50 of 124 μg/mL; interestingly, this inhibition
is specific for CD45. Since CD45 participates in activation of both B and T cells via their antigen-specific
receptors, pulchellalactam (1a) might be expected as a drug intervention in various auto-immune and/or
inflammatory diseases.1,2 Quite recently, the synthesis of 1a has been reported by Li et al.3 The key step
of the sequence involves addition and elimination of an enolic lactam with an organocuprate reagent.
Their total synthesis afforded (Z)-1a in 6 steps and 32% overall yield from Boc-glycine. Independently,
we have recently accomplished convenient synthesis of 1a and its related compounds (1b) from
commercially available and quite inexpensive citraconic anhydride (2), employing regioselective
reduction of citraconimide (3) (Scheme 1).
O
O
O
H
N
H
O
N
O
(Z)-Pulchellalactam (1a)
Scheme 1. Synthetic plan of (Z)-1a from citraconic anhydride.

A conventional method for the preparation of citraconimide (3) is heating the anhydride (2) under reflux
in the presence of NH4OAc/AcOH,4 however, giving 3 in 41% yield. A higher yield was achieved by
using 1,1,1,3,3,3-hexamethyldisilazane (HMDS) as ammonia equivalent in DMF at 100 °C as shown in
Scheme 2. In 1999, we succeeded in synthesis of an antitumor alkaloid (jatropham) through regioselective
reduction of 3, in which NaBH4 reduction afforded 1,5-dihydro-5-hydroxy-4-methyl-2H-pyrrol-2-one
(4a), whereas NaBH4/CeCl3 or DIBAL-H reduction gave the regioisomer (4b) with extremely high
regioselectivities.5 We obtained 4a as a single regioisomer by recrystallization. Reductive deoxygenation6
of 4a following Boc-protection7 furnished N-tert-butoxycarbonyl-1,5-dihydro-4-methyl-pyrrol-2(2H)-one
(5a)8 in 91% yield. The pyrrol-2(2H)-one (5a) was treated with LDA at -78 °C, and the resulting anion
was reacted with isobutyraldehyde. Interestingly, the product highly depended on the reaction time and
temperature. Longer reaction time and higher reaction temperature gave the desirable (Z)-1a as a single
geometrical isomer in a single step. On the other hand, the aldol reaction at -78 °C for 15 min gave the
aldol product (6a) in 80% yield, which was easily transformed into (E)-1a in an E/Z ratio of 96/4 by
elimination and deprotection.9
O
O
O
R1
R2
NaBH4 → 4a
HMDS
O
N H
N H
OH
DMF, 100 °C
DIBAL-H → 4b
1) BF3·OEt2
O
O
Et3SiH
2
3 (y. 80%)
4a (y. 92%)
CH2Cl2, -78 °C → rt
2) (Boc)2O
4b (y. 85%)
O
R1
R2
DMAP, CH2Cl2, rt
LDA
isobutyraldehyde
O
R1
R2
N H
N Boc
THF, -78 → 0 °C
24 h
5a (y. 91%)
5b (y. 70%)
(Z)-1a (y. 82%, E : Z = 0 : 100)
(Z)-1b (y. 98%, E : Z = 54 : 46)
LDA
I2, benzene
reflux
isobutyraldehyde
THF, -78 °C, 15 min
(Z)-1b (E : Z = 22 : 78)
O
O
R1
R1
R2
1) Ac2O
DMAP
N H
Boc
N
R2
2) DBU
3) TFA
HO
(E)-1a (y. 81%, E : Z = 96 : 4)
(E)-1b (y. 90%, E : Z = 97 : 3)
6a (y. 80%)
6b (y. 79%)
a: R1 = H, R2 = Me, b: R1 = Me, R2 = H
Scheme 2. Synthesis of pulchellalactam (1a) and its analogue (1b).

Structural determinations of both isomers, (Z)- and (E)-1a, were performed by X-Ray crystallographic
analysis as shown in Figure 1.10 We also tried to determine the structure of synthetic pulchellalactam (1a)
1
13
by NMR spectral analyses compared with the reported value of the natural product. H and C NMR
chemical shifts are given in Table 1. Most of the signals were in agreement with the reported values of
(E)-pulchellalactam; however, unfortunately signals for H-6 proton and C-6 carbon were found to differ.
(Z)-Pulchellalactam
(E)-Pulchellalactam
Figure 1. X-Ray structure of pulchellalactam.
Table 1. 1H and 13C NMR chemical shifts (ppm).
No.
Naturala)
9.41 (br s)
5.94 (br s)
(Z)-1a
(E)-1a
H-1
H-3
H-6
H-7
H-8,9
H-10
C-2
C-3
C-4
C-5
C-6
C-7
C-8,9
C-10
7.65 (br s)
5.86 (br s)
8.10 (br s)
5.91 (br s)
5.40 (d, 9.87 Hz) 5.11 (d, 9.80 Hz) 5.30 (d, 10.60 Hz)
2.60-2.67 (m) 2.51-2.64 (m) 2.89-3.10 (m)
1.12 (d, 6.63 Hz) 1.11 (d, 6.80 Hz) 1.08 (d, 6.60 Hz)
2.11 (d, 1.16 Hz)
173.1
119.7
2.08 (s)
173.0
120.7
148.5
137.7
120.7
27.3
2.28 (s)
171.7
125.7
146.9
136.6
124.5
26.6
150.2
137.0
125.5
28.0
22.7
11.9
22.8
11.7
23.4
16.1
a) Revised spectrum data provided by Dr. Khisal A. Alvi.
(Z)-Pulchellalactam is thermodynamically quite stable at least for a half-year, while the (E)-isomer is
relatively unstable. An E/Z ratio of 99/1 of (E)-1a decreased to 70/30 after 1 week, then 24/76 after 3
months, even stored in a freezer at –20 °C. It took one week or more to isolate natural pulchellalactam
from the marine fungus; therefore, it seemed reasonable to assume that the stereochemistry of 1a was E.
One principal advantage of our synthetic method is a convenient approach to the related compounds,
1,5-dihydro-3-methyl-5-(2-methylpropylidene)pyrrol-2(2H)-one (1b). In a similar way to the synthesis of
(Z)-1a, 1b was obtained as a mixture of geometrical isomers in an E/Z ratio of 54/46 in 47% overall yield.

Refluxing in benzene with iodine afforded the thermodynamically more stable geometrical isomer
((Z)-1b) in an E/Z ratio of 22/78. Similarly, (E)-1b was obtained in an E/Z ratio of 97/3 in 34% overall
yield, which was relatively unstable to show an E/Z ratio of 82/18 after being stored at –20 °C for 1 week.
In conclusion, we have shown a synthesis of (Z)- and (E)-pulchellalactams (1a) from commercially
available citraconic anhydride. Interestingly, the product ratio in the aldol reaction highly depended on
the reaction time and temperature to give the (E)- and (Z)-isomers, regioselectively.
ACKNOWLEDGEMENTS
We thank Dr. Khisal A. Alvi of AMRI Bothell Research Center for providing the NMR spectra data of
pulchellalactam and for scientific discussions on the stereochemistry of pulchellalactam. This work was
supported in part by a Grant-in-Aid (No.14540491) for Scientific Research from Japan Society for the
Promotion of Science.
REFERENCES AND NOTES
1. K. A. Alvi, A. Casey, and B. G. Nair, J. Antibiotics, 1998, 51, 515.
2. A. M. S. Mayer and V. K. B. Lehmann, The Pharmacologist, 2000, 42, 62.
3. W.-R. Li, S. T. Lin, N.-M. Hsu, and M.-S. Chern, J. Org. Chem., 2002, 67, 4702.
4. P. Kuehne and M. Hesse, Tetrahedron, 1993, 49, 4575; G. B. Gill, G. D. James, K. V. Oates, and G.
Pattenden, J. Chem. Soc., Perkin Trans. 1, 1993, 2567; K. R. Shah and C. D. Blanton, Jr., J. Org.
Chem., 1982, 47, 502.
5. N. Mase, T. Nishi, Y. Takamori, H. Yoda, and K. Takabe, Tetrahedron: Asymmetry, 1999, 10, 4469;
N. Mase, T. Nishi, M. Hiyoshi, K. Ichihara, J, Bessho, H. Yoda, and K. Takabe, J. Chem. Soc.,
Perkin Trans. 1, 2002, 707.
6. C. Pedregal, J.-A. Escribano, M. C. Carreño, and J. L. G. Ruano, Tetrahedron Lett., 1994, 35, 2053.
7. D. L. Flynn, R. E. Zelle, and P. A. Grieco, J. Org. Chem., 1983, 48, 2424.
8. In a previous synthesis of the pyrrol-2(2H)-one (5a) was obtained as a by-product in low yield, see; I.
Ojima, A. Korda, and W. R. Shay, J. Org. Chem., 1991, 56, 2024; I. Ojima and A. Korda,
Tetrahedron Lett., 1989, 30, 6283; K. Clauss, Tetrahedron Lett., 1974, 15, 1271.
9. The geometrical selectivity was determined by 1H NMR spectrometry before deprotection of the Boc
group due to instability of (E)-1a.
10. (Z)-1a: monoclinic (P21/c); a=11.9896(45)Å, b=6.9290(34)Å, c=12.5863(32)Å; β= 115.698(20)°,
Z=4, R1(wR2)=0.0662(0.1679). (E)-1a: monoclinic (P21/c); a=9.6464(158)Å, b=8.4541(63)Å,
c=11.7772(54)Å; β= 107.423(67)°, Z=4, R1(wR2)=0.0591(0.0809).