Syntheses of S-enantiomers of hanishin, longamide B, and longamide B methyl ester from L-aspartic acid β-methyl ester: Establishment of absolute stereochemistry

Article abstract of DOI:10.1021/jo051555l

Total syntheses of enantiopure hanishin, longamide B, and longamide B methyl ester are described. Absolute configurations of these natural products have been established.

Full text of DOI:10.1021/jo051555l

SCHEME 1
Syntheses of S-Enantiomers of Hanishin,
Longamide B, and Longamide B Methyl
Ester from L-Aspartic Acid â-Methyl Ester:
Establishment of Absolute Stereochemistry
Jignesh Patel, Nadia Pelloux-Le´on,*
Fre´de´ric Minassian, and Yannick Valle´e
LEDSS, UMR 5616 CNRS, Institut de Chimie Mole´culaire
de Grenoble - FR 2607, Universite´ Joseph Fourier, B.P. 53,
38041 Grenoble, France
of our studies on pyrrole alkaloids,7 we have decided to
synthesize nonracemic (S)-hanishin, (S)-longamide B,
and (S)-longamide B methyl ester.
nadia.pelloux-leon@ujf-grenoble.fr
Received July 26, 2005
We chose the commercially available L-aspartic acid
â-methyl ester as starting material for the introduction
of the stereogenic center. According to a procedure
described by Jefford and co-workers,8 L-aspartic acid
â-methyl ester was condensed with tetrahydro-2,5-
dimethoxyfuran to give the pyrrole 1 (Scheme 2). Reac-
tion of this compound with N,N-dibenzylamine, using
DCC as the coupling reagent and HOBt/CuCl2 as a
deracemizing additive,9 gave the corresponding amide in
poor yield. Replacement of the secondary amine with
N-benzylamine led to the desired pyrrole which was then
treated with a solution of boran tetrahydrofuran complex
in THF. Concomitant reduction of ester and amide
functions was observed.10 The obtained secondary amine
2 was subsequently transformed into a tertiary amine
by subsequent treatment with benzyl bromide. In the
next step, protection of the alcohol function was per-
formed under Williamson’s conditions using methyl
iodide and sodium hydride.11 At this stage, the enantio-
meric excess of compound 3 was determined by chiral
HPLC (column Chiralpak OD) using a racemic standard.
It was found to be >99%. Considering the fact that the
proton at the chiral center is no longer acidic, further
chemical transformations should not induce racemiza-
tion. Subsequently, trichloroacetylation at the C2 position
on the pyrrole ring was carried out. Treatment of the
crude material with sodium methoxide in methanol gave
the corresponding methyl ester 4.12 Deprotection of the
amino group under classical conditions13 followed by
heating led to formation of bicyclic compound 5, which
reacts readily with 2 equiv of N-bromosuccinimide14 to
give the corresponding 4,5-dibromo derivative. The re-
gioselectivity of this reaction was confirmed by 2D NMR
correlation experiments (GCOSY, GHMQC, and GH-
MBC). Demethylation using boron tribromide afforded
Total syntheses of enantiopure hanishin, longamide B, and
longamide B methyl ester are described. Absolute configura-
tions of these natural products have been established.
A large number of bioactive bromopyrrole alkaloids
have been isolated from marine sponges.1 Examples
include hanishin,2 longamide B,2,3 and longamide B
methyl ester4 (Scheme 1), which have been isolated as a
racemic mixture from Acanthella carteri, Agelas dispar,
and Homaxinella sp., respectively. Recently, Venkat-
teswarlu and co-workers published the isolation of (+)-
methyl ester of longamide B from Agelas ceylonica.5 All
of these three natural products exhibit interesting bio-
logical properties. For example, hanishin is cytotoxic
toward NSCLC-N6 human non-small-cellung carcinoma
(IC50 9.7 µg‚mL-1), longamide B displays antibiotic activ-
ity against Gram-positive bacteria (Bacillus subtilis
ATCC #6538; MIC 50 µg‚mL-1), and its methyl ester
shows an activity against P388 lymphocytic leukemia
cells (ED50 30 µg‚mL-1).
Racemic syntheses of hanishin, longamide B, and its
methyl ester have been previously reported;6 however,
enantiopure syntheses have not been described. As part
(7) (a) Sayah, B.; Pelloux-Le´on, N.; Valle´e, Y. J. Org. Chem. 2000,
65, 2824-2826. (b) Sayah, B.; Pelloux-Le´on, N.; Milet, A.; Pardillos-
Guindet, J.; Valle´e, Y. J. Org. Chem. 2001, 66, 2522-2525.
(8) Jefford, C. W.; Villedon de Naide, F.; Sienkiewicz, K. Tetrahe-
dron: Asymmetry 1996, 7, 1069-1076.
(1) (a) Christophersen, C. In The Alkaloids; Brossi, A., Ed.; Academic
Press: Orlando, 1985; Vol. 24, pp 25-98. (b) Hao, E.; Fromont, J.;
Jardine, D.; Karuso, P. Molecules 2001, 6, 130-141 and references
therein.
(9) Saravanan, P.; Bisai, A.; Baktharaman, S.; Chandrasekhar, M.;
Singh, V. K. Tetrahedron 2002, 58, 4693-4706.
(2) Mancini, I.; Guella, G.; Amade, P.; Roussakis, C.; Pietra, F.
Tetrahedron Lett. 1997, 38, 6271-6274.
(10) Knapp, S.; Hale, J. J.; Bastos, M.; Molina, A.; Chen, K. Y. J.
Org. Chem. 1992, 57, 6239-6256.
(3) Cafieri, F.; Fattorusso, E.; Taglialatela-Scafati, O. J. Nat. Prod.
1998, 61, 122-125.
(11) Greene, T. W. In Protective Groups in Organic Chemistry; John
Wiley & Sons, Inc.: New York, 1999; Chapter 2.
(4) Umeyama, A.; Ito, S.; Yuasa, E.; Arihara, S.; Yamada, T. J. Nat.
Prod. 1998, 61, 1433-1434.
(12) Kumar, R.; Lown, J. W. Org. Biom. Chem. 2003, 1, 2630-2647.
(13) Greene, T. W. In Protective Groups in Organic Chemistry; John
Wiley & Sons, Inc.: New York, 1999; Chapter 7.
(5) Srinivasa Reddy, N.; Venkateswarlu, Y. Biochem. Syst. Ecol.
2000, 28, 1035-1037.
(6) Banwell, M. G.; Bray, A. M.; Willis, A. C.; Wong, D. J. New J.
Chem 1999, 23, 687-690.
(14) He, R. H.-Y.; Jiang, X.-K. J. Chem. Res., Synop. 1998, 786-
787.
10.1021/jo051555l CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/06/2005
J. Org. Chem. 2005, 70, 9081-9084
9081

SCHEME 2
alcohol 6. In a first attempt, we tried to oxidize the
primary alcohol to the corresponding carboxylic acid
using a sodium periodate-ruthenium trichloride combi-
nation; however, these conditions led to the acid in low
yield.15 A two-step strategy was then envisaged involving
the treatment of alcohol 6 with o-iodoxybenzoic acid16
followed by oxidation of the resulting aldehyde with
sodium chlorite to give longamide B.17 Spectral properties
of the acid were in good agreement with those described
in the literature for a racemic sample.3,6 As described by
Banwell et al., we did not observe an infrared absorption
band at 1780 cm-1.6 The optical rotation was -5.5 (c 1,
MeOH). Finally, to confirm the ee determined for com-
pound 3 (>99%), we synthesized a chiral ester by reacting
our longamide B sample with (S)-phenylethanol. The
corresponding ester exhibited a de >99%, which was
determined by HPLC using a standard.
previously described for a sample which was assumed to
be (S)-(+).4,5 Our synthetic compound has indeed the
absolute S configuration18 but exhibits a (-) sign for its
specific rotation. Several measurements were performed
using the previously described concentrations.4,5 In all
cases, we found nearly the same absolute value but the
opposite sign (experimental results: [R]D ) -2.8 (c 0.49,
MeOH) and -7.7 (c 1, MeOH) compared to the litera-
ture: [R]D ) +2.6 (c 0.39, MeOH)4 and [R]D ) +7 (c 1,
MeOH)5).
In conclusion, the first syntheses of enantiopure (S)-
(-)-longamide B, (S)-(-)-hanishin, and (S)-(-)-longamide
B methyl ester are reported. Their optical rotations are
-5.5 (c 1, MeOH), -4.3 (c 1, MeOH), and -7.7 (c 1,
MeOH), respectively. Finally, we showed that the levoro-
tary enantiomers of these three natural products have
the S configuration.
The second target molecule, e.g., hanishin, was pre-
pared by reacting longamide B with acidic ethanol.6 Once
again, spectroscopic data for this product were in agree-
ment with those reported in the literature.2,6 The optical
rotation exhibited by this bromopyrrole alkaloid was -4.3
(c 1, MeOH).
Finally, longamide B methyl ester was also prepared
from longamide B by treatment with diazomethane in
diethyl ether.6 X-ray analysis and spectroscopic data
confirmed the structure of this alkaloid.3,6 However, we
observed an [R]D value with the sign opposite to the one
Experimental Section
All commercial solvents were distilled before use. Tetrahy-
drofuran (THF) was distilled from sodium benzophenone ketyl
under nitrogen atmosphere. Column chromatography purifica-
tions were carried out using silica gel (70-230 mesh). 1H and
13C NMR were recorded at 300 and 75 MHz, respectively. Peak
assignments were determined using DEPT and two-dimensional
experiments. HPLC analyses were performed on a Shimadzu
apparatus (UV diodes array detector) equipped with a chiral
column Daicel Chiralpak OD (25 cm) for the separation of
enantiomers or a reversed-phase column Kromasil C18 (25 cm)
for the separation of diastereoisomers.
(15) Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K. B.
J. Org. Chem. 1981, 46, 3936-3938.
(16) More, J. D.; Finney, N. S. Org. Lett. 2002, 4, 3001-3003.
(17) Al-Awar, R. S.; Ray, J. E.; Schultz, R. M.; Andis, S. L.; Kennedy,
J. H.; Moore, R. E.; Liang, J.; Golakoti, T.; Subbaraju, G. V.; Corbett,
T. H. J. Med. Chem. 2003, 46, 2985-3007.
(2S)-2-Pyrrol-1-ylsuccinic Acid 4-Methyl Ester (1). So-
dium acetate (3.385 g, 41.26 mmol) and acetic acid (3 mL) were
(18) An anomalous dispersion X-ray analysis of longamide B methyl
ester confirmed the S configuration of our sample.
9082 J. Org. Chem., Vol. 70, No. 22, 2005

added to a solution of L-aspartic acid â-methyl ester hydrochlo-
ride (7.5 g, 40.85 mmol) in 1,2-dichloroethane (75 mL) and water
(45 mL). The resulting mixture was heated at 80 °C, and
tetrahydro-2,5-dimethoxyfuran (5.67 g, 42.89 mmol) was then
added. After 30 min at 80 °C, the solution was cooled. The
aqueous layer was washed with dichloromethane. The combined
organic layers were washed with brine and then dried, filtered,
and evaporated. The crude residue was purified by chromatog-
raphy on silica gel (pentane/EtOAc, 7:3) to give compound 1 as
resulting mixture was stirred at room temperature for 24 h. The
solution was cooled at 0 °C and quenched with water. The layers
were separated, and the aqueous one was extracted twice with
EtOAc (2 × 50 mL). The organic layer was dried, the solvent
was removed, and the residue was purified by a column
chromatography to give a colorless oil (pentane/EtOAc, 5:5, 2.61
g, 82% yield). The ee of this pure compound (99%) was deter-
mined by chiral HPLC (debit: 0.3 mL‚min-1; cyclohexane/
propan-2-ol 99.8:0.2) using a racemic standard (retention
times: 17.0 min for the (S)-enantiomer and 19.1 min for the (R)-
a colorless viscous oil (4.91 g, 61% yield): [R]20 ) -47.9 (c 1,
D
CH2Cl2); IR (KBr, cm-1) 3101 (OH), 1732 (CO), 1700 (CO); 1H
NMR (300 MHz, CDCl3) δ (ppm) 2.99 (dd, J ) 16.8, 7.0 Hz, 1H,
H-3), 3.26 (dd, J ) 16.8, 7.6 Hz, 1H, H-3), 3.69 (s, 3H, CH3),
5.13-5.18 (m, 1H, H-2), 6.19 (t, J ) 2.1 Hz, 2H, Hpyr), 6.71 (t, J
) 2.1 Hz, 2H, Hpyr); 13C NMR (75 MHz, CDCl3) δ (ppm) 37.5
(C-3), 52.8 (CH3), 58.0 (C-2), 109.8 (CAr), 120.6 (CAr), 170.8 (Cd
O), 175.2 (CdO); MS (DCI) m/z 198 (M + H+). Anal. Calcd for
C9H11NO4: C, 54.82; H, 5.62; N, 7.10. Found: C, 54.91; H, 5.83;
N, 6.98.
enantiomer): [R]20 ) -3.7 (c 1, CH2Cl2); IR (KBr, cm-1) 3028,
D
2873, 2807, 1493, 1120; 1H NMR (300 MHz, CDCl3) δ (ppm)
1.60-1.71 (m, 1H, H-3), 2.03-2.09 (m, 1H, H-3), 2.65-2.79 (m,
2H, H-1), 2.90-2.97 (m, 1H, H-4), 3.13-3.20 (m, 1H, H-4), 3.21
(s, 3H, OCH3), 3.51 (s, 2H, CH2Ph), 3.52 (s, 2H, CH2Ph), 4.14-
4.23 (m, 1H, H-2), 6.12 (t, J ) 2.0 Hz, 2H, Hpyr), 6.52 (t, J ) 2.2
Hz, 2H, Hpyr), 7.21-7.28 (m, 10H, HPh). 13C NMR (75 MHz,
CDCl3) δ (ppm) 34.5 (C-3), 58.9 (C-2), 59.2 (CH2Ph), 59.4 (C-1),
60.2 (C-4), 108.0 (Cpyr), 119.6 (Cpyr), 127.3 (CAr), 128.6 (CAr), 129.2
(CAr), 139.6 (CAr); MS (DCI) m/z 349 (M + H+). Anal. Calcd for
C23H28N2O: C, 79.27; H, 8.10; N, 8.04. Found: C, 79.00; H, 8.25;
N, 8.02.
(3S)-4-Benzylamino-3-pyrrol-1-ylbutan-1-ol (2). A suspen-
sion of 1 (4.9 g, 24.97 mmol), anhydrous CuCl2 (4.01 g, 29.85
mmol), and HOBt (4.03 g, 29.85 mmol) in dry DMF (80 mL) was
cooled at 0 °C. DCC (6.16 g, 29.85 mmol) in DMF (5 mL) was
then added. After the mixture was stirred for 30 min, N-
benzylamine (5.06 g, 47.26 mmol) was added, and the stirring
was continued for 24 h at room temperature. Ethyl acetate (50
mL) was added, and the resulting organic layer was washed
subsequently with aqueous HCl (0.1 N), saturated aqueous
NaHCO3, and brine. It was then dried, filtered, and concentrated
under reduced pressure.
(1S)-1-{1-[(Dibenzylamino)methyl]-3-methoxypropyl}-
1H-pyrrole-2-carboxylic Acid Methyl Ester (4). Compound
3 (2.61 g, 7.5 mmol) in dry THF (10 mL) was added to a solution
of trichloroacetyl chloride (3.41 g, 18.75 mmol) and 2,6-lutidine
(2.01 g, 18.75 mmol) in THF (70 mL). The resulting mixture was
stirred under reflux for 20 h. Then, the reaction was quenched
with water and extracted with dichloromethane. The organic
layers were successively washed with a saturated aqueous
solution of NaHCO3, water, and brine. After drying, filtration,
and evaporation of the organic solvent, the corresponding crude
trichloroacetyl derivative was obtained and directly dissolved
in dry methanol in the presence of sodium methoxide (2.23 g,
41.28 mmol). The reaction was performed at room temperature
for 3 h and stopped by adding a saturated aqueous solution of
NH4Cl. Ethyl acetate was added, and the organic layer washed
with brine, dried, and evaporated under reduced pressure. The
resulting oil was purified by column chromatography (pentane/
CH2Cl2, 6:4). Ester 4 was obtained as a colorless oil in 63%
The resulting crude ester was dissolved in dry THF (40 mL).
A THF solution of BH3 (90 mL, 1 M solution) was then added,
and the resulting mixture was stirred at room temperature for
12 h. The organic layer was removed in vacuo, and water (25
mL) was added. The aqueous layer was neutralized with aqueous
NaOH (10%) and the desired compound extracted with EtOAc.
Evaporation of the solvent gave a residue which was purified
by column chromatography on silica gel (pentane/EtOAc, 5:5).
The amino alcohol (3.16 g, 12.98 mmol) was obtained as a red
oil (52% yield for two steps): [R]20 ) -10.3 (c 1, CH2Cl2); IR
D
(KBr, cm-1) 3319, 3020, 2929; 1H NMR (300 MHz, CDCl3) δ
(ppm) 1.95-1.99 (m, 2H, H-2), 2.40 (bs, 2H, OH and NH), 2.86-
3.00 (m, 2H, H-4), 3.41-3.49 (m, 1H, H-1), 3.57-3.64 (m, 1H,
H-1), 3.74 (d, 2H, CH2Ph), 4.19 (m, 1H, H-3), 6.15 (t, J ) 2.1
Hz, 2H, Hpyr), 6.67 (t, J ) 2.1 Hz, 2H, Hpyr), 7.21-7.30 (m, 5H,
yield: [R]20 ) -3.85 (c 1, CH2Cl2); IR (KBr, cm-1) 3058, 3021,
D
1
2916, 1496; H NMR (300 MHz, CDCl3) δ (ppm) 1.77-1.83 (m,
1H, H-2′), 2.04-2.15 (m, 1H, H-2′), 2.59-2.80 (m, 2H, CH2N),
3.11-3.20 (m, 6H, 2 CH3), 3.41-3.45 (m, 2H, H-3′), 6.11-6.13
(dd, J ) 3.9, 2.7 Hz, 1H, Hpyr), 6.64 (bs, 1H, Hpyr), 6.95-6.97
(dd, J ) 3.9, 1.7 Hz, 1H, Hpyr), 7.15-7.25 (m, 10H, HPh); 13C NMR
(75 MHz, CDCl3) δ (ppm) 34.9 (C-2′), 51.3 (C-1′), 58.2 (CH2N),
58.8 (CH2Ph), 58.9 (CH2Ph), 59.2 (2 CH3), 109.0 (CAr), 118.0 (CAr),
123.0 (CAr), 127.2 (CAr), 128.7 (CAr), 128.9 (CAr), 129.4 (CAr), 130.1
(CAr), 139.7 (CAr), 162.2 (CO); MS (DCI) m/z 407 (M + H+). Anal.
Calcd for C25H30N2O3: C, 73.87; H, 7.44; N, 6.90. Found: C,
73.63; H, 7.46; N, 6.84.
H
Ph); 13C NMR (75 MHz, CDCl3) δ (ppm) 38.2 (C-2), 53.9 (C-3),
54.7 (CH2Ph), 58.1 (C-1), 59.7 (C-4), 108.7 (Cpyr), 119.3 (Cpyr),
127.6 (CAr), 128.5 (CAr), 128.9 (CAr), 139.7 (CAr); MS (DCI) m/z
245 (M + H+).
(3S)-4-Dibenzylamino-3-pyrrol-1-ylbutan-1-ol. A mixture
of compound 2 (2.97 g, 12.17 mmol), K2CO3 (5.05 g, 36.51 mmol),
and benzyl bromide (3.96 g, 23.13 mmol) in acetonitrile (50 mL)
was stirred at room temperature for 24 h. The reaction was
stopped by the addition of water (30 mL). Ethyl acetate (50 mL)
was used twice for the extraction of the product. The organic
layer was then dried to give the desired crude tertiary amine
after evaporation. Purification by column chromatography
(CH2Cl2) yielded the desired amino alcohol as a colorless viscous
(4S)-4-(2-Methoxy-ethyl)-3,4-dihydro-2H-pyrrolo[1,2-a]-
pyrazin-1-one (5). To a solution of 4 (2.3 g, 5.66 mmol) in
methanol (75 mL) was added Pd(OH)2/C. After the reaction flask
was purged with argon, the reaction mixture was stirred for 12
h under hydrogen. It was then filtered through Celite and
concentrated in vacuo. The residue was dissolved in toluene (50
mL) and heated under reflux in an argon atmosphere for 20 h.
The reaction mixture was then cooled and the solvent removed
under reduced pressure. Column chromatography (pentane/
EtOAc, 8:2) yielded the desired compound as a colorless oil (89%
oil (3.05 g, 9.13 mmol): [R]20 ) -20.6 (c 1, CH2Cl2); IR (KBr,
D
cm-1) 3392 (OH), 3020, 2939, 2802; 1H NMR (300 MHz, CDCl3)
δ (ppm) 1.74 (qd, J ) 9.3, 4.7 Hz, 1H, H-2), 1.95-2.04 (m, 1H,
H-2), 2.76 (ddd, J ) 17.9, 13.2, 7.0 Hz, 2H, H-4), 3.33 (ddd, J )
11.0, 8.8, 4.5 Hz, 1H, H-1), 3.43-3.53 (m, 1H, H-1), 3.54-3.59
(m, 4H, CH2Ph), 4.06-4.16 (m, 1H, H-3), 6.12 (t, J ) 2.0 Hz,
2H, Hpyr), 6.53 (t, J ) 2.2 Hz, 2H, Hpyr), 7.21-7.37 (m, 10H, HPh);
13C NMR (75 MHz, CDCl3) δ (ppm) 37.7 (C-2), 56.1 (C-3), 59.6
(CH2Ph), 59.9 (C-4), 60.2 (C-1), 108.4 (Cpyr), 119.4 (Cpyr), 127.6
(CAr), 128.7 (CAr), 129.5 (CAr), 139.1 (CAr); MS (DCI) m/z 335 (M
+ H+). Anal. Calcd for C22H26N2O: C, 79.01; H, 7.84; N, 8.38.
Found: C, 79.30; H, 8.17; N, 8.18.
yield): [R]20 ) -5.6 (c 1, CH2Cl2); IR (KBr, cm-1) 3198, 2924,
D
1
2879, 1662; H NMR (300 MHz, CDCl3) δ (ppm) 1.98-2.11 (m,
2H, H-1′), 3.17-3.28 (m, 1H, H-3), 3.34 (s, 3H, CH3), 3.38-3.45
(m, 2H, H-2′ and H-3), 3.87-3.92 (dd, J ) 11.9, 4.6 Hz, 1H, H-2′),
4.36-4.43 (m, 1H, H-4), 6.21-6.23 (dd, J ) 3.8, 2.6 Hz, 1H, Hpyr),
6.49 (bs, 1H, NH), 6.82-6.83 (m, 1H, Hpyr), 6.94-6.96 (m, 1H,
Hpyr); 13C NMR (75 MHz, CDCl3) δ (ppm) 33.2 (C-1′), 45.2 (C-3),
51.7 (CH3), 59.0 (C-4), 68.9 (C-2′), 109.7 (Cpyr), 114.2 (Cpyr), 123.7
(Cpyr), 125.0 (Cpyr), 161.7 (CO); MS (DCI) m/z 195 (M + H+). Anal.
Calcd for C10H14N2O2: C, 61.84; H, 7.27; N, 14.43. Found: C,
61.97; H, 7.47; N, 14.01.
(2S)-Dibenzyl(4-methoxy-2-pyrrol-1-ylbutyl)amine (3).
The previously obtained alcohol (3.05 g, 9.13 mmol) was treated
with sodium hydride in dry THF (60 mL) at 0 °C for 30 min.
Methyl iodide (6.48 g, 45.65 mmol) was then added, and the
J. Org. Chem, Vol. 70, No. 22, 2005 9083

(4S)-6,7-Dibromo-4-(2′-methoxyethyl)-3,4-dihydro-2H-
pyrrolo[1,2-a]pyrazin-1-one. NBS (1.798 g, 10.1 mmol) was
added to a solution of compound 5 (0.89 g, 5.05 mmol) in THF
(35 mL). The resulting mixture was then stirred for 50 min at
room temperature. Then, an aqueous solution of NaHCO3 (5%)
and dichloromethane were added. The layers were separated,
and the organic phase was successively washed with aqueous
NaHCO3 (5%), water, and brine. After evaporation of the organic
solvent, the residue was purified by column chromatography on
silica gel to give a white solid (CH2Cl2/MeOH, 98:2, yield 83%):
a white powder (667 mg, 1.92 mmol): mp 208-210 °C; [R]20
)
D
-5.51 (c 1, CH3OH); IR (KBr, cm-1) 3270, 3214, 3044, 1728,
1643; 1H NMR (300 MHz, CD3OD) δ (ppm) 2.57 (ddd, J ) 16.3,
3.4, 1.4 Hz, 1H, H-10), 2.87 (dd, J ) 16.4, 10.8 Hz, 1H, H-10),
3.67 (dd, J ) 13.5, 1.3 Hz, 1H, H-8), 3.91 (ddd, J ) 13.6, 4.1, 1.5
Hz, 1H, H-8), 4.81 (m, 1H, H-9), 6.96 (s, 1H, Hpyr); 13C NMR (75
MHz, CD3OD) δ (ppm) 36.7 (C-10), 44.2 (C-8), 52.4 (C-9), 102.1
(C-4), 108.4 (C-5), 117.0 (C-3), 126.7 (C-2), 161.4 (CdO), 173.2
(CdO); MS (DCI) m/z 349.5, 351.1, 352.9; HRMS (EI) m/z calcd
for C9H8N2O379Br81Br 351.88812, found 351.8889. Anal. Calcd
for C9H8Br2N2O3: C, 30.72; H, 2.3; N, 7.96, Found: C, 30.56; H,
2.29; N, 7.65.
mp 109-110 °C; [R]20 ) -31.6 (c 1, CH3OH); IR (KBr, cm-1
)
D
1
3220 (NH), 3077, 2918, 1652 (CO); H NMR (300 MHz, CDCl3)
δ (ppm) 1.87-1.98 (m, J ) 14.3, 11.1, 5.5 Hz, 1H, H-1′), 2.02-
2.17 (m, 1H, H-1′), 3.35 (s, 3H, CH3), 3.40-3.48 (m, 2H, H-2′),
3.64 (ddd, J ) 13.1, 5.4, 1.3 Hz, 1H, H-3), 3.84 (dd, J ) 13.1, 4.1
Hz, 1H, H-3), 4.45-4.53 (m, 1H, H-4), 6.96 (bs, 1H, NH), 6.98
(s, 1H, H-8); 13C NMR (75 MHz, CDCl3) δ (ppm) 31.9 (C-1′), 43.2
(C-3), 52.4 (CH3), 58.8 (C-4), 68.9 (C-2′), 100.7 (C-7), 106.7 (C-
6), 115.9 (C-8), 124.9 (C-8a), 159.4 (CdO); MS (DCI) m/z 351,
353, 355; HRMS (EI) m/z calcd for C10H1279Br2N2O2 349.9265,
found 349.9270.
(S)-(-)-Hanishin. We followed the procedure described by
Banwell et al.,6 starting from 30 mg of longamide B: mp 158-
160 °C; [R]20D ) -4.3 (c 1, CH3OH); IR (KBr, cm-1) 3180, 3064,
2978, 2924, 1722, 1684; 1H NMR (300 MHz, (CD3)2CO) δ (ppm)
1.27 (t, J ) 7.1 Hz, 3H, CH3), 2.68 (ddd, J ) 15.9, 3.8, 1.3 Hz,
1H, H-10), 3.00 (ddd, J ) 15.9, 10.0, 1.3 Hz, 1H, H-10), 3.67 (s,
3H, OCH3), 3.71 (ddd, J ) 13.4, 5.3, 1.3 Hz, 1H, H-8), 4.03 (ddd,
J ) 13.4, 4.1, 1.1 Hz, 1H, H-8), 4.19 (q, J ) 7.1 Hz, 2H, CH2),
4.86 (m, 1H, H-9), 6.89 (s, 1H, Hpyr), 6.94 (bs, 1H, NH); 13C NMR
(75 MHz, CDCl3) δ (ppm) 14.3 (CH3), 36.5 (C-10), 43.6 (C-8), 51.6
(C-9), 61.5 (OCH2), 100.6 (C-4), 106.1 (C-5), 115.2 (C-3), 127.0
(C-2), 158.6 (CdO), 170.2 (CdO); MS (DCI) m/z 379.0, 381, 383;
HRMS (EI) m/z calcd for C11H12N2O379Br2 377.92146, found
377.9224.
(4S)-6,7-Dibromo-4-(2′-hydroxyethyl)-3,4-dihydro-2H-
pyrrolo[1,2-a]pyrazin-1-one (6). A 1 M solution of BBr3 (42.32
mmol) in dichloromethane was slowly added to a solution of the
previous ether (1.49 g, 4.23 mmol) in CH2Cl2 (50 mL) at -20
°C. The resulting mixture was stirred at room temperature for
20 h and treated with water. The aqueous phase was extracted
twice with ethyl acetate, and the combined organic layers were
dried and concentrated under vacuum. Chromatography of the
residue gave pure title alcohol as a white powder in 84% yield:
(S)-(-)-Longamide B Methyl Ester. We followed the pro-
cedure described by Banwell et al.6 starting from 80 mg of
longamide B: mp 150-151 °C; [R]20D ) -7.7 (c 1, CH3OH), [R]20
D
) -2.8 (c 0.49, CH3OH); CD (c 0.013, CH3OH, 25 °C) (∆ꢀ) 226
nm (+3.02); IR (KBr, cm-1) 3218, 1728, 1671; 1H NMR (300 MHz,
CDCl3) δ (ppm) 2.54 (ddd, J ) 16.8, 3.1, 1.5 Hz, 1H, H-10), 2.95
(dd, J ) 16.7, 10.8 Hz, 1H, H-10), 3.61 (ddd, J ) 13.4, 5.3, 1.3
Hz, 1H, H-8), 3.67 (s, 3H, OCH3), 3.86 (ddd, J ) 13.3, 4.0, 1.4
Hz, 1H, H-8), 4.68 (m, 1H, H-9), 6.26 (bs, 1H, NH), 6.93 (s, 1H,
Hpyr); 13C NMR (75 MHz, CDCl3) δ (ppm) 35.6 (C-10), 43.5 (C-
8), 50.7 (C-9), 52.6 (OCH3), 101.7 (C-4), 107.0 (C-5), 116.7 (C-3),
125.1 (C-2), 159.9 (CdO), 170.5 (CdO); MS (DCI) m/z 363.3,
365.2, 367.1; HRMS (EI) m/z calcd for C10H19N2O379Br2 363.90581,
found 363.9052.
mp 138-140 °C; [R]20 ) -32.8 (c 1, CH3OH); IR (KBr, cm-1
)
D
3434, 3245, 3129, 1648, 1617; 1H NMR (300 MHz, CD3OD) δ
(ppm) 1.75-1.86 (m, 1H, H-1′), 2.90-2.02 (m, 1H, H-1′), 3.59-
3.65 (m, 3H, 2 H-3 and H-2′), 3.77 (ddd, J ) 13.5, 4.1, 0.9 Hz,
1H, H-2′), 3.75-3.81 (m, 1H, H-4), 6.88 (s, 1H, H-8); 13C NMR
(75 MHz, CD3OD) δ (ppm) 34.7 (C-1′), 43.3 (C-3), 52.9 (C-4), 59.4
(C-2′), 101.7 (C-7), 108.5 (C-6), 116.2 (C-8), 126.4 (C-8a), 161.2
(CdO); MS (DCI) m/z 337, 339, 341; HRMS (EI) m/z calcd
for C9H1079Br2N2O2 335.91090, found 335.9116; calcd for
C9H1079Br81BrN2O2 337.90885, found 337.9072.
(S)-(-)-Longamide B. The alcohol 6 (1.2 g, 3.55 mmol) was
dissolved in ethyl acetate (60 mL), and o-iodoxybenzoic acid (2.98
g, 10.65 mmol) was added. The resulting suspension was heated
at 80 °C for 3 h. The solution was then cooled to room
temperature, filtered through Celite, and concentrated in vacuo.
Purification of the resulting residue on silica gel (MeOH/EtOAc,
5:95) gave the corresponding aldehyde which was immediately
dissolved in THF (50 mL) and water (50 mL) at 0 °C, in the
presence of 2-methylbut-2-ene (50 mL), NaClO2 (2.05 g, 20.62
mmol), and NaH2PO4‚H2O (3.51 g, 25.44 mmol). The mixture
was then stirred at room temperature for 5 h and then diluted
with dichloromethane. The layers were separated, and the
aqueous one was extracted with EtOAc. The combined organic
layers were dried, filtered, and concentrated in vacuo. Purifica-
tion after a classical acid-base treatment gave longamide B as
Acknowledgment. We are grateful to the Univer-
sity Joseph Fourier and CNRS (UMR 5616, FR 2607)
for financial support. Dr. C. Philouze is thanked for
performing the X-ray analysis of longamide B methyl
ester.
1
Supporting Information Available: Copies of H NMR
spectra of all compounds, CD spectrum of (S)-(-)-longamide
B methyl ester, and crystallographic data for (S)-(-)-longamide
B methyl ester. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO051555L
9084 J. Org. Chem., Vol. 70, No. 22, 2005