Total synthesis of the cytostatic marine natural product dibromophakellstatin via three-component imidazolidinone anellation

Article abstract of DOI:10.1021/jo061813u

(Chemical Equation Presented) The tetracyclic pyrrole-imidazole alkaloid dibromophakellstatin from the marine sponge Phakellia mauritiana has been synthesized within seven steps from pyrrole in an 18% overall yield. The key step is a three-component assem

Full text of DOI:10.1021/jo061813u

Total Synthesis of the Cytostatic Marine Natural Product
Dibromophakellstatin via Three-Component Imidazolidinone
Anellation
Michael Zo¨llinger, Peter Mayer, and Thomas Lindel*
Ludwig Maximilian UniVersity, Department of Chemistry and Biochemistry, Butenandtstrasse 5-13,
D-81377 Munich, Germany
thomas.lindel@cup.uni-muenchen.de
ReceiVed September 1, 2006
The tetracyclic pyrrole-imidazole alkaloid dibromophakellstatin from the marine sponge Phakellia
mauritiana has been synthesized within seven steps from pyrrole in an 18% overall yield. The key step
is a three-component assembly of a tricyclic enamide, a nitrene, and a carbamoyl building block, affording
the imidazolidinone ring of dibromophakellstatin in one step. Notably, it is possible to employ the reagent
EtO2CNHOTs in a double function as a source of the electrophilic nitrene and of a dipolar carbamoyl
component. Use of debrominated precursor dipyrrolopyrazinones leads to much higher anellation yields
and allowed us to develop a second generation synthesis. The cytostatic activity of dibromophakellstatin
is confirmed.
Introduction
cycle. Ring D obtains a 70° angle referred to as the almost planar
ABC tricycle. While the approaches by Horne4 and Feldman5
adapt the biomimetic synthesis of rac-dibromophakellin reported
by Bu¨chi,6 the “ABC strategy” (Figure 1) employs tricyclic
pyrrolopyrazinones as precursors.7-10
In the biomimetic strategy, rings C and B are formed in one
step starting from open-chain precursors where rings A and D
are connected by an amide-containing chain. Our strategy toward
the synthesis of dibromophakellstatin aimed at the installation
of the imidazolidinone ring D in one step from an ABC tricyclic
precursor. Ideally, a three-component assembly should include
Pyrrole-imidazole alkaloids constitute a unique family of
natural products exclusively found in marine sponges. Struc-
tures1 and total syntheses2 have been reviewed. We started a
program on the total synthesis of the phakellin-type pyrrole-
imidazole alkaloid dibromophakellstatin (1) from Phakellia
mauritiana, because 1 is among the relatively few family
members for which cytostatic activity has been reported in the
course of its isolation in 1997.3 Nothing is known on the
mechanism of action. An efficient total synthesis of 1 should
allow for the confirmation of that biological activity and pave
the way toward functionalization of dibromophakellstatin for
studies in chemical biology.
(4) Wiese, K. J.; Yakushijin, K.; Horne, D. A. Tetrahedron Lett. 2002,
43, 5135-5136.
(5) Feldman, K. S.; Skoumbourdis, A. P. Org. Lett. 2005, 7, 929-931.
(6) Foley, L. H.; Bu¨chi, G. J. Am. Chem. Soc. 1982, 104, 1776-1777.
(7) Poullennec, K. G.; Romo, D. J. Am. Chem. Soc. 2003, 125, 6344-
6345.
The key problem associated with the total synthesis of
dibromophakellstatin originates in the ring strain of the tetra-
(1) Al-Mourabit, A.; Potier, P. Eur. J. Org. Chem. 2001, 237-243.
(2) (a) Hoffmann, H.; Lindel, T. Synthesis 2003, 1753-1783. (b) Jacquot,
D. E. N.; Lindel, T. Curr. Org. Chem. 2005, 9, 1551-1565.
(3) Pettit, G. R.; McNulty, J.; Herald, D. L.; Doubek, D. L.; Chapuis, J.
C.; Schmidt, J. M.; Tackett, L. P.; Boyd, M. R. J. Nat. Prod. 1997, 60,
180-183.
(8) Chung, R.; Yu, E.; Incarvito, C. D.; Austin, D. J. Org. Lett. 2004, 6,
3881-3884.
(9) Jacquot, D. E. N.; Zo¨llinger, M.; Lindel, T. Angew. Chem., Int. Ed.
2005, 44, 2295-2298.
(10) Travert, N.; Martin, M.-T.; Bourguet-Kondracki, M.-L.; Al-Mourabit,
A. Tetrahedron Lett. 2005, 46, 249-252.
10.1021/jo061813u CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/16/2006
J. Org. Chem. 2006, 71, 9431-9439
9431

Zo¨llinger et al.
SCHEME 1. Functionalization of the Dipyrrolopyrazinone
3a
a (a) POCl3 (1.5 equiv), pyridine, 0 °C, 2 h, 85%. (b) DEAD (1 equiv),
CHCl3, 70 °C, sealed tube, 12 h, 95%. (c) NBS (1 equiv), DCM, 0 °C, 1
h, 97%. (d) NBS (1.1 equiv), pyridine, 0 °C, 90 min, 85%.
FIGURE 1. Retrosynthesis of dibromophakellstatin (1).
SCHEME 2. Diamination of the Enamide 3 with
Chloramine T
an enamide, a nitrene, and a carbamoyl building block (Figure
1). Use of an electrophilic nitrene offers the advantage of
partially defining the order by which the condensation would
take place. In the beginning, the nitrene is neutral. After being
attacked by the enamide component, a negatively charged
nitrogen would arise, capable of reacting with an activated
carbamoyl component (Figure 1). Of course, the stereochemical
course of our planned condensation was unclear in the beginning
of our studies. The choice of reagents and variation in the pyrrole
bromination would have some influence. In this paper, we will
show that it is possible to anellate the imidazolidinone ring (D)
in one step.
Results and Discussion
Orienting Experiments. We decided to first investigate
reactions between a tricyclic enamide and various electrophiles.
Scheme 1 outlines a few experiments on the reactivity of the
tricyclic dipyrrolopyrazinone 3, which was synthesized in five
steps from pyrrole via the N,O-acetal 2.11
On treatment of 3 with mCPBA in the presence of water, we
observed anti dihydroxylation of the C10,C10a double bond.11
Deuteration of 3 on reaction with D2O/D+ at room temperature
takes place first at C10, quantitatively, and then at the pyrrole
ring. Deuteration of C1 was not observed. On reaction of 3 with
NBS in pyridine at 0 °C, position C10 is brominated, affording
6 in 85% yield. Pyridine acts as an HBr scavenger and
suppresses the radical process favoring an ionic mechanism.
Changing the solvent to dichloromethane at 0 °C leads to
exclusive radical substitution in the allylic position C1 (97%
of 5). Introduction of a nitrogen-containing substituent at C10
was possible on reaction of 3 with diethyl azodicarboxylate
(DEAD) in refluxing CHCl3 providing hydrazine 4 in 95% yield.
The tendency of our proton-poor pyrrolopyrazinone intermedi-
ates to crystallize is fortunate for structural assignments.12
Diamination. A total synthesis of dibromophakellstatin (1)
would require the introduction of two nitrogen substituents at
C10 and C10a of enamide 3. In a model study, reaction of 3
with chloramine T trihydrate (TosNClNa‚3H2O, 1.5 equiv) in
THF at room temperature afforded the C10,C10a diaminated
product 9 (55%), together with the C10 monoamination product
8 (15%, Scheme 2). The anti stereochemistry of 9 was
determined by X-ray analysis.12 With dry chloramine T,
monoaminated 8 (45%) and the C1 allylic amination product 7
(20%) were formed and the diaminated product 9 was absent.
Bis-adduct 9 is formed by nucleophilic attack of toluene
sulfonamide at C10a of the intermediate acyliminium ion 10.
It is known that, in the presence of water, chloramine T exists
in equilibrium with toluene sulfonamide and hypochlorite.13 For
the formation of 9, TosNClNa‚3H2O should act as water source,
(11) Jacquot, D. E. N.; Hoffmann, H.; Polborn, K.; Lindel, T. Tetrahedron
Lett. 2002, 43, 3699-3702.
(12) CCDC numbers: 614045 (4), 614046 (8), 614047 (9), 614042 (12),
614041 (13), 252066 (15), 252067 (16), 252091 (18), 252090 (19), 614040
(20), 614039 (32), 614044 (33), 614043 (37).
9432 J. Org. Chem., Vol. 71, No. 25, 2006

Total Synthesis of Dibromophakellstatin
SCHEME 3. Reaction of 3 with the Bis-Functional
because reaction of 3 with dry TosNClNa does not lead to the
formation of 9.
Reagents 11 and 14 (Ar ) 2-m-Xylyl)a
In a competitive process, 10 would undergo deprotonation,
affording the monoaminated product 8. Amination in the allylic
position C1, affording 7, may arise from a radical process, paral-
leling allylic bromination. Chloramine T forms N-centered radi-
cals at pH 5 in aqueous solutions at room temperature.14 Allylic
aminations of olefins by chloramine T have also been observed
as side reactions of metal-mediated aziridination reactions.15
The most facile ionic process (path A, Scheme 2) would arise
if the nitrogen of chloramine T acted as the electrophile with
displacement of chloride. In the crystal, TosNClNa‚3H2O is a
chloroimine rather than a chloroamine,16 and it is difficult to
judge on the polarization of that NsCl bond. After attack by
C10, deprotonation would immediately afford intermediate 10.
A more complicated ionic mechanism may commence with
the chlorination of C10 via a chloronium intermediate (path B).
Because we did not observe any chlorinated products, a C10
chloro substituent would have to be displaced by a tosylamino
group that would first attack at the intermediate acyliminium
ion (C10a), followed by intramolecular aziridine formation, ring
opening, and either deprotonation or attack of a second
equivalent of TosNH2. A 1,2-rearrangement of the amino group
of enamines on treatment with chloramine T has been observed
even in the absence of metals.17 In our case, the tosylamino
group would have to migrate. Stradi et al. explored related reac-
tions of N-chlorocarbamates with 1,2-diaminoalkenes affording
imidazolidines and with 1-aminoalkenes leading to aminochlo-
rinations with the chloro substituent in the â position.18
Although diamination of 3 was achieved with chloramine T,
the anti stereochemistry of the product 9 prevented its use as a
starting material for the assembly of the imidazolidinone ring
D of dibromophakellstatin (1). We did not observe any syn
product.
a (a) 11 (1 equiv), 1,4-dioxane, reflux, 12 h, 55% (12), 15% (13). (b)
HOAc-TFA (9:1), 50 °C, 24 h, 35%. (c) 14 (1.1 equiv), 1,4-dioxane, reflux,
8 h, 35% (15), 10% (16).
On treatment of tricycle 3 with 1 equiv of the mesylated
isourea derivative 14 under thermal conditions, the desired
cyclization to an imidazolidinone ring indeed occurred but with
the cost of ring B opening. We obtained the ring B-opened spiro-
tricycle 1612 incorporating a second equivalent of reagent 14
as a side product (10%), together with the C10-aminated product
15.12
Reaction of 3 with Ethyl-N-tosyloxycarbamate. In our
hands, it was not possible to cyclize enamines like 13 and 15
by formation of a bond between C10a and an the isourea
nitrogen. Alternatively, the final step of ring D formation could
be a condensation reaction to the urea unit.
We decided to investigate ethyl-N-tosyloxycarbamate (17)20
for electrophilic amination of C10,21 because we expected the
anion of 17 to undergo R-elimination slowly enough to use the
reagent as an electrophilic nitrene and as an N-nucleophile in
the same reaction. For the synthesis of 17, regioselective
tosylation of the distilled N-hydroxycarbamate proceeded best
in the presence of NaHCO3 as the base (95% yield) when
formation of the N,O-bistosylated side product was suppressed.
Reaction of the dibrominated pyrrolopyrazinone 3 was
performed under conditions reported by Tardella et al. employ-
ing 7 equiv of CaO as a heterogeneous base in dichloromethane,
together with 7 equiv of 17 (Scheme 4).22 Reagent 17 can also
be applied in two-phase aqueous systems in the presence of
phase-transfer catalysts with NaHCO3 as the base.23 We
observed that addition of water led to rapid initiation of the
reaction without influencing the yield or the composition of the
product mixture. The three stable products 18 (25%), 19 (20%),
and 20 (5%) were isolated, together with a fourth, less stable
product. For the workup, partitioning between saturated aqueous
Reaction with Isourea Precursors. We turned to the isourea-
derived nitrene sources 11 and 1419 anticipating that, after
electrophilic attack, a zwitterionic intermediate would be formed
that could undergo ring closure to an imidazoline. On irradiation
of 3 in the presence of 11 or 14 employing a medium-pressure
Hg lamp for 48 h, monoamination at C10 took place, affording
13 and 15 in low yields (Scheme 3). We went to thermal
conditions and treated tricycle 3 with 1 equiv of N-cyanoisourea
11 in refluxing 1,4-dioxane. To our surprise, we obtained the
unprecedented spiro-imidazoline 12 (structure confirmed by
X-ray analysis12) in 55% yield, together with 15% of its putative
precursor 13. It was possible to cyclize the N-cyanoisourea 13
to 12 on treatment with HOAc-TFA (9:1) at 50 °C. Compound
12 is formed from 1312 by nucleophilic attack of the enamide
portion at the cyano carbon. The percentage of 13 could be
enhanced to 25% when the reaction time was reduced from 12
to 4 h.
(13) (a) Campbell, M. M.; Johnson, G. Chem. ReV. 1978, 78, 65-79.
(b) Bowda, B. T.; Rao, R. V. J. Chem. Soc., Perkin Trans. 2 1988, 355-
361.
(14) Evans, J. C.; Jackson, S. K.; Rowlands, C. C.; Barratt, M. D.
Tetrahedron 1985, 41, 5191-5194.
(20) (a) Lwowski, W.; Maricich, T. J. J. Am. Chem. Soc. 1963, 85, 1200-
1202. (b) Lwowski, W.; Maricich, T. J. J. Am. Chem. Soc. 1965, 87, 3630-
3637.
(21) For a review on electrophilic amination of carbonyl compounds,
see: Erdik, E. Tetrahedron 2004, 60, 8747-8782.
(22) (a) Barani, M.; Fioravanti, S.; Pellacani, L.; Tardella, P. A.
Tetrahedron 1994, 50, 3829-3834. (b) Fioravanti, S.; Colantoni, D.;
Pellacani, L.; Tardella, P. A. J. Org. Chem. 2005, 70, 3296-3298, and
references cited therein.
(15) (a) Albone, D. P.; Aujila, P. S.; Taylor, P. C. J. Org. Chem. 1998,
63, 9569-9571. (b) Chanda, B. M.; Vyas, R.; Bedekar, A. V. J. Org. Chem.
2001, 66, 30-34.
(16) Olmstaed, M. M.; Power, P. P. Inorg. Chem. 1986, 25, 4057-4058.
(17) Dyong, I.; Lam-Chi, Q. Angew. Chem. 1979, 91, 997-998.
(18) Rossi, E.; Stradi, R.; Benedusi, A. Tetrahedron 1987, 43, 4785-
4791, and references cited therein.
(19) Subbaraj, A.; Rao, S.; Lwowski, W. J. Org. Chem. 1989, 54, 3945-
3952.
(23) Seno, M.; Namba, T.; Kise, H. J. Org. Chem. 1978, 43, 3345-
3348.
J. Org. Chem, Vol. 71, No. 25, 2006 9433

Zo¨llinger et al.
SCHEME 4. Synthesis of rac-Dibromophakellstatin (1)a
SCHEME 5. Possible Mechanisms for the Formation of the
Tetracycle 18a
a With NEt3 as base, no tetracycle is formed.
investigated. It is likely that the acyliminium ion 25 (Scheme
5) is an intermediate of this novel three-component reaction,
which itself could be formed by electrophilic attack of a nitrene
at C10.
Interestingly, no reaction took place on replacing CaO by
NEt3, although it has been shown that NEt3 is capable of
generating nitrenes from 17 by slow R-elimination.20 Nitrenes
are probably not involved in the reaction of 3 with 17 in the
presence of CaO. The acyliminium ion may alternatively be
formed by direct nucleophilic attack of C10 of 3 at an sp2-
hybridized nitrogen and substitution of the tosyloxy group. There
is precedence for the bimolecular reaction of enamines with
tosyloximes.26 A tosyloxime complex like 26 could be formed
from reagent 3 and Ca2+ liberated from CaO and traces of water.
NEt3H+ would fail to activate 17 toward direct attack by the
nucleophilic enamide 3.
Tetracycle 18 would be formed by nucleophilic syn attack
of the reagent 17 at the acyliminium ion 25, followed by
condensation to the imidazolidinone with loss of ethanol
(Scheme 5). It cannot be expected that an anti adduct (28) would
cyclize to an imidazolidinone, which would be about 20 kcal/
mol higher in energy than 18. Instead, competing elimination
of the tosyloxycarbamate 17 from 28 will occur, affording the
alkenyl carbamate 19. Although there is no direct proof of an
equilibrium between the anomeric syn and anti adducts,
exchange of nucleophiles at C10a has been shown for several
alcohols and for water.11
a (a) 17 (7 equiv), CaO (7 equiv), DCM, 23 °C, 24 h, 25% (18), 20%
(19), 5% (20). (b) SmI2 (5 equiv), THF, 23 °C, 24 h, then MeOH, 23 °C,
12 h, 76%. (c) SmI2 (2.5 equiv), THF, 23 °C, 15 min, 99%. (d) SmI2 (7.5
equiv), THF, 23 °C, 24 h, then MeOH, 23 °C, 12 h, 60%. (e) 17 (7 equiv),
CaO (7 equiv), DCM, 23 °C, 24 h, 50%.
NaHCO3 and dichloromethane was preferred over tedious
filtration.
The structures of products 18, 19, and 20 were secured by
crystal structure analysis.12 We did not succeed in characterizing
the fourth reaction product, which presumably is the open anti
adduct (Scheme 5), which, on treatment with aqueous ammonia,
afforded the alkenylcarbamate 19 by elimination.
On addition of 2.5 equiv of SmI2 to a solution of 18 in THF,
exclusive reduction of the NsOsbond occurs, affording tet-
racycle 21. The ethoxycarbonyl group is cleaved off on treatment
with an additional 2.5 equiv of SmI2 and methanolic workup,
affording dibromophakellstatin (1). With 7.5 equiv of SmI2, the
bromo substituent in the pyrrole R-position can be removed
selectively, providing monobromophakellstatin (22).
The minor product 20 (Scheme 4) is formed after a second
attack at C10, followed by deprotonation generating the C10ad
C1 double bond. Indeed, we were able to independently convert
19 to 20 in 50% yield on treatment with 7 equiv of 17.
Mechanism. â-Enaminoesters have been R-aminated em-
ploying NsONHCO2Et,24 and trialkylenamines may react as
ambident N/C nucleophiles.25 However, enamides have not been
Isolation of Bis-Adducts. We wondered whether it would
be possible to characterize ring-opened syn and anti bis-adducts
related to the putative intermediates 27 and 28 (Scheme 6).
An N-mesylated compound obtained from rac-longamide A
(29)27 did not react at all with the N-tosyloxycarbamate 17 in
(24) Felice, E.; Fioravanti, S.; Pellacani, L.; Tardella, P. A. Tetrahedron
Lett. 1999, 40, 4413-4416.
(25) Fioravanti, S.; Loreto, M. A.; Pellacani, L.; Tardella, P. A. J. Org.
Chem. 1985, 50, 5365-5368.
(26) (a) Schoeni, J.-P.; Fleury, J.-P. Tetrahedron 1975, 31, 671-678.
(b) Lang, M.; Schoeni, J.-P.; Pont, C.; Fleury, J.-P. HelV. Chim. Acta. 1986,
69, 793-802.
9434 J. Org. Chem., Vol. 71, No. 25, 2006

Total Synthesis of Dibromophakellstatin
SCHEME 6. Model Study with Bicyclic
Pyrrolopyrazinonesa
SCHEME 7. Access to rac-Dibromophakellstatin (1) via
Nonbrominated Intermediates.a
a (a) H2, Pd-C (5 mol %), NEt3 (2 equiv), MeOH-DCM (1:1), 6 h,
98%. (b) MsCl (2 equiv), DBU (4 equiv), DCM, 0 °C to rt, 24 h, 60%. (c)
17 (7 equiv), CaO (7 equiv), DCM, rt, 24 h, 60%. (d) NBS (2 equiv), DCM,
0 °C, 4 h, 92%. (e) SmI2 (5 equiv), THF, rt, 24 h, then MeOH, rt, 12 h,
85%.
The nonbrominated ABC tricycle 36 can be obtained from
pyrrole in four steps.8,10 However, because of the greater stability
of the dibrominated vs the non-brominated intermediates, we
chose to start from dibrominated N,O-acetal 2, which was
hydrogenated and then dehydrated.8 The reaction of 36 with
the TsONHCO2Et (17)/CaO proceeded completely, but the ratio
of products was different from the nonbrominated case. We were
pleased to observe that the yield of the nonbrominated tetracycle
37 was 60%, much higher than that for the dibrominated
tetracycle 22. We conclude that, fortunately, the absence of
pyrrole bromination indeed favors the syn over the anti anomer
(see Scheme 5). rac-phakellstatin was obtained on treatment of
37 with 5 equiv of SmI2. rac-dibromophakellstatin (1) was
synthesized from 37 by dibromination and deprotection with
SmI2 in the final step. Overall, our second generation synthesis
provides rac-dibromophakellstatin (1) in an 18% yield over 7
steps, calculated starting from readily available 4,5-dibromopy-
rrolyltrichloromethylketone28 as a precursor of tricycle 2.11 Our
procedure gives access to gram quantities of the natural product.
We probably would not have found this special synthesis if we
had first focused on the development of a general method.
Related Work with Carbamates. Pellacani et al. reported
the formation of structurally different imidazolidinones on
a (a) TosCl (2 equiv), DCM, NEt3 (6 equiv), rt, 24 h, 53%. (b) NaH (2
equiv), MeI (2 equiv), DMF, 0 °C to rt, 40 h, 73%. (c) 17 (7 equiv), CaO
(7 equiv), DCM, rt, 24 h, 32 (44%), 33 (11%). (d) DMAP (1 equiv),
pyridine, rt, 16 h, 80%.
the presence of CaO. Therefore, we investigated the behavior
of the N-methylated AB system 31, which was obtained in two
steps from 29 (Scheme 6). On treatment of 31 with 17, we
obtained the diastereomers 32 (44%) and 33 (11%), which were
separated by column chromatography and whose structures were
both confirmed by crystal structure analyses.12 However, there
was neither an alkenyl carbamate formed that would be an
analog of 19 (Scheme 4) nor any ABD tricycle that would
correspond to the tetracycle 18.
The absence of an alkenyl carbamate suggests that nucleo-
philic attack at C10a is faster at the bicycle AB than at the ABC
tricyclic acyliminium ion and overwhelms competing deproto-
nation. The absence of an ABD tricycle reflects the sterically
more relaxed situation in the bicyclic compared to the tricyclic
system.
t
reaction of NsONHCO2 Bu with â-ketoesters in the presence
of CaO.29 Mechanistically, Pellacani’s reaction is different from
the formation of our tetracycles 18 and 37 and presumably
involves a diaziridinone intermediate formed from tert-butoxy-
isocyanate and an unchanged reagent.
It was not possible to cyclize 32 or 33 to an ABD tricycle.
Competing â-elimination of TsOH occurs in DMAP/pyridine,
followed by tautomerization, affording the tetraaminoalkene 34
(Scheme 6).
The dependence of the behavior of arylsulfonyloxycarbamates
on the reaction conditions may further be illustrated by the
R-carbamoylation of â-ketoesters in the presence of NaH instead
of CaO, as discovered by Hanessian.30 tert-Butoxyisocyanate,
formed via a Lossen rearrangement, is assumed to be the
reacting species. Representing another mode of reactivity,
R-amination of carbonyl compounds may occur (Genet had
earlier identified BocNLiOTs,31 crystallized by Boche,32 as a
reagent for this).
Debromination Is the Key. The short total synthesis shown
in Scheme 4 already allowed the production of more than 200
mg of dibromophakellstatin (1). However, the low yield of 25%
of tetracycle 18 prompted us to investigate the effect of pyrrole
bromination on the imidazolidinone anellation. Molecular
modeling indicates that the energy differences between the syn
and anti adducts should be close to 1 kcal/mol for the di- and
nonbrominated compounds. Thus, we anticipated that subtle
effects such as pyrrole bromination might influence the syn/
anti ratio and perhaps even favor the syn adduct.
(28) Bailey, D. M.; Johnson, R. E. J. Med. Chem. 1973, 16, 1300-1302.
(29) Fioravanti, S.; Marchetti, F.; Morreale, A.; Pellacani, L.; Tardella,
P. A. Org. Lett. 2003, 5, 1019-1021.
(27) Lindel, T.; Hoffmann, H.; Hochgu¨rtel, M.; Pawlik, J. R. J. Chem.
Ecol. 2000, 26, 1477-1496.
(30) Hanessian, S.; Johnstone, S. J. Org. Chem. 1999, 64, 5896-5903.
J. Org. Chem, Vol. 71, No. 25, 2006 9435

Zo¨llinger et al.
evaporated to dryness. Recrystallization from MeOH provided
dipyrrolopyrazinone 3 (6.61 g, 85%) as a colorless solid. Mp: 156
1
°C. Rf (silica gel, EtOAc): 0.55. H NMR (CDCl3, 250 MHz): δ
2.20 (qi, 2H, 3J ) 7.1 Hz), 3.01 (dt, 2H, 3J ) 7.1, 7.1 Hz, 4J ) 1.4
3
Hz), 4.04 (t, 2H, J ) 7.1 Hz), 6.98 (s, 1H), 7.14 (br s, 1H). 13C
NMR (CDCl3, 63 MHz): δ 22.5, 28.6, 46.8, 100.8, 101.0, 103.3,
111.0, 124.8, 131.3, 153.1. MS (EI, 70 eV): m/z (%) 330/332/334
(50.0/100/50.0) [M+], 329/331/333 (10.0/25.0/20.0), 251/253 (27.1/
20.9). IR (KBr): ν˜ 3421, 3105, 2961, 2924, 1635, 1406, 1327, 1096,
795, 733. UV (CF3CH2OH): λmax (log ꢀ) 198 (4.00), 236 (4.67),
294 (4.31). Anal. Calcd for C10H8Br2N2O: C, 36.18; H, 2.43; N,
8.44. Found: C, 36.15; H, 2.58; N, 8.37.
(2,3-Dibromo-10-oxo-7,8-dihydro-6H,10H-dipyrrolo[1,2-a;1′,2′-
d]pyrazin-5-yl)-hydrazine-1,2-dicarboxylic Acid Diethyl Ester
(4). To a stirred solution of 3 (332 mg, 1.0 mmol) in CHCl3 (10
mL) was added DEAD (454 µL, 1.0 mmol, 40% in toluene). The
mixture was heated to 70 °C in a sealed tube for 12 h. After the
reaction was complete, the solution was evaporated to yield a yellow
crude product. After chromatography (silica gel, EtOAc/isohexane,
1:1), 4 (481 mg, 95%) was obtained as yellow crystals. Mp: 183
°C. Rf (silica gel, EtOAc): 0.80. The data for the major rotamer
follow. 1H NMR (CDCl3, 400 MHz): δ 1.23 (t, 3H, 3J ) 7.1 Hz),
1.29 (t, 3H, 3J ) 7.1 Hz), 2.13-2.23 (m, 2H), 3.00-3.08 (m, 1H),
3.63-3.73 (m, 1H), 4.00-4.06 (m, 1H), 4.09-4.16 (m, 1H), 4.23
FIGURE 2. CD spectra and assignment of both enantiomers of
dibromophakellstatin (1).
Biological Activity. When it comes to biological activity of
the pyrrole-imidazole alkaloids, knowledge is still limited.
Publications on the isolation and structure elucidation of the
pyrrole-imidazole alkaloids contain more information on their
biological activity than those reporting total syntheses. In theory,
total synthesis should overcome problems associated with
limited quantities of a natural product. We were pleased to
confirm the cytostatic activity reported by Pettit et al. for (-)-
dibromophakellstatin obtained by isolation from Phakellia
mauritiana. In a preliminary investigation, our synthesized
sample of rac-dibromophakellstatin (1) showed cytotstatic
effects against the cell lines SF-268 (brain cancer, ED50 1.4 µM),
NCI-H460 (large cell lung cancer, 2.1 µM), and KM20L2 (colon
cancer, 0.4 µM). This is important for the further development
of the pyrrole-imidazole alkaloids at the border between the
chemical and biological sciences.
3
3
(q, 2H, J ) 7.1 Hz), 4.26 (q, 2H, J ) 7.1 Hz), 6.89 (br s, 1H),
7.25 (s, 1H). 13C NMR (CDCl3, 100 MHz): δ 14.3, 14.6, 22.0,
29.1, 47.9, 62.6, 64.1, 97.4, 105.9, 113.1, 113.3, 126.6, 133.5, 152.5,
1
154.6, 155.9. The data for the minor rotamer follow. H NMR
(CDCl3, 400 MHz): δ 1.29 (t, 3H, 3J ) 7.1 Hz), 1.34 (t, 3H, 3J )
7.1 Hz), 2.13-2.23 (m, 2H), 3.10-3.17 (m, 1H), 3.58-3.63 (m,
3
1H), 4.00-4.06 (m, 1H), 4.09-4.16 (m, 1H), 4.26 (q, 2H, J )
3
7.1 Hz), 4.32 (q, 2H, J ) 7.1 Hz), 6.96 (br s, 1H), 7.24 (s, 1H).
13C NMR (CDCl3, 100 MHz): δ 14.4, 14.5, 21.9, 28.9, 47.9, 62.5,
64.1, 97.0, 105.9, 113.0, 113.2, 126.7, 134.1, 152.5, 154.1, 155.3.
MS (EI, 70 eV): m/z (%) 503.9/505.9/507.9 (54.3/100/53.4) [M+],
415.9/417.9/419.9 (27.1/55.7/28.0), 343.9/345.9/347.9 (40.4/74.2/
38.6), 328.9/330.9/332.9 (44.0/76.1/37.1), 308/310 (31.5/44.0). IR
(ATR): ν˜ 3330, 3030, 2957, 2872, 1790, 1700, 1650, 1350, 1300,
1194, 1180, 1001, 804, 710. UV (CHCl3): λmax (log ꢀ) 244 (4.53),
287 (3.87). HRMS (EI) calcd for C16H18Br2N4O5: 503.9644.
Found: 503.9650.
Separation of Enantiomers. Baseline separation of the
enantiomers was possible on a Chiralpak AD-H/45 HPLC
column.
By comparing the circular dichroism (CD) spectra with those
of other pyrrolopyrazinones,33 we identified the later eluting
compound as (-)-dibromophakellstatin ((-)-1). In 2,2,2-tri-
fluoroethanol, (-)-1 shows a trough at 248 nm (∆ꢀ ) -3.5),
together with a negative shoulder at 278 nm (∆ꢀ ) -1.0, Figure
2). As expected, the CD spectra of the urea (-)-dibro-
mophakellstatin and guanidine analogue (-)-dibromophakellin
are very similar above 240 nm, because the negative helicity
and syn position of substituents of the pyrrolopyrazinone partial
structure strongly dominate over the influence of more distant
functionalities. The values of optical rotations ([RD20] ) -74.1°
for (-)-1) were in agreement with our assignment.
1,7,8-Tribromo-2,3-dihydro-1H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-
5-one (5). To a stirred solution of 3 (332 mg, 1.0 mmol) in DCM
(30 mL) was added NBS (187 mg, 1.05 mmol) at 0 °C. After 1 h,
the reaction mixture was quenched with 2 N NaOH (25 mL) and
extracted with DCM (3 × 100 mL). The combined organic layers
were dried over MgSO4 and evaporated. The crude product was
purified by column chromatography (silica gel, EtOAc) and
obtained as a colorless solid (400 mg, 97%). Mp: 69 °C. Rf
1
(EtOAc): 0.75. H NMR (CDCl3, 400 MHz): δ 2.53-2.69 (m,
2H), 4.08-4.15 (m, 1H), 4.30-4.35 (m, 1H), 5.38-5.40 (m, 1H),
7.23 (s, 1H), 7.30 (br s, 1H). 13C NMR (CDCl3, 100 MHz): δ
35.2, 44.7, 44.8, 102.6, 103.8, 104.8, 112.5, 124.9, 132.4, 152.6.
MS (EI, 70 eV): m/z (%) 407.9/409.9/411.9/413.9 (35.5/100/96.5/
33.3) [M+], 330/332/334 (24.5/36.6/15.1), 329/331/333 (55.4/97.9/
50.6), 250/252 (15.6/17.1). IR (KBr): ν˜ 3445, 3118, 2925, 1684,
1639, 1408, 1366, 1290, 1172, 967, 539. UV (EtOH): λmax (log ꢀ)
200 (3.97), 252 (4.45), 290 (4.23). HRMS (EI) calcd for C10H7-
Br3N2O: 407.8108. Found: 407.8096.
Experimental Section
7,8-Dibromo-2,3-dihydro-1H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-
5-one (3). To a stirred solution of N,O-acetal 2 (8.0 g, 22.9 mmol)11
in pyridine (180 mL) was slowly added POCl3 (3.2 mL, 34.3 mmol)
at 0 °C under an argon atmosphere. After 2 h, water (10 mL) was
added and the reaction mixture was diluted with 6 N HCl (250
mL). The aqueous layer was extracted with DCM (3 × 300 mL).
The combined organic phases were dried over MgSO4 and
7,8,10-Tribromo-2,3-dihydro-1H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-
5-one (6). To a stirred solution of 3 (664 mg, 2.0 mmol) in pyridine
(5 mL) was added NBS (392 mg, 2.2 mmol) at 0 °C. After 90 min,
a second portion of NBS (178 mg, 1.0 mmol) was added, and the
mixture was stirred for 30 min and evaporated to dryness. The crude
product was purified by column chromatography (silica gel, EtOAc).
6 (698 mg, 85%) was obtained as a colorless solid. Mp: 101-102
°C. Rf (EtOAc): 0.63. 1H NMR (CDCl3, 400 MHz): δ 2.17-2.25
(31) Genet, J. P.; Mallart, S.; Greck, C.; Piveteau, E. Tetrahedron Lett.
1991, 32, 2359-2362.
(32) Boche, G.; Boie, C.; Bosold, F.; Harms, K.; Marsch, M. Angew.
Chem., Int. Ed. 1994, 33, 115-117.
(m, 2H), 3.01-3.05 (m, 2H), 4.11-4.14 (m, 2H), 7.25 (s, 1H). 13
NMR (CDCl3, 100 MHz): δ 21.5, 32.0, 48.5, 90.3, 102.4, 107.4,
C
(33) Jacquot, D. E. N.; Mayer, P.; Lindel, T. Chem. Eur. J. 2004, 10,
1141-1148.
9436 J. Org. Chem., Vol. 71, No. 25, 2006

Total Synthesis of Dibromophakellstatin
113.5, 127.5, 131.7, 152.3. MS (EI, 70 eV): m/z (%) 407.9/409.9/
411.9/413.9 (35.4/100/96.6/33.5) [M+], 330/332/334 (24.3/36.2/
14.9), 329/331/333 (54.4/98.7/49.7), 250/252 (15.6/17.0). IR
(ATR): ν˜ 3200, 2957, 2872, 1650, 1445, 1376, 1241, 1160, 1107,
806, 710. UV (EtOH): λmax (log ꢀ) 202 (3.91), 248 (4.36). HRMS
(EI) calcd for C10H7Br3N2O: 407.8108. Found: 407.8099.
(m, 3H), 8.67 (br s, 1H), 9.03 (br s, 1H). 13C NMR (DMSO-d6,
100 MHz): δ 16.0, 27.1, 44.3, 84.0, 103.0, 106.1, 108.1, 113.7,
125.4, 126.4, 128.4 (2C), 129.4 (2C), 138.9, 149.9, 151.5, 175.1,
183.2. MS (EI, 70 eV): m/z (%) 516.9/518.9/520.9 (45.0/90.8/44.1)
[M+], 438.0/440.0 (88.8/89.0), 439.0/ 441.0 (39.9/24.0), 369.9/
371.9/373.9 (16.8/30.0/13.0), 307.0/309.0 (36.9/36.9), 268.1 (82.6),
175.0 (100). IR (KBr): ν˜ 3434, 3064, 2925, 1640, 1556, 1474,
1436,1414, 1376, 1329, 1302. UV (DMSO): λmax (log ꢀ) 236 (3.32),
249 (2.92). HRMS (EI) calcd for C20H17Br2N5O2: 516.9749.
N-(7,8-Dibromo-5-oxo-2,3-dihydo-1H,5H-dipyrrolo[1,2-a;1′,2′-
d]pyrazin-1-yl)-4-methylbenzenesulfonamide (7), N-(2,3-Dibromo-
10-oxo-7,8-dihydro-6H,10H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-5-yl)-
4-methylbenzenesulfonamide (8), and Diaminated Product 9.
To a stirred solution of 3 (332 mg, 1.0 mmol) in THF (10 mL)
was added chloramine T trihydrate (844 mg, 3.0 mmol) at room
temperature. After 4 h, the reaction was quenched by the addition
of aqueous sodium carbonate (30 mL). The layers were separated,
and the aqueous layer was extracted with DCM (3 × 100 mL).
The combined organic layers were dried over MgSO4, filtered off,
and evaporated to yield a yellow crude product. Chromatography
(silica gel, EtOAc/isohexane, 1:1) gave the diaminated product 9
(369 mg, 55%), alkenylsulfonamide 8 (75 mg, 15%), and sulfona-
mide 7 (50 mg, 10%) as colorless solids. Mp: >190 °C decom-
position (7), >240 °C decomposition (8), >130 °C decomposition
(9). Rf (silica gel, EtOAc): 0.77 (9), 0.58 (8), 0.63 (7). Data for 7
1
Found: 516.9759. Data for E-/Z-isomers of 13 follow. H NMR
(DMSO-d6, 400 MHz): δ 1.95-2.08 (br s, 3H), 2.13-2.20 (m,
2H), 2.24 (s, 3H), 2.89-3.08 (m, 2H), 3.91-4.07 (m, 2H), 7.09-
7.18 (m, 3H), 7.25/7.27 (s, 1H), 11.0/11.4 (s, 1H). 13C NMR
(DMSO-d6, 100 MHz): δ 15.9 (br), 21.3/21.6, 27.7/28.0, 47.6/
47.7, 98.6/99.1, 105.2/105.3, 106.8/107.8, 111.1/113.1, 111.9/112.0,
126.1/126.3, 126.5/126.5, 128.8/128.8 (2C), 129.4/129.7 (2C),
131.4/132.7, 147.9/148.1, 151.4/151.6, 159.6/161.1. MS (EI, 70
eV): 516/518/520 (2.88/7.04/5.02) [M+], 438/440 (20.3/25.4), 329/
331/333 (15.2/27.5/20.9), 330/332/334 (55.9/100/55.9). IR (KBr):
ν˜ 3468, 3126, 2956, 2923, 2198, 1692, 1621, 1586, 1474, 1406,
1379, 1327, 1272, 1243. UV (CHCl3): λmax (log ꢀ) 243 (3.52), 286
(2.94). HRMS (EI) calcd for C20H17Br2N5O2: 516.9749. Found:
516.9767.
1
follow. H NMR (DMSO-d6, 400 MHz): δ 1.85-1.96 (m, 1H),
2.14-2.22 (m, 1H), 2.37 (s, 3H), 3.70 (m, 1H), 3.93-4.00 (m,
N-[1-(2,3-Dibromo-10-oxo-7,8-dihydro-6H,10H-dipyrrolo[1,2-
a;1′,2′-d]pyrazin-5-ylamino)-1-(2,6-dimethylphenoxy)methylidene]-
methanesulfonamide (15) and spiro-Cycle 16. Compound 3 (332
mg, 1.0 mmol) was suspended in dioxane (5 mL) and heated to 60
°C until the solution became clear. A solution of azide 14 (289
mg, 1.08 mmol)19 in dioxane (2 mL) was added. After refluxing
for 8 h, the reaction mixture was cooled to room temperature and
poured into brine (10 mL). The resulting solution was extracted
with DCM (3 × 100 mL). The combined organic layers were dried
over MgSO4 and evaporated. The crude product was purified by
column chromatography on silica gel, CHCl3/MeOH, (40:1). 15
(140 mg, 35%) and 16 (50 mg, 10%) were obtained as yellowish
solids. Mp: 210 °C (15), 130 °C (16). Rf (CHCl3/MeOH (9.75:
0.25)): 0.43 (15), 0.82 (16). Data for 15 follow. 1H NMR (CDCl3,
400 MHz): δ 2.06 (br s, 6H,), 2.25 (qi, 2H, 3J ) 7.3 Hz), 2.97 (s,
3H), 3.12 (dddd, 1H, 2J ) 9.9 Hz, 3J ) 8.1, 7.3 Hz, 5J ) 1.5 Hz),
3.16 (dddd, 1H, 2J ) 9.9 Hz, 3J ) 8.1, 7.3 Hz, 5J ) 1.1 Hz), 4.12
1H), 4.84-4.90 (m, 1H), 6.60 (s, 1H), 7.12 (s, 1H), 7.41 (d, 2H, 3J
3
3
) 8.0 Hz), 7.75 (d, 2H, J ) 8.0 Hz), 8.51 (d, 1H, J ) 8.2 Hz).
13C NMR (DMSO-d6, 100 MHz): δ 20.8, 29.9, 44.1, 53.7, 101.3,
101.4, 102.8, 110.3, 124.6, 126.2 (2C), 129.7 (2C), 132.2, 138.3,
143.0, 151.6. MS (ESI+, FT): m/z (%) 499.9/501.9/503.9 (51.5/
100/43.9) [M + H+]. IR (KBr): ν˜ 3436, 2923, 2854, 1634, 1435,
1409, 1392, 1339, 1159, 1090, 812, 739, 667, 546. UV (CHCl3):
λ
max (log ꢀ) 285 (3.78). HRMS (ESI) calcd for C17H1579Br81BrN3O3S
1
+ H: 501.9259. Found: 501.9256. Data for 8 follow. H NMR
(DMSO-d6, 400 MHz): δ 1.33-1.58 (m, 1H), 1.65-1.79 (m, 2H),
2.30-2.40 (m, 1H), 2.41 (s, 3H), 3.78-3.82 (m, 2H), 7.19 (s, 1H),
7.42 (d, 2H, 3J ) 8.3 Hz), 7.65 (d, 2H, 3J ) 8.3 Hz), 10.2 (s, 1H).
13C NMR (DMSO-d6, 100 MHz): δ 20.9, 21.6, 27.5, 47.2, 100.5,
105.3, 108.5, 111.5, 126.3, 126.8 (2C), 129.8 (2C), 132.3, 137.7,
143.6, 151.3. MS (EI, 70 eV): m/z (%) 498.9/500.9/502.9 (0.97/
2.32/1.08) [M+], 343.9/345.9/347.9 (51.1/88.2/45.3), 249.9/251.9/
253.9 (21.2/39.8/20.8), 91.1 (100). IR (ATR): ν˜ 3030, 3000, 2957,
2871, 1615, 1430, 1376, 1324, 1156, 1074, 804, 668. UV
(DMSO): λmax (log ꢀ) 293 (3.95). HRMS (EI) calcd for C17H15
79Br81BrN3O3S + H+: 501.9259. Found: 501.9256. Data for 9
3
(t, J ) 8.1 Hz, 2H), 7.00-7.07 (m, 3H), 7.28 (s, 1H), 8.96 (br s,
1H). 13C NMR (CDCl3, 100 MHz): δ 16.7, 22.2, 28.8, 43.1, 47.8,
99.2, 106.5, 107.7, 113.5, 126.6, 126.7, 128.9 (2C), 130.1 (2C),
131.5, 148.0, 152.4, 155.5. MS (EI, 70 eV): m/z (%) 570/572/574
(11.4/22.5/9.5) [M+], 491/493 (7.1/11), 369/371/373 (63.2/100/
49.4), 370/372/374 (13.7/21.3/7.3). IR (KBr): ν˜ 3436, 3260, 3120,
2926, 1692, 1654, 1618, 1587, 1476, 1369, 1328, 1283, 1165, 1133,
1091, 970, 900, 792, 770, 738, 623, 605, 562, 527. UV (CHCl3):
λmax (log ꢀ) 242 (1.45), 287 (0.86). HRMS (EI) calcd for C20H20-
1
follow. H NMR (DMSO-d6, 400 MHz): δ 1.30-1.42 (m, 1H),
1.70-1.72 (m, 1H), 1.90-1.97 (m, 1H), 2.00-2.08 (m, 1H), 2.37
(s, 6H), 3.23-3.32 (m, 1H), 3.44-3.52 (m, 1H), 6.30 (d, 1H, 3J )
9.5 Hz), 6.75 (s, 1H), 7.31-7.35 (m, 4H), 7.54-7.57 (m, 4H), 8.57
3
(s, 1H), 9.29 (d, 1H, J ) 9.5 Hz). 13C NMR (DMSO-d6, 100
1
MHz): δ 19.8, 20.8, 20.9, 33.1, 44.3, 66.6, 80.4, 100.5, 107.4,
114.4, 125.1, 125.7 (2C), 126.0 (2C), 129.2 (2C), 129.3 (2C), 138.5,
139.4, 142.6, 142.8, 153.9. MS (ESI+, FT): m/z (%) 668.9/670.9/
672.9 (66.9/100/53.0) [M + H+]. IR (KBr): ν˜ 3434, 3219, 1648,
1432, 1408, 1340, 1326, 1159, 1089, 812, 668, 555. UV (DMSO):
Br2N4O4S: 569.9572. Found: 569.9575. Data for 16 follow. H
NMR (CDCl3, 400 MHz): δ 1.87 (br s, 6H), 2.25-2.33 (m, 1H),
2.27 (s, 3H), 2.37 (s, 3H), 2.39-2.48 (m, 1H), 2.71 (s, 3H), 2.82-
2.87 (m, 1H), 2.94-3.05 (m, 1H), 3.29 (s, 3H), 3.94-4.01 (m,
4
1H), 4.11-4.15 (m, 1H), 6.74 (d, 1H, J ) 2.5 Hz), 6.89-6.96
λ
max (log ꢀ) 287 (3.96). HRMS (ESI-) calcd for C24H23Br2N4O5S2
(m, 3H), 7.09-7.18 (m, 3H), 9.51 (d, 1H, 4J ) 2.5 Hz). 13C NMR
(CDCl3, 100 MHz): δ 15.9, 16.1, 16.4, 23.6, 36.3, 41.9, 43.1, 49.8,
89.6, 100.6, 106.8, 116.4, 125.6, 125.8, 127.2, 128.3 (2C), 128.9,
129.0, 129.6, 129.9, 130.5 (2C), 148.7, 149.9, 158.4, 165.9, 170.9
175.6. MS (EI, 70 eV): m/z (%) 810/812/814 (0.27/0.53/0.34) [M+],
811/813/815 (0.06/0.14/0.0.07), 731/733/735 (0.22/0.34/0.12), 732/
734 (0.13/0.21), 689/691/693 (2.16/4.17/2.68), 367/369/370 (31.3/
44.6/18.1), 368/370 (10.2/12.9), 121/122/123 (41.4/100/8.5), 107
(40.6), 77 (30.3). IR (KBr): ν˜ 3405, 3189, 3144, 3007, 2958, 2926,
2888, 1710, 1622, 1598, 1581, 1528, 1475, 1417, 1367, 1305, 1274,
1238, 1171, 1144, 1096, 1057, 968, 808, 780, 764, 616, 564, 540,
528, 502. UV (CHCl3): λmax (log ꢀ) 283 (0.85). HRMS (EI) calcd
for C30H32Br2N6O7S2: 810.0141. Found: 810.0141.
- H: 668.9477. Found: 668.9441.
spiro-Tetracycle 12 and 1-(2,3-Dibromo-10-oxo-7,8-dihydro-
6H,10H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-5-yl)-2-(2,6-dimethylphe-
nyl)-3-cyanoisourea (13). To a stirred solution of 3 (332 mg, 1.0
mmol) in 1,4-dioxane (10.0 mL) was added 11 (215 mg, 1.0
mmol)19 at room temperature. The reaction mixture was refluxed
for 12 h. The solvent was evaporated to yield a brown precipitate,
which was purified by column chromatography (silica gel, EtOAc/
isohexane, 8:2). spiro-Tetracycle 12 (286 mg, 55%) and alkenyli-
sourea 13 (78 mg, 15%) were obtained as colorless solids. Mp:
>199 °C decomposition (12), >186 °C decomposition (13). Rf
1
(silica gel, EtOAc): 0.23 (12), 0.40 (13). Data for 12 follow. H
NMR (DMSO-d6, 400 MHz): δ 2.14 (s, 6H), 2.56-2.76 (m, 2H),
3.83-4.00 (m, 2H), 5.13-5.14 (m, 1H), 6.98 (s, 1H), 7.00-7.07
(2,3-Dibromo-10-oxo-7,8-dihydro-6H,10H-dipyrrolo[1,2-a;1′,2′-
d]pyrazin-5-yl)carbamic Acid Ethyl Ester (19), rac-N-Ethoxy-
J. Org. Chem, Vol. 71, No. 25, 2006 9437

Zo¨llinger et al.
carbonyl-N′tosyloxydibromophakellstatin (18), and (2,3-Dibromo-
5-ethoxycarbonylamino-10-oxo-7,8-dihydro-5H,10H-dipyrrolo[1,2-
a;1′,2′-d]pyrazin-5-yl)carbamic Acid Ethyl Ester (20). CaO (1.06
g, 19.0 mmol) was added to a solution of pyrazinone 3 (0.9 g, 2.7
mmol) and urethane 17 (4.90 g, 19.0 mmol)20 in DCM (250 mL)
under an argon atmosphere. Within 15 min, the suspension became
orange. After stirring at room temperature for 24 h, the reaction
mixture was filtered off and the precipitate was washed with DCM.
The combined organic layers were evaporated to a yield a yellow
solid. Chromatography (silica gel, EtOAc/isohexane, 7:3) gave
tetracycle 18 (427 mg, 25%), carbamate 19 (226 mg, 20%), and
orthoamidine 20 (68 mg, 5%) as colorless solids. Mp: 193 °C (18),
187 °C (19), 196 °C (20). Rf (silica gel, EtOAc): 0.49 (18), 0.38
reaction mixture was stirred for 36 h and then quenched with MeOH
(5.0 mL). After an additional 4 h, the reaction mixture was
evaporated to dryness. Purification by flash chromatography (silica
gel, CHCl3/MeOH, 9:1) afforded rac-dibromophakellstatin (rac-1,
179 mg, 76%) as a colorless solid. Rf (silica gel, CHCl3/MeOH,
9:1): 0.28. 1H NMR (DMSO-d6, 400 MHz): δ 1.93-2.01 (m, 1H),
2.07-2.14 (m, 2H), 2.26-2.33 (m, 1H), 3.39-3.46 (m, 1H), 3.53-
3
3.58 (m, 1H), 5.99 (d, 1H, J ) 2.2 Hz), 6.91 (s, 1H), 7.98 (br s,
1H), 8.28 (br s, 1H). 13C NMR (DMSO-d6, 100 MHz): δ 18.7,
38.7, 44.0, 68.5, 78.9, 101.0, 105.4, 113.7, 125.3, 153.9, 157.7.
MS (EI, 70 eV): m/z (%) 388/390/392 (10.9/22.5/10.0) [M+], 317/
319/321 (2.25/6.07/2.69), 250/252/254 (4.02/7.36/4.30), 139 (100).
HRMS (EI) calcd for C11H10Br2N4O2: 387.9170. Found: 387.9154.
rac-Monobromophakellstatin (22). To a solution of 18 (63 mg,
0.1 mmol) in THF (10 mL) was added SmI2 (7.5 mL, 0.1 M in
THF) at room temperature under an argon atmosphere. The reaction
mixture was stirred for 36 h and then quenched with MeOH (1.0
mL). After an additional 4 h, the reaction mixture was evaporated
to dryness. Purification by flash chromatography (silica gel, CHCl3/
MeOH, 9:1) yielded monobromophakellstatin (22, 18 mg, 60%)
as a colorless solid. Mp: 185 °C. Rf (silica gel, CHCl3/MeOH,
9:1): 0.22. 1H NMR (DMSO-d6, 400 MHz): δ 1.93-2.01 (m, 1H),
2.07-2.14 (m, 2H), 2.26-2.33 (m, 1H), 3.39-3.46 (m, 1H), 3.53-
1
(19), 0.33 (20). Data for 18 follow. H NMR (CDCl3, 400 MHz):
δ 1.36 (t, 3H, 3J ) 7.2 Hz), 2.09 (ddd, 1H, 2J ) 12.4 Hz, 3J ) 8.0,
4.0 Hz), 2.24-2.39 (m, 2H), 2.42 (s, 3H), 2.43-2.50 (m, 1H),
2
3
3.70-3.78 (m, 1H), 3.87 (ddd, 1H, J ) 12.0 Hz, J ) 8.0, 4.0
Hz), 4.36 (qd, 1H, 2J ) 10.6 Hz, 3J ) 7.2 Hz), 4.43 (qd, 1H, 2J )
3
3
10.6 Hz, J ) 7.2 Hz), 6.27 (s, 1H), 7.12 (s, 1H), 7.22 (d, 2H, J
) 8.2 Hz), 7.57 (d, 2H, J ) 8.2 Hz). 13C NMR (CDCl3, 100
3
MHz): δ 14.2, 20.8, 21.8, 34.1, 46.1, 65.1, 68.3, 86.0, 104.4, 105.6,
117.0, 126.7, 129.3 (2C), 129.9 (2C), 146.9, 149.9, 150.4, 153.9.
MS (FAB, NBA): m/z (%) 631/633/635 (30/60/30) [M + H+]. IR
(KBr): ν˜ 3436, 2983, 1803, 1743, 1670, 1552, 1388, 1273, 1015,
722, 545. UV (CH3OH): λmax (log ꢀ) 289 (3.92), 231 (4.27). HRMS
(FAB) calcd for C21H20Br2N4O7S + H: 629.9420. Found: 629.9429.
Data for 19 follow. 1H NMR (CDCl3, 400 MHz): δ 1.35 (t, 3H, 3J
4
3.58 (m, 1H), 5.77 (br s, 1H), 6.71 (d, 1H, J ) 1.8 Hz), 7.24 (d,
4
1H, J ) 1.8 Hz), 7.88 (br s, 1H), 8.08 (br s, 1H). 13C NMR
(DMSO-d6, 100 MHz): δ 19.1, 38.4, 44.4, 67.9, 78.4, 97.1, 112.4,
121.1, 124.0, 154.3, 158.3. MS (EI, 70 eV): m/z (%) 310/312 (62.9/
60.5) [M+], 282/284 (5.64/5.17), 293/241 (9.69/9.68), 172/174
(21.7/21.0), 139 (100). IR (KBr): ν˜ 3422, 3255, 2926, 1724, 1627,
1551, 1481, 1438, 1188, 926. UV (DMSO): λmax (log ꢀ) 275 (7.26).
HRMS (EI) calcd for C11H11BrN4O2: 310.0065. Found: 310.0072.
6,7-Dibromo-2H-pyrrolo[1,2-a]pyrazin-1-one (30). rac-Lon-
gamide A (29, 7.10 g, 22.9 mmol)27 was suspended in dry DCM
(190 mL) and treated with p-TsCl (8.80 g, 46.2 mmol) at room
temperature. Triethylamine (15.6 mL, 112 mmol) was added via a
dropping funnel within 30 min at 0 °C. After 24 h at room
temperature, the solvent was evaporated. Purification by column
chromatography (silica gel, CHCl3/MeOH, 20:1) yielded 30 as
colorless crystals (3.54 g, 53%). Mp: 140-142 °C. Rf (silica gel,
3
3
) 7.2 Hz), 2.19 (qi, 2H, J ) 7.3 Hz), 2.97 (t, 2H, J ) 7.3 Hz),
4.08 (t, 2H, 3J ) 7.3 Hz), 4.28 (q, 2H, 3J ) 7.2 Hz), 6.29 (s, 1H),
7.23 (s, 1H). 13C NMR (CDCl3, 100 MHz): δ 14.6, 21.9, 28.3,
47.8, 62.5, 106.1, 109.0, 111.2, 113.0, 117.1, 126.6, 152.7, 154.4.
MS (EI, 70 eV): m/z (%) 417/419/421 (45/85/42) [M+], 344/346/
348 (30/59/30), 339/341 (27/24), 338/340 (48/49), 250/252/254 (33/
60/31), 106 (30), 95 (100), 83 (44), 41 (26). IR (KBr): ν˜ 3411,
3264, 2981, 1737, 1622, 1525, 1414, 1382, 1330, 1236, 1067, 738.
UV (CH3CN): λmax (log ꢀ) 283 (3.66), 236 (4.11). HRMS (EI)
calcd for C13H13Br2N3O3: 416.9324. Found: 416.9298. Data for
20 follow. 1H NMR (DMSO-d6, 400 MHz): δ 1.00-1.20 (m, 6H),
2.61-2.66 (m, 2H), 3.90-3.94 (m, 6H), 5.41 (t, 1H, 3J ) 2.8 Hz),
6.80 (s, 1H), 8.64 (br s, 2H). 13C NMR (DMSO-d6, 100 MHz): δ
14.0, 26.8, 44.4, 60.8, 75.9, 102.6, 102.7, 108.4, 111.7, 128.9, 142.5,
150.8, 152.4. MS (FAB, NBA): m/z (%) 505.3/507.3/509.3 (6.52/
8.67/4.32) [M + H+]. IR (KBr): ν˜ 3422, 3286, 2981, 1742, 1629,
1561, 1481, 1377, 1252. UV (DMSO): λmax (log ꢀ) 266 (4.08),
231 (4.08), 300 (4.05). HRMS (FAB) calcd for C16H18Br2N4O5 +
H: 504.9692. Found: 504.9722.
1
CHCl3/MeOH, 10:1): 0.43. H NMR (DMSO-d6, 300 MHz): δ
6.78 (dd, 1H, 3J ) 5.7, 5.7 Hz), 7.15 (s, 1H), 7.16 (d, 1H, 3J ) 5.6
Hz), 10.88 (br s, 1H). 13C NMR (DMSO-d6, 63 MHz): δ 101.6,
102.8, 105.6, 111.0, 116.5, 125.6, 154.1. MS (EI, 70 eV): m/z (%)
290.0/292.0/294.0 (51.9/100.0/49.3) [M+], 235.0/237.0/239.0 (5.31/
10.0/5.22). IR (KBr): ν˜ 3116, 3048, 2956, 2908, 1656, 1441, 1416,
1376, 1356, 1285, 1202, 1130, 973, 903, 806, 719. UV (CHCl3):
λmax (log ꢀ) 204 (3.82), 234 (4.35), 278 (3.95). HRMS (EI) calcd
for C7H479Br81BrN2O: 291.8670. Found: 291.8669.
rac-N-Ethoxycarbonyldibromophakellstatin (21). To a solution
of 18 (63 mg, 0.1 mmol) in dry THF (10 mL) was added SmI2
(2.5 mL, 0.1 M solution in THF) at room temperature under an
argon atmosphere. The reaction mixture was stirred for 10 min and
then quenched with MeOH (1.0 mL). The reaction mixture was
evaporated to dryness, and the crude product was purified by flash
chromatography (silica gel, CHCl3/MeOH, 9:1), yielding rac-N-
ethoxycarbonyldibromophakellstatin (21, 46 mg, 99%) as a colorless
solid. Mp: 201 °C. Rf (silica gel, CHCl3/MeOH, 9:1): 0.36. Data
6,7-Dibromo-2-methyl-2H-pyrrolo[1,2-a]pyrazin-1-one (31).
To a stirred suspension of 30 (1.50 g, 5.14 mmol) in DMF (15.0
mL) was added NaH (250 mg, 10.3 mmol, 60% in mineral oil) at
0 °C under an argon atmosphere. After 30 min, MeI (650 µL, 10.3
mmol) was added via cannula. The reaction mixture was stirred
for an additional 40 h at room temperature. It was quenched by
the addition of water (50 mL). The suspension was extracted with
DCM (3 × 150 mL), and the combined organic layers were
evaporated to dryness. The crude product was suspended in water
(10 mL). Filtration gave 31 (1.10 g, 73%) as a colorless solid. Mp:
145-146 °C. Rf (silica gel, CHCl3/MeOH (10:1)): 0.78. 1H NMR
(DMSO-d6, 400 MHz): δ 3.33 (s, 3H), 7.03 (d, 1H, 3J ) 5.9 Hz),
1
3
for 21 follow. H NMR (DMSO-d6, 400 MHz): δ 1.28 (t, 3H, J
) 7.2 Hz), 1.97-2.05 (m, 2H), 2.19-2.24 (m, 1H), 2.26-2.35
(m, 1H), 3.37-3.44 (m, 1H), 3.58-3.63 (m, 1H), 4.23-4.33 (m,
2H), 6.69 (d, 1H, J ) 0.7 Hz), 7.00 (s, 1H), 9.06 (s, 1H). 13C
4
NMR (DMSO-d6, 100 MHz): δ 13.9, 18.9, 37.7, 44.8, 62.7, 71.3,
77.5, 102.7, 105.6, 114.6, 126.4, 150.1, 150.8, 153.4. MS (EI, 70
eV): m/z (%) 464/462/460 (10.9/22.3/10.2) [M+], 421/419/417
(27.3/58.4/29.6), 375/373/371 (11.4/19.2/9.85), 95.1 (100). IR
(KBr): ν˜ 3430, 3195, 3133, 2991, 1767, 1727, 1637, 1556, 1408,
1303, 1124, 1016, 738. UV (DMSO): λmax (log ꢀ) 283 (6.90).
HRMS (EI) calcd for C14 H14Br2N4O4: 459.9382. Found: 459.9370.
rac-Dibromophakellstatin (rac-1). To a solution of 18 (380 mg,
0.60 mmol) in dry THF (60 mL) was added SmI2 (30 mL, 0.1 M
in THF) at room temperature under an argon atmosphere. The
5
3
5
7.15 (d, 1H, J ) 0.7 Hz), 7.27 (dd, 1H, J ) 5.9 Hz, J ) 0.7
Hz). 13C NMR (DMSO-d6, 100 MHz): δ 34.5, 100.9, 102.9, 105.4,
110.6, 121.0, 124.9, 153.4. MS (EI, 70 eV): m/z (%) 304.1/306.1/
308.0 (48.9/100/45.0) [M+], 225.1/227.1 (26.4/25.7). IR (KBr): ν˜
3118, 1709, 1665, 1448, 1406, 1381, 1364, 1245, 1077, 730. UV
(DMSO): λmax (log ꢀ) 285 (3.89). HRMS (EI) calcd for C8H6-
Br2N2O: 303.8847. Found: 303.8841.
Syn-AB-Adduct (33) and anti-AB-Adduct (32). To a stirred
solution of 31 (800 mg, 2.62 mmol) in DCM (43 mL) was added
9438 J. Org. Chem., Vol. 71, No. 25, 2006

Total Synthesis of Dibromophakellstatin
EtO2CNHOTs (17, 4.67 g, 18.3 mmol) at room temperature. After
20 min, CaO (1.03 g, 18.3 mmol) was added and the suspension
was stirred for an additional 24 h. Filtration and evaporation of the
mixture yielded a pale yellow crude product, which was purified
by column chromatography (silica gel, EtOAc/isohexane, 1:1).
Products 32 (752 mg, 44%) and 33 (188 mg, 11%) were obtained
as colorless foams. Mp: 156-158 °C (32), 159-161 °C (33). Rf
(silica gel, DCM/EtOAc, 9:1): 0.45 (33), 0.15 (32). Data for 32
124.1, 156.5. MS (EI, 70 eV): m/z (%) 192.0 (56) [M+], 95 (14),
94 (47), 70 (100). IR (KBr): ν˜ 3125, 2967, 2887, 1610, 1547, 1446,
1368, 1329, 1269, 1220, 1155, 1112, 1061, 1026, 963, 744, 653.
UV (CF3CH2OH): λmax (log ꢀ) 198 (3.90), 230 (3.92), 278 (3.98).
HRMS (EI) calcd for C10H12N2O2 192.0899. Found: 192.0904.
2,3-Dihydro-1H-dipyrrolo[1,2-a;1′,2′-d]pyrazin-5-one (36). To
a stirred suspension of the N,O-acetal 35 (750 mg, 3.9 mmol) in
dry DCM was added MsCl (604 µL, 7.8 mmol) at 0 °C. DBU (2.32
mL, 15.6 mmol) was added within 30 min, and the solution was
allowed to warm to room temperature. After 24 h, 2 N HCl (50
mL) was added and the layers were separated. The aqueous layer
was extracted with DCM (3 × 100 mL) and the combined organic
phases were dried over MgSO4. Evaporation of the solvent gave a
brown crude product, which was purified by column chromatog-
raphy (silica gel, EtOAc) to yield dipyrrolopyrazinone 36 (407 mg,
60%)8 as colorless crystals. Mp: 143 °C. Rf (silica gel, EtOAc):
0.34. H NMR (CDCl3, 400 MHz): δ 2.16 (q, 2H, J ) 7.3 Hz),
2.93 (dt, 2H, 3J ) 7.3 Hz, 4J ) 1.4 Hz), 4.02 (t, 2H, 3J ) 7.1 Hz),
6.50-6.52 (m, 1H), 6.91 (br s, 1H), 7.03-7.05 (m, 2H). 13C NMR
(CDCl3, 100 MHz): δ 22.8, 28.2, 46.3, 102.6, 108.8, 112.0, 117.7,
123.9, 129.8, 155.2. MS (EI, 70 eV): m/z (%) 174/175 (100/14)
[M+], 145 (7), 118 (12). IR (KBr): ν˜ 3432, 3013, 2971, 1691, 1630,
1372, 1284, 1183, 1022, 876, 729, 595. UV (CF3CH2OH): λmax
(log ꢀ) 200 (4.76), 228 (4.64), 284 (3.94). EA calcd for
C10H10N2O: C, 68.95; H, 5.79; N, 16.08. Found: C, 68.75; H, 5.77;
N, 15.99.
1
3
follow. H NMR (DMSO-d6, 400 MHz): δ 0.95 (t, 3H, J ) 7.0
3
Hz), 1.18 (t, 3H, J ) 7.0 Hz), 2.42 (s, 3H), 2.93 (s, 3H), 3.89-
4.15 (m, 4H), 5.81 (s, 1H), 6.05 (br s, 1H), 6.90 (s, 1H), 7.47 (d,
2H, 3J ) 8.5 Hz), 7.65 (d, 2H, 3J ) 8.5 Hz), 8.75 (br d, 1H, 3J )
8.0 Hz). 13C NMR (DMSO-d6, 100 MHz): δ 13.3, 14.2, 21.1, 32.2,
60.6, 61.6, 64.3, 77.9, 100.8, 105.4, 114.3, 125.3, 127.8, 128.8 (2C),
129.9 (2C), 146.5, 154.4, 155.3, 155.8. MS (ESI+, FT): m/z (%)
651/653/655 (50.0/100/47.3) [M+]. IR (KBr): ν˜ 3192, 2983, 1732,
1704, 1651, 1521, 1435, 1394, 1370, 1313, 1237, 1194, 1048, 660.
UV (DMSO): λmax (log ꢀ) 278 (4.01). HRMS (ESI+) calcd for
C21H24Br2N4O8S + H: 652.9741. Found: 652.9736. Data for 33
1
3
1
3
follow. H NMR (DMSO-d6, 400 MHz): δ 0.91 (t, 3H, J ) 7.1
Hz), 1.19 (t, 3H, 3J ) 7.1 Hz), 2.43 (s, 3H), 2.96 (s, 3H), 3.81 (q,
2H, 3J ) 7.1 Hz), 4.11 (q, 2H, 3J ) 7.1 Hz), 5.91 (d, 1H, 3J ) 5.8
Hz), 6.51 (dd, 1H, 3J ) 10.1, 5.8 Hz), 6.89 (s, 1H), 7.49 (d, 2H, 3J
) 8.4 Hz), 7.77 (d, 2H, J ) 8.4 Hz), 7.83 (br s, 1H). 13C NMR
3
(DMSO-d6, 100 MHz): δ 13.3, 14.2, 21.1, 30.5, 58.5, 60.8, 62.1,
102.3, 105.4, 112.7, 123.7, 127.8, 125.3 (2C), 127.8 (2C), 146.4,
154.4, 154.6, 154.9. MS (ESI+, FT): m/z (%) 651/653/655 (52.0/
100/49) [M + H+]. IR (KBr): ν˜ 3233, 2980, 1723, 1650, 1593,
1530, 1422, 1435, 1387, 1371, 1296, 1193, 1180, 1053, 812, 724,
652. UV (DMSO): λmax (log ꢀ) ) 282 (3.71). HRMS (ESI+) calcd
for C21H24Br2N4O8S + H: 652.9741. Found: 652.9738.
rac-N-Ethoxycarbonyl-N′-tosyloxyphakellstatin (37). CaO (1.68
g, 30 mmol) was added to a solution of pyrazinone 36 (870 mg,
5.0 mmol) and EtO2CNHOTs (17, 7.77 g, 30.0 mmol)20 in DCM
(150 mL) at room temperature, followed by water (270 µL, 15.0
mmol). Within 15 min, the suspension became orange. After stirring
at room temperature for 24 h, a saturated solution of NaHCO3 (200
mL) was added. The separated aqueous layer was extracted with
DCM (3 × 150 mL). The combined organic layers were dried over
MgSO4 and evaporated to a yellow solid. The crude product was
purified by column chromatography (silica gel, EtOAc/isohexane,
1:1) to give tetracycle 37 (1.42 g, 60%) as a light yellow solid.
Mp: 197-199 °C. Rf (silica gel, EtOAc): 0.65. 1H NMR (CDCl3,
400 MHz): δ 1.39 (t, 3H, 3J ) 7.1 Hz), 2.04-2.14 (m, 1H), 2.27-
2.37 (m, 2H), 2.41 (s, 3H), 2.48-2.54 (m, 1H), 3.66-3.73 (m,
1H), 3.78-3.84 (m, 1H), 4.36-4.47 (m, 2H), 6.08 (s, 1H), 6.32-
6.34 (m, 1H), 6.84-6.85 (m, 1H), 7.02-7.04 (m, 1H), 7.21 (d,
(6,7-Dibromo-4-ethoxycarbonylamino-2-methyl-1-oxo-1,2-di-
hydropyrrolo[1,2-a]pyrazin-3-yl)-carbamic Acid Ethyl Ester
(34). To a stirred solution of the 1:4 mixture of 32 and 33 (100
mg, 0.15 mmol) in pyridine (2 mL) was added DMAP (18.3 mg,
0.15 mmol) at room temperature. After 16 h, the mixture was diluted
with 2 N HCl and then extracted with DCM (3 × 50 mL). The
combined organic layers were dried over MgSO4 and filtered off.
The solvent was evaporated under reduced pressure, and a brown
precipitate was obtained as crude product. Chromatography (silica
gel, EtOAc/isohexane, 1:1) gave 34 (58 mg, 80%) as a colorless
1
solid. Mp: 190-193 °C. Rf (silica gel, EtOAc): 0.80. H NMR
3
3
(CDCl3, 400 MHz): δ 1.24-1.38 (m, 6H), 3.41 (s, 3H), 4.17-
4.27 (m, 4H), 6.71 (br s, 1H), 6.87 (br s, 1H), 7.25 (s, 1H). 13C
NMR (CDCl3, 100 MHz): δ 13.4, 14.1, 28.3, 58.3, 59.1, 92.1,
101.2, 103.1, 108.3, 114.3, 127.9, 152.3, 153.1, 158.2. MS (ESI+,
FT): m/z (%) 478.9/480.9/482.9 (55.4/100/45.5) [M + H+]. IR
(KBr): ν˜ 3233, 2980, 1723, 1650, 1593, 1530, 1422, 1435, 1387,
1371, 1296, 1193, 1180, 1053, 812, 724, 652. UV (CHCl3): λmax
(log ꢀ) 251 (4.13). HRMS (ESI+) calcd for C14H16Br2N4O5 + H:
478.9566. Found: 478.9557.
2H, J ) 8.4 Hz), 7.58 (d, 2H, J ) 8.4 Hz). 13C NMR (CDCl3,
100 MHz): δ 14.1, 21.0, 21.9, 35.1, 45.7, 65.0, 67.0, 84.1, 112.0,
114.3, 122.4, 124.1, 129.3 (2C), 129.4, 129.9 (2C), 146.7, 149.6,
151.1, 155.7. MS (ESI+, FT): m/z (%) 576.2/577.2 (100/26.4) [M
+ TEA + H+]. IR (KBr): ν˜ 3123, 2985, 1785, 1720, 1600, 1553,
1338, 1275. UV (DMSO): λmax (log ꢀ) 229 (4.28), 276 (3.96).
HRMS (ESI+) calcd for C14H16Br2N4O5+C6H15N + H: 576.2492.
Found: 576.2472.
Conversion of 37 to 18. To a solution of 37 (475 mg, 1.0 mmol)
in DCM (10 mL) was added NBS (365 mg, 2.05 mmol) at 0 °C.
After 4 h, the reaction mixture was evaporated to dryness and the
crude product was purified by column chromatography (silica gel,
EtOAc/isohexane, 6:4). Tetracycle 18 (578 mg, 92%) was obtained
as a colorless solid. For characterization data, see above.
10-Hydroxy-2,3,10,10a-tetrahydro-1H-dipyrrolo[1,2-a;1′,2′-
d]pyrazin-5-one (35). To a stirred solution of 2 (4.0 g, 11.4 mmol)33
in DCM/MeOH (200 mL, 1:1) was added NEt3 (3.2 mL, 22.8 mmol)
and 10% Pd/C (600 mg, 0.57 mmol, 5 mol %) at room temperature.
The mixture was kept under hydrogen until the reaction was
complete (ca. 6 h). Brine (150 mL) was added, followed by
extraction with DCM (3 × 300 mL). The combined organic layers
were dried over MgSO4, and the solvent was evaporated under
reduced pressure. Recrystallization of the crude product from MeOH
yielded 35 (2.15 g, 98%) as colorless crystals. Mp: 180-181 °C.
Acknowledgment. We thank Xaver Wurzenberger for
experimental assistance and George R. Pettit for performing the
biological assays. Heiko Tietgen (Sanofi-Aventis GmbH) is
thanked for preparative chromatography on a chiral HPLC
column.
23
Rf (silica gel, EtOAc): 0.25. [R]D ) +116° (c ) 10.0 mg/mL,
1
MeOH). H NMR (DMSO-d6, 400 MHz): δ 1.76-2.26 (m, 4H),
Supporting Information Available: Complete experimental
details and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
3.30-3.44 (m, 1H), 3.50-3.58 (m, 1H), 3.98 (m, 1H), 5.56 (dd,
3
1H, J ) 7.0, 2.8 Hz), 6.13-6.15 (m, 1H), 6.58-6.60 (m, 1H),
6.64 (d, 1H, 3J ) 7.0), 7.01-7.05 (m, 1H). 13C NMR (DMSO-d6,
100 MHz): δ 22.6, 26.7, 43.8, 60.4, 75.4, 109.1, 110.9, 122.7,
JO061813U
J. Org. Chem, Vol. 71, No. 25, 2006 9439