Carbodiimide-mediated preparation of the tricyclic pyrido[3′,2′:4,5]pyrrolo[1,2-c]pyrimidine ring system and its application to the synthesis of the potent antitumoral marine alkaloid variolin B and analog

Article abstract of DOI:10.1021/jo026508x

A total synthesis of the marine alkaloid variolin B has been completed in 13 steps in an overall yield of 6.5% from 3-formyl-4-methoxypyridine. Our approach is based on the sequential formation of the 7-azaindole ring, the tricyclic pyrido[3′,2′:4,5]pyrrolo[1,2-c]pyrimidine ring system, and finally installation of the 2-aminopyrimidine ring at C5. The required 7-azaindole ring appropriately substituted is formed by a modified indole synthesis involving a nitrene insertion process (two steps). Formation of the annelated pyrimidine ring is achieved by two routes both involving a carbodiimide-mediated cyclization process, which allow incorporation of the amine functionality at C9 of the core tricyclic (six steps). Installation of the northeast 2-aminopyrimidine ring at C5 is performed using the Bredereck protocol (three steps). Ultimate, thermal decarboxylation with concomitant O-methyl deprotection and further N-benzyl deprotection by the action of triflic acid completed the synthesis of the target natural product variolin B.

Full text of DOI:10.1021/jo026508x

Ca r bod iim id e-Med ia ted P r ep a r a tion of th e Tr icyclic
P yr id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in e Rin g System a n d Its
Ap p lica tion to th e Syn th esis of th e P oten t An titu m or a l Ma r in e
Alk a loid Va r iolin B a n d An a log
Pedro Molina,* Pilar M. Fresneda,* and Santiago Delgado
Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de Murcia,
Campus de Espinardo, E-30100 Murcia, Spain
pmolina@um.es
Received October 2, 2002
A total synthesis of the marine alkaloid variolin B has been completed in 13 steps in an overall
yield of 6.5% from 3-formyl-4-methoxypyridine. Our approach is based on the sequential formation
of the 7-azaindole ring, the tricyclic pyrido[3’,2’:4,5]pyrrolo[1,2-c]pyrimidine ring system, and finally
installation of the 2-aminopyrimidine ring at C5. The required 7-azaindole ring appropriately
substituted is formed by a modified indole synthesis involving a nitrene insertion process (two
steps). Formation of the annelated pyrimidine ring is achieved by two routes both involving a
carbodiimide-mediated cyclization process, which allow incorporation of the amine functionality
at C9 of the core tricyclic (six steps). Installation of the northeast 2-aminopyrimidine ring at C5 is
performed using the Bredereck protocol (three steps). Ultimate, thermal decarboxylation with
concomitant O-methyl deprotection and further N-benzyl deprotection by the action of triflic acid
completed the synthesis of the target natural product variolin B.
In tr od u ction
Marine organisms are among the most promising
sources of new biologically active molecules.1 Certain
secondary metabolites are nontraditional guanidine-
based alkaloids2 that possess a broad spectrum of power-
ful biological activities. The guanidine moiety is fre-
quently found in the guise of a 2-aminoimidazole ring3
or a 2-aminopyrimidine ring,4 which represent an emerg-
ing structural class of marine alkaloids based upon their
high degree of biological activity. In 1994, Blunt and
Munro reported the isolation and structural elucidation
of the variolins a new class of marine alkaloids from the
rare, difficult to access Antartic sponge Kirkpatrickia
variolosa.5
This new class of alkaloids are interesting from both
the structural and biological points of view. All the
variolins have a common pyridopyrrolopyrimidine core,
strictly a pyrido[3′,2′:4,5]pyrrolo[1,2-c]pyrimidine, with
either a heterocyclic or methoxycarbonyl group attached
at C5, which has no precedents in either terrestrial or
marine natural products. Variolins can also be considered
as guanidine-based alkaloids in which the guanidine
moiety is found in the guise of a 2-aminopyrimidine ring.
An important feature of variolins is significant bioac-
tivity; variolin B is the most active, having citotoxic
activity against the P388 murine leukemia cells, and also
being effective against Herpes simples type I, it is inactive
against a range of other microorganisms.5a Variolin A also
showed important cytotoxic activity against the P388 cell
line. N-3’-Methyl-3′,4′,5′,6′-tetrahydrovariolin B inhibited
the grow of Sacharomyces cerevisiae and showed in vitro
activity against the HCT 116 cell line. The differential
biological activity of these alkaloids is believed to show
the importance of the 2-aminopyrimidine ring at C5.
(1) Foulkner, D. J . Nat. Prod. Rep. 2001, 18, 1.
(2) Albizati, K. F.; Martin, V. A.; Agharahimi, M. R.; Stolze, D. A.
Synthesis of Marine Natural Product in Biorganic Marine Chemistry;
Scheuer, P. J ., Ed.; Springer: Berlin, 1992; vol 6, p 158.
(3) Molina, P., Fresneda, P. M.; Sanz, M. A. J . Org. Chem. 1999,
64, 2540 and references therein.
(4) Franco, I. H.; J offe´, E. B. K.; Puricelli, L.; Tatian, M.; Seldes,
A., M.; Palermo, J . A. J . Nat. Prod. 1998, 61, 1130.
(5) Perry, N. B. Ettouati, L.; Litaudon, M.; Blunt, J . W., Munro, M.
H. G.; J ameson, G. B. Tetrahedron 1994, 50, 3987. (b) Trimurtulu, G.;
Faulkner, D. J .; Perry, N. B.; Ettouati, L.; Litaudon, M.; Blunt, J . W.;
Munro, M. H. G.; J ameson, G. B. Tetrahedron 1994, 50, 3993.
10.1021/jo026508x CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/05/2002
J . Org. Chem. 2003, 68, 489-499
489

Molina et al.
SCHEME 1
SCHEME 2a
a
Reagents and conditions: (a) N3CH2COOEt, NaEtO, EtOH,
-15 °C (61%); (b) o-xylene, reflux (67%).
reaction of appropriately substituted 1,2,4-triazines9 and
direct ortho-lithiation of aminopyridine derivatives10 have
been also used for the preparation of substituted 7-aza-
indoles. Recently, palladium-catalyzed heteroannelations
of 2-amino-3-iodopyridine derivatives with internal alkynes
based on modified synthetic procedures for indoles11 have
been useful for the preparation of 2, 3-disubstituted
7-azaindole derivatives.12 Moreover, functionalization at
the 2-, 3-, and 6-position of the 7-azaindole derivatives
through palladium-mediated coupling reactions has been
reported.13 Nevertheless, reports on the functionalization
at the 4-position are very limited in the literature and
only has briefly been reported the preparation of 4-C-
substituted-7-azaindoles through palladium-catalyzed
Suzuki, Sonogashira, and Heck coupling reactions.13d
The first facet of the synthesis, the preparation of the
appropriately 2,4-disubstituted-7-azaindole, was accom-
plished by taking advantage of a modified Hemetsberger
indole synthesis which has been successfully applied for
the synthesis of a number of indole alkaloids.14 For this
purpose the required 3-formyl-4-methoxypyridine 2 was
prepared in 77% yield following the Comins protocol15 by
ortho-lithiation of 4-methoxypyridine with mesityllithium
as the metalating base followed by reaction with N,N-
dimethylformamide. Condensation of 2 with ethyl azi-
doacetate in the presence of NaEtO at -15 °C pro-
vided the vinyl azide 3 in 61% yield. When compound 3
was exposed to heat in o-xylene at reflux temperature
for a short period of time, indolization took place by a
nitrene insertion process to give the key intermediate
2-ethoxycarbonyl-4-methoxy-7-azaindole 4 in 67% yield.
(Scheme 2).
Consequently, owing to the intriguing structure of
these alkaloids, significant bioactivity, and also its low
natural occurrence, the elaboration of the central tricyclic
core has stimulated interest in the scientific community
and attracted assorted synthetic efforts. Recently, the
first total synthesis of the architecturally sophisticated
alkaloid variolin B has been reported;6 this synthesis
starts from commercially available 4-iodo-2-methylthio-
pyrimidine involving as a key step a tandem deoxygen-
ation/cyclization of an appropriate triheteroaryl methanol
using a combination of triethylsilane and trifluoroacetic
acid. This work prompted us to report as a preliminary
communication7 a new synthesis of variolin B using our
experience in the field of iminophosphorane. In view of
our interest in the synthesis of natural marine alkaloids
as lead compounds to new and more biologically active
agents, we present now the full account of a general
synthetic method for building the central heterocyclic
moiety presents in variolins, which is essential for
making this family of compounds and a wide variety of
analogues readily available.
Our approach to the synthesis of variolin B is depicted
in the retrosynthetic analysis shown in Scheme 1. The
key steps were: (a) the synthesis of an appropriately
functionalized 2,4-disubstituted-7-azaindole, (b) construc-
tion of the central core pyrido[3′,2′:4,5]pyrrolo[1,2-c]-
pyrimidine ring that typify this compound by two differ-
ent routes involving a carbodiimide-mediated pyrimido
annelation process, and (c) installation of the 2-aminopy-
rimidine ring at C5.
Before our first synthesis of the pyrido[3′,2′:4,5]pyrrolo-
[1,2-c]pyrimidine ring16 bearing suitable functionalities
(9) Taylor, E. C.; Macor, J . E.; Pont, J . L. Tetrahedron 1987, 43,
5145.
(10) Turner, J . A. J . Org. Chem. 1983, 48, 3401. (b) Hands, D.;
Bishop, B.; Cameron, M.; Edwads, J . S.; Cotrell, I. F.; Wright, S. H. B.
Synthesis 1996, 877.
(11) Larock, R. C.; Yum, E. K.; Refvik, M. D. J . Org. Chem. 1988,
53, 7652. (b) Larock, R. C.; Yum, E. K. J . Am. Chem. Soc. 1991, 113,
6689.
Critical to the success of this strategy was the forma-
tion of a 4-methoxy-7-azaindole equipped with appropri-
ate functionalization at C2 for subsequent manipulation
to generate the fused pyrimidine ring.
(12) Rodriguez, A. L.; Koradin, C.; Dohle, W.; Knochel, P. Angew.
Chem., Int. Ed. 2000, 39, 2488. (b) Kang, S. K.; Park, S. S.; Kim, S. S.;
Choi, J .-K.; Yum, E. K. Tetrahedron Lett. 1999, 4379. (c) Mazeas, D.;
Guillaument, G.; Viaud, M.-C Heterocycles 1999, 50, 1065. (d) Park,
S. S.; Choi, J .-K.; Yum, E. K. Tetrahedron Lett. 1998, 627. (e)
Uijainwalla, F.; Warner, D. Tetrahedron Lett. 1998, 5355. (f) Wensbo,
D.; Annby, U.; Gronowitz, S.; Cohen, L. A. Tetrahedron Lett. 1993, 627.
(g) J eschke, T.; Wensbo, D.; Annby, U.; Gronowitz, S.; Cohen, L. A.
Tetrahedron Lett. 1993, 6471. (h) Kumar, V.; Dority, J . A.; Bacon, E.
R.; Singh, B.; Lesher, G. Y. J . Org. Chem. 1992, 57, 6995.
(13) Chi, S. M.; Choi, J .-K.; Yum, E. K. Tetrahedron Lett. 2000, 919.
(b) Alvarez, M.; Ferna´ndez, D.; J oule, J . A. Synthesis 1999, 615. (c)
J oseph, B.; Da Costa, H.; Merour, J .-Y.; Leonce, S. Tetrahedron 2000,
56, 3189. (d) Allegretti, M.; Arcadi, A.; Marinelli, F.; Nicolini, L. Synlett
2001, 609.
Resu lts a n d Discu ssion
Limited general synthetic pathways exist in the lit-
erature for the preparation of 7-azaindoles; they involve
classical methods such as Fisher, Madelung, and Reissert
procedures, which despite their synthetic value, generally
suffer from harsh reaction conditions and modest yields.8
The intramolecular inverse electron demand Diels-Alder
(6) Anderson, R. J .; Morris, J . C. Tetrahedron Lett. 2001, 8697.
(7) Molina, P.; Fresneda, P. M.; Delgado, S.; Bleda, J . A. Tetrahedron
Lett. 2002, 1005.
(8) Yakhontov, L. N.; Prokov, A. A. Russian Chem. Rev. 1980, 49,
428. (b) Willete, R. E. Adv. Heterocycl. Chem. 1968, 9, 27.
(14) Gribble, G. W. J . Chem. Soc., Perkin Trans 1 2000, 1045.
(15) Comins, D. L.; La Munyon, D. H. Tetrahedron Lett. 1988, 773.
490 J . Org. Chem., Vol. 68, No. 2, 2003

Synthesis of Marine Alkaloid Variolin B
SCHEME 3a
for the preparation of the natural product, there was only
one report dealing with the preparation of a thio-
functionalized derivative of this ring system.17 Almost at
the same time of our approach three syntheses appeared
employing different solutions to the challenging problem
of constructing the core tricyclic of variolin B. The first
one involves the reaction of N-methoxycarbonyl-2-bro-
momethyl-4-methoxy-7-azaindole with tosylmethyl iso-
cyanide (TOSMIC) to give the target tricyclic ring al-
though without any kind of substituent introduced at
C9.18 The second way is based on the introduction of an
aminomethyl side-chain at C2 of the 7-azaindole ring
followed by closure and aromatization to give the tricyclic
ring bearing an oxo functionality at C9.19 The third way
involves the deoxygenation and concomitant cyclization
of a bis(2-methylthiopyrimidyl)pyridylmethanol using the
combination of triethylsilane and trifluoroacetic acid.20
In the two latter approaches the amine functionality at
C9 required is absent but elements of its eventual
formation are present.
In our first approach, the pyrimido annelation reaction,
which allows incorporation of the amine functionality is
accomplished by taking advantage of our developed
tandem aza Wittig/heterocumulene-mediated ring closure
applied to fused pyrimidines.21 To this end, we required
the 2-formyl-4-methoxy-7-azaindole 8, which was pre-
pared from 4, by the following sequence: reduction with
LiAlH4/THF gave 6 in 93% yield, which in turn was
converted into 8 in 60% yield by reaction with MnO2.
However, from the reaction of 8 with ethyl azidoacetate
under basic conditions, only numerous intractable de-
composition products were formed. Therefore, we decided
to protect the 7-azaindole ring prior to condensation with
ethyl azidoacetate. Although, the N-SEM-protected
2-formyl-4-methoxy-7-azaindole 9 was obtained in 93%
yield from 8 using standar conditions, the best results
for the preparation of this compound were obtained by
the following three-step sequence: (a) N-SEM protection
of 4 provided 5 in 94% yield, (b) reduction with LiAlH4/
THF at reflux temperature gave 7 in 93% yield, and (c)
oxidation with MnO2 in CH2Cl2 at room temperature
afforded 9 in 86% yield.
Condensation of the N-SEM-protected 7-azaindole 9
with ethyl azidoacetate under the same conditions used
for the preparation of 3 furnished the vinyl azide 10 in
85% yield. The Staudinger reaction of 10 with triphen-
ylphosphine in dry CH2Cl2 at room temperature gave the
expected iminophosphorane derivative 11 in 82% yield.
Treatment of 11 with tetrabutylammonium fluoride
(TBAF)-SiO2 under microwave heating for 1 min proved
advantageous and cleanly removed the SEM group to
give 12 in 70% yield, without affecting the iminophos-
phorane group. The use of TBAF/THF as deprotecting
agent resulted even after an extended reaction time in a
low yield of 12 with a considerable amount of the product
a
Reagents and conditions: (a) SEM-Cl, NaH, DMF (5, 94%),
(9, 93%); (b) LiAlH4, THF, reflux (6, 93%), (7, 93%); (c) MnO2,
CH2Cl2, rt (8, 60%; 9, 86%); (d) BF3‚Et2O, CH2Cl2 (95%); (e)
N3CH2COOEt, NaEtO, EtOH, -15 °C (85%) (f) Ph3P, CH2Cl2, rt
(82%); (g) TBAF-SiO2, THF, MW (70%); (h) BnNCO, THF, 50 °C
(14, 97%); (15, 90%).
derived from the hydrolytic cleavage of the iminophos-
phorane moiety. Iminophosphorane 12 was more ap-
propriately prepared by changing the reaction sequence.
Thus sequential treatment of the azide 10 with BF3‚Et2O
and triphenylphosphine allowed the one-flask conversion
of 10 into 12 in 88% yield.
Aza-Wittig reaction of iminophosphorane 12 with ben-
zyl isocyanate in dry THF at 50 °C proceeded unevent-
fully to afford directly the desired pyrimido annelation
product 14 in almost quantitative yields, thus completing
the tricyclic pyridopyrrolopyrimidine ring bearing oxo
and nitrogen functionalities placed at suitable positions
for the preparation of the target molecule (Scheme 3).
(16) Fresneda, P. M.; Molina, P.; Delgado, S.; Bleda, J . A. Tetrahe-
dron Lett. 2000, 4777.
(17) Capuano, L.; Schepfer, H. J .; Mu¨ller, K.; Roos, H. Chem Ber.
1974, 107, 929.
(18) Mendiola, J .; Minguez, J . M.; Alvarez-Builla, J .; Vaquero, J . J .
Org. Lett. 2000, 2, 3253.
(19) Alvarez, M.; Fernandez, D.; J oule, J . A. Tetrahedron Lett. 2001,
315.
The regioselective cyclization of the non isolable inter-
mediate carbodiimide 13 to afford the angular fused
pyrimidine 14 instead of the linear tricyclic compound
deserves some comments. We have previously reported
(20) Anderson, R. J .; Morris, J . C. Tetrahedron Lett. 2001, 311.
(21) Molina, P.; Vilaplana, M. J . Synthesis 1994, 1197.
J . Org. Chem, Vol. 68, No. 2, 2003 491

Molina et al.
SCHEME 4a
that â-(indol-2-yl)vinyl heterocumulenes under thermal
conditions undergo electrocyclic ring-closure to give
γ-carbolines,22 whereas when the indole ring is substi-
tuted at C3, heterocyclization reaction takes place by
nucleophilic attack of the amino group on the central
carbon of the heterocumulene moiety to give pyrimido-
[1,6-a]indoles.23 In the present case the preferential
formation of the intramolecular amination product 14
with respect to the electrocyclic ring-closure product could
be explained by the lower reactivity of R-vinyl-7-azain-
doles than the same indole derivatives in electrocyclic
process due to decrease of the energy level of the HOMO
of the diene.13c The linear tricyclic compound 15 was
obtained in 90% yield by treatment of the N-SEM-
protected 7-azaindole 11 with benzyl isocyanate.
The second approach to prepare the tricyclic core of
variolins without the undesired ester group at C7 is based
on our finding that carbodiimides resulting from the aza
Wittig reaction of the iminophosphorane derived from
2-(2-azidoethyl)indole and isocyanates undergo regiose-
lective intramolecular cyclization under acid (SnCl4),
basic (KHMDS) and thermal conditions (160° C) to give
dihydropyrimido[3,4-a]indoles.24 The starting material of
choice was the 2-formyl-4-methoxy-7-azaindole 8 pre-
pared from 9 in 95% yield by N-SEM deprotection with
BF3‚Et2O. Condensation of 8 with nitromethane under
Henry conditions led to the 2-nitrovinyl-7-azaindole 16
in 80% yield. When compound 16 was submitted to react
with LiAlH4 the 2-(2-aminoethyl)-7-azaindole 17 was
obtained as a solid. However, its purification was tedious
and the isolated solid was found to be sunlight unstable,
for these reasons compound 17 was used without isola-
tion for the next step. The direct conversion of 16 into
the urea derivative 18 was achieved in 60% yield by
sequential treatment with LiAlH4 and benzyl isocyanate.
Dehydration of 18 and concomitant cyclization of the
resulting carbodiimide to give 19 was accomplished in
90% yield under mild conditions by using the Appel
reagent (CCl4/PPh3/Et3N). The fact that the cyclization
of the intermediate carbodiimide takes place under mild
conditions (DCM, 50 °C) is in clear contrast with the
results found for related carbodiimides in the indole
series, which need drastic conditions. All attempts to
promote the aromatization of the dihydropyrimidine ring
in 19 failed and only complex mixtures were obtained
(Scheme 4).
a
Reagents and conditions: (a) CH3NO2, NH4AcO, EtOH, 75
°C (80%); (b) LiAlH4, THF, 0 °C to rt; (c) PhCH2NCO, CH2Cl2, rt
(60%); (d) CCl4/Ph3P/Et3N, CH2Cl2, 50 °C (90%).
C5 in the tricyclic ring would provide the site for the
formation of the acetyl appendage. Since, bromination
of 7-azaindole has precedent to proceed at C3,27 we were
reasonably optimistic about the chances of success for the
regioselective bromination at C5 of compound 19.
The bromination reaction was carried out under clas-
sical conditions (Br2, pyridine, 0 °C or NBS at -60 °C),
thus allowing the predictable and exclusive incorporation
of the bromine atom at C5 of the tricyclic ring, as
evidenced by the disappearance of H-5 triplet at δ 6.31
1
ppm with J ) 1.2 Hz in the H NMR spectrum of 19, to
give 20 in 70% yield. The replacement of the bromine
atom in 20 by the acetyl functionality to give 21 was
achieved in 65% yield by coupling with (R-ethoxyvinyl)-
tributyltin28 in the presence of dichlorobis (triphenylphos-
phine)palladium (II).29 Oxidative conversion of the dihydro-
heterocyclic compound 21 to the corresponding heteroaro-
matic 22 was achieved either by using bromotrichlo-
romethane in combination with DBU30 or DDQ in dichlo-
romethane at room temperature, albeit in very disapoint-
ing yield (32-40%).
The next task was the installation of the 2-aminopy-
rimidine ring at C5, which was effected along the lines
previously employed in our meridianins (3-aminopyrim-
idylindoles) synthesis,25 using an acetyl side chain as C2
moiety for the construction of the 2-aminopyrimidine
ring.26 Acylation of compound 19 proved to be more
problematic than we have anticipated, because the
tricyclic pyridopyrrolodihydropyrimidine ring did not
survive under the conditions employed for acylation of
the 7-azaindole.25 We then decided to switch our plans,
one could envisage that the introduction of a bromine at
On the other hand, the reaction of compound 14 with
N,N-dimethylacetamide in the presence of POCl3 allowed
the direct introduction of the acetyl group at C5 to give
23 in 90% yield, which was converted into 22 in some-
what better yield: ester hydrolysis provided the acid 24
in quantitative yield which under thermal treatment
(22) Molina, P.; Fresneda, P. M. J . Chem. Soc., Perkin Trans. 1 1988,
1819.
(26) Bredereck, M.; Effenberger, F.; Botsch, H.; Rehn, H. Chem. Ber.
1965, 98, 1081.
(23) Molina, P.; Alajarin, M.; Vidal, A. Tetrahedron 1990, 46,
1063.
(27) Robinson, M. M.; Robinson, B. L. J . Am. Chem. Soc. 1956, 78,
1247.
(24) Molina, P.; Alcantara, J .; Lopez-Leonardo, C. Tetrahedron 1996,
52, 5833.
(25) Fresneda, P. M.; Molina, P. M.; Bleda, J . A. Tetrahedron 2001,
57, 2355.
(28) Sodersquist, J . A.; Hsu, G. J -H Organometallics 1982, 1, 830.
(29) Solberg, J .; Undheim, K. Acta Chim. Scand. 1989, 52.
(30) Williams, D. R.; Lowder, P. D.; Gu, Y.-G.; Brooks, D. Tetrahe-
dron Lett. 1997, 331.
492 J . Org. Chem., Vol. 68, No. 2, 2003

Synthesis of Marine Alkaloid Variolin B
SCHEME 5a
All that remained for realization of the final goal were
decarboxylation and O- and N-deprotection. To remove
the O-methyl group, compound 27 was treated with an
excess of sodium methanethiolate31 in dry DMF to give
29 in 85% yield. Decarboxylation of 28, essential to our
objective of preparing variolin B, proved to be difficult.
After several trials with various reagents and conditions
(Cu/quinoline, ∆; Cu2Cr2O5/BaO, ∆, and Barton radical
decarboxylation procedure32), we were unable to ac-
complish this transformation. This series of frustrating
results was finally broken by using only thermal condi-
tions. Thus when compound 28 was heated in diphenyl
ether at 280 °C for 5 h, a 1:1 mixture of the desired
decarboxylation product 27 and, much to our surprise,
decarboxylation/O-methyl deprotection product 29 was
obtained. Traces of water present in compound 28 are
apparently enough to inhibit the O-methyl deprotection
since when this reaction was carried out under rigorous
anhydrous conditions, compound 29 was obtained as the
only reaction product. Thus, when acid 28, previously
dried over anhydrous magnesium sulfate, was heated in
dry diphenyl ether under the same reaction conditions
under nitrogen, compound 29 was obtained in 67% yield.
Despite the moderate yield and considering that this
conversion concatenates two transformations, the yield
must be considered as good.
To the best of our knowledge, this serendipitously
thermal O-methyl deprotection is unprecedented, and we
are currently exploring the scope and applicability of this
deprotection method. Finally, the N-benzyl protecting
group was removed by treatment of 29 in neat triflic acid
at 50 °C to give the variolin B 1 in 74% yield, whose
spectra are in excellent agreement with those reported
for natural 1 (Scheme 6).
The convenience of the above synthetic pathway to
variolin B makes this approach very attractive for the
preparation of structurally related compounds. Thus, the
key intermediate 14 was converted into 35 a regioisomer
of variolin B by a six-step sequence in an overall yield of
28%. Hydrolysis of compound 14 provided the acid 30 in
95% yield, which in turn was converted into the 3-acetyl
derivative 31 in 62% yield by the action of methyllithium
at -15 °C. Formation of enaminone 32 (84% yield)
followed by formation of the 2-aminopyrimidine ring
afforded 33 in 90% yield. The O-methyl deprotection gave
34 in 83% yield, which by N-benzyl deprotection under
acid conditions yielded 35 in 75% yield (Scheme 7). From
these results, it is important to point out that the
introduction of the 2-aminopyrimidine ring at C7 takes
place under milder reaction conditions and higher yield
than at C5.
a
Reagents and conditions: (a) Br2 pyridine, 0 °C (50%); NBS,
THF, CH2Cl2 (70%); (b) R-(ethoxyvinyl)tributyltin, DMF, PdCl2-
(PPh3)2, 70 °C; acetone, 1 M HCl, rt (65%); (c) i: DBU, CBrCl3
(32%); ii: DDQ, CH2Cl2 (40%); (d) DMA, POCl3 (90%); (e) LiOH,
THF-H2O (100%); (f) Ph2O, 250 °C (50%).
underwent decarboxylation to give 22 in moderate yield
(50%). (Scheme 5)
The acetyl derivatives 22 and 23 were reacted with
N,N-dimethylformamide dimethylacetal (DMF-DMA) in
dimethylformamide at 110 °C to give the enaminones 25
and 26 in moderate yields (45%). Switching to the more
reactive N,N-dimethylformamide di-tert-butylacetal (DMF-
DtBA) produced the best yields at lower reaction tem-
perature. Thus, when compounds 22 and 23 were treated
with DMF-DtBA in DMF at 80°C the enaminones 25 and
26 was obtained in 85-70% yield.
Conversion of 25 to 27, which involves the formation
of the substituted 2-aminopyrimidine ring, was achieved
in 90% yield by treatment with guanidine hydrochloride
in 2-methoxyethanol in the presence of anhydrous potas-
sium carbonate. Under the same reaction conditions,
enaminone 26 underwent ring-closure and concomitant
ester hydrolysis to give 28 in 93% yield. It must be
mentioned that a previous work19 related on the synthesis
of deoxyvariolin B, the introduction of the 2-aminopyri-
midine ring at C5 of the tricyclic core was achieved in
four steps in 36% overall yield, by heteroaryl palladium
(0)-catalyzed coupling followed by oxidation with MCPBA
and further treatment with ammonium hydroxide. In the
present approach, using the Bredereck protocol,26 this
chemical operation is performed in three steps in 56%
overall yield.
Con clu sion s
A total synthesis of the marine alkaloid variolin B has
been achieved in 13 steps in an overall yield of 6.5%. The
required 2-ethoxycarbonyl-4-methoxy-7-azaindole was
formed from 3-formyl-4-methoxypyridine by condensation
with ethyl azidoacetate followed by thermal treatment
(31) Hegde, S. G. J . Org. Chem. 1991, 56, 5726.
(32) Barton, D. H. R.; Crich, D.; Motherwell, W. B. J . Chem. Soc.,
Chem. Commun. 1983, 939. (b) Barton, D. H. R.; Crich, D.; Motherwell,
W. B. Tetrahedron 1985, 41, 3901. (c) Crich, D. Aldrichimica Acta 1987,
20, 35.
J . Org. Chem, Vol. 68, No. 2, 2003 493

Molina et al.
SCHEME 6a
involves condensation with ethyl azidoacetate and N-
SEM deprotection, Staudinger reaction with triphenyl-
phosphine and then aza-Wittig reaction of the imino-
phosphorane with benzylisocyanate (six steps). The sec-
ond route is based on the formation of a fused dihydro-
pyrimidine ring by condensation with nitromethane,
reduction, and condensation with benzyl isocyanate fol-
lowed by dehydration and concomitant cyclization of the
resulting carbodiimide. These pyrimido annelation pro-
cesses allow the direct introduction of the amine func-
tionality at C9 of the tricyclic ring system. Installation
of the 2-aminopyrimidine ring involves regioselective
acylation at C5 of the tricyclic ring, followed by conden-
sation of the resulting acetyl derivative with DMF-DtBA
and cyclization by reaction with guanidine (three steps).
Eventually, thermal decarboxylation with concomitant
O-methyl deprotection followed by N-benzyl deprotection
affords the target variolin B, that was found to be
identical in all respects with the natural product.
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were carried under N2 and
using solvents that were dried by routine procedures. Column
chromatography was performed with the use of silica gel (60
Ac.c 70-200 µm, SDS) as the stationary phase. All melting
points were determined on a Kofler hot-plate melting point
apparatus and are uncorrected. IR spectra were determined
as Nujol emulsions or films. NMR spectra were obtained at
200, 300, or 400 MHz (1H) and 50, 75, or 100 MHz (13C).
Chemical shifts are reported in ppm relative to trimethylchlo-
rosilane. Reactions under microwave irradiation were carried
out in a microwave reactor (2.45 GHz, adjustable power within
the range 0-300 W with a simple mode focused system) fitted
with a rotational system and a IR detector of temperature.
The microwave oven is monitored by a computer that allows
the temperature of the reaction mixture to be adjusted.
Ma ter ia ls. The 3-formyl-4-methoxypyridine 2,15 ethyl azi-
doacetate33 and R-(ethoxyvinyl)tributylltin28 were synthesized
according to already reported procedures.
a
Reagents and conditions: (a) DMF-DtBA, DMF, 70-80 °C (25,
85%), (26, 70%); (b) H2N(CdNH)NH2‚HCl, K2CO3, 2-methoxy-
ethanol, reflux (27, 90%), (28, 93%); (c) i: NaMeS, DMF, 80 °C
(85%); ii: Ph2O, 280 °C, 4 h (67%); (d) triflic acid, 50 °C (74%).
SCHEME 7a
P r ep a r a tion of Eth yl R-Azid o-â-(4-m eth oxyp yr id -3-yl)-
a cr yla te (3). A mixture of ethyl azidoacetate (4.15 g, 32 mmol)
and 3-formyl-4-methoxypyridine 2 (1.1 g, 8.02 mmol) in
anhydrous EtOH (50 mL) was added dropwise under N2 at
-15 °C to a well-stirred solution containing Na (0.74 g, 32.1
mmol) in anhydrous EtOH (30 mL). The mixture was stirred
at that temperature for 72 h. The resultant solution was
poured into aqueous 30% ammonium chloride (50 mL). The
separated solid was washed with H2O, air-dried, and recrystal-
lized from EtOAc/n-hexane (1:1) to give 3 (1.21 g, 61% yield)
as colorless needles: mp 106-108°C; IR (Nujol) ν: 2129 (N3),
1
1710 (s), 1593 (s), 1287 (s), 1098 (s), 1028 (s) cm-1. H NMR
(300 MHz, CDCl3) δ: 1.41 (t, 3H, J ) 7.2 Hz), 3.92 (s, 3H,),
4.38 (q, 2H, J ) 7.2 Hz), 6.81 (d, 1H, J ) 5.7 Hz), 7.19 (s, 1H),
8.43 (d, 1H, J ) 5.7 Hz), 9.25 (s, 1H). 13C NMR (75 MHz,
CDCl3) δ: 14.1, 55.5, 62.3, 105.8, 115.9, 118.8, 126.7, 151.3,
151.4, 162.7, 163.1. EIMS: m/z (%) 248 (M+, 49), 220 (85), 192
(30), 174 (82), 147 (100), 132 (50), 119 (68), 117 (92), 107 (52),
90 (95). Anal. Calcd for C11H12N4O3: C, 53.22; H, 4.87; N, 22.57.
Found: C, 53.10; H, 4.70; N, 22.41.
P r ep a r a tion of 2-Eth oxyca r bon yl-4-m eth oxyp yr r olo-
[2,3-b]p yr id in e (4). A stirred solution of vinyl azide 3 (1.0 g,
4.03 mmol) in dry o-xylene (140 mL) was heated in a molten
salts bath at 170 °C for 25 min. After cooling at -10 °C the
precipitated white solid was collected by filtration, washed
with Et2O and recrystallized from CH2Cl2 to give 4 (0.59 g,
67% yield) as white needles; mp 216-217 °C; IR (Nujol) ν: 3416
a
Reagents and conditions: (a) LiOH, THF-H2O (95%); (b)
MeLi, THF, -15 °C (62%); (c) DMF-DtBA, DMF, 80 °C (84%); (d)
H2N(CdNH)NH2‚HCl, K2CO3, 2-methoxyethanol, 110 °C (90%);
(e) NaMeS, DMF, 80 °C (83%); (f) triflic acid, rt (75%).
(two steps). Formation of the core tricyclic was achieved
by two routes; both of them involve initial N-SEM
protection of the 7-azaindole and reduction followed by
oxidation to give 2-formyl-7-azaindole. The first one
(NH), 1723 (CO), 1583 (s), 1524 (s), 1345 (s) cm-1 1H NMR
.
(33) Hemestsberger, H.; Knittel, D. Monatsh. Chem. 1972, 103, 194.
494 J . Org. Chem., Vol. 68, No. 2, 2003

Synthesis of Marine Alkaloid Variolin B
(300 MHz, CDCl3) δ: 1.32 (t, 3H, J ) 6.9 Hz), 3.97 (s, 3H),
4.32 (q, 2H, J ) 6.9 Hz), 6.71 (d, 1H, J ) 5.7 Hz), 7.09 (s, 1H),
8.28 (d, 1H, J ) 5.7 Hz), 12.45 (brs, 1H). 13C NMR (75
MHz, CDCl3) δ: 13.9, 55.5, 60.2, 98.3, 103.4, 109.7, 125.7,
148.4, 150.4, 160.4, 160.5. EIMS: m/z (%) 220 (M+, 98), 221
(38), 192 (28), 175 (58), 174 (100), 145 (35). Anal. Calcd for
was recrystallized from Et2O/n-hexane (1:1) to give the corre-
sponding 2-formyl-7-azaindole derivative.
8: (0.68 g, 60% yield); mp 250-252 °C (white prisms). IR
(Nujol) ν: 1672 (s), 1610 (m), 1584 (s) cm-1. 1H NMR (300 MHz,
DMSO-d6) δ: 4.0 (s, 3H), 6.74 (d, 1H, J ) 5.4 Hz), 7.33 (s,
1H), 8.34 (d, 1H, J ) 5.4 Hz), 9.79 (s, 1H), 12.50 (brs, 1H). 13
C
C
11H12N2O3: C, 59.99; H, 5.49; N, 12.72. Found: C, 59.79; H,
NMR (75 MHz, CDCl3) δ: 55.6, 98.5, 109.5, 110.0, 134.3, 150.1,
151.2, 161.1, 182.3. EIMS: m/z (%) 177 (M+ + 1, 29), 176 (M+,
100), 161 (21), 147 (21), 133 (79), 119 (26), 105 (60). Anal. Calcd
for C9H8N2O2: C, 61.36; H, 4.58; N, 15.90. Found: C, 61.14;
H, 4.73; N, 15.71.
5.64; N, 12.60.
P r ep a r a t ion of N-(Tr im et h ylsilyl)et h oxym et h yl-2-
eth oxyca r bon yl-4-m eth oxy p yr r olo[2,3-b]p yr id in e (5). To
a suspension of sodium hydride (0.44 g, 11.1 mmol) in dry DMF
(18 mL) was added dropwise under N2 a solution of the
7-azaindole 4 (1.77 g, 8.03 mmol) in the same solvent (15 mL).
The mixture was stirred at room temperature for 45 min. After
this time, the solution was cooled at 0 °C, and 2-(trimethyl-
silyl)ethoxymethyl chloride (2 mL, 11.2 mmol) was slowly
added. The solution was allowed to warm at room temperature
and stirred for 12 h. Then, it was poured into H2O (20 mL)
and stirred for 30 min, and the precipitated solid was collected
by filtration, air-dried, washed with Et2O, and recrystallized
from n-hexane to give 5 (2.7 g, 94% yield) as white prisms:
mp 74-75 °C. IR (Nujol) ν: 1716 (CO), 1601 (s), 1571 (s), 1503
(s), 1332 (s), 1297 (s) cm-1. 1H NMR (300 MHz, CDCl3) δ: -0.12
(s, 9H), 0.87 (t, 2H, J ) 8.4 Hz), 1.39 (t, 3H, J ) 7.2 Hz), 3.52
(t, 2H, J ) 8.4 Hz), 3.98 (s, 3H), 4.36 (q, 2H, J ) 7.2 Hz), 6.09
(s, 2H), 6.54 (d, 1H, J ) 5.7 Hz), 7.37 (s, 1H), 8.33 (d, 1H, J )
5.7 Hz). 13C NMR (75 MHz, CDCl3) δ: -1.6, 14.2, 17.8, 55.6,
60.7, 66.0, 71.1, 98.8, 107.6, 109.5, 126.1, 148.7, 151.8, 161.1,
161.3. EIMS: m/z (%) 350 (M+, 7), 351 (9), 307 (42), 277 (41),
249 (94), 232 (94), 219 (96), 204 (99), 174 (98), 161 (99), 148
(38), 72 (100). Anal. Calcd for C17H26N2O4Si: C, 58.26; H, 7.48;
N, 7.99. Found: C, 58.10; H, 7.30; N, 7.81.
P r ep a r a tion of 2-Hyd r oxym eth ylp yr r olo[2,3-b]p yr i-
d in es (6 a n d 7). To a solution of the appropriate 7-azaindole
4 or 5 (4.85 mmol) in anhydrous THF (40 mL) was added
LiAlH4 (5.33 mL of a 1 M solution in THF, 5.33 mmol). The
mixture was stirred at reflux temperature for 30 min. After
cooling, it was poured into cool H2O (20 mL) and extracted
with EtOAc (4 × 15 mL). The combined organic layers were
washed with brine (3 × 20 mL) and dried (MgSO4). After
filtration, the filtrate was concentrated to dryness and the
residue was chromatographed on a silica gel column with Et2O
as eluent.
9: (1.7 g, 86% yield) mp 80-81 °C (white prisms). IR (Nujol)
ν: 1680 (CO), 1661 (s), 1598 (s), 1569 (s), 1496 (s) cm-1 1H
.
NMR (300 MHz, DMSO-d6) δ: -0.16 (s, 9H), 0.76 (t, 2H, J )
7.8 Hz), 3.47 (t, 2H, J ) 7.8 Hz), 4.02 (s, 3H), 5.92 (s, 2H),
6.85 (d, 1H, J ) 5.4 Hz), 7.54 (s, 1H), 8.42 (d, 1H, J ) 5.4 Hz),
9.89 (s, 1H). 13C NMR (75 MHz, DMSO-d6) δ: -1.5, 17.1, 56.1,
65.3, 70.7, 99.8, 109.4, 113.6, 133.5, 150.7, 151.8, 161.4, 183.0.
EIMS: m/z (%) 306 (M+, 29), 307 (27), 277 (23), 263 (46), 233
(55), 220 (29), 205 (53), 189 (99), 176 (38), 162 (100). Anal.
Calcd for C15H22N2O3Si: C, 58.79; H, 7.24; N, 9.14. Found: C,
58.60; H, 7.03; N, 9.05.
This compound was also prepared from 8 in 93% yield,
using the same method as described for the preparation of 5
from 4.
P r ep a r a tion of r-Azid o-â[(1-(tr im eth ylsilyl)eth oxy-
m eth yl)-4-m eth oxy-pyr r olo[2,3-b]pyr id-2-yl]pr open ic Acid
Eth yl Ester (10). This compound was prepared from 9 and
ethyl azidoacetate in 85% yields using the same method as
described for 3. The titled compound was purified by chroma-
tography on a silica gel column using CH2Cl2/EtOAc (9:1) as
eluent: mp 106-108 °C (yellow prisms from CH2Cl2/Et2O). IR
(Nujol) ν: 2127 (s), 1709 (s), 1597 (s) cm-1. 1H NMR (300 MHz,
CDCl3) δ: -0.10 (s, 9H), 0.87 (t, 2H, J ) 8.1 Hz), 1.38 (t, 3H,
J ) 7.2 Hz), 3.51 (t, 2H, J ) 8.1 Hz), 4.00 (s, 3H), 4.36 (q, 2H,
J ) 7.2 Hz), 5.76 (s, 2H), 6.52 (d, 1H, J ) 5.4 Hz), 7.15 (s,
1H), 7.53 (s, 1H), 8.21 (d, 1H, J ) 5.4 Hz). 13C NMR (75 MHz,
CDCl3) δ: -1.4, 14.2, 17.6, 55.6, 62.2, 65.8, 70.2, 98.7, 104.6,
111.0, 112.6, 125.7, 130.7, 146.9, 150.8, 160.2, 163.1. EIMS:
m/z (%) 417 (M+, 2), 389 (41), 359 (7), 344 (11), 331 (20), 273
(37), 258 (40), 249 (25), 226 (41), 199 (34), 175 (32), 73 (100).
Anal. Calcd for C19H27N5O4Si: C, 54.65; H, 6.52; N, 16.77.
Found: C, 54.46; H, 6.71; N, 16.59.
6: (0.8 g, 93% yield); mp 213-215 °C (white prisms). IR
(Nujol) ν: 3920 (s), 3356 (s), 1604 (s), 1546 (m), 1160(s), 1006
P r ep a r a t ion of t h e N-P r ot ect ed Im in op h osp h or a n e
(11). To a solution of the azide 9 (1.03 g, 2.46 mmol) in
anhydrous CH2Cl2 (15 mL) was added a solution of triphen-
ylphosphine (0.64 g, 2.46 mmol) in the same solvent (15 mL)
dropwise under N2. The resultant solution was stirred at room
temperature for 24 h. Afterward, the solution was concentrated
to dryness and the residue was chromatographed on a silica
gel column using Et2O/n-hexane (8:2) as eluent to give 11 (1.31
g, 82% yield); mp 103-104 °C (yellow prisms from Et2O). IR
(Nujol) ν: 1698 (s), 1566 (s), 1500 (s), 1462 (s), 1434 (s), 1379
1
(s) cm-1. H NMR (300 MHz, DMSO-d6) δ: 3.92 (s, 3H), 4.56
(d, 2H, J ) 5.7 Hz), 5.23 (t, 1H, J ) 5.7 Hz), 6.29 (s, 1H), 6.60
(d, 1H, J ) 5.4 Hz), 8.04 (d, 1H, J ) 5.4 Hz), 11.51 (s, 1H). 13
C
NMR (75 MHz, CDCl3) δ: 55.3, 56.8, 94.4, 97.8, 109.7, 138.4,
143.9, 150.4, 158.5. EIMS: m/z (%) 179 (M+ + 1, 13), 178 (M+,
100), 161 (98), 149 (25), 134(40), 118 (21), 105 (10). Anal. Calcd
for C9H10N2O2: C, 60.66; H, 5.66; N, 15.72. Found: C, 60.44;
H, 5.82; N, 15.65.
1
(s) cm-1. H NMR (300 MHz, CDCl3) δ: -0.07 (s, 9H), 0.95 (t,
7: (1.39 g, 93% yield); yellow oil. IR (film) ν: 3370 (m), 1612
(s), 1576 (s), 1548 (s), 1292 (s) cm-1. 1H NMR (300 MHz, CDCl3)
δ: -0.08 (s, 9H), 0.90 (t, 2H, J ) 8.1 Hz), 3.27 (t, 1H, J ) 6.3
Hz), 3.55 (t, 2H, J ) 8.1 Hz), 4.0 (s, 3H), 4.79 (d, 2H, J ) 6.3
Hz), 5.79 (s, 2H), 6.55 (d, 1H, J ) 5.7 Hz), 6.56 (s, 1H), 8.19
(d, 1H, J ) 5.7 Hz). 13C NMR (75 MHz, CDCl3) δ: -1.6, 17.8,
55.5, 57.2, 66.2, 70.3, 98.6, 98.7, 109.8, 137.3, 145.2, 151.0,
159.9. EIMS: m/z (%) 308 (M+, 58), 309 (38), 290 (8), 265 (39),
249 (49), 235 (91), 232 (51), 205 (22), 189 (87), 177 (99), 159
(89), 144 (58), 43 (100). Anal. Calcd for C15H24N2O3Si: C, 58.41;
H, 7.84; N, 9.08. Found: C, 58.30; H, 7.92; N, 8.92.
P r ep a r a t ion of 2-F or m ylp yr r olo[2,3-b]p yr id in es (8
a n d 9). To a solution of the appropriate 2-hydroxymethyl-7-
azaindole 6 or 7 (6.48 mmol) in anhydrous CH2Cl2 (20 mL)
was added active MnO2 (2.81 g, 32.4 mmol). The mixture
was stirred at room temperature for 72 h under N2. After-
ward, it was filtered over a Celite pad, which was washed with
CH2Cl2 (2 × 10 mL) and Et2O (2 × 10 mL). The combined
filtrates were concentrated to dryness, and the resulting solid
2H, J ) 8.0 Hz), 1.00 (t, 3H, J ) 6.9 Hz), 3.61 (t, 2H, J ) 8.0
Hz), 3.89 (q, 2H, J ) 6.9 Hz), 3.93 (s, 3H), 5.80 (s, 2H), 6.47
(d, 1H, J ) 5.5 Hz), 7.00 (d, 1H, J HP ) 7.5 Hz), 7.39-7.48 (m,
9H), 7.73-7.80 (m, 7H), 8.09 (d, 1H, J ) 5.5 Hz). 13C NMR
(75 MHz, CDCl3) δ: -1.4, 14.0, 17.7, 55.4, 60.8, 65.4, 70.0, 98.8,
3
3
100.6, 104.3 (d, J ) 20.7 Hz), 111.4, 128.1 (d, J ) 12.1 Hz),
4
2
1
130.9 (d, J ) 2.2 Hz), 132.4 (d, J ) 9.8 Hz), 132.8 (d, J )
3
102.7 Hz), 136.5, 137.8, 143.8, 150.8, 158.9, 167.0 (d, J ) 7.5
Hz). 31P NMR (125 MHz, CDCl3) δ: 9.4. EIMS: m/z (%) 652
(M+, 88), 653 (15), 550 (53), 535 (9), 522 (50), 277 (28), 262
(100), 183 (97), 152 (20), 108 (42), 73 (91). Anal. Calcd for
C
37H42N3O4PSi: C, 68.18; H, 6.49; N, 6.45. Found: C, 68.31;
H, 6.64; N, 6.28.
P r ep a r a tion of Im in op h osp h or a n e (12). Meth od A. A
mixture of N-protected iminophosphorane 11 (0.15 g, 0.23
mmol) and TBAF-SiO2 (4 g, 1 mmol F-/g SiO2) was placed in
a cylindrical quart tube. The tube was introduced into micro-
wave reactor (2.45 GHz, adjustable power with the range
J . Org. Chem, Vol. 68, No. 2, 2003 495

Molina et al.
0-300 W) and irradiated at 90% power for 90 s. After cooling,
the solid mixture was chromatographed on a silica gel column
using Et2O/n-hexane (7:3) as eluent to give 12 (84 mg, 70%
yield) as yellow prisms: mp 195-196 °C (CH2Cl2/Et2O). IR
(Nujol) ν: 3302 (m), 1686 (s), 1608 (m), 1559 (m), 1457 (s),
Hz). 13C NMR (75 MHz, CDCl3) δ: -1.6, 14.3, 17.6, 45.8, 55.5,
61.1, 66.2, 70.4, 99.1, 99.8, 103.6, 104.0, 127.1, 128.1, 128.4,
139.7, 143.4, 143.7, 150.8, 152.9, 159.1, 166,1. EIMS: m/z (%)
508 (M+ + 2, 27), 506 (M+,100), 476 (15), 447 (15), 402 (21),
389 (53), 376 (36), 342 (26), 301 (31), 268 (33), 106 (31), 91
(56). Anal. Calcd for C27H34N4O4Si: C, 64.00; H, 6.76; N, 11.06.
Found: C, 63.89; H, 6.64; N, 11.17
1
1382 (s) cm-1. H NMR (300 MHz, CDCl3) δ: 1.02 (t, 3H, J )
7.2 Hz), 3.88 (q, 2H, J ) 7.2 Hz), 3.97 (s, 3H), 6.45 (d, 1H, J )
5.4 Hz), 6.48 (d, 1H, J ) 1.8 Hz), 6.80 (d, 1H, J HP ) 7.8 Hz),
7.44-7.54 (m, 9H), 7.70-7.80 (m, 6H), 8.07 (d, 1H, J ) 5.4
Hz), 11.78 (brs, 1H). 13C NMR (75 MHz, CDCl3) δ: 14.1, 55.4,
P r ep a r a tion of 4-Meth oxy-2-(2-n itr ovin yl)-7-p yr r olo-
[2,3-b]p yr id in e (16). To a solution of NH4AcO (0.197 g, 2.55
mmol) in anhydrous EtOH (22 mL) were added compound 8
(0.45 g, 2.55 mmol) and nitromethane (3.1 g, 51 mmol) at room
temperature under N2. The reaction mixture was stirred at
75 °C for 2 h and then allowed to cool to 0 °C. After dilution
with Et2O (20 mL), the precipitate solid was separated by
filtration and washed with H2O (5 mL) and Et2O (2 × 5 mL).
The filtrated and the combined organic layers were concen-
trated to dryness under reduced pressure. The residue was
purified by chromatography on a silica gel column using MeOH
(9.5:0.5) as eluent and further recrystallized from EtOH to give
16 (0.45 g, 80% yield); mp > 300 °C. IR (Nujol) ν: 3089 (m),
3
3
61.0, 97.1, 97.6, 107.7 (d, J ) 20.7 Hz), 111.2, 128.5 (d, J )
4
2
12.2 Hz), 131.3 (d, J ) 2.3 Hz), 132.1 (d, J ) 9.8 Hz), 132.4
4
(d, J ) 103.2 Hz), 135.8, 136.3, 144,4, 149.8, 158.6, 167.1 (d,
3J ) 7.5 Hz). 31P NMR (125 MHz, CDCl3) δ: 12.6. EIMS: m/z
(%) 522 (M+ + 1, 28), 521 (M+, 78), 464 (15), 277 (12), 262
(91), 237 (51), 201 (31), 186 (81), 183 (100), 152 (15), 108 (37),
77 (91). Anal. Calcd for C31H28N3O3P: C, 71.39; H, 5.41; N,
8.06. Found: C, 71.19; H, 5.59; N, 7.85.
Meth od B. To a solution cooled at 0 °C of vinyl azide 10
(1.45 g, 3.42 mmol) in anhydrous CH2Cl2 (60 mL) was added
dropwise BF3‚Et2O (1.49 mL, 12.14 mmol) under N2. The
resultant solution was allowed to warm to room temperature
and stirred for 3 h protected from the sunlight. Afterward, an
aqueous 5% solution of NaOH was added until basic pH, and
stirring was continued for additional 3 h. The organic layer
was separated and the aqueous layer was extracted with
CH2Cl2 (3 × 15 mL). The combined organic layers were dried
(MgSO4) and concentrated to dryness at room temperature
under reduced pressure. The crude N-deprotected azide was
dissolved in anhydrous CH2Cl2 (60 mL), and a solution of
triphenylphosphine (0.90 g, 3.47 mmol) in the same solvent
(10 mL) was added under N2. The reaction mixture was stirred
at room temperature for 12 h. The solvent was removed off
under reduced pressure, and the residue was chromatographed
on a silica gel column using Et2O as eluent to give 12 (1.59 g,
88% yield).
1
1629 (s), 1589 (s), 1468 (s), 1147 (s) cm-1. H NMR (300 MHz,
DMSO-d6) δ: 3.95 (s, 3H), 6.68 (d, 1H, J ) 5.4 Hz), 7.21 (s,
1H), 8.07 (m, 2H), 8.24 (d, 1H, J ) 5.4 Hz), 12.22 (s, 1H). 13C
NMR (75 MHz, DMSO-d6) δ: 55.8, 98.8, 109.6, 111.1, 127.7,
129.9, 135.3, 149.4, 152.2, 160.1. EIMS: m/z (%) 220 (M+ + 1,
15), 219 (M+, 100), 189 (7), 172 (53), 158 (32), 148 (16), 129
(25). Anal. Calcd for C10H9N3O3: C, 54.79; H, 4.45; N, 14.54.
Found: C, 54.85; H, 4.34; N, 14.68.
P r ep a r a tion of th e Ur ea Der iva tive (18). To a suspen-
sion of the nitrovinyl compound 16 (0.5 g, 2.28 mmol) in
anhydrous THF (50 mL) was added LiAlH4 (5.69 mL of a 1 M
solution in THF, 5.69 mmol) at 0 °C under N2. The mixture
was stirred at room temperature for 2 h. Afterward, 5%
aqueous solution of NaOH (40 mL) was added, and stirring
was continued for additional 15 min. The mixture was
extracted with EtOAc (4 × 20 mL), and the combined organic
layers were washed with H2O (2 × 10 mL), and dried (MgSO4).
After filtration, the solution was concentrated to dryness to
give the crude amine 17, which was used in the next step
without purification. To a solution of the amine 17 in anhy-
drous CH2Cl2 (25 mL) was added benzyl isocyanate (0.35 g,
2.61 mmol) dropwise at 0 °C under N2. Then, the mixture was
stirred at room temperature for 4 h. The solvent was removed
off under reduced pressure, and the residue was purified by
column chromatography on a silica gel column using CH2Cl2/
EtOH (9:1) as eluent to give 18 (0.44 g, 60% yield); mp 234-
235 °C. IR (Nujol) ν: 3323 (m), 3139 (m), 3096 (m), 1581 (s),
P r ep a r a t ion of 9-Ben zyla m in o-7-et h oxyca r b on yl-4-
m eth oxyp yr id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in e (14). To
a solution of iminophosphorane 12 (0.8 g, 1.54 mmol) in
anhydrous THF (25 mL) was added benzyl isocyanate (0.24 g,
1.84 mmol) dropwise under N2. The reaction mixture was
stirred at 50 °C for 24 h. The solvent was removed off under
reduced pressure, and the residue was chromatographed on a
silica gel column with CH2Cl2 as eluent to give 14 (0.55 g, 97%
yield); mp 167-169 °C (yellow needles from CH2Cl2/Et2O); IR
(Nujol) ν: 3269 (m), 1730 (s), 1713 (s), 1699 (s), 1615 (s) cm-1
.
1H NMR (300 MHz, CDCl3) δ: 1.44 (t, 3H, J ) 7.2 Hz), 4.03
(s, 3H), 4.41 (q, 2H, J ) 7.2 Hz), 5.02 (q, 2H, J ) 5.4 Hz), 6.72
(s, 1H), 6.73 (d, 1H, J ) 6.0 Hz), 7.28-7.31 (m, 1H), 7.36 (tm,
2H, J ) 7.2 Hz), 7.55 (dm, 2H, J ) 7.2 Hz), 7.61 (s, 1H), 8.19
(d, 1H, J ) 6.0 Hz), 10.0 (t, 1H, J ) 5.4 Hz). 13C NMR (75
MHz, CDCl3) δ: 14.3, 45.0, 55.7, 61.2, 91.3, 100.3, 106.5, 114.4,
127.3, 128.0, 128.5, 134.3, 137.1, 138.7, 142.7, 143.9, 147.9,
159.2, 165.8. EIMS: m/z (%) 377 (M+ + 1, 17), 376 (M+, 40),
347 (7), 302 (28), 205 (22), 149 (72), 105 (20), 57 (100). Anal.
Calcd for C21H20N4O3: C, 67.01; H, 5.36; N, 14.88. Found: C,
66.82; H, 5.54; N, 14.70.
1
1515 (m) cm-1. H NMR (300 MHz, DMSO-d6) δ: 2.83 (t, 2H,
J ) 6.0 Hz), 3.40 (m, 2H), 3.92 (s, 3H), 4.21 (d, 2H, J ) 5.4
Hz), 6.01 (brs, 1H), 6.20 (s, 1H), 6.39 (brs, 1H), 6.59 (d, 1H, J
) 5.2 Hz), 7.21-7.29 (m, 5H), 8.01 (d, 1H, J ) 5.2 Hz), 11.44
(s, 1H). 13C NMR (75 MHz, DMSO-d6) δ: 28.9, 38.8, 42.9, 55.2,
94.7, 97.8, 110.1, 126.5, 126.9, 128.1, 136.0, 140.8, 143.4 150.4,
158.0. EIMS: m/z (%) 325 (M+ + 1, 9), 324 (M+, 100), 217 (21),
174 (100), 161 (39), 147 (27), 106 (22), 91 (28). Anal. Calcd for
C
18H20N4O2: C, 66.65; H, 6.21; N, 17.27. Found: C, 66.52; H,
6.08; N, 17.38.
P r ep a r a tion of 5-Ben zyla m in o-7-eth oxyca r bon yl-
4-m e t h o x y -9-[(t r im e t h y ls ily l)e t h o x y m e t h y l]p y r id o -
[3′,2′:4,5]p yr r olo[3,2-c]p yr id in e (15). To a solution of imi-
nophosphorane 11 (0.1 g, 0.15 mmol) in anhydrous THF (7
mL) was added benzyl isocyanate (19 µL, 0.15 mmol) dropwise
under N2. The reaction mixture was stirred at 50 °C for 24 h.
The solvent was removed off under reduced pressure and the
residue was chromatographed on a silica gel column with Et2O/
n-hexane (7:3) as eluent to give 15 (70 mg, 90% yield); IR
P r ep a r a tion of 9-Ben zyla m in o-4-m eth oxyp yr id o-
[3′,2′:4,5]p yr r olo[1,2-c]-6,7-d ih yd r op yr im id in e (19). To a
solution of urea 18 (0.15 g, 0.46 mmol) in anhydrous CH2Cl2
(25 mL), triphenylphosphine (0.24 g, 0.92 mmol), triethylamine
(0.14 g, 1.38 mmol), and CCl4 (0.36 g, 2.31 mmol) were added
at room temperature under N2. The reaction mixture was
heated at reflux temperature for 24 h. After cooling, the solvent
was removed under high vacuum and the residue was chro-
matographed on a silica gel column using EtOAc/MeOH (8:2)
as eluent to give 19 (0.13 g, 90% yield); mp 119-121 °C (yellow
prisms from CH2Cl2/Et2O). IR (Nujol) ν: 3299 (m), 1658 (s),
(Nujol) ν: 3398 (s), 1736 (s), 1707 (s), 1604 (s), 1089 (s) cm-1
.
1H NMR (300 MHz, CDCl3) δ: -0.094 (s, 9H), 0.91 (t, 2H, J )
8.1 Hz), 1.46 (t, 3H, J ) 7.2 Hz), 3.57 (t, 2H, J ) 8.1 Hz), 3.77
(s, 3H), 4.46 (q, 2H, J ) 7.2 Hz), 4.87 (d, 2H, J ) 4.8 Hz), 5.81
(s, 2H), 6.62 (d, 1H, J ) 5.7 Hz), 7.27-7.3 (m, 1H), 7.35-7.40
(m, 2H), 7.47-7.52 (m, 3H), 7.72 (s, 1H), 8.29 (d, 1H, J ) 5.7
1
1605 (m), 1561 (s), 1291 (s) cm-1. H NMR (300 MHz, CDCl3)
δ: 2.95 (td, 2H, J ) 6.6, 1.2 Hz), 3.63 (t, 2H, J ) 6.6 Hz), 3.96
(s, 3H),, 4.69 (d, 2, J ) 4.8 Hz), 6.31 (t, 1H, J ) 1.2 Hz), 6.56
(d, 1H, J ) 5.7 Hz), 7.23-7.28 (m, 1H), 7.32-7.37 (m, 2H),
496 J . Org. Chem., Vol. 68, No. 2, 2003

Synthesis of Marine Alkaloid Variolin B
7.45-7.48 (m, 2H), 8.03 (d, 1H, J ) 5.7 Hz), 8.82 (brs, 1H).
13C NMR (50 MHz, CDCl3) δ: 23.1, 42.0, 45.2, 55.6, 95.9, 100.3,
111.4, 127.1, 127.6, 128.5, 134.3, 138.5, 143.9, 146.8, 148.2,
159.4. EIMS: m/z (%) 307 (M+ + 1, 36), 306 (M+, 100), 176
(50), 161 (73), 91 (61). Anal. Calcd for C18H18N4O: C, 70.57;
H, 5.92; N, 18.29. Found: C, 70.42; H, 5.83; N, 18.37.
P r ep a r a tion of 9-Ben zyla m in o-5-a cetyl-4-m eth oxyp y-
r id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in e (22). Meth od A. To
a solution of compound 24 in anhydrous Ph2O (15 mL) was
heated in a molten salts bath at 250 °C for 24 h under N2.
After cooling, the solution was chromatographed on a silica
gel column first with CH2Cl2 as eluent to separate the Ph2O
and then with CH2Cl2/EtOAc (7:3) to give 22 (44 mg, 50%
yield); mp 150-151 °C (orange prisms from CH2Cl2/Et2O). IR
P r ep a r a tion of 9-Ben zyla m in o-5-br om o-4-m eth oxyp y-
r id o[3′,2′:4,5]p yr r olo[1,2-c]-6,7-d ih yd r op yr im id in e (20).
Meth od A. To a solution of compound 19 (0.19 g, 0.62 mmol)
in anhydrous pyridine (6 mL) was added bromine (32 µL, 0.62
mmol) slowly at 0 °C under N2. The reaction mixture was
stirred at 0 °C for 1 h and then poured into ice-water and
stirred for 20 min. After this time, the mixture was extracted
with ethyl acetate (3 × 10 mL) and the combined organic
extracts were washed with H2O (2 × 10 mL) and dried
(MgSO4). The solvent was removed under reduced pressure,
and the residue was chromatographed on a silica gel column
using EtOAc/MeOH (8:2) as eluent to give 21 (0.12 g, 50%
yield); mp 129-130 °C (white prisms from CH2Cl2/Et2O). mp
129-130 °C (white prisms from CH2Cl2/Et2O). IR (Nujol) ν:
3258 (m), 1661 (s), 1604 (m), 1555 (s) cm-1. 1H NMR (200 MHz,
CDCl3) δ: 2.81 (t, 2H, J ) 6.7 Hz), 3.55 (t, 2H, J ) 6.7 Hz),
3.90 (s, 3H), 4.57 (d, 2, J ) 5.4 Hz), 6.51 (d, 1H, J ) 5.6 Hz),
7.18-7.38 (m, 5H), 7.97 (d, 1H, J ) 5.6 Hz), 8.60 (t, 1H, J )
5.4 Hz). 13C NMR (50 MHz, CDCl3) δ: 21.2, 42.0, 45.0, 55.6,
85.8, 100.2, 109.4, 127.0, 127.4, 128.5, 133.0, 139.0, 144.4,
145.5, 146.8, 159.9. EIMS: m/z (%) 386 (M+ + 2, 90), 384 (M+,
91), 305 (M+-Br, 49), 256 (19), 241 (40), 239 (43), 91 (100). Anal.
Calcd for C18H17BrN4O: C, 56.12; H, 4.45; N, 14.54. Found:
C, 56.00; H, 4.35; N, 14.64.
Meth od B. To a solution of compound 19 (0.3 g, 0.98 mmol)
in THF/CH2Cl2 (1:1) (50 mL), NBS (0.183 g, 1.03 mmol) was
added in three portions at -20 °C under N2. The reaction
mixture was stirred at -20 °C for 24 h and then poured into
ice-water. The mixture was extracted with CH2Cl2 (3 × 10
mL). The combined organic extracts were washed with H2O
(2 × 10 mL) and dried (MgSO4). The solvent was removed
under reduced pressure, and the residue was chromatographed
on a silica gel column using AcOEt/MeOH (8:2) as eluent to
give 21 (0.26 g, 70% yield).
P r ep a r a tion of 9-Ben zyla m in o-5-a cetyl-4-m eth oxyp y-
r id o[3′,2′:4,5]p yr r olo[1,2-c]-6,7-d ih yd r op yr im id in e (21). A
mixture of the bromo compound 20 (0.238 g, 3.1 mmol), (R-
ethoxyvinyl)tributyltin (0.79 g, 2.19 mmol), and dichlorobis-
(triphenylphosphine)palladium (II) (0.087 g, 0.123 mmol) in
anhydrous DMF (15 mL) was stirred at 70 °C for 30 h under
N2. After cooling, the mixture was poured into H2O (15 mL)
and extracted with AcOEt (3 × 10 mL). The combined organic
extracts were washed with H2O (2 × 10 mL) and dried
(MgSO4). The solvent was removed under reduced pressure,
and the residue was chromatographed on a silica gel column
using AcOEt/MeOH (8:2) as eluent to give as mixture of 21
and the vinyl ether. A solution of this mixture in acetone/1 M
HCl (4:1) (37.5 mL) was stirred at room temperature for 48 h.
Afterward, H2O (30 mL) and K2CO3 were added until pH ) 7
and extracted with AcOEt (3 × 10 mL). The combined organic
layers were washed with H2O (2 × 10 mL) and dried (MgSO4).
The solvent was removed under reduced pressure, and the
residue was chromatographed on a silica gel column using
AcOEt/MeOH (8:2) as eluent to give 21 (0.14 g, 65%); mp 137-
138 °C (white prisms from CH2Cl2/Et2O). IR (Nujol) ν: 3330
(m), 1669 (s), 1596 (s), 1574 (m), 1531 (m) cm-1. 1H NMR (400
MHz, CDCl3) δ: 2.66 (s, 3H), 3.20 (t, 2H, J ) 6.8 Hz), 3.60 (t,
2H, J ) 6.8 Hz), 4.01 (s, 3H), 4.66 (d, 2, J ) 4.8 Hz), 6.71 (d,
1H, J ) 5.6 Hz), 7.26-7.30 (m, 1H), 7.36 (t, 2H, J ) 7.5 Hz),
7.44 (d, 2H, J ) 7.5 Hz), 8.11 (d, 1H, J ) 5.6 Hz), 9.01 (brs,
1H). 13C NMR (100 MHz, CDCl3) δ: 22.2, 32.0, 41.6, 45.0, 55.5,
101.1, 108.3, 113.5, 127.0, 127.4, 128.5, 138.8, 141.0, 144.5,
145.3, 147.8, 159.7, 196.9. EIMS: m/z (%) 349 (M+ + 1, 21),
348 (M+, 100), 305 (60), 203 (55), 91 (90). Anal. Calcd for
1
(Nujol) ν: 3208 (m), 1621 (s), 1595 (s), 1463 (s) cm-1. H NMR
(400 MHz, CDCl3) δ: 2.74 (s, 3H), 4.07 (s, 3H), 4.94 (d, 2H, J
) 5.6 Hz), 6.87 (d, 1H, J ) 5.6 Hz), 7.30-7.33 (m, 1H), 7.39 (t,
2H, J ) 7.4 Hz), 7.47 (d, 2H, J ) 7.4 Hz), 7.65 (d, 1H, J ) 6.4
Hz), 7.74 (d, 1H, J ) 6.4 Hz), 8.171 (d, 1H, J ) 5.6 Hz), 10.61
(t, 1H, J ) 5.6 Hz). 13C NMR (100 MHz, CDCl3) δ: 32.3, 44.8,
55.6, 102.3, 102.4, 104.5, 111.2, 127.3, 127.4, 128.7, 138.2,
140.9, 141.8, 144.6, 144.9, 148.1, 159.1, 195.2. EIMS: m/z (%)
347 (M+ + 1, 30), 346 (M+, 100), 331 (54), 198 (62), 91 (71).
Anal. Calcd for C20H18N4O2: C, 69.35; H, 5.24; N, 16.17.
Found: C, 69.24; H, 5.38; N, 16.29.
Meth od B. To a solution of dihydropyrimidine derivative
21 (40 mg, 0.115 mmol) in anhydrous CH2Cl2 (5 mL) was added
DDQ (29 mg, 0.126 mmol) in dry CH2Cl2 (3 mL) at 0 °C under
N2. The reaction mixture was stirred at room temperature for
24 h. Afterward, the solvent was removed under reduced
pressure and the residue was chromatographed on a silica gel
column first with EtOAc/n-hexane (6:4) as eluent and then
EtOAc/MeOH (8:2) to give 22 (12 mg, 48%) and compound 21
(15 mg).
Meth od C. To a solution of dihydropyrimidine derivative
21 (50 mg, 0.143 mmol) in anhydrous CH2Cl2 (8 mL) were
slowly added DBU (42 µL, 0.43 mmol) and CBrCl3 (86 µL, 0.272
mmol) at room temperature under N2. The reaction mixture
was stirred at room temperature for 24 h. Afterward, aqueous
saturated solution of NH4Cl was added and extracted with
EtOAc (3 × 10 mL). The combined organic layers were washed
with H2O (2 × 10 mL) and dried (MgSO4). The solvent was
removed under reduced pressure, and the residue was chro-
matographed on a silica gel column using EtOAc/n-hexane (8:
2) as eluent to give 22 (16 mg, 32%).
P r ep a r a tion of 5-Acetyl-9-ben zyla m in o-7-eth oxyca r -
b on yl-4-m et h oxyp yr id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im i-
d in e (23). To a solution of POCl3 (1.78 mL, 19.1 mmol) in
anhydrous CHCl3 (5 mL) was added DMA (1.98 mL, 21.3
mmol) at 0 °C under N2. The mixture was stirred at room
temperature for 25 min. Then, a solution of compound 14 (0.4
g, 1.06 mmol) in CHCl3 (6 mL) was added, and the reaction
mixture was heated at 70 °C for 30 h. After cooling, the
mixture was poured into a saturated solution of NaOAc in H2O
(159 mL) and then extracted with CH2Cl2 (5 × 30 mL). The
combined organic layers were washed with H2O (2 × 10 mL)
and dried (MgSO4). The solvent was removed under reduced
pressure, and the residue was chromatographed on a silica
gel column using EtOAc/CH2Cl2 (1:20) as eluent to give 23
(0.33 g, 90% yield) along with starting material (0.13 g); mp
223-225 °C. IR (Nujol) ν: 3229 (m), 1734 (s), 1715 (s), 1632
1
(m), 1607 (s) cm-1. H NMR (300 MHz, CDCl3) δ: 1.35 (t, 3H,
J ) 7.2 Hz), 2.65 (s, 3H), 3.97 (s, 3H), 4.35 (q, 2H, J ) 7.2 Hz),
4.92 (d, 2H, J ) 5.4 Hz), 6.76 (d, 1H, J ) 5.7 Hz), 7.21 (m,
1H), 7.29 (tm, 2H, J ) 6.9 Hz), 7.44 (dm, 2H, J ) 6.9 Hz),
8.11 (d, 1H, J ) 5.7 Hz), 8.26 (s, 1H), 10.37 (t, 1H, J ) 5.4
Hz). 13C NMR (75 MHz, CDCl3) δ: 14.3, 32.3, 45.1, 55.7, 61.5,
102.3, 105.7, 107.8, 111.5, 127.4, 128.0, 128.6, 138.1, 139.1,
142.2, 143.1, 144.3, 148.0, 159.4, 165.1, 195.3. EIMS: m/z (%)
418 (M+, 100), 403 (18), 389 (21), 344 (51), 91 (55). Anal. Calcd
for C23H22N4O4: C, 66.02; H, 5.30; N, 13.39. Found: C, 65.92;
H, 5.39; N, 13.31.
P r ep a r a tion of 5-Acetyl-9-ben zyla m in o-4-m eth oxyp y-
r id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in e-7-ca r boxylic Acid
(24). To a solution of ester 23 (0.44 g, 1.05 mmol) in THF/H2O
(4:1) (175 mL) was added LiOH‚H2O (0.154 g, 3.67 mmol) at
room temperature, and the resulting mixture was stirred at
room temperature for 2 h. Then, the THF was removed under
C
20H20N4O2: C, 68.95; H, 5.79; N, 16.08. Found: C, 70.15; H,
5.91; N, 16.17.
J . Org. Chem, Vol. 68, No. 2, 2003 497

Molina et al.
reduced pressure, and 1 N solution of HCl (10 mL) was added
until acid pH. The separated solid was washed with H2O (3 ×
5 mL) dissolved in CH2Cl2 (150 mL) and dried (MgSO4). The
solvent was removed under reduced pressure, and the residue
was recrystallyzed from CH2Cl2/Et2O to give 24 (0.41 g, 100%);
mp 231-232 °C (yellows needles). IR (Nujol) ν: 3209 (m), 1765
399 (M+ + 2, 7), 398 (M+ + 1, 42), 399 (M+, 100), 382 (25), 266
(46), 91 (47). Anal. Calcd for C22H19N7O: C, 66.49; H, 4.82; N,
24.67. Found: C, 66.40; H, 4.92; N, 24.78.
28. (27 mg, 93% yield) mp 200 °C (decomposes)(orange
prisms). IR (Nujol) ν: 3350 (m), 1656 (s), 1108 (s) cm-1. 1H NMR
(300 MHz, DMSO-d6) δ: 3.99 (s, 3H), 4.97 (d, 2H, J ) 5.4 Hz),
6.47 (s, 2H, NH2), 6.83 (d, 1H, J ) 5.7 Hz), 7.13 (d, 1H, J )
5.5 Hz), 7.25-7.38 (m, 3H), 7.49 (d, 2H, J ) 7.2 Hz), 7.99 (s,
1H), 8.20 (d, 1H, J ) 5.7 Hz), 8.31 (d, 1H, J ) 5.5 Hz), 10.23
(brs, 1H). 13C NMR (75 MHz, CDCl3) δ: 44.1, 55.9, 102.3, 103.4,
103.6, 111.1, 111.8, 127.1, 127.5, 128.5, 135.2, 138.8, 142.7,
143.5, 143.8, 147.4, 156.6, 159.4, 160.9, 163.1, 168.4. EIMS:
m/z (%) 442 (M+ + 1, 45), 441 (M+, 75), 426 (48), 395 (54), 381
(72), 304 (31), 277 (34), 265 (48), 196 (28), 91 (100). Anal. Calcd
for C23H19N7O3: C, 62.58; H, 4.34; N, 22.21. Found: C, 62.40;
H, 4.45; N, 22.33.
(s), 1733 (m), 1646 (m), 1632 (s) cm-1 1H NMR (300 MHz,
.
DMSO-d6) δ: 2.61 (s, 3H), 4.04 (s, 3H), 4.93 (d, 2H, J ) 5.5
Hz), 7.14 (d, 1H, J ) 5.7 Hz), 7.24-7.37 (m, 3H), 7.49 (d, 2H,
J ) 7.2 Hz), 8.09 (s, 1H), 8.30 (d, 1H, J ) 5.7 Hz), 10.37 (t,
1H, J ) 5.5 Hz). 13C NMR (75 MHz, CDCl3) δ: 32.1, 44.0, 56.1,
103.3, 104.4, 106.7, 110.6, 127.2, 127.6, 128.5, 138.4, 138.5,
142.2, 143.6, 143.7, 147.6, 159.1, 165.7, 194.1. EIMS: m/z (%)
391 (M+ + 1, 40), 390 (M+, 100), 346 (40), 344 (50), 331 (21),
91 (71). Anal. Calcd for C21H18N4O4: C, 64.61; H, 4.65; N, 14.35.
Found: C, 64.72; H, 4.51; N, 14.49.
P r ep a r a tion of En a m in on es 25 a n d 26. A mixture of the
appropriate acetyl derivative 22 or 23 (1.6 mmol), N,N-
dimethylformamide-di-tert-butylacetal (2.56 g, 12.6 mmol), and
anhydrous DMF (33 mL) was stirred at 70-80 °C for 16-24
h under N2. After cooling, the solution was poured into H2O
(100 mL) and then extracted with EtOAc (2 × 25 mL). The
combined organic layers were dried (MgSO4) and concentrated
to dryness under reduced pressure. The residue was chro-
matographed on a silica gel column using CH2Cl2/MeOH
(9.5:0.5) as eluent for 25 and CH2Cl2/MeOH (9:1) for 26.
25: (0.40 g, 85% yield) mp.166-167 °C; IR (Nujol) ν: 3223
(m), 1639 (m), 1577 (m), 1534 (m), 1460 (s) cm-1. 1H NMR (300
MHz, CDCl3) δ: 2.97 (brs, 6H), 4.01 (s, 3H), 4.91 (d, 2H, J )
5.4 Hz), 5.81 (d, 1H, J ) 12.6 Hz), 6.81 (d, 1H, J ) 5.7 Hz),
7.27-7.38 (m, 4H), 7.45 (d, 2H, J ) 6.9 Hz), 7.53 (d, 2H, J )
6.6 Hz), 7.66 (d, 2H, J ) 12.6 Hz), 8.13 (d, 1H, J ) 5.7 Hz),
10.47 (t, 1H, J ) 5.4 Hz). 13C NMR (75 MHz, CDCl3) δ: 29.6,
44.7, 55.5, 99.3, 101.8, 102.4, 106.0, 111.8, 127.2, 127.3, 128.6,
138.1, 138.4, 141.3, 141.4, 144.3, 148.6, 152.2, 159.4, 185.6.
EIMS: m/z (%) 403 (M+ + 2, 6), 402 (M+ + 1, 37), 401 (M+,
100), 385 (19), 384 (60), 304 (72), 198 (22), 91 (96). Anal. Calcd
for C23H23N5O2: C, 68.81; H, 5.77; N, 17.44. Found: C, 68.70;
H, 5.87; N, 17.57.
P r ep a r a tion of 9-Ben zyla m in o-va r iolin B (29). Meth od
A. To a solution of compound 27 (21 mg, 0.053 mmol) in
anhydrous DMF (7 mL) was added NaMeS (37 mg, 0.53 mmol)
at 80 °C under N2. The reaction mixture was stirred at 80 °C
for 3 h. After cooling, the mixture was poured into a saturated
solution of NH4Cl (100 mL), and 1 N HCl (4 mL) was added
until pH ) 4-5. The resultant mixture was extracted with
EtOAc (5 × 30 mL), the combined organic extracts were
washed with H2O (2 × 10 mL) and dried (MgSO4). The solvent
was removed under reduced pressure, and the residue was
chromatographed on a silica gel column using CH2Cl2/MeOH
(9.5:0.5) as eluent to give 29 (17 mg, 85% yield); mp 298-300
°C (oranges prisms from CH2Cl2/Et2O). IR (Nujol) ν: 3416 (m),
3308 (m), 3157 (m), 1657 (s), 1570 (s) cm-1. 1H NMR (300 MHz,
DMSO-d6) δ: 4.88 (d, 2H, J ) 6.0 Hz), 6.79 (d, 1H, J ) 5.6
Hz), 7.01 (brs, 2H), 7.13 (d, 1H, J ) 5.4 Hz), 7.24 (d, 1H, J )
6.6 Hz), 7.25 (m, 1H), 7.35 (m, 2H), 7.43 (m, 2H), 7.65 (d, 1H,
J ) 6.6 Hz), 8.13 (d, 1H, J ) 5.6 Hz), 8.27 (d, 1H, J ) 5.4 Hz),
10.82 (brs, 1H), 16.05 (s, 1H). 13C NMR (75 MHz, DMSO-d6)
δ: 43.9, 99.9, 100.6, 106.0, 107.4, 110.9, 127.1, 127.2, 128.5,
136.7, 138.7, 143.0, 143.7, 144.9, 149.2, 158.2, 159.9, 160.0,
161.3. EIMS: m/z (%) 385 (M+ + 2, 12), 383 (M+, 100), 306
(12), 292 (25), 278 (50), 252 (35), 210 (25), 91 (52). Anal. Calcd
for C21H17N7O: C, 65.79; H, 4.47; N, 25.57. Found: C, 65.60;
H, 4.38; N, 25.45.
26: (0.53 g, 70% yield)mp 203-205 °C (orange prisms from
CH2Cl2/Et2O). IR (Nujol) ν: 1783 (s), 1644 (s), 1619 (m), 1580
(s) cm-1 1H NMR (300 MHz, CDCl3) δ: 1.41 (t, 3H, J ) 7.2
.
Meth od B. A degassed suspension of the variolinic acid 28
(0.13 g, 0.295 mmol) in dry Ph2O (15 mL) was heated in a
molten salts bath at 280 °C for 4 h under N2. After cooling, to
room temperature the solution was chromatographed on a
silica gel column first with CH2Cl2 as eluent to separate the
solvent and then with CH2Cl2/MeOH (9:1) to give 29 (76 mg,
67% yield).
Hz), 2.99 (brs, 6H), 4.02 (s, 3H), 4.40 (q, 2H, J ) 7.2 Hz),, 5.02
(d, 2H, J ) 5.4 Hz), 5.77 (d, 1H, J ) 12.9 Hz), 6.82 (d, 1H, J
) 5.7 Hz), 7.28-7.39 (m, 3H), 7.52 (m, 2H), 7.67 (d, 2H, J )
12.9 Hz), 8.19 (s, 1H), 8.20 (d, 1H, J ) 5.7 Hz), 10.35 (t, 1H, J
) 5.4 Hz). 13C NMR (75 MHz, CDCl3) δ: 14.3, 37.2, 44.9, 45.0,
55.5, 61.2, 99.4, 101.6, 106.4, 109.9, 111.8, 127.3, 127.9, 128.5,
136.2, 138.4, 139.2, 142.8, 144.1, 147.1, 152.7, 159.9, 165.6,
185.2. EIMS: m/z (%) 474 (M+ + 1, 37), 473 (M+, 73), 456 (54),
376 (74), 301 (100), 186 (55), 98 (71), 91 (85). Anal. Calcd for
P r ep a r a tion of Va r iolin B (1). A solution of compound
29 (59 mg, 0.154 mmol) in triflic acid (3 mL) was stirred at 50
°C for 150 min under N2. After cooling at 0 °C, MeOH (5 mL)
was added and the resulting mixture was stirred at that
temperature for 5 min. Afterward, a solution of 30% am-
monium hydroxide was added until basic pH. The solvent was
removed off at room temperature under reduced pressure until
a yellow solid began to separate. The solid was collected by
filtration and then washed with H2O (3 × 5 mL) and Et2O (2
× 5 mL) and air-dried to give crude variolin B (36 mg). The
combined mother liquors and the filtrate were saturated with
NaCl and then extracted with EtOAc (2 × 10 mL). The
combined organic layers were concentrated to dryness to give
a second crop of variolin B (8 mg). After purification by
chromatography on a silica gel column using CH2Cl2/MeOH
as eluent 9.5/0.5 and then 8.5/1.5, variolin B was obtained (33
mg, 74%) in a high state of purity. IR (Nujol) ν: 3290 (m), 3256
(m), 3102 (m), 1669 (m), 1573 (m), 1465 (s), 1385 (m), 1300
C
26H27N5O4: C, 65.95; H, 5.75; N, 14.79. Found: C, 65.80; H,
5.84; N, 14.61.
P r ep a r a tion of Com p ou n d s 27 a n d 28. Gen er a l P r o-
ced u r e. A mixture of the enaminone 25 or 26 (0.062 mmol),
guanidine hydrochloride (18 mg, 0.186 mmol), anhydrous
K2CO3 (30 mg, 0.217 mmol), and dry 2-methoxyethanol (7 mL)
was heated at reflux temperature for 24 h under N2. After
cooling, the solvent was removed under high vacuum and the
residue was chromatographed on a silica gel column using
EtAcO/MeOH (8:2) as eluent for 27 and CH2Cl2/MeOH (8:2)
for 28.
27: (22 mg, 90% yield) mp 237-238 °C (yellow prisms). IR
(Nujol) ν: 3456 (m), 3317 (m), 3190 (m), 1719(m), 1603 (s), 1575
1
(s) cm-1. H NMR (300 MHz, CDCl3) δ: 3.93 (s, 3H), 4.87 (d,
2H, J ) 5.5 Hz), 5.06 (s, 2H), 6.76 (d, 1H, J ) 5.7 Hz), 6.98 (d,
1H, J ) 5.4 Hz), 7.22-7.32 (m, 3H), 7.39 (d, 2H, J ) 6.6 Hz),
7.40 (d, 1H, J ) 6.6 Hz), 7.49 (d, 1H, J ) 6.6 Hz), 8.11 (d, 1H,
J ) 5.7 Hz), 8.17 (d, 1H, J ) 5.4 Hz), 10.40 (t, 1H, J ) 5.5
Hz). 13C NMR (75 MHz, CDCl3) δ: 44.8, 55.5, 101.2, 101.5,
101.6, 111.1, 113.2, 127.3, 127.4, 128.7, 137.0, 138.5, 141.5,
141.6, 144.6, 148.8, 156.3, 159.5, 162.4, 162.7. EIMS: m/z (%)
1
(m), 1271 (m), 1237 (m) cm-1. H NMR (300 MHz, DMSO-d6)
δ: 6.79 (d, 1H, J ) 5.7 Hz, H-3), 6.99 (brs, 2H, NH2-C-2′),
7.13 (d, 1H, J ) 5.5 Hz, H-5’), 7.22 (d, 1H, J ) 6.7 Hz, H-6),
7.63 (d, 1H, J ) 6.7 Hz, H-7), 8.15 (d, 1H, J ) 5.7 Hz, H-2),
8.26 (d, 1H, J ) 5.5 Hz, H-6’), 8.5 (brs, 1H, NH-C-9), 9.7 (brs,
1H, NH-C-9), 16.03 (s, 1H, OH). 13C NMR (75 MHz, DMSO-
498 J . Org. Chem., Vol. 68, No. 2, 2003

Synthesis of Marine Alkaloid Variolin B
d6) δ: 99.5 (C-5), 100.3 (C-6), 106.0 (C-5’), 107.4 (C-3), 111.1
(C-4a), 137,0 (C-5a), 143.1 (C-2), 144.5 (C-7), 144.9 (C-10a),
150.2 (C-9), 158.3 (C-2’), 159.8 (C-4), 160.0 (C-6’), 161.4 (C-4’).
HREIMS C14H11N7O calcd 293.1025, found 293.1021.
CDCl3) δ: 29.6, 37.3, 44.8, 55.6, 90.2, 92.4, 100.1, 102.0, 114.5,
127.0, 127.5, 128.5, 135.6, 139.3, 141.8, 143.8, 144.9, 147.1,
154.1, 158.9, 186.7. EIMS: m/z (%) 402 (M+ + 1, 32), 401 (M+,
100), 384 (45), 383 (37), 318 (59), 227 (45), 186 (30), 91 (56).
Anal. Calcd for C23H23N5O2: C, 68.81; H, 5.77; N, 17.44.
Found: C, 67.53; H, 5.01; N, 18.15.
P r ep a r a tion of Com p ou n d 33. This compound was pre-
pared from 32 using the same method as described for the
preparation of 27 from 25.
P r ep a r a tion of 9-Ben zyla m in o-4-m eth oxyp yr id o-
[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in -7-ca r boxylic Acid (30).
To a solution of ester 14 (0.5 g, 1.33 mmol) in THF/H2O (4:1)
(175 mL) was added LiOH‚H2O (0.195 g, 4.65 mmol) at room
temperature. The resulting mixture was stirred at that tem-
perature for 2 h. Afterward, the THF was removed under
reduced pressure and a 1 N solution of HCl (12 mL) was added
until acid pH. The separated solid was washed with H2O (3 ×
5 mL), dissolved in CH2Cl2 (150 mL), and dried (MgSO4). The
solvent was removed under reduced pressure, and the residue
was recrystallyzed from CH2Cl2/Et2O to give 30 (0.44 g, 95%);
mp 231-232 °C (yellows needles). IR (Nujol) ν: 3253 (m), 1692
33: (90% yield) mp 222-224 °C (orange prisms from
CH2Cl2/Et2O). IR (Nujol) ν: 1615 (m), 1598 (m), 1463 (s) cm-1
.
1H NMR (400 MHz, CDCl3) δ: 4.02 (s, 3H), 5.01 (d, 2H, J )
5.7 Hz), 5.03 (brs, 2H), 6.66 (s, 1H), 6.72 (d, 1H, J ) 5.6 Hz),
7.26 (m, 1H), 7.35 (d, 2H, J ) 7.4 Hz), 7.50 (d, 2H, J ) 7.4
Hz), 7.59 (d, 1H, J ) 5.2 Hz), 7.85 (s, 1H), 8.15 (d, 1H, J ) 5.6
Hz), 8.36 (d, 1H, J ) 5.2 Hz), 10.00 (t, 1H, J ) 5.7 Hz). 13C
NMR (75 MHz, CDCl3) δ: 44.8, 55.7, 90.1, 100.3, 100.9, 108.4,
114.6, 127.2, 127.6, 128.6, 135.6, 139.1, 142.8, 142.9, 144.1,
147.7, 158.9, 159.1, 162.7, 164.1. EIMS: m/z (%) 399 (M+ + 2,
4), 398 (M+ + 1, 26), 397 (M+, 100), 382 (25), 91 (40). Anal.
Calcd for C22H19N7O: C, 66.49; H, 4.82; N, 24.67. Found: C,
66.35; H, 4.97; N, 24.53.
1
(s), 1610 (s), 1598 (s) cm-1. H NMR (300 MHz, DMSO-d6) δ:
4.04 (s, 3H), 4.96 (d, 2H, J ) 5.6 Hz), 6.84 (s, 1H), 7.04 (d, 1H,
J ) 5.7 Hz), 7.28-7.38 (m, 3H), 7.50 (m, 2H), 7.59 (s, 1H),
8.31 (d, 1H, J ) 5.7 Hz), 9.90 (t, 1H, J ) 5.6 Hz). 13C NMR (75
MHz, CDCl3) δ: 43.8, 56.0, 91.3, 101.3, 105.8, 113.6, 127.12,
127.5, 128.5, 133.8, 136.6, 138.9, 143.1, 143.2, 147.1, 158.9,
166.0. EIMS: m/z (%) 350 (M+ + 2, 13), 348 (M+, 88), 303 (100),
289 (30), 213 (50), 186 (87), 171 (63), 91 (94). Anal. Calcd for
P r ep a r a tion of Com p ou n d 34. This compound was pre-
pared from 33 using the same method as described for the
preparation of 29 from 27.
C
19H16N4O3: C, 65.41; H, 4.63; N, 16.08. Found: C, 65.30; H,
1
4.51; N, 16.22.
34: (83% yield). H NMR (300 MHz, DMSO-d6) δ: 4.96 (d,
P r ep a r a tion of 7-Acetyl-9-ben zyla m in o-4-m eth oxyp y-
r id o[3′,2′:4,5]p yr r olo[1,2-c]p yr im id in e (31). To a solution
of acid 30 (0.5 g, 1.33 mmol) in anhydrous THF (15 mL) was
added MeLi (0.72 mL, 1.148 mmol, 1.6M in ethyl ether) at -15
°C under N2, and the resulting mixture was stirred at that
temperature for 2 h. Afterward, the reaction mixture was
poured into ice-H2O (10 mL), and solid NH4Cl was added until
pH ) 7 and then extracted with CH2Cl2 (4 × 15 mL). The
combined organic layers were concentrated to dryness, and the
solvent was removed under reduced pressure. The remaining
solid was chromatographed on a silica gel column using
CH2Cl2/EtOAc (20:1) as eluent to give 31 (61 mg, 62% yields).
IR (Nujol) ν: 3237 (m), 1683 (s), 1602 (s), 1503 (s), 1381 (s)
cm-1. 1H NMR (300 MHz, CDCl3) δ: 2.63 (s, 3H), 4.04 (s, 3H),
4.97 (d, 2H, J ) 5.6 Hz), 6.73 (d, 1H, J ) 5.4 Hz), 6.74 (s, 1H),
7.28-7.41 (m, 3H), 7.48 (s, 1H), 7.49-7.54 (m, 2H), 8.21 (d,
1H, J ) 5.4 Hz), 10.02 (t, 1H, J ) 5.6 Hz). 13C NMR (50 MHz,
CDCl3) δ: 26.6, 44.8, 55.7, 92.1, 100.3, 102.7, 114.5, 127.2,
127.7, 128.5, 134.6, 139.0, 142.6, 142.7, 143.8, 147.5, 159.2,
200.0. EIMS: m/z (%) 346 (M+, 100), 331 (33), 256 (29), 213
(25), 91 (45). Anal. Calcd for C20H18N4O2: C, 69.35; H, 5.24;
N, 16.17. Found: C, 69.22; H, 5.37; N, 16.29.
2H, J ) 5.8 Hz), 6.55 (brs, 2H), 6.81 (s, 1H), 6.82 (d, 1H, J )
5.4 Hz), 7.25 (m, 1H), 7.35 (t, 2H, J ) 7.5 Hz), 7.39 (d, 1H, J
) 5.1 Hz), 7.50 (d, 2H, J ) 6.9 Hz), 7.78 (s, 1H), 8.13 (d, 1H,
J ) 5.4 Hz), 8.34 (d, 1H, J ) 5.1 Hz), 10.02 (t, 1H, J ) 5.8
Hz), 11.20 (brs, 1H). 13C NMR (75 MHz, DMSO-d6) δ: 43.9,
90.2, 99.9, 105.1, 106.2, 113.7, 127.0, 127.4, 128.5, 134.2, 139.3,
142.3, 142.4, 144.9, 147.2, 157.6, 159.2, 162.5, 163.3. EIMS:
m/z (%) 385 (M+ + 2, 8), 384 (M+ + 1, 42), 383 (M+, 100), 277
(25), 91 (54). Anal. Calcd for C21H17N7O: C, 65.79; H, 4.47; N,
25.57. Found: C, 65.63; H, 4.31; N, 25.66.
P r ep a r a tion of Com p ou n d 35. This compound was pre-
pared from 34 using the same method as described for the
preparation of variolin B from 29.
35: (75% yield). 1H NMR (400 MHz, DMSO-d6) δ: 6.54 (brs,
2H), 6.80 (s, 1H), 6.88 (d, 1H, J ) 5.4 Hz), 7.36 (d, 1H, J ) 5.0
Hz), 7.40 (brs, 1H), 7.77 (s, 1H), 8.15 (d, 1H, J ) 5.4 Hz), 8.36
(d, 1H, J ) 5.0 Hz), 8.6 (brs, 1H), 11.35 (brs, 1H). 13C NMR
(100 MHz, DMSO-d6) δ: 89.8, 99.7, 105.1, 106.0, 113.8, 134,2,
142.3, 142.9, 144.0, 148.1, 157.5, 159.2, 162.5, 163.4. EIMS:
m/z (%) 294 (M+ + 1, 24), 293 (M+, 73), 207 (51), 90 (38), 83
(100). Anal. Calcd for C14H11N7O: C, 57.33; H, 3.78; N, 33.43.
Found: C, 57.25; H, 3.87; N, 33.55.
En a m in on e 32. This compound was prepared from 31
using the same method as described for the preparation of 25
from 22. The residue was chromatographed on a silica gel
column using CH2Cl2/MeOH (9.5:0.5) as eluent to give 32 (59
mg, 84% yield); mp 240 °C (orange prisms from CH2Cl2/Et2O).
Ack n ow led gm en t . We thank the Direccio´n Gen-
eral de Investigacio´n (MCYT, Spain) project number
BQU2001-0014, Foundation Seneca (CARM) project
number PI-48/00740/FS/01 and PharmaMar S. A. (Tres
Cantos, Spain) for financial support. S.D. thanks the
Foundatio´n Seneca (CARM) and CajaMurcia for a
studentship.
1
IR (Nujol) ν: 3248 (m), 1645 (s), 1557 (m), 1380 (m) cm-1. H
NMR (200 MHz, CDCl3) δ: 2.94 (brs, 3H), 3.10 (brs, 3H), 4.03
(s, 3H), 5.99 (d, 2H, J ) 5.6 Hz), 6.43 (d, 1H, J ) 12.8 Hz),
6.68 (s, 1H), 6.72 (d, 1H, J ) 5.4 Hz), 7.29-7.38 (m, 3H), 7.51-
7.55 (m, 2H), 7.62 (s, 1H), 7.87 (d, 1H, J ) 12.8 Hz), 8.16 (d,
1H, J ) 5.4 Hz), 9.96 (t, 1H, J ) 5.6 Hz). 13C NMR (50 MHz,
J O026508X
J . Org. Chem, Vol. 68, No. 2, 2003 499