Synthesis of variolin B

Article abstract of DOI:10.1016/S0040-4039(03)01551-X

The total synthesis of variolin B from 4-methoxy-7-azaindole is described. The preparation of the protected amino derivative 10 and a coupling reaction of the iodo derivative 12 with 2-acetylamino-4-trimethylstannylpyrimidine are the key steps of the sequ

Full text of DOI:10.1016/S0040-4039(03)01551-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6191–6194
Synthesis of variolin B
Abderaouf Ahaidar,a,b David Ferna´ndez,b Olga Pe´rez,a Gerardo Danelo´n,a Carmen Cuevas,c
e
a,b,
Ignacio Manzanares,c Fernando Albericio,a,d John A. Joule and Mercedes Alvarez,
*
´
aBiomedical Research Institute, Barcelona Scientific Park, University of Barcelona, Josep Samitier 1-5, E 08028 Barcelona, Spain
bLaboratory of Organic Chemistry, Faculty of Pharmacy, University of Barcelona, Avda. Joan XXIII s/n, E 08028 Barcelona,
Spain
cPharma Mar, Avda Reyes Cato´licos 1, 28770 Colmenar Viejo, Madrid, Spain
dDepartment of Organic Chemistry, Universitat de Barcelona, Mart´ı Franque´s 1–11, E 08028 Barcelona, Spain
eChemistry Department, The University of Manchester, Manchester M13 9PL, UK
Received 13 June 2003; revised 20 June 2003; accepted 20 June 2003
Abstract—The total synthesis of variolin B from 4-methoxy-7-azaindole is described. The preparation of the protected amino
derivative 10 and a coupling reaction of the iodo derivative 12 with 2-acetylamino-4-trimethylstannylpyrimidine are the key steps
of the sequence. The use of N-tosyldichloromethanimine for the cyclisation step afforded a good entry to the 9-aminopy-
rido[3%,2%:4,5]pyrrolo[1,2-c]pyrimidine system. Variolin B was obtained from the triply protected tetracyclic compound 13 in two
steps.
© 2003 Elsevier Ltd. All rights reserved.
Variolins 1–4 comprise a group of marine heterocyclic
substances isolated from the Antarctic sponge Kirk-
patrickia variolosa1,2 They have a common tricyclic
skeleton, which has no precedents in either terrestrial or
marine natural products, a pyrido[3%,2%:4,5]pyrrolo[1,2-
c]pyrimidine, substituted at position 5. Pharmacological
evaluation of these compounds showed important
antiviral and antiproliferative activity against P388
leukaemia cells.1,2 Variolin B (1) is the most active of
the family, oxidation or reduction of the isolated D ring
as in variolin A (2) or N-3%-methyl-3%,4%,5%,6%-tetra-
hydrovariolin B (3) reduces the activity. The impor-
tance of the aminopyrimidine ring at C5 of the above
mentioned tricyclic system is corroborated by the lack
of activity of variolin D (4).
Two total syntheses of variolin B,3,4 and two syntheses
of deoxyvariolin B,5,6 as well as several methods7–9 for
the construction of the common tricyclic skeleton of
these compounds have been reported. Our synthetic
approach for the preparation of deoxyvariolin B6
started from 7-azaindole and had two key steps: the
introduction of the amino substituent of 9-aminopy-
rido[3%,2%:4,5]pyrrolo[1,2-c]pyrimidine from a precursor
pyrimidone by nucleophilic substitution of its O-silyl
derivative with ammonia under strong conditions of
temperature and pressure. Secondly, a Stille coupling of
a tricyclic iodo derivative with a 4-trimethylstannyl-2-
methylthiopyrimidine allowed introduction of the 5-
substituent; subsequent oxidation of methylthio group
followed by displacement with ammonia gave deoxy-
variolin B.
* Corresponding author. Tel.: +34 93 403 70 86; fax: +34 03 403 71
26; e-mail: malvarez@pcb.ub.es
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01551-X

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A. Ahaidar et al. / Tetrahedron Letters 44 (2003) 6191–6194
In this paper we describe a total synthesis of variolin B
itself starting from 4-methoxy-7-azaindole.10 The route
used for the preparation of deoxyvariolin B6 has been
improved in this work with important changes in the
above mentioned two key steps. The first change was in
the reagent used for the formation of the tricyclic
system, now with concomitant introduction of a pro-
tected amino group. This avoids pyrimidone formation
and the need for a functional group transformation and
hence a reduction in the number of synthetic steps.
Secondly, the use of 2-acetylamino-4-trimethylstan-
nylpyrimidine, not previously described, for the cou-
pling reaction created a more convergent process
avoiding two functional group interchanges at the end
of the sequence.
Reaction of amine 8 with the Cl2CꢀNTs using diiso-
propylethylamine (DIPEA) as a base in DCM gave the
tricyclic compound 9 as a diastereomeric mixture in a
ratio of 1:1. The two singlets at 6.67 and 6.69 ppm for
the proton at C5 were the most characteristic signals
for assigning this ratio.14 This cyclisation procedure
avoids the strong conditions used for the introduction
of the 9-amino group in our previous deoxyvariolin
synthesis6 and also more importantly avoided any pos-
sibility that the methoxy group might be substituted by
amino as an undesirable side reaction. Removal of the
O-THP protecting group of 9 by refluxing in 4N HCl
gave the alcohol 10 characterised by its strong absorp-
tion at 3315 cm−1 and the presence of a singlet at 6.52
ppm for its C5 proton. The NH signals from 9 and 10
were broad which may indicate mixtures of tautomers,
i.e. those shown as 9/10 and others in which the hydro-
gen resides on the endocyclic nitrogen. Elimination of
the hydroxy group of 10 by formation of its mesylate
and treatment with triethylamine (TEA) afforded the
4-Methoxy-7-azaindole was transformed into alcohol 6
by formation of a 2-lithio-7-azaindole followed by con-
densation with 2-phthalimidoacetaldehyde.11 A lithium-
carboxylate was used as N-protecting and ortho-
directing substituent for the preparation of the 2-lithio-
7-azaindole as described by Katritzky for indole 2-lithi-
ation.12 Protection of alcohol 6 by reaction with
dihydropyran and deprotection of the amino group by
hydrazinolysis gave the amino-acetal 8 as a mixture of
diastereomers. This mixture was used without separa-
tion in the cyclisation reaction outlined in Scheme 1.
1
pyridopyrrolopyrimidine 11. The H NMR spectrum of
11 was characterised by the two singlets at 2.35 and
4.02 ppm for the C-methyl and O-methyl groups, the
singlet for the proton at C5 at 6.57 ppm, and the three
AB systems of the aromatic protons.15
Other alternatives to N-tosyl protection were tried.
Thus, an N-dichloroacetyl protected, cyclised com-
pound analogous to 9 was prepared (25%) by reaction
with Cl2CꢀNCOCHCl2. However, it was not possible to
bring about the necessary selective O-deprotection of
the pyranyl group. Furthermore, an N-trityl protected,
cyclised compound analogous to 9 was also prepared
We tested several N-substituted C,C-dichloro methan-
imines for the cyclisation of 8, namely N-dichloro-
acetyl-, N-trityl- and N-tosyldichloro methanimines.13
Only the N-tosyl-substituted dichloro methanimine
(Cl2CꢀNTs) was suitable for the synthetic sequence.
Scheme 1. Reagents and conditions: (i) (a) n-BuLi, THF, −78°C; (b) CO2, −78°C; (c) t-BuLi, THF, −78°C; (d) 2-phthalimidoacet-
aldehyde, THF, −78°C to rt (43%); (ii) DHP, HCl, benzene, CHCl3, D (63%); (iii) NH2NH2·H2O, EtOH, D (quant.); (iv)
TsNꢀC(Cl)2, DIPEA, DCM (65%); (v) 4N HCl, DCM, rt (95%); (vi) MsCl, TEA, DCM, rt (71%); (vii) NIS, DCM, −30°C (95%);
(viii) 2-acetylamino-4-trimethylstannylpyrimidine, Pd2(dba)3, PPh3, LiCl, CuI, dioxane, D (75%); (ix) aq. HBr, D (60%); (x) hw,
p-(OMe)2C6H4, NH2NH2·H2O, MeOH (30%).

A. Ahaidar et al. / Tetrahedron Letters 44 (2003) 6191–6194
6193
(45%) by reaction with Cl2CꢀNCPh3. Although selec-
References
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several different reaction conditions applied. We specu-
late that the failure of that elimination process was
probably due to the less acidic character of the proton
at C7 as a consequence of the presence of the trityl
substituent.
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pound 13a,16 variolin B in heavily protected form.
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The N-tosyl deprotection was a more difficult process.
Treatment with aqueous HBr,19 HBr and AcOH in
phenol at reflux temperature,20 aqueous HI, HF,21 Mg
and NH4Cl in EtOH,22 Red-Al in toluene at different
temperatures,23 NaOH in MeOH or DCM, Na in liquid
ammonia,24,25 Na in naphthalene,26 all failed to bring
about N9-tosyl deprotection of 13. Finally, the tosyl
group of 13a was removed with Li–naphthalene in
THF27 but in only 3% yield—traces of 13b were
detected by HPLC-MS. A reductive photolysis of the
tosyl group was possible using a high pressure Hg lamp
with a Pyrex filter, NaBH4 as a reducing agent and
1,4-dimethoxybenzene as an electron source.28 This pro-
cedure allowed the removal of the tosyl group from 11
in a 64% yield. Finally, the preparation of variolin B 1
was completed, starting from 14, using the reductive
photolysis procedure with H2NNH2. H2O as a reducing
agent instead of NaBH4, in a 30% yield.29
14. 1H NMR (CDCl3, 200 MHz) l 1.18–1.82 (m, 6H, H3%,
H4%, and H5%); 2.38 (s, 3H, Me); 3.40–3.60 (m, 2H, H6%);
3.64–3.80 (m, 2H, H-7); 3.97 (s, 3H, Me); 4.41 and 4.90
(2dd, J=3.2 and 3.4 Hz and 2.6 and 1.9 Hz, 1H, H2%);
4.90–5.05 (m, 1H, H6); 6.67 and 6.69 (2s, 1H, H5); 7.26
(d, J=8.4 Hz, 2H, Ts); 8.10 (d, J=8.4 Hz, 2H, Ts); 8.40
(d, J=2.8 Hz, 1H, H3); 8.42 (d, J=2.8 Hz, 1H, H2). 13C
NMR (CDCl3, 75 MHz) 18.6, 19.3 (t); 21.6 (q); 25.2, 25.3
(t); 30.0, 30.2 (d); 43.3 and 44.9 (t); 55.6 (q); 61.9 and 62.3
(t); 62.7 and 63.5 (d); 95.5 and 96.4 (d); 101.3 and 101.4
(d); 102.9 (d); 111.9 (s); 126.2 (s); 126.4 (d); 129.2 (d);
129.3 (s); 132.2 (s); 142.5 (s); 147.6 and 150.0 (d); 159.7
(s).
15. 1H NMR (CD3OD, 300 MHz) l 2.35 (s, 3H, Me); 4.02 (s,
3H, OMe); 6.57 (s, 1H, H5); 6.63 (d, J=7.5 Hz, 1H, H6);
6.94 (d, J=6.0 Hz, 1H, H3); 6.98 (d, J=7.5 Hz, 1H, H7);
7.29 (d, J=8.1 Hz, 2H, Ts); 8.01 (d, J=8.1 Hz, 2H, Ts);
8.30 (d, J=6.0 Hz, 1H, H2). 13C NMR (DMSO-d6, 75
MHz): 21.0 (q); 55.9 (q); 93.0 (d); 102.1 (d); 113.9 (s);
120.2 (d); 124.4 (d); 125.5 (s); 125.9 (d); 128.2 (s); 129.3
(d); 132.6 (s); 142.3 (s); 143.9 (s); 145.1 (d); 158.4 (s).
16. 1H NMR (CDCl3, 400 MHz) l 2.40 (s, 3H, Me); 2.45 (s,
3H, Me); 4.05 (s, 3H, OMe); 6.92 (d, J=5.2 Hz, 1H, H3);
7.30 (d, J=8.4 Hz, 2H, Ts); 7.43 (d, J=5.2 Hz, 1H, H5%);
7.56 (d, J=6.8 Hz, 1H, H7); 7.95 (bs, 1H, H6); 8.13 (d,
J=8.4 Hz, 2H, Ts); 8.32 (d, J=5.2 Hz, 1H, H6%); 8.49 (d,
J=5.2 Hz, 1H, H2); 8.69 (bs, 1H, NH); 13.28 (bs, 1H,
NH). 13C NMR (CDCl3, 100 MHz) 21.9 (q); 25.3 (q);
56.2 (q); 93.9 (s); 94.1 (s); 102.5 (d, C3); 107.8 (d, C6);
117.3 (d, C5%); 129.0 (d, Ts); 129.5 (d, Ts); 136.3 (s); 136.5
(s); 140.1 (d, C7); 141.9 (s); 142.9 (d, C6%); 143.5 (s); 144.9
(s); 156.9 (d, 2); 158.5 (s); 160.4 (s); 162.0 (s).
The described procedure constitutes a versatile route
for the synthesis of variolin B and will be used for the
preparation of analogues of the natural product. The
coupling of iodo compound 12 with other organometal-
lic derivatives will afford analogues of variolin B differ-
ing in ring D. Nucleophilic substitution of the methoxy
group will give a series of compounds differing from
the natural compound in the substituent at position 4
of the tricyclic system.
Acknowledgements
Financial support from the DGICYT, Spain (Project
BQU2000-0235), and from Biomar (Leo´n) and Pharma
Mar (Madrid) is gratefully acknowledged.

6194
A. Ahaidar et al. / Tetrahedron Letters 44 (2003) 6191–6194
17. 1H NMR (CDCl3, 300 MHz) l 0.36 (s, 9H, SnMe3); 2.53
(s, 3H, Me); 7.13 (d, J=4.8 Hz, 1H, H5); 7.98 (bs, 1H,
NH); 8.35 (d, J=4.8 Hz, 1H, H6); 13C NMR (CDCl3, 75
MHz) 9.5 (q, 3Me), 25.3 (q, Me), 124.4 (d, C5); 130.3 (q,
C4); 154.7 (d, C6); 155.4 (s); 186.4 (s). MS (EI) m/z 300
E.; Habgood, G. J.; Hubbard, C. D.; Young, K. M. J.
Org. Chem. 1991, 56, 6508–6516.
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Chem. Soc. 1987, 109, 6493–6502.
26. Nagashima, H.; Ozaki, N.; Washiyama, M.; Itoh, K.
Tetrahedron Lett. 1985, 26, 657–660 The tricyclic deriva-
tive 11 with Na–naphthalene in THF at −78°C was
N-deprotected in a 25% yield.
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(
120SnM+, 1), 285 (120SnM–Me, 34) 270 (120SnM–2Me, 1),
255 (120SnM–3Me, 6), 244 (120SnM–3Me–Ac, 30), 136
(M–SnMe3, 100).
18. 1H NMR (CDCl3–CD3OD, 400 MHz) l 2.40 (s, 3H,
Me); 6.81 (d, J=7.0 Hz, 1H, H-6); 7.05–7.09 (m, 2H, H-3
and H-5%); 7.28 (d, J=8.0 Hz, 2H, Ts); 7.71 (d, J=6.2
Hz, 1H, H-6%); 8.03 (d, J=8.0 Hz, 2H, Ts); 8.09 (d,
J=7.0 Hz, 1H, H-7); 8.29 (d, J=5.6 Hz, 1H, H-2). 13C
NMR (CDCl3–CD3OD, 100 MHz) 20.1 (q); 101.8 (d,
C3); 103.9 (s); 105.0 (s); 106.0 (d, C6); 110.1 (d, C5%);
123.5 (s); 128.0 (d, Ts); 128.9 (d, Ts); 129.3 (d, C7); 134.3
(d, C6%); 138.3 (s); 139.0 (s); 142.9 (s); 143.1 (s); 156.2 (s);
159.0 (s); 160.7 (d, C2).
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29. The synthetic product had identical spectroscopic data to
those described for variolin B and showed identical TLC
and HPLC behaviour as a sample of the natural product
supplied by Pharma Mar.
21. Andersen, K. K.; Bray, D. D.; Chumpradit, S.; Clark, M.