Concise synthesis of guanidine-containing heterocycles using the Biginelli reaction

Article abstract of DOI:10.1021/jo061199m

(Chemical Equation Presented) Two general methods for the synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines were developed using the three-component Biginelli reaction. The first method utilizes pyrazole carboxamidine, a β-ketoester, and an aldehyde in an initial Biginelli reaction. After Boc protection, these products undergo aminolysis and acidic deprotection to generate 2-imino-5-carboxy-3,4-dihydropyrimidines in a four-step sequence. The second method utilizes a triazone-protected guanidine, a β-ketoester, and an aldehyde in a Biginelli reaction. Acidic cleavage of the triazone yields 2-imino-5-carboxy-3,4-dihydropyrimidines in a two-step sequence. We also describe the further elaboration of several of these products using a tethered Biginelli reaction to give triazaacenaphthalene structures similar to those found in crambescidin and batzelladine alkaloids.

Full text of DOI:10.1021/jo061199m

Concise Synthesis of Guanidine-Containing Heterocycles Using the
Biginelli Reaction
Bradley L. Nilsson and Larry E. Overman*
Department of Chemistry, 1102 Natural Sciences II, UniVersity of California,
IrVine, California 92697-2025
leoVerma@uci.edu
ReceiVed June 9, 2006
Two general methods for the synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines were developed
using the three-component Biginelli reaction. The first method utilizes pyrazole carboxamidine, a
â-ketoester, and an aldehyde in an initial Biginelli reaction. After Boc protection, these products undergo
aminolysis and acidic deprotection to generate 2-imino-5-carboxy-3,4-dihydropyrimidines in a four-step
sequence. The second method utilizes a triazone-protected guanidine, a â-ketoester, and an aldehyde in
a Biginelli reaction. Acidic cleavage of the triazone yields 2-imino-5-carboxy-3,4-dihydropyrimidines in
a two-step sequence. We also describe the further elaboration of several of these products using a tethered
Biginelli reaction to give triazaacenaphthalene structures similar to those found in crambescidin and
batzelladine alkaloids.
Introduction
(2), and the nucleoside base guanine. There are several structur-
ally novel marine alkaloids that contain a guanidine unit within
complex polycyclic architectures. These include the crambes-
cidin4 (examples 3-5) and batzelladine4i,5 (6) families, which
are related by their triazaacenaphthalene core structures. These
guanidine alkaloids, and some of their synthetic derivatives,
display promising anticancer4a-h,k,6 and antiviral activities.4b-d,h,j,6c
They have also been shown to inhibit important protein-protein
Guanidine functional groups are found in numerous biologi-
cally active natural products and several drugs and drug
candidates.1,2 These guanidine-containing therapeutics include
cardiovascular, antihistamine, anti-inflammatory, antidiabetic,
antibacterial, antiviral, and antineoplastic drugs.1 The diversity
in activity of these compounds likely derives, in part, from the
ability of guanidinium cations to recognize receptors by a variety
of noncovalent interactions, including hydrogen-bonding, elec-
trostatic, and π-stacking associations.3
(4) (a) Kashman, Y.; Hirsh, S.; McConnell, O. J.; Ohtani, I.; Kusumi,
T.; Kakisawa, H. J. Am. Chem. Soc. 1989, 111, 8925-8926. (b) Ohtani, I.;
Kusumi, T.; Kakisawa, H.; Kashman, Y.; Hirsh, S. J. Am. Chem. Soc. 1992,
114, 8372-8479. (c) Jares-Erijman, E. A.; Sakai, R.; Rinehart, K. L. J.
Org. Chem. 1991, 56, 5712-5715. (d) Jares-Erijman, E. A.; Ingrum, A.
L.; Carney, J. R.; Rinehart, K. L.; Sakai, R. J. Org. Chem. 1993, 58, 4805-
4808. (e) Rinehart, K. L.; Jares-Erijman, E. A. U.S. Patent 5,756,734, 1998.
(f) Shi, J.-G.; Sun, F.; Rinehart, K. L. WO Patent 3,756,734, 1998. (g)
Berlinck, R. G. S.; Braekman, J. C.; Daloze, D.; Bruno, I.; Riccio, R.; Ferri,
S.; Spampinato, S.; Speroni, E. J. Nat. Prod. 1993, 56, 1007-1015. (h)
Palagiano, E.; De Marino, S.; Minale, L.; Riccio, R.; Zollo, F.; Iorizzi, M.;
Carre´, J. B.; Debitus, C.; Lucarain, L.; Provost, J. Tetrahedron 1995, 51,
3675-3682. (i) Braekman, J. C.; Daloze, D.; Tavares, R.; Hajdu, E.; Van
Soest, R. W. M. J. Nat. Prod. 2000, 63, 193-196. (j) Chang, L. C.;
Whittaker, N. F.; Bewley, C. A. J. Nat. Prod. 2003, 66, 1490-1494. (k)
Aoki, S.; Kong, D.; Matsui, K.; Kobayashi, M. Anticancer Res. 2004, 24,
2325-2330.
Various natural products contain the guanidine functional
group embedded in a six-membered ring, such as the Na+
channel blocker saxitoxin (1), the puffer fish poison tetrodotoxin
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(b) Berlinck, R. G. S. Nat. Prod. Rep. 2002, 19, 617-649. (c) Faulkner, D.
J. Nat. Prod. Rep. 1999, 16, 155-198. (d) Berlinck, R. G. S. Nat. Prod.
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250 and references contained therein.
10.1021/jo061199m CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/26/2006
7706
J. Org. Chem. 2006, 71, 7706-7714

Synthesis of Guanidine-Containing Heterocycles
and their analogues, although highly refined, requires many
steps. We have, therefore, directed some effort to the develop-
ment of new chemistry that will allow access to analogues of
these guanidine-containing compounds in just a few steps. The
Biginelli reaction20 utilizes urea, an aldehyde, and a â-ketoester
in a three-component condensation giving rise to dihydropyri-
midinone products.14 The related condensation of guanidine, an
aldehyde, and a â-ketoester to form six-membered guanidine-
containing heterocycles, 2-imino-5-carboxy-3,4-dihydropyrim-
idines 10, (eq 1) is much less well developed.14,21 Cho21a and
Atwal21b both demonstrated that reactions between the Knoev-
enagel product of ethyl acetoacetate and 3-nitrobenzaldehyde
with guanidine or methylguanidine give Biginelli adducts, albeit
in low yields (14-25%). Milcent showed that yields are
improved when the R1 substituent of the â-ketoester is phenyl
rather than methyl (eq 2).21c In these latter examples, double
Biginelli adducts 12 are also observed as byproducts when
guanidine is used. Kappe also showed that guanidine can be
successfully used in a traditional three-component Biginelli
reaction when the â-ketoester possesses a phenyl substituent
(as in the case of ethyl benzoyl acetate, R1 ) Ph) to give
Biginelli adducts in satisfactory yields while avoiding double
Biginelli byproducts.21d Finally, multistep Biginelli-aminolysis
strategies in which isoureas or isothioureas are used as guanidine
precursors to access 2-imino-3,4-dihydropyrimidines are pre-
cedented in the literature.22 For example, Atwal demonstrated
the aminolysis of Biginelli adducts containing methyl isourea22a
FIGURE 1.
interactions,4g,5a,b,7 voltage-sensitive Ca2+ channels,4j and Na+,
K+, and Ca2+ ATPases.8 The complex architecture of the cram-
bescidins and the batzelladines and their compelling biological
activities have sparked intense effort toward their total chemical
synthesis.9 Notably, the Snider10 and Murphy11 groups developed
biomimetic approaches, the Nagasawa12 group used a 1,3-dipolar
cycloaddition approach, and the Overman13 group pioneered the
use of tethered Biginelli9b,14 reactions for assembly of cramb-
escidin alkaloids. The batzelladines and their analogues have
also attracted considerable synthetic attention from the Overman,15
Snider,16 Murphy,17 and Nagasawa18 groups, among others.19
The existing chemistry for the synthesis of complex guanidine
natural products of the crambescidin and batzelladine families
(13) (a) Overman, L. E.; Rabinowitz, M. H. J. Org. Chem. 1993, 58,
3235-3237. (b) Overman, L. E.; Rabinowitz, M. H.; Renhowe, P. A. J.
Am. Chem. Soc. 1995, 117, 2657-2658. (c) Coffey, D. S.; McDonald, A.
I.; Overman, L. E.; Stappenbeck, F. J. Am. Chem. Soc. 1999, 121, 6944-
6945. (d) Coffey, D. S.; McDonald, A. I.; Overman, L. E.; Rabinowitz, M.
H.; Renhowe, P. A. J. Am. Chem. Soc. 2000, 122, 4893-4903. (e) Coffey,
D. S.; Overman, L. E.; Stappenbeck, F. J. Am. Chem. Soc. 2000, 122, 4904-
4914. (f) Aron, Z. F.; Overman, L. E. J. Am. Chem. Soc. 2005, 127, 3380-
3390. (g) Rhee, Y. H.; Overman, L. E. J. Am. Chem. Soc. 2005, 127, 15652-
15658.
(5) (a) Patil, A. D.; Kumar, N. V.; Kokke, W. C.; Bean, M. F.; Freyer,
A. J.; De Brosse, C.; Mai, S.; Truneh, A.; Faulkner, D. J.; Carte, B.; Breen,
A. L.; Hertzberg, R. P.; Johnson, R. K.; Westley, J. W.; Potts, B. C. M. J.
Org. Chem. 1995, 60, 1182-1188. (b) Patil, A. D.; Freyer, A. J.; Taylor,
P. B.; Carte´, B.; Zuber, G.; Johnson, R. K.; Faulkner, D. J. J. Org. Chem.
1997, 62, 1814-1819.
(6) (a) Moore, C. G.; Murphy, P. J.; Williams, H. L.; McGown, A. T.;
Smith, N. K. Tetrahedron Lett. 2003, 44, 251-254. (b) Black, G. P.; Coles,
S. J.; Hizi, A.; Howard-Jones, A. G.; Hursthouse, M. B.; McGown, A. T.;
Loya, S.; Moore, C. G.; Murphy, P. J.; Smith, N. K.; Walshe, N. D.
Tetrahedron Lett. 2001, 42, 3377-3381. (c) Aron, Z. D.; Pietraszkiewicz,
H.; Overman, L. E.; Valeriote, F.; Cuevas, C. Bioorg. Med. Chem. Lett.
2004, 14, 3445-3449.
(7) (a) Olszewski, A.; Sato, K.; Aron, Z. D.; Cohen, F.; Harris, A.;
McDougall, B. R.; Robinson, W. E., Jr.; Overman, L. E.; Weiss, G. A.
Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 14079-14084. (b) Olszewski, A.;
Weiss, G. A. J. Am. Chem. Soc. 2005, 127, 12178-12179.
(8) (a) Ohizumi, Y.; Sasaki, S.; Kusumi, T.; Ohtani, I. I. Eur. J.
Pharmacol. 1996, 310, 95-98. (b) Georgieva, A.; Hirai, M.; Hashimoto,
Y.; Nakata, T.; Ohizumi, Y.; Nagasawa, K. Synthesis 2003, 1427-1432.
(9) For brief reviews of the synthesis and isolation of crambescidins,
see: (a) Reference 2b. (b) Aron, Z. D.; Overman, L. E. Chem. Commun.
2004, 253-265. (c) Heys, L.; Moore, C. G.; Murphy, P. J. Chem. Soc.
ReV. 2000, 29, 57-67.
(14) For a comprehensive review of the Biginelli reaction, see: Kappe,
C. O.; Stadler, A. Org. React. 2004, 63, 1-116.
(15) (a) Franklin, A. S.; Ly, S. K.; Mackin, G. H.; Overman, L. E.; Shaka,
A. J. J. Org. Chem. 1999, 64, 1512-1519. (b) Cohen, F.; Overman, L. E.;
Sakata, S. K. L. Org. Lett. 1999, 1, 2169-2172. (c) Cohen, F.; Overman,
L. E. J. Am. Chem. Soc. 2001, 123, 10782-10783. (d) Cohen, F.; Collins,
S. K.; Overman, L. E. Org. Lett. 2003, 5, 4485-4488. (e) Cohen, F.;
Overman, L. E. J. Am. Chem. Soc. 2006, 128, 2594-2603. (f) Cohen, F.;
Overman, L. E. J. Am. Chem. Soc. 2006, 128, 2604-2608.
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Lett. 1996, 37, 6977-6980. (b) Snider, B. B.; Chen, J. Tetrahedron Lett.
1998, 39, 5697-5700.
(17) (a) See ref 11a. (b) Black, G. P.; Murphy, P. J.; Walshe, N. D. A.;
Hibbs, D. E.; Hursthouse, M. B.; Abdul Malik, K. M. Tetrahedron Lett.
1996, 37, 6043-6046. (c) Black, G. P.; Murphy, P. J.; Walshe, N. D. A.
Tetrahedron 1998, 54, 9481-9488. (d) Black, G. P.; Murphy, P. J.;
Thornhill, A. J.; Walshe, N. D. A.; Zanetti, C. Tetrahedron 1999, 55, 6547-
6554.
(18) (a) Nagasawa, K.; Koshino, H.; Nakata, T. Org. Lett. 2001, 3, 4155-
4158. (b) Ishiwata, T.; Hino, T.; Koshino, H.; Hasimoto, Y.; Nakata, T.;
Nagasawa, K. Org. Lett. 2002, 4, 2921-2924.
(19) (a) Louwrier, S.; Ostendorf, M.; Tuynman, A.; Hiemstra, H.
Tetrahedron Lett. 1996, 37, 905-908. (b) Duron, S. G.; Gin, D. Y. Org.
Lett. 2001, 3, 1551-1554.
(10) (a) Snider, B. B.; Shi, Z. Tetrahedron Lett. 1993, 34, 2099-2102.
(b) Snider, B. B.; Shi, Z. J. Am. Chem. Soc. 1994, 116, 549-557.
(11) (a) Murphy, P. J.; Williams, H. L.; Hursthouse, M. B.; Abdul Malik,
K. M. J. Chem. Soc., Chem. Commun. 1994, 119-120. (b) Murphy, P. J.;
Williams, H. L.; Hibbs, D. E.; Hursthouse, M. B.; Abdul Malik, K. M.
Tetrahedron 1996, 52, 8315-8332. (c) Moore, C. G.; Murphy, P. J.;
Williams, H. L.; McGown, A. T.; Smith, A. K. Tetrahedron Lett. 2003,
44, 251-254.
(20) Biginelli, P. Gazz. Chim. Ital. 1893, 23, 360.
(21) (a) Cho, H.; Shima, K.; Hayashimatsu, M.; Ohnaka, Y.; Mizuno,
A.; Takeuchi, Y. J. Org. Chem. 1985, 50, 4227-4230. (b) Atwal, K. S.;
Rovnyak, G. C.; Schwartz, J.; Moreland, S.; Hedberg, A.; Gougoutas, J.
Z.; Malley, M. F.; Floyd, D. M. J. Med. Chem. 1990, 33, 1510-1515. (c)
Milcent, R.; Malanda, J.-C.; Barbier, G. J. Heterocycl. Chem. 1997, 34,
329-336. (d) Vanden Eynde, J. J.; Hecq, N.; Kataeva, O.; Kappe, C. O.
Tetrahedron 2001, 57, 1785-1791.
(22) (a) Atwal, K. S.; Rovnyak, G. C.; O’Reilly, B. C.; Schwartz, J. J.
Org. Chem. 1989, 54, 5898-5907. (b) Kappe, C. O. Bioorg. Med. Chem.
Lett. 2000, 10, 49-51.
(12) (a) Nagasawa, K.; Georgieva, A.; Nakata, T. Tetrahedron 2000,
56, 187-192. (b) Nagasawa, K.; Georgieva, A.; Nakata, T.; Kita, T.;
Hasimoto, Y. Org. Lett. 2002, 4, 177-180.
J. Org. Chem, Vol. 71, No. 20, 2006 7707

Nilsson and Overman
SCHEME 2
fragments, whereas Kappe prepared 2-imino-3,4-dihydropyri-
midine salts by aminolysis of resin-bound isothiourea Biginelli
adducts.22b
In the course of our efforts to prepare focused libraries of
polycyclic guanidines, we discovered that direct three-compo-
nent Biginelli reactions with guanidine are useful only with
benzoyl acetates and aryl aldehydes. Attempted reactions using
acetoacetates (R1 ) Me) failed to give useful yields of the
desired Biginelli adducts. We thus undertook the development
of more general Biginelli-based methods for preparing 2-imino-
5-carboxy-3,4-dihydropyrimidines.
TABLE 1. Synthesis of Biginelli Adducts 18 Using Pyrazole
Carboxamidine Hydrochloride 17
entry
R′
R
product
yield (%)
1
2
3
4
5
6
Et
Et
Bn
Bn
Bn
Et
Ph
18a
18b
18c
18d
18e
18f
73
58
74
65
63
60
o-vinyl-C6H4
i-Pr
n-Pr
cyclohexyl
m-NO2-C6H4
Results
Synthesis of 2-Imino-5-carboxy-3,4-dihydropyrimidines:
Aminolysis Strategy. Our initial efforts focused on Biginelli
reactions with guanidine surrogates followed by aminolysis of
the resulting products (Scheme 1). Atwal-type Biginelli reactions
to give substrates of type 15 would improve aminolysis rates,
similar to effects reported by Atwal.22a However, <10%
conversion of Boc-protected methoxydihydropyrimidines 15 to
aminolysis products 16 was observed, even after 45 h at 70 °C.
In an attempt to identify Biginelli products in which
subsequent aminolysis to generate a guanidine functional group
would be more facile, pyrazole carboxamidine hydrochloride
(17) was chosen as a coupling partner for Biginelli condensations
(Scheme 2). It was reasoned that Biginelli adducts 18 would
be efficient substrates for aminolysis based on the common use
of 17 as a guanylating agent.24 The initial Biginelli condensation
in this sequence when carried out in DMF at 70 °C proceeded
in moderate to good yields (58-73%, Table 1). Reaction times
of 48 h were required, as the final dehydration step to form the
SCHEME 1
∆
5,6 alkene was slow relative to the comparable step in Biginelli
reactions employing urea. Both aromatic and aliphatic aldehydes
could be effectively used as reaction substrates. However, three
of the representative aldehydes we examined did not give rise
to the desired products: 2-furaldehyde and pivaldehyde failed
to give isolable Biginelli adducts, whereas p-nitrobenzaldehyde
provided marginal yields of Biginelli products as part of
intractable mixtures. Biginelli products 18 are depicted in
Scheme 2 as the ∆2,3 tautomers; however, NMR analysis shows
that these heterocycles are mixtures of the ∆2,3 and the ∆1,2
tautomers in variable ratios.
with O-methylisourea 13 and Knoevenagel precursors 11 to give
dihydropyrimidines 14 are well established.23 However, ami-
nolysis of these methoxy-1,4-dihydropyrimidines proved to be
problematic in our hands. Specifically, it was found that direct
aminolysis of methoxydihydropyrimidines 14 was prohibitively
sluggish. We hoped that installation of an acyloxy group at N3
We initially examined direct aminolysis of Biginelli adducts
18 using the conditions described by Atwal22a for related methyl
isourea adducts. These Biginelli products were dissolved in THF,
(24) (a) Bernatowicz, M. S.; Wu, Y.; Matsueda, G. R. J. Org. Chem.
1992, 57, 2497-2502. (b) Bernatowicz, M. S.; Wu, Y.; Matsueda, G. R.
Tetrahedron Lett. 1993, 34, 3389-3392.
(23) O’Reilly, B. C.; Atwal, K. S. Heterocycles 1987, 26, 1185-1188.
7708 J. Org. Chem., Vol. 71, No. 20, 2006

Synthesis of Guanidine-Containing Heterocycles
TABLE 2. Conversion of Pyrazole Biginelli Adducts 18 to Guanidines 10 by the Sequence Depicted in Scheme 2
entry
R′
R
product
yield (%)
product
yield (%)
product
yield (%)
1
2
3
4
5
6
Et
Et
Bn
Bn
Bn
Et
Ph
19a
19b
19c
19d
19e
19f
67
86
68
99
89
53
16a
16b
16c
16d
16e
16f
77
60
63
70
86
75
10a
10b
10c
10d
10e
10f
90
65
86
68
95
94
o-vinyl-C6H4
i-Pr
n-Pr
cyclohexyl
m-NO2-C6H4
SCHEME 3
cooled to 0 °C, and saturated with ammonia by bubbling
ammonia gas through the solution for 15 min. The resulting
solution was then heated at 70 °C in a sealed tube. Direct
aminolysis under these conditions was too slow to be practical.
However, introduction of a Boc group significantly improved
the rate of aminolysis. Substrates 18a-f were Boc-protected
using standard conditions (DMAP, Boc2O) to give compounds
19a-f in good yields (Table 2). These products exist as mixtures
of tautomers, with the Boc substituent assumed to be at N3 based
on precedent for related functionalizations of 2-methoxy- and
2-[[(4-methoxyphenyl)methyl]thio]-1,4-dihydropyrimidines.22a
Substrates 19a-f typically underwent aminolysis at 70 °C within
24 h to give good yields of Boc-protected guanidine products
16a-f (Table 2).25
Removal of the Boc group from aminolysis products 16 was
facile. Exposing compounds 16a-f to 50% TFA in dichlo-
romethane at room temperature for 1 h gave 2-imino-5-carboxy-
3,4-dihydropyrimidines 10a-f as their trifluoroacetate salts
(Table 2). Recrystallization of these salts provided pure products
in good to excellent yields.
The pyrazole Biginelli reaction-aminolysis sequence outlined
in Scheme 2 provides a reasonably general method for preparing
2-imino-5-carboxy-3,4-dihydropyrimidines in four steps from
a â-ketoester, an aldehyde, and a pyrazole carboxamidine
hydrochloride (17). For the substrates examined, overall yields
ranged from 19 to 46%. Of most significance, this strategy can
be used with aliphatic as well as aromatic aldehydes and is not
limited to the use of benzoyl acetates as the â-ketoester
component.
Protected Guanidine Strategy. Although Biginelli reactions
with guanidines containing a single alkyl substituent proceed
in low to moderate yields,21a-c we postulated that N,N-
dialkylguanidines might give improved yields in Biginelli
condensations. Initial support for this supposition came from
the Biginelli reaction of N,N-diallylguanidine,26 benzaldehyde,
and benzyl acetoacetate in sodium bicarbonate-buffered DMF,
which gave the Biginelli adduct in 80% yield (70 °C, 11 h).
However, preliminary attempts to remove the allyl protecting
groups from the resulting product were not encouraging.
On the basis of this result, other substituted guanidines were
surveyed in order to identify a protected guanidine that would
both participate in Biginelli condensations and allow subsequent
deprotection to be accomplished efficiently. Neither Boc-guani-
dine27 nor p-tosylguanidine28 were reactive in Biginelli conden-
sations using numerous reaction conditions (acidic, basic, Lewis
acidic).14 The reduced basicity of these guanidines compared
to N,N-diallylguanidine likely accounts for this lack of reactiv-
ity.29 1,3,5-Triaz-4-ones have been employed as masked primary
amines.30 We reasoned that the related triazone derivative 20
(Scheme 3) would more closely approximate the basicity and
nucleophilicity of N,N-dialkylguanidines during Biginelli con-
densations to give products that potentially could be converted
to simple guanidines under acidic conditions (Scheme 3).
The preparation of 3,5-dimethyl-4-oxo-[1,3,5]triazinane-1-
carboxamidine (20) is summarized in Scheme 4. Benzylamine
was first converted to triazone derivative 25 by standard
condensation with N,N′-dimethylurea and aqueous formalde-
hyde. The benzyl group was subsequently removed by high-
pressure hydrogenation to give 1,3-dimethyl-[1,3,5]triazinan-
2-one (26) in 96% yield. Upon heating triazone 26 and N,N′-
bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine (27) at
70 °C in THF, the di-Boc-protected guanidine precursor 28 was
formed in 53% yield (80% based on consumed starting material);
extended reaction times or elevated temperatures gave rise to
unwanted byproducts. The Boc-protecting groups of product 28
were easily removed by reaction with 50% TFA in dichlo-
romethane at room temperature to give the triazone-protected
guanidine 20 in quantitative yield.
Triazone-protected guanidine 20 performed well in three-
component Biginelli condensations. Heating an aldehyde (aro-
matic or aliphatic), â-ketoester, and guanidine 20 at 70 °C for
12 h in sodium bicarbonate-buffered DMF gave good yields
(62-86%) of Biginelli adducts 21a-j (Table 3). These Biginelli
condensations reached completion more quickly than the
corresponding reactions of pyrazole carboxamidine 17. In
addition, 2-furaldehyde, pivaldehyde, and p-nitrobenzaldehyde
were used successfully in Biginelli reactions with 20.
(25) We initially included ammonium chloride (0.5 equiv) in the reaction
mixture to serve as a proton source. Later, it was found that omitting
ammonium chloride had no deleterious effect on the outcome of these
aminolysis reactions.
(26) Schaefer, F. C.; Wright, A. C. U.S. Patent 3,734,939, 1973.
(27) Zapf, C. W.; Creighton, C. J.; Tomioka, M.; Goodman, M. Org.
Lett. 2001, 3, 1133-1136.
(28) Qi, Y.; Gao, H.; Yang, M.; Xia, C.-G.; Suo, J. Synth. Commun.
2003, 33, 1073-1079.
(29) The pKa of N,N′-dimethylguanidinium sulfate is 13.4 and that of
carbamate-substituted guanidines is ∼7.0; the pKa of N-tosylguanidines
should be lower, pKa ∼2.0. See: Taylor, P. J.; Wait, A. R. J. Chem. Soc.,
Perkin Trans. 2 1986, 1765-1770.
(30) (a) Knapp, S.; Hale, J. J.; Bastos, M.; Gibson, F. S. Tetrahedron
Lett. 1990, 31, 2109-2112. (b) Knapp, S.; Hale, J. J.; Bastos, M.; Molina,
A.; Chen, K. Y. J. Org. Chem. 1992, 57, 6239-6256. (c) Knight, S. D.;
Overman, L. E.; Pairaudeau, G. J. Am. Chem. Soc. 1995, 117, 5776-5788.
J. Org. Chem, Vol. 71, No. 20, 2006 7709

Nilsson and Overman
SCHEME 4
TABLE 4. Deprotection of Triazone-Protected Biginelli Adducts
entry
R′
R
product
yield (%)
1
2
3
4
5
6
7
8
9
Et
Et
Bn
Bn
Bn
Et
Et
Bn
Et
Ph
10a
10b
10c
10d
10e
10f
10g
10h
10i
84
69
84
76
83
84
o-vinyl-C6H4
i-Pr
n-Pr
cyclohexyl
m-NO2-C6H4
2-furan
t-Bu
p-NO2-C6H4
2,4-di-MeO-C6H4
79
10
Et
10j
SCHEME 5
TABLE 3. Biginelli Reactions with Triazone-Protected Guanidine
20
entry
R′
R
product
yield (%)
1
2
3
4
5
6
7
8
9
Et
Et
Bn
Bn
Bn
Et
Et
Bn
Et
Ph
21a
21b
21c
21d
21e
21f
21g
21h
21i
73
73
86
76
91
71
67
43
86
62
o-vinyl-C6H4
i-Pr
n-Pr
cyclohexyl
m-NO2-C6H4
2-furanyl
t-Bu
limited to substrates that survive the strongly acidic conditions
required to remove the triazone-protecting group from the
Biginelli product.
p-NO2-C6H4
2,4-di-MeO-C6H4
10
Et
21j
Transformation of 2-Imino-5-carboxy-3,4-dihydropyri-
midinones Derived From Masked Dialdehydes to Hexahy-
drotriazaacenaphthalenes. With two methods for the prepa-
ration of 2-imino-5-carboxy-3,4-dihydropyrimidines in hand, we
applied these procedures in the synthesis of hexahydrotriaza-
acenaphthalene structures, envisioning the use of a masked
dialdehyde 29 (Scheme 5) in an initial Biginelli reaction to give
2-iminodihydropyrimidine products 30. The protected aldehyde
functional group would then be unmasked to generate pyrrol-
opyrimidinium hemiaminals of type 31. These cyclic hemiami-
nals would finally be used in tethered Biginelli reactions9b with
a second â-ketoester to give hexahydrotriazaacenaphthalenes
32.
A variety of Boc-protected 2-imino-5-carboxy-3,4-dihydro-
pyrimidines 30a-e were synthesized from masked dialdehydes
29a,b31 using the pyrazole variant of our Biginelli-aminolysis
strategy (Scheme 6). The initial Biginelli adducts 33a-e were
synthesized in yields ranging from 61 to 73%. These Biginelli
condensations were successful with the monodimethyl acetal-
protected dialdehydes 29a,b and â-ketoesters having various
We turned to examine conditions for removal of the triazone
group from compounds 21a-j in order to reveal the unprotected
guanidine functional group. Unfortunately, the triazone ring of
these guanidines was not unraveled under the mildly acidic
conditions previously used for cleaving triazone derivatives of
alkylamines.30b For example, exposure of these products to 2-6
N HCl in EtOH at room temperature resulted in no deprotection
after 16 h; various other acids gave similar results. Heating
adducts 21a-j in 6 N HCl in EtOH to 60 °C gave incomplete
deprotection in the same time period. Finally, it was discovered
that heating these substrates in 6 N aqueous HCl at 60 °C for
24 h in a sealed tube (to prevent loss of HCl) gave complete
deprotection in most cases (Table 4). Not surprisingly, these
relatively harsh acidic conditions were problematic with some
substrates; compounds 21g, 21h, and 21j underwent extensive
decomposition under these conditions.
This triazone-protected guanidine Biginelli strategy provides
2-imino-5-carboxy-3,4-dihydropyrimidines in only two steps
with good overall yields (53-76%). However, this sequence is
7710 J. Org. Chem., Vol. 71, No. 20, 2006

Synthesis of Guanidine-Containing Heterocycles
TABLE 5. Triazaacenaphthalene and Related Structures
entry
R1
Et
Et
Et
Et
Et
Et
allyl
R2
R3
R4
n
product
yield (%)
dr (syn:anti)
1
2
3
4
5
6
7
Me
Ph
Ph
Me
allyl
Et
allyl
Bn
Me
Me
1
1
1
1
1
1
2
32a
32b
32c
32d
32e
32f
70
68
86
90
54
51
27
3:1
1:1.3
3.2:1
1:1.3
3.2:1
1:1
C6F5
p-NO2-C6H4
p-NO2-C6H4
p-NO2-C6H4
Me
Me
Me
allyl
Et
(CH2)4OBn
p-NO2-Ph
32g
1:0
SCHEME 6
SCHEME 7
linium acetate as a promoter and sodium sulfate as a desiccant
to give hexahydrotriazaacenaphthalene tethered-Biginelli prod-
ucts 32.
This five-step sequence was employed to prepare six hexahy-
drotriazaacenaphthalenes (32a-f, Table 5). The overall yields
for these syntheses ranged from 11 to 42%. For the hexahy-
drotriazaacenaphthalene products (n ) 1), the yields for the
tethered Biginelli reactions ranged from 51 to 90%. However,
little diastereoselectivity was observed, a result that stands in
contrast to the tethered Biginelli reactions reported previously
from our laboratories.9b,13,15 The syn relative configuration was
1
slightly favored (3:1, verified by H NMR NOE correlations)
groups at R1 (ethyl and allyl esters) and R2 (methyl, phenyl,
p-nitrophenyl). Boc protection and subsequent aminolysis of
these Biginelli products proceeded in good yields to give
products 30a-e.
The Boc-protected 2-imino-5-carboxy-3,4-dihydropyrimidine
products were converted to hexahydrotriazaacenaphthalenes as
summarized in Scheme 7. Treatment of compounds 30a-e with
trifluoroacetic acid at room temperature resulted in cleavage of
the Boc and dimethylacetal groups and ring closure to give
tetrahydropyrrolopyrimidinium salts of type 31. The solvent and
excess TFA were removed from these products under reduced
pressure; these crude intermediates were then redissolved in
trifluoroethanol containing an excess of a second â-ketoester.
These solutions were then heated in the presence of morpho-
for several of these products (32a,c, and e). Other hexahydro-
triazaacenaphthalene adducts were isolated as 1:1 mixtures of
epimers. In all cases, the syn and anti epimers were cleanly
separated using standard flash chromatography techniques.
In addition to the preparation of triazaacenaphthalene struc-
tures, one derivative that incorporates a fused piperidine ring,
hexahydro-1H-1,9,9b-diazaphenalene 32g, was also synthesized.
The inclusion of the six-membered piperidine ring resulted in
a lower yield for the tethered Biginelli reaction (27%). However,
this reaction proceeded with high diastereoselection to produce
exclusively the syn product.
Discussion
Biginelli strategies utilizing pyrazole carboxamidine 17 or
triazone-protected guanidine 20 enable the concise and general
synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines. Only
a few members of this class of heterocycles had previously been
prepared using Biginelli reactions, which suffered from low
yields or lack of generality in one or more of the reactants.21,22
(31) The monoprotected dialdehyde precursors were synthesized as
described in: Davis, F. A.; Zhang, H.; Lee, S. H. Org. Lett. 2001, 3, 759-
762. Spectral data were consistent with previously published data. For
28a: Griesbaum, K.; Jung, I. C.; Mertens, H. J. Org. Chem. 1990, 55, 6024-
6027. For 28b: Boeckman, R. K., Jr.; Ko, S. S. J. Am. Chem. Soc. 1982,
104, 1033-1041.
J. Org. Chem, Vol. 71, No. 20, 2006 7711

Nilsson and Overman
decidedly non-natural analogues of the crambescidin and the
batzelladine alkaloids. In addition to alternative sizes for the
central fused ring and the substituents R1-R4, a variety of other
analogues undoubtedly can be synthesized from core structures
of type 32.
FIGURE 2.
Conclusion
Natural products that contain guanidine units within six-
membered heterocyclic rings possess diverse biological activi-
ties. The short synthesis of 2-imino-5-carboxy-1,4-dihydropy-
rimidines and hexahydrotriazaacenaphthalenes reported herein
will likely find use for the synthesis of focused libraries of
guanidine alkaloid-like structures. We are applying and extend-
ing the strategies described herein to the preparation of such
libraries in order to more completely probe the biology of these
fascinating guanidine-containing heterocycles.
There are advantages and limitations to each method developed
during the present study. The Biginelli reaction with pyrazole
carboxamidine 17 followed by aminolysis utilizes relatively mild
conditions throughout. However, the initial Biginelli step
involves a slow dehydration that requires prolonged heating to
drive to completion. In addition, efficient aminolysis is depend-
ent upon the introduction of a Boc substituent. This protection
step and the subsequent deprotection reaction add two steps to
the route. Despite these limitations, this method is preferable
for substrates having sensitive functionality, as most of the
chemistry is relatively mild. In contrast, the synthesis of 2-imino-
5-carboxy-3,4-dihydropyrimidines from triazone guanidine 20
requires only two steps: the Biginelli reaction, which is
complete within 12 h, and the final deprotection step. This
Biginelli reaction is more general in scope than that employing
pyrazole carboxamidine 17, as every aldehyde surveyed pro-
vided the desired adduct. The major limitation of this second
strategy is the harsh deprotection step (strong acid, heat), which
is incompatible with some functionality. Taken together, these
two strategies provide ready access to a wide range of 2-imino-
5-carboxy-3,4-dihydropyrimidines. Both strategies provide these
products as racemates.32
We have applied the pyrazole carboxamidine 17 Biginelli-
aminolysis sequence to masked dialdehydes to ultimately
generate tetrahydropyrrolopyrimidinium intermediates 31, which
were transformed by tethered Biginelli reactions to a variety of
hexahydrotriazaacenaphthalenes 32. This chemistry provides
access to the core structures of the crambescidin and the
batzelladine alkaloids in a succinct five-step sequence from
readily available starting materials.
The standard Knoevenagel conditions used in this study for
the tethered Biginelli condensation (1 equiv of morpholinium
acetate, trifluoroethanol) were previously shown to favor syn
stereoselection with similar substrates.15a Thus, tethered Biginelli
reactions of hexahydropyrrolopyrimidinium substrates 35 (Fig-
ure 2) proceeded with up to 9:1 stereoselectivity under es-
sentially identical conditions.15 These studies showed that the
stereochemical outcome of this reaction can be rather solvent
and temperature dependent, with lower temperatures and more
polar solvents improving stereoselectivity.15a We were, therefore,
somewhat surprised by the low selectivities observed for tethered
Biginelli reactions of tetrahydropyrrolopyridinium substrates 31.
The incorporation of a second double bond in substrates of type
31 compared to those of type 33 (Figure 2) leads to some
flattening of the bicyclic ring system. This change may account
for the erosion in stereoselectivity.
Experimental Section33
6-Methyl-4-phenyl-2-pyrazol-1-yl-1,4-dihydropyrimidine-5-
carboxylic acid ethyl ester (18a). Representative Procedure for
Synthesis of Biginelli Products 18. Benzaldehyde (1.14 mL, 11.3
mmol), ethyl acetoacetate (1.45 mL, 11.3 mmol), pyrazole car-
boxamidine hydrochloride 17 (1.86 g, 12.5 mmol), and NaHCO3
(3.8 g, 45 mmol) were heated in DMF (16.5 mL) at 70 °C for 48
h. After cooling to room temperature, the solution was diluted with
water (50 mL) and washed with Et2O (3 × 50 mL). The combined
organic layers were dried over anhydrous MgSO4, filtered, and
concentrated. The residue was purified by flash chromatography
(silica gel, 20% Et2O in hexanes f 40% Et2O in hexanes) to give
compound 18a as a pale yellow oil (2.65 g, 73% yield, a 1.6:1
mixture of tautomers): 1H NMR (500 MHz, DMSO-d6) major
tautomer δ 9.56 (d, J ) 3.5 Hz, 1 H), 8.46 (d, J ) 3 Hz, 1 H), 7.86
(s, 1 H), 7.37-7.31 (m, 4 H), 7.28-7.23 (m, 1 H), 6.58-6.57 (m,
1 H), 5.59 (d, J ) 3.5 Hz, 1 H), 4.08-4.03 (m, 2 H), 2.42 (s, 3 H),
1.14 (t, J ) 7 Hz, 3 H) ppm; minor tautomer δ 9.88 (s, 1 H), 8.34
(d, J ) 2.5 Hz, 1 H), 7.83 (s, 1 H), 7.37-7.31 (m, 4 H), 7.28-
7.23 (m, 1 H), 6.54-6.53 (m, 1 H), 5.65 (s, 1 H), 4.08-4.03 (m,
2 H), 2.47 (s, 3 H), 1.13 (t, J ) 7 Hz, 3 H) ppm; 13C NMR (125
MHz, DMSO-d6) all peaks from both tautomers δ 165.9, 165.8,
156.6, 146.9, 146.2, 145.5, 144.5, 143.1, 141.5, 141.3, 128.8, 128.4,
128.3, 128.0, 127.6, 127.0, 126.8, 126.5, 109.1, 108.6, 103.8, 99.8,
59.3, 59.3, 58.0, 52.7, 23.0, 17.6, 14.1, 14.0 ppm; IR (thin film)
3381, 3335, 2983, 2931, 1743, 1716, 1702, 1628, 1532, 1485, 1395,
1370, 1263, 1240, 1200, 1171, 1151, 1091, 1307 cm-1; HRMS
+
(ESI) m/z 311.1513 (311.1508 calcd for C17H19N4O2 [MH]+).
4-Methyl-6-phenyl-2-pyrazol-1-yl-6H-pyrimidine-1,5-dicar-
boxylic acid 1-tert-butyl ester 5-ethyl ester (19a). Representative
Procedure for Synthesis of Boc-Protected Biginelli Adducts 19.
Compound 18a (262 mg, 0.84 mmol) and di-tert-butyl dicarbonate
(Boc2O, 285 mg, 1.01 mmol) were dissolved in MeCN (3.0 mL)
under a N2 atmosphere. DMAP (10 mg, 78 µmol) was added, and
the solution was maintained at room temperature for 12 h. The
solvent was removed in vacuo, and the residue was recrystallized
from EtOH to give 19a as a colorless solid (230 mg, 67%, mp
144-145 °C): 1H NMR (500 MHz, CDCl3) δ 8.10 (s, 1 H), 7.71
(s, 1 H), 7.47 (d, J ) 7.5 Hz, 2 H), 7.32-7.29 (m, 2 H), 7.27 (m,
1 H), 6.45 (s, 1 H), 6.39 (s, 1 H), 4.31-4.22 (m, 2 H), 2.61 (s, 3
H), 1.35 (s, 9 H), 1.29 (t, J ) 7 Hz, 3 H) ppm; 13C NMR (125
MHz, CDCl3) δ 165. 7, 153.5, 151.6, 143.3, 143.1, 138. 5, 129.5,
128.5, 128.1, 126.9, 113.6, 108.6, 83.7, 60.7, 55.0, 27.6, 21.2, 14.3
ppm; IR (thin film) 2983, 2935, 1732, 1707, 1702, 1623, 1564,
1461, 1397, 1370, 1300, 1267, 1231, 1143, 1090, 1038, 963, 908
cm-1; HRMS (ESI) m/z 433.1867 (433.1852 calcd for C22H26N4O4-
The chemistry reported herein can likely be used to synthesize
several heterocyclic architectures in addition to hexahydrotri-
azaacenaphthalenes. One example was demonstrated, the short
construction of hexahydro-1H-1,9,9b-triazaphenalene 32g. This
example demonstrates the utility of this chemistry for preparing
(32) Some progress in orchestrating asymmetric Biginelli reactions has
been described. See: Huang, Y.; Yang, F.; Zhu, C. J. Am. Chem. Soc. 2005,
127, 16386-16387.
(33) General experimental details are described in the Supporting
Information.
7712 J. Org. Chem., Vol. 71, No. 20, 2006

Synthesis of Guanidine-Containing Heterocycles
Na+ [MNa]+). Anal. Calcd for C22H26N4O4: C, 64.37; H, 6.38; N,
13.65. Found: C, 64.24; H, 6.45; N, 13.65.
(567 mg, 96% yield): 1H NMR (500 MHz, CDCl3) δ 3.99 (s, 4
H), 2.68 (s, 6 H) ppm; 13C NMR (125 MHz, CDCl3) δ 155.8, 63.1,
31.5 ppm; IR (thin film) 3420, 3283, 2939, 2871, 1624, 1523, 1418,
1405, 1301, 1029, 906 cm-1; HRMS (ESI) m/z 152.0801 (152.0800
calcd for C5H11N3ONa+ [MNa]+).
2-Amino-4-methyl-6-phenyl-6H-pyrimidine-1,5-dicarboxyl-
ic acid 1-tert-butyl ester 5-ethyl ester (16a). Representative
Procedure for Synthesis of Aminolysis Products 16. A solution
of compound 19a (219 mg, 0.53 mmol), ammonium chloride (14
mg, 0.27 mmol), and THF (2 mL) was cooled to 0 °C, and ammonia
gas was bubbled through for 30 min. The resulting solution was
heated in a sealed tube at 70 °C for 12 h. After cooling to room
temperature, the solvent was removed in vacuo, and the residue
was purified by recrystallization (EtOH) to give 16a as a colorless
solid (147 mg, 77% yield, mp 169-170 °C): 1H NMR (500 MHz,
CDCl3) δ 7.71 (br s, 1 H), 7.40-7.38 (m, 2 H), 7.35-7.29 (m, 3
H), 6.25 (s, 1 H), 4.17 (q, J ) 6.5 Hz, 2 H), 2.37 (s, 3 H), 1.55 (s,
9 H), 1.29 (t, J ) 7 Hz, 3 H) ppm; 13C NMR (125 MHz, CDCl3)
δ 166.5, 156.0, 153.3, 151.8, 142.3, 128.6, 128.0, 127.0, 104.4,
84.7, 59.8, 55.7, 28.1, 22.2, 14.5 ppm; IR (thin film) 3370, 3011,
2986, 1717, 1697, 1651, 1607, 1519, 1370, 1342, 1337, 1297, 1236,
1148, 1115, 1063 cm-1; HRMS (ESI) m/z 360.1929 (360.1923 calcd
for C19H26N3O4+ [MH]+). Anal. Calcd for C19H25N3O4: C, 63.49;
H, 7.01; N, 11.69. Found: C, 63.08; H, 7.12; N, 11.85.
5-Ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihydro-1H-pyrimi-
din-2-ylidene ammonium trifluoroacetate (10a). Representative
Procedure for Synthesis of Heterocyclic Guanidines 10. Method
A: Compound 16a (674 mg, 1.88 mmol) was dissolved in CH2Cl2
(2.5 mL) under nitrogen. TFA (2.5 mL) was added, and the resulting
solution was stirred for 1 h at room temperature. The solvent and
excess TFA were removed under reduced pressure, and the resulting
residue was purified by trituration with Et2O to give trifluoroacetate
salt 10a as a colorless solid (629 mg, 90%): 1H NMR (500 MHz,
CDCl3:CD3OD (1:1)) δ 7.34-7.31 (m, 2 H), 7.28-7.26 (m, 3 H),
5.43 (s, 1 H), 4.10-4.04 (m, 2 H), 2.42 (s, 3 H), 1.14 (t, J ) 7 Hz,
3 H) ppm; 13C NMR (125 MHz, CDCl3:CD3OD (1:1)) δ 164.9,
151.0, 143.4, 141.6, 128.8, 128.4, 126.4, 103.6, 60.5, 53.3, 17.2,
13.6 ppm; IR (thin film) 3247, 3065, 2984, 2875, 1690, 1637, 1556,
1496, 1457, 1386, 1370, 1329, 1273, 1241, 1090, 910 cm-1; HRMS
(ESI) m/z 260.1400 (260.1399 calcd for C14H19N3O2+ [MH]+). Anal.
Calcd for C16H18N3O6F3: C, 51.48; H, 4.86; N, 11.26. Found: C,
51.59; H, 4.87; N, 11.19. Method B: Compound 21a (189 mg,
0.51 mmol) was dissolved in 6 N HCl (10 mL) and heated to 60
°C in a sealed tube for 24 h. After cooling to room temperature,
the reaction solution was then extracted with CH2Cl2 (8 × 10 mL).
The organic layers were dried over MgSO4, filtered, and concen-
trated. The residue was purified by flash chromatography (silica
gel, 10% MeOH in CH2Cl2) to give the HCl salt of 10a as a
colorless solid (126 mg, 84%).
5-Benzyl-1,3-dimethyl-[1,3,5]triazinan-2-one (25). Benzyl-
amine (5 mL, 45.8 mmol), formaldehyde (37% w/w solution, 7.5
mL, 91.6 mmol), and N,N′-dimethylurea (4.03 g, 45.8 mmol) were
charged to a reaction flask equipped with a reflux condenser and
heated to 100 °C under an argon atmosphere for 16 h. After cooling
to room temperature, the reaction was quenched by the addition of
H2O (25 mL) and CH2Cl2 (50 mL). The organic layer was separated
and washed with brine (1 × 25 mL), dried over anhydrous MgSO4,
filtered, and concentrated. The residue was purified by flash
chromatography (silica gel, Et2O f 10% MeOH in Et2O) to give
25 as a colorless solid (7.46 g, 74% yield): 1H NMR (500 MHz,
CDCl3) δ 7.25-7.16 (m, 5 H), 4.00 (s, 4 H), 3.80 (s, 2 H), 2.74 (s,
6 H) ppm; 13C NMR (125 MHz, CDCl3) δ 155.9, 137.5, 129.0,
128.5, 127.6, 67.7, 55.3, 32.4 ppm; IR (thin film) 3452, 2924, 2869,
1629, 1514, 1453, 1417, 1404, 1297, 1151, 1024 cm-1; HRMS
(ESI) m/z 242.1276 (242.1269 calcd for C12H17N3ONa+ [MNa]+).
1,3-Dimethyl-[1,3,5]triazinan-2-one (26). 5-Benzyl-1,3-dimeth-
yl-[1,3,5]triazinan-2-one 25 (1.0 g, 4.56 mmol) was dissolved in
EtOH (25 mL). To this solution was added Pd/C (10%) (140 mg),
and the resulting mixture was heated to 65 °C under an H2
atmosphere (750 psi) for 18 h. After cooling to room temperature,
the suspension was then filtered through Celite, and the eluent was
concentrated by rotary evaporation to give 26 as a colorless solid
tert-Butyloxycarbonylimino-[(3,5-dimethyl-4-oxo-[1,3,5]triazi-
nan-1-yl)methyl]carbamic acid tert-butyl ester (28). N,N′-Bis(t-
butoxycarbonyl)-1H-pyrazole-1-carboxamidine 27 (7.46 g, 24.1
mmol) and 1,3-dimethyl-[1,3,5]triazinan-2-one 26 (3.73 g, 28.9
mmol) were dissolved in dry THF (10 mL) under argon in a flame-
dried flask. This solution was heated at 70 °C for 24 h. After cooling
to room temperature, the solvent was removed in vacuo, and the
residue was purified by flash chromatography (silica gel, 4% MeOH
in CH2Cl2) to give 28 as a colorless solid (4.63 g, 53% yield): 1H
NMR (500 MHz, CDCl3) δ 10.00 (br s, 1 H), 4.67 (s, 4 H), 2.89
(s, 6 H), 1.46 (s, 18 H) ppm; 13C NMR (125 MHz, CD3OD) δ
157.1, 153.7, 81.6, 62.1, 33.0, 28.1 ppm; IR (thin film) 3179, 2981,
2934, 1750, 1636, 1607, 1517, 1392, 1288, 1254, 1226, 1131, 1124
cm-1; HRMS (ESI) m/z 394.2064 (394.2066 calcd for C16H29N5O5-
Na+ [MNa]+).
3,5-Dimethyl-4-oxo-[1,3,5]triazinane-1-carboxamidine trifluo-
roacetate salt (20). Compound 28 (4.93 g, 13.3 mmol) was
dissolved in CH2Cl2 (18.6 mL) under argon, and TFA (18.6 mL)
was added. The resulting solution was maintained at room tem-
perature for 1 h, the solvent was removed under reduced pressure,
and the resulting residue was crystallized from Et2O to give 20 as
a colorless solid (3.8 g, 100% yield, mp 179-181 °C): 1H NMR
(500 MHz, CD3OD) δ 4.73 (s, 4 H), 2.88 (s, 6 H) ppm; 13C NMR
(125 MHz, CD3OD) δ 161.3, 158.4, 63.3, 33.1 ppm; IR (thin film)
3336, 2931, 2882, 2476, 1602, 1527, 1423, 1406, 1313, 1120, 1035,
975 cm-1; HRMS (ESI) m/z 172.1204 (172.1198 calcd for
C6H14N5O+ [MH]+). Anal. Calcd for C8H14F3N5O3: C, 33.69; H,
5.00; N, 24.55. Found: C, 33.83; H, 5.00; N, 24.43.
2-(3,5-Dimethyl-4-oxo-[1,3,5]triazinan-1-yl)-6-methyl-4-phen-
yl-1,4-dihydropyrimidine-5-carboxylic acid ethyl ester (21a).
Representative Procedure for Synthesis of Biginelli Products
21. Compound 20 (293 mg, 1.71 mmol), ethyl acetoacetate (0.2
mL, 1.56 mmol), benzaldehyde (0.16 mL, 1.56 mmol), and NaHCO3
(575 mg, 6.85 mmol) were added to DMF (2.5 mL) under a nitrogen
atmosphere. This mixture was heated at 70 °C for 22 h. The reaction
was cooled to room temperature and poured over crushed ice (25
g). The resulting suspension was extracted with Et2O (1 × 75 mL)
and CH2Cl2 (3 × 25 mL), and the organic layers were dried over
anhydrous MgSO4, filtered, and concentrated. The resulting residue
was recrystallized from Et2O to give 21a as a colorless solid (425
mg, 73% yield, mp 198-200 °C): 1H NMR (500 MHz, CDCl3) δ
7.30-7.21 (m, 5 H), 5.39 (s, 1 H), 4.79-4.72 (m, 4 H), 4.06-
4.02 (m, 2 H), 2.77 (s, 6 H), 2.39 (s, 3 H), 1.19 (t, J ) 7 Hz, 3 H)
ppm; 13C NMR (125 MHz, CDCl3) δ 167.3, 158.7, 156.2, 153.0,
145.2, 128.5, 127.6, 126.5, 102.2, 61.8, 60.0, 54.2, 32.7, 24.0, 14.4
ppm; IR (thin film) 3252, 2981, 2932, 2904, 2886, 2880, 1664,
1629, 1599, 1522, 1503, 1372, 1339, 1303, 1217, 1126, 1061, 1033,
968, 910 cm-1; HRMS (ESI) m/z 372.2030 (372.2036 calcd for
C19H26N5O3+ [MNa]+). Anal. Calcd for C19H25N5O3: C, 61.44; H,
6.78; N, 18.85. Found: C, 61.22; H, 6.94; N, 18.67.
4-(3,3-Dimethoxypropyl)-6-methyl-2-pyrazol-1-yl-1,4-dihy-
dropyrimidine-5-carboxylic acid ethyl ester (33a). Representa-
tive Procedure for Synthesis of Biginelli Products 33. Compound
17 (1.24 g, 8.32 mmol), ethyl acetoacetate (0.96 mL, 7.57 mmol),
and aldehyde 29a (1.0 g, 7.57 mmol) were dissolved in DMF
buffered with NaHCO3 (2.54 g, 30.3 mmol) and heated under a N2
atmosphere for 48 h. The mixture was cooled to room temperature,
diluted with water (20 mL), and the resulting solution was extracted
with Et2O (3 × 15 mL). The combined organic layers were dried
over MgSO4, filtered, and concentrated. The resulting residue was
purified by flash chromatography (silica gel, 100% Et2O) to give
33a as a yellow oil (1.58 g, 62% yield, a 1.3:1 mixture of
tautomers): 1H NMR (500 MHz, CDCl3) major tautomer δ 8.11
(d, J ) 5 Hz, 1 H), 7.87 (s, 1 H), 7.43 (d, J ) 1 Hz, 1 H), 6.25-
J. Org. Chem, Vol. 71, No. 20, 2006 7713

Nilsson and Overman
6.24 (m, 1 H), 4.55 (t, J ) 5 Hz, 1 H), 4.23 (t, J ) 5 Hz, 1 H),
4.09-4.00 (m, 2 H), 3.12 (s, 6 H), 2.21 (s, 3 H), 1.71-1.65 (m, 1
H), 1.59-1.49 (m, 3 H), 1.13 (q, J ) 7 Hz, 3 H) ppm; minor
tautomer δ 8.20 (d, J ) 5 Hz, 1 H), 7.48 (d, J ) 1 Hz, 1 H), 7.35
(d, J ) 2.5 Hz, 1 H), 6.25-6.24 (m, 1 H), 4.48-4.47 (m, 1 H),
4.18 (t, J ) 5 Hz, 1 H), 4.09-4.00 (m, 2 H), 3.14 (s, 3 H), 3.12 (s,
3 H), 2.20 (s, 3 H), 1.71-1.65 (m, 1 H), 1.59-1.49 (m, 3 H), 1.14
(q, J ) 7 Hz, 3 H) ppm; 13C NMR (125 MHz, CDCl3) all peaks
from both tautomers δ 166.5, 157.2, 147.4, 144.8, 142.6, 141.1,
141.0, 128.4, 127.1, 108.7, 108.5, 104.9, 104.5, 104.4, 101.3, 59.68,
59.7, 54.4, 53.0, 52. 6, 52.5, 52.4, 50.2, 32.0, 31.8, 27.6, 27.2, 23.2,
18.6, 14.3, 14.3 ppm; IR (thin film) 3389, 3320, 2982, 2955, 2937,
1697, 1627, 1531, 1484, 1394, 1377, 1241, 1200, 1126, 1105, 1071,
1038 cm-1; HRMS (ESI) m/z 359.1689 (359.1695 calcd for
C16H24N4O4Na+ [MNa]+).
408.2104 (408.2111 calcd for C18H31N3O6Na+ [MNa]+). Anal.
Calcd for C18H31N3O6: C, 56.09; H, 8.11; N, 10.90. Found: C,
56.39; H, 8.05; N, 10.85.
3-Ethoxycarbonyl-8-methoxycarbonyl-4,7-dimethyl-1,2,2a,5,6,-
8a-hexahydro-5,6,8b-triazaacenaphthalene (32a). Representative
Procedure for Synthesis of Triazaacenaphthalenes 32. Com-
pound 30a (200 mg, 0.52 mmol) was stirred in 50% TFA:CH2Cl2
(1.4 mL) under a N2 atmosphere for 1 h. The solvent and excess
TFA were removed under reduced pressure, and the residue was
placed under high vacuum for 8 h. The residue was then dissolved
in trifluoroethanol (1 mL), and to this solution were added
morpholinium acetate (84 mg, 0.57 mmol), Na2SO4 (81 mg, 0.57
mmol), and methyl acetoacetate (0.16 mL, 1.56 mmol). This mixture
was heated at 70 °C for 72 h and then filtered to remove Na2SO4,
concentrated, and the residue was purified by chromatography (silica
gel, 1% MeOH in CH2Cl2 to 5% MeOH in CH2Cl2) to give
compound 32a as a mixture of diastereomers. These epimers were
separated by chromatography (silica gel, EtOAc) to give syn-32a
(119 mg, 52% yield) and anti-32a (40 mg, 18% yield) as tan
trifluoroacetate salt solids. The relative configuration of the angular
hydrogens flanking the pyrrolidine nitrogen was verified by NOESY
cross-peaks. syn-32a: 1H NMR (500 MHz, CDCl3) δ 4.31 (q, J )
5 Hz, 2 H), 4.27-4.20 (m, 1 H), 4.18-4.12 (m, 1 H), 2.65-2.62
(m, 2 H), 2.29 (s, 6 H), 1.94-1.90 (m, 2 H), 1.30 (t, J ) 7 Hz, 3
H) ppm; 13C NMR (125 MHz, CDCl3) δ 166.3, 165.9, 153.1, 152.3,
149.6, 103.7, 103.4, 60.1, 55.8, 51.1, 33.1, 33.0, 20.3, 20.1, 14.6
ppm; IR (thin film) 3283, 3145, 2983, 2948, 2872, 2840, 1691,
1685, 1617, 1522, 1443, 1372, 1338, 1317, 1290, 1269, 1248, 1203,
1183, 1130, 1104, 1079 cm-1; HRMS (ESI) m/z 320.1604
(320.1610 calcd for C16H22N3O4+ [M]+). anti-32a: 1H NMR (500
MHz, CDCl3) δ 4.27-4.15 (m, 4 H), 3.75 (s, 3 H), 2.40-2.38 (m,
2 H), 2.37 (s, 6 H), 1.63-1.60 (m, 2 H), 1.30 (t, J ) 7 Hz, 3 H)
ppm; 13C NMR (125 MHz, CDCl3) δ 166.3, 165.8, 153.1, 152.2,
149.6, 103.7, 103.4, 60.1, 55.8, 51.1, 33.1, 33.0, 20.3, 20.1, 14.6
ppm; IR (thin film) 3190, 3078, 2983, 2908, 2779, 1702, 1697,
1619, 1569, 1541, 1433, 1378, 1320, 1280, 1268, 1238, 1201, 1187,
1166, 1089 cm-1; HRMS (ESI) m/z 320.1616 (320.1610 calcd for
6-(3,3-Dimethoxypropyl)-4-methyl-2-pyrazol-1-yl-6H-pyrimi-
dine-1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (34a).
Representative Procedure for Synthesis of Boc-Protected Bigi-
nelli Products 34. Compound 33a (1.45 g, 4.31 mmol) and di-
tert-butyl dicarbonate (Boc2O, 1.13 g, 5.17 mmol) were dissolved
in MeCN (17.0 mL) under a N2 atmosphere. DMAP (53 mg, 0.43
mmol) was added, and the solution was stirred at room temperature
for 12 h. The solvent was removed in vacuo, and the residue was
recrystallized from Et2O/hexanes to give 34a as a colorless solid
(1.36 g, 73%, mp 114-115 °C): 1H NMR (500 MHz, CDCl3) δ
8.15 (d, J ) 2.5 Hz, 1 H), 7.72 (s, 1 H), 6.44 (s, 1 H), 5.19 (dd, J
) 10, 4 Hz, 1 H), 4.39 (t, J ) 6 Hz, 1 H), 4.32-4.20 (m, 2 H),
3.30 (s, 3 H), 3.27 (s, 3 H), 2.43 (s, 3 H), 1.94-1.87 (m, 1 H),
1.80-1.73 (m, 1 H), 1.65-1.58 (m, 1 H), 1.54-1.46 (m, 1 H),
1.34 (t, J ) 7 Hz, 3 H), 1.25 (s, 9 H) ppm; 13C NMR (125 MHz,
CDCl3) δ 165.4, 152.5, 151.4, 143.0, 129.5, 114.9, 108.6, 104.18,
83.3, 60.6, 53.2, 52.9, 52.6, 27.8, 27.7, 27.6, 26.8, 21.2, 14.4 ppm;
IR (thin film) 3394, 2982, 2935, 2831, 1774, 1732, 1709, 1650,
1623, 1567, 1460, 1398, 1370, 1301, 1288, 1230, 1142, 1070 cm-1
;
HRMS (ESI) m/z 459.2212 (459.2220 calcd for C21H32N4O6Na+
[MNa]+).
2-Amino-6-(3,3-dimethoxypropyl)-4-methyl-6H-pyrimidine-
1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (30a).
Representative Procedure for Synthesis of Products 30. Com-
pound 34a (1.15 g, 2.63 mmol) and ammonium chloride (71 mg,
1.32 mmol) were dissolved in THF (17 mL), cooled to 0 °C, and
ammonia gas was bubbled through for 30 min. The resulting
solution was sealed in a glass tube and stirred at 70 °C for 12 h.
After cooling to room temperature, the solvent was removed in
vacuo, and the residue was purified by flash chromatography (silica
gel, 40% Et2O in hexanes f 100% Et2O f 10% MeOH in Et2O)
to give product 30a as a colorless solid (0.90 g, 88% yield, mp
110-111 °C): 1H NMR (500 MHz, CDCl3) δ 7.87 (br s, 2 H),
5.21 (s, 1 H), 4.30 (s, 1 H), 4.16 (q, J ) 7 Hz, 2 H), 3.26 (s, 6 H),
2.21 (s, 3 H), 1.55-1.54 (m, 4 H), 1.52 (s, 9 H), 1.26 (t, J ) 7 Hz,
3 H) ppm; 13C NMR (125 MHz, CDCl3) δ 166.3, 156.1, 153.0,
151.6, 104.1, 84.0, 59.5, 52. 7, 52.65, 51.9, 28.9, 28.0, 27.9, 21.5,
14.4 ppm; IR (thin film) 3410, 2982, 2936, 1716, 1645, 1597, 1514,
1371, 1343, 1289, 1265, 1241, 1141, 1064 cm-1; HRMS (ESI) m/z
+
C16H22N3O4 [M]+).
Acknowledgment. This research was supported by NIH
(HL-25854), and Pharma Mar. Postdoctoral fellowship support
for B.L.N. from the Canadian Institutes of Health Research
Institute of Infection and Immunity is gratefully acknowledged.
NMR and mass spectra were obtained at UC Irvine using
instrumentation acquired with the assistance of NSF and NIH
Shared Instrumentation programs.
Supporting Information Available: Experimental procedures,
1
characterization data, and copies of H and 13C NMR spectra for
all new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO061199M
7714 J. Org. Chem., Vol. 71, No. 20, 2006