Synthesis and inverse electron demand Diels-Alder reactions of 3,6-bis(3,4-dimethoxybenzoyl)-1,2,4,5-tetrazine

Article abstract of DOI:10.1021/jo020713v

The synthesis of 3,6-bis(3,4-dimethoxybenzoyl)-1,2,4,5-tetrazine (2) and the scope of its reactivity in inverse electron demand Diels-Alder reactions are disclosed representing the first systematic study of the [4 + 2] cycloaddition reactions of 3,6-diacy

Full text of DOI:10.1021/jo020713v

Syn th esis a n d In ver se Electr on Dem a n d Diels-Ald er Rea ction s of
3,6-Bis(3,4-d im eth oxyben zoyl)-1,2,4,5-tetr a zin e
Danielle R. Soenen, J effrey M. Zimpleman, and Dale L. Boger*
Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La J olla, California 92037
boger@scripps.edu
Received November 26, 2002
The synthesis of 3,6-bis(3,4-dimethoxybenzoyl)-1,2,4,5-tetrazine (2) and the scope of its reactivity
in inverse electron demand Diels-Alder reactions are disclosed representing the first systematic
study of the [4 + 2] cycloaddition reactions of 3,6-diacyl-1,2,4,5-tetrazines.
1,2,4,5-Tetrazines serve as useful precursors for the
preparation of a variety of natural products bearing
pyrrole ring systems.1,2 The methodology is based on an
inverse electron demand Diels-Alder reaction of an
electron-deficient 1,2,4,5-tetrazine with an electron-rich
dienophile to form a 1,2-diazine, followed by a reductive
ring contraction to yield the corresponding pyrrole (Fig-
ure 1).2 Typically, this has been conducted utilizing the
readily accessible dimethyl 1,2,4,5-tetrazine-3,6-dicar-
boxylate (1)3 which exhibits exceptional reactivity par-
ticipating in [4 + 2] cycloaddition reactions with electron-
rich, unactivated, and even electron-deficient dienophiles
including a select set of heterodienophiles.4 In the course
of studies on the total synthesis of ningalin D and
purpurone,5 we elected to examine the extension of these
studies to the novel 3,6-dibenzoyl-1,2,4,5-tetrazine 2,
thereby incorporating 3,6-dicarboxylate functionalization
into the substrate prior to cycloaddition. We are aware
of only two earlier reports on the synthesis of 3,6-
dibenzoyltetrazines, and their reactivity as azadienes was
F IGURE 1.
not investigated.6,7 More recently, Snyder and co-workers
have reported the synthesis of methyl 6-acetyl-1,2,4,5-
tetrazine-3-carboxylate and 3,6-diacetyl-1,2,4,5-tetrazine,
the only known additional acyl-substituted 1,2,4,5-tet-
razines, and have examined their reactivity in a select
series of Diels-Alder cycloadditions.8 Thus, prior reports
detailing the synthesis of 3,6-diacyl-1,2,4,5-tetrazines and
their participation in Diels-Alder reactions are limited.
Herein, we report our findings regarding the synthesis
of 2 and its reactivity in Diels-Alder cycloadditions.
Tetrazine 2 was synthesized in five steps and in 28%
overall yield from commercially available 3,4-dimethoxy-
benzaldehyde (3) as depicted in Scheme 1. The prepara-
tion commenced with the formation of TMS-protected 3,4-
dimethoxybenzaldehyde cyanohydrin 49 (TMSCN, ZnI2)
in quantitative yield.10 Treatment of 4 with saturated
ethanolic HCl11 promoted ethanol addition to the nitrile
(1) Reviews: Boger, D. L. Tetrahedron 1983, 39, 2869. Boger, D. L.
Chem. Rev. 1986, 86, 781. Boger, D. L. Bull. Soc. Chim. Belg. 1990,
99, 599. Boger, D. L. Chemtracts: Org. Chem. 1996, 9, 149. Boger, D.
L.; Weinreb, S. M. Hetero Diels-Alder Methodology in Organic
Synthesis; Academic: San Diego, 1987.
(2) Boger, D. L.; Coleman, R. S.; Panek, J . S.; Yohannes, D. J . Org.
Chem. 1984, 49, 4405. Boger, D. L.; Patel, M. J . Org. Chem. 1988, 53,
1405. Boger, D. L.; Baldino, C. M. J . Org. Chem. 1991, 56, 6942. Boger,
D. L.; Baldino, C. M. J . Am. Chem. Soc. 1993, 115, 11418. Boger, D.
L.; Boyce, C. W.; Labroli, M. A.; Sehon, C. A.; J in, Q. J . Am. Chem.
Soc. 1999, 121, 54. Boger, D. L.; Soenen, D. R.; Boyce, C. W.; Hedrick,
M. P.; J in, Q. J . Org. Chem. 2000, 65, 2479. Boger, D. L.; Wolkenberg,
S. E. J . Org. Chem. 2000, 65, 9120. Boger, D. L.; Hong, J . J . Am. Chem.
Soc. 2001, 123, 8515.
(3) Boger, D. L.; Coleman, R. S.; Panek, J . S.; Huber, F. X.; Sauer,
J . J . Org. Chem. 1985, 50, 5377. Boger, D. L.; Panek, J . S.; Patel, M.
Org. Synth. 1992, 70, 79.
(4) Boger, D. L.; Panek, J . S. Tetrahedron Lett. 1983, 24, 4511. Boger,
D. L.; Panek, J . S. J . Am. Chem. Soc. 1985, 107, 5745.
(5) Ningalin D: Fenical, W.; Kang, H. J . Org. Chem. 1997, 62, 3254.
Purpurone: Chan, G. W.; Francis, T.; Thureen, D. R.; Offen, P. H.;
Pierce, N. J .; Westley, J . W.; J ohnson, R. K.; Faulkner, D. J . J . Org.
Chem. 1993, 58, 2544. For the total synthesis of purpurone, see:
Peschko, C.; Steglich, W. Tetrahedron Lett. 2000, 41, 9477.
(6) Asinger, F.; Neuray, D.; Leuchtenberger, W.; Lames, U. Liebigs
Ann. Chem. 1973, 879.
(8) Girardot, M.; Nomak, R.; Snyder, J . K. J . Org. Chem. 1998, 63,
10063.
(9) J ackson, W. R.; J acobs, H. A.; J ayatilake, G. S.; Matthews, B.
R.; Watson, K. G. Aust. J . Chem. 1990, 43, 2045.
(10) Usami, K.; Isobe, M. Tetrahedron 1996, 52, 12061.
(11) Schnur, R. C.; Morville, M. J . Med. Chem. 1986, 29, 770.
(7) Yates, P.; Meresz, O. Tetrahedron Lett. 1967, 77.
10.1021/jo020713v CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/05/2003
J . Org. Chem. 2003, 68, 3593-3598
3593

Soenen et al.
TABLE 1. Diels-Ald er Rea ction s of 3,6-Bis(3,4-d im eth oxyben zoyl)-1,2,4,5-tetr a zin e (2) w ith Alk en es
and deprotection of the benzylic alcohol to provide 5.12,13
Imidate 5 was most conveniently purified by repeated
washings with Et2O or EtOAc, and the off-white solid was
then added in portions to neat hydrazine hydrate7,14-16
at 0 °C to afford the 1,4-dihydrotetrazine 6. Like 5, the
dihydrotetrazine 6 could be conveniently purified by
repeated washings with EtOAc14 to afford 6 as a white
solid, although air oxidation to the more soluble tetrazine
during this purification may result in a loss of material
in the EtOAc trituration. Deliberate oxidation of the 1,4-
dihydrotetrazine 6 was accomplished by treatment with
ferric chloride17 in EtOH/H2O to provide tetrazine 7 as a
bright pink solid (41%, two steps). Final oxidation of the
benzylic alcohols with Dess-Martin periodinane18 then
afforded 2 (90%) as a bright orange solid. Tetrazine 2 was
SCHEME 1
(12) Fulton, J . D.; Robinson, R. J . Chem. Soc. 1933, 1463. Bristow,
N. W. J . Chem. Soc. 1957, 513.
(13) Neilson, D. G. In The Chemistry of Amidines and Imidates;
Patai, S., Ed.; Wiley: New York, 1975; p 391.
(14) Hunter, D.; Neilson, D. G. J . Chem. Soc., Perkin Trans. 1 1981,
1371.
(15) Gerard, J . F.; Lions, F. J . Org. Chem. 1965, 30, 318.
(16) Pinner, A. Chem. Ber. 1897, 30, 1871. Pinner, A. Ann. 1897,
297, 221. Pinner, A. Ann. 1897, 298, 1.
(17) Sandstro¨m, J . Acta Chem. Scand. 1961, 15, 1575.
(18) Dess, D. B.; Martin, J . C. J . Am. Chem. Soc. 1991, 113, 7277.
3594 J . Org. Chem., Vol. 68, No. 9, 2003

Synthesis of 3,6-Bis(3,4-dimethoxybenzoyl)-1,2,4,5-tetrazine
TABLE 2. Diels-Ald er Rea ction s of 3,6-Bis(3,4-d im eth oxyben zoyl)-1,2,4,5-tetr a zin e (2) w ith Alk yn es
slightly soluble in CHCl3, CH2Cl2, toluene, and DMF
above 23 °C but insoluble in diethyl ether, benzene,
tetrahydrofuran, and 1,4-dioxane at ambient tempera-
tures. Exposure of 2 to protic solvents including methanol
resulted in its rapid decomposition while 7 was stable to
methanol over prolonged exposure times (>2 days).
Neither tetrazine 7 nor 2 was stable to conventional silica
gel chromatography; however, both could be purified by
simple trituration with CH2Cl2 or MeOH (for 7) or EtOAc
(for 2) due to their minimal solubilities in these solvents.
An alternative synthesis of 2 via base-catalyzed dimer-
ization of R-diazo-3,4-dimethoxyacetophenone7 was un-
successful, although this was not investigated in detail.
aromatization step involving the loss of an alcohol or
secondary amine was often incomplete under mild reac-
tion conditions. This aromatization could be facilitated
by treatment of the intermediate crude product mixture
with 10% HOAc/C6H6 (v:v). For example, the elimination
of morpholine from entry 1 (Table 1) was effected by
treatment with 10% HOAc/C6H6, whereas the aromati-
zation for the entry 2 enamine was completed by extend-
ing the reaction time under more vigorous conditions
(toluene, 60 °C, 30 min) to provide diazine 9a . Notably,
the initial [4 + 2] cycloadditions of the enamines 8a and
8b with tetrazine 2 were complete at 23 °C (<5 min).
Similarly, entry 3 employed acid treatment to complete
the loss of ethanol, while entries 4 and 5 (Table 1) simply
required additional reaction time (2-3 h) at ambient
temperature to complete aromatization following a cy-
cloaddition that was complete in 5 min (50 °C). Thus, as
expected, the electron-rich alkenes 8a -g, including
enamines, enol ethers, and ketene acetals, undergo facile
[4 + 2] cycloadditions under mild conditions. Although
longer reaction times were required, Diels-Alder reac-
tions with 2-methoxypropene (entry 6) could be observed
even at temperatures as low as 4 °C. Not surprisingly,
the [4 + 2] cycloaddition with the less electron-rich vinyl
acetate (8h , 110 °C, 17 h) required higher reaction
temperatures.
The reactivity of 2 in Diels-Alder cycloadditions with
a variety of dienophiles was examined, and representa-
tive results are summarized in Tables 1 and 2. Tetrazine
2 participated in inverse electron demand Diels-Alder
reactions with both electron-rich and neutral alkenes
8a -h and alkynes 14a -d , providing the 1,2-diazine
cycloadducts in good yields.19 As previously observed with
Diels-Alder cycloadditions of 3,6-bis(methylthio)-1,2,4,5-
tetrazine20 and 6-[(tert-butyloxycarbonyl)amino]-3-(me-
thylthio)-1,2,4,5-tetrazine,21 the slow step of the reaction
cascade with electron-rich olefinic dienophiles typically
was the aromatization step. Enamines, enol ethers, and
ketene acetals underwent rapid, room-temperature [4 +
2] cycloaddition with concomitant loss of N2, and the
Although not extensively investigated, tetrazine 2 was
found to participate in effective [4 + 2] cycloadditions
with selected heterodienophiles including S-alkyl thio-
imidates4 providing the corresponding 1,2,4-triazine
(eq 1). Analogous to prior observations with 1,4 thioimi-
dates proved more effective than either imidates or
amidines (X ) SCH3 > OCH3, NH2) that in our prior
studies was shown to be the result of an optimal
combination of properties. That is, the thioimidates are
(19) The reactions were monitored by the disappearance of baseline
tetrazine on TLC and dissolution of the initial orange slurry to provide
a clear solution.
(20) Boger, D. L.; Sakya, S. M. J . Org. Chem. 1988, 53, 1415. Boger,
D. L.; Coleman, R. S. J . Org. Chem. 1984, 49, 2240. For the preparation
of 3-methoxy-6-methylthio-1,2,4,5-tetrazine, see: Sakya, S. M.; Grosk-
opf, K. K.; Boger, D. L. Tetrahedron Lett. 1997, 38, 3805.
(21) Boger, D. L.; Schaum, R. P.; Garbaccio, R. M. J . Org. Chem.
1998, 63, 6329.
J . Org. Chem, Vol. 68, No. 9, 2003 3595

Soenen et al.
F IGURE 2. Relative dienophile reactivity toward 2.
sufficiently electron-rich to participate effectively in the
[4 + 2] cycloaddition reaction (NH2 > OCH3 g SCH3) and
the thiomethyl group is a sufficiently good leaving group
Diels-Alder cycloadditions were also observed with
alkynyl dienophiles (Table 2), as demonstrated by forma-
tion of the 1,2-diazines 15 (100%) and 9c (66%) upon
reaction with the ynamine 14a (entry 1, 23 °C, 2 h) and
ethoxyacetylene (entry 2, 60 °C, 3.5 d), respectively.
Higher reaction temperatures were required for [4 + 2]
cycloaddition with phenylacetylene (14c, 110 °C, 2 d).
Tetrazine 2 even exhibited limited reactivity toward the
electron-poor alkyne methyl propiolate although satisfac-
tory conversions were observed only after prolonged
reaction times (14d , 95 °C, 5 d, 65%). Typically, the
alkyne dienophiles were not as reactive as the corre-
sponding alkenes (e.g., 8g > 14b) but they follow reactiv-
ity trends expected on the basis of the electronic prop-
erties of their substituents (Figure 2).
to permit subsequent aromatization (SCH3 > OCH3
>
NH2) competitive with further dihydrotriazine reaction
with a substrate. Tetrazine 2 was also found to serve as
an effective trap of typically inaccessible electron-rich
dienophiles generated by in situ tautomerization of a
more stable precursor (eq 2). Such tactics enlisting the
in situ generation of reactive electron-rich dienophiles
substantially increases the scope of cycloadditions avail-
able to tetrazine 2. For example, 2 underwent facile
Diels-Alder cycloaddition with dimethylhydrazone 12a
(eq 3) and the oxime 12b upon in situ tautomerization
of the carbon-nitrogen double bond (eq 2) to allow
cycloaddition across the carbon-carbon double bond.
Finally, it was evident from attempted cycloadditions
with a variety of dienophiles bearing bulky substituents
that steric hindrance can be a factor limiting the reactiv-
ity of 2. For example, cycloadditions with enol ether 1322
could not be effected even at temperatures in excess of
140 °C (eq 4). In this regard, tetrazine 2 is not as broadly
reactive as the diester 1.
Con clu sion s
The synthesis of tetrazine 2, by an approach applicable
to a range of 3,6-dibenzoyl-1,2,4,5-tetrazines, and its
reactivity in inverse electron demand Diels-Alder reac-
tions were described representing the first systematic
study of the [4 + 2] cycloaddition reactions of a 3,6-diacyl-
1,2,4,5-tetrazine. Tetrazine 2 was shown to readily
undergo [4 + 2] cycloadditions with electron-rich and
neutral alkenes and alkynes and even a select electron-
deficient dienophile to provide substituted 1,2-diazines
in good yields defining a broadly reactive and useful class
of heterocyclic azadienes.
Exp er im en ta l Section
3,4-Dim eth oxym a n d elim id ic Acid Eth yl Ester Hyd r o-
ch lor id e (5). 2-(3,4-Dimethoxyphenyl)-2-(trimethylsilyloxy)-
acetonitrile 49 (1.60 g, 6.02 mmol) as prepared by the procedure
of Isobe10 was treated with saturated HCl-EtOH (20 mL) at
0 °C for 16 h. Removal of the solvent and washing of the crude
solid with Et2O (2 × 30 mL) provided 5 (1.26 g, 76%) as an
off-white solid: mp 125-127 °C (lit. mp12 129-149 °C); 1H
NMR (DMSO-d6, 400 MHz) δ 11.68 (br s, 2H), 7.23 (br s, 1H),
7.08 (s, 1H), 6.98 (s, 2H), 5.47 (s, 1H), 4.46 (m, 2H), 3.76 (s,
3H), 3.75 (s, 3H), 1.25 (t, J ) 7.0 Hz, 3H); 13C NMR (DMSO-
d6, 100 MHz) δ 179.5, 149.2, 148.7, 129.7, 119.3, 111.6, 110.4,
70.6, 69.7, 55.5 (2C), 13.3; IR (film) νmax 3234, 1643, 1514 cm-1
ESIMS m/z 274 (C12H18ClNO4 - H- requires 274).
;
(3,4-Dim et h oxyp h en yl)[6-[(3,4-d im et h oxyp h en yl)h y-
d r oxym eth yl]-1,2,4,5-tetr a zin -3-yl]m eth a n ol (7). Imidate
5 (3.28 g, 11.90 mmol) was added in portions over 15 min to a
stirring solution of hydrazine monohydrate (2.6 mL) at 0 °C.
(22) Krow, G. R.; Michener, E. Synthesis 1974, 572.
3596 J . Org. Chem., Vol. 68, No. 9, 2003

Synthesis of 3,6-Bis(3,4-dimethoxybenzoyl)-1,2,4,5-tetrazine
After 3 h, the hydrazine was removed to provide crude 1,4-
dihydrotetrazine 6. The 1,4-dihydrotetrazine could be purified
by trituration with EtOAc14 to provide 6 as a white solid as a
near 1:1 mixture of two diastereomers: 1H NMR (DMSO-d6/
D2O, 400 MHz) δ 7.01 (s, 1H), 6.97 (s, 1H), 6.87 (s, 2H), 6.84
(s, 2H), 5.05 (s, 1H), 5.03 (s, 1H), 3.72 (s, 3H), 3.71 (s, 3H),
3.70 (s, 3H), 3.67 (s, 3H); MALDIFTMS (DHB) m/z 417.1761
(C20H24N4O6 + H+ requires 417.1769). A slurry of the crude
1,4-dihydrotetrazine in EtOH (80 mL) at 75 °C was treated
with a solution of FeCl3 (2.89 g, 17.8 mmol) in H2O (27 mL).
The reaction solubilized and became bright pink within 15 min.
After an additional 15 min, the reaction was cooled to 25 °C,
diluted with CH2Cl2, washed with H2O (3 × 100 mL), dried
(Na2SO4), and concentrated in vacuo. Trituration of the crude
solid with CH2Cl2 or MeOH (400 mL) afforded 7 (1.02 g, 41%)
as a bright pink solid: mp 171-172 °C; 1H NMR (CDCl3, 400
MHz) δ 7.10 (d, J ) 1.8 Hz, 2H), 7.04 (dd, J ) 1.8, 8.2 Hz,
2H), 6.84 (d, J ) 8.2 Hz, 2H), 6.34 (s, 2H), 3.88 (s, 6H), 3.86
(s, 6H); 13C NMR (CDCl3, 100 MHz) δ 170.5 (2C), 149.7 (2C),
149.6 (2C), 132.1 (2C), 119.5 (2C), 111.5 (2C), 109.7 (2C), 74.5
(2C), 56.2 (4C); IR (film) νmax 3377, 1591, 1512 cm-1; MALDIFT-
MS (DHB) m/z 437.1414 (C20H22N4O6 + Na+ requires 437.1431).
to 23 °C after an additional 9 min. The solvent was removed
under reduced pressure, and the residue was redissolved in
10% HOAc-benzene (500 µL) and stirred at 23 °C for 1.5 h to
induce aromatization before reconcentration. Flash chroma-
tography (SiO2, 1.5 × 8 cm, 0-20% EtOAc/CH2Cl2 gradient
elution) provided 9c (6.4 mg, 64%) as yellow needles: mp 171-
173 °C; 1H NMR (CDCl3, 400 MHz) δ 8.06 (dd, J ) 1.8, 8.5
Hz, 1H), 7.87 (d, J ) 1.8 Hz, 1H), 7.67 (d, J ) 1.8 Hz, 1H),
7.65 (s, 1H), 7.33 (dd, J ) 1.8, 8.5 Hz, 1H), 6.95 (d, J ) 8.5
Hz, 1H), 6.86 (d, J ) 8.5 Hz, 1H), 4.26 (q, J ) 7.0 Hz, 2H),
3.982 (s, 3H), 3.976 (s, 3H), 3.97 (s, 3H), 3.96 (s, 3H), 1.42 (t,
J ) 7.0 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ 189.8, 189.7,
159.5, 157.0, 154.8, 154.5, 153.0, 149.6, 149.2, 129.0, 128.4,
128.2, 127.1, 113.0, 111.0, 110.3, 110.2, 109.4, 65.3, 56.4 (2C),
56.3 (2C), 14.2; IR (film) νmax 1661, 1590, 1515 cm-1; MALDIFT-
MS (DHB) m/z 453.1666 (C24H24N2O7 + H+ requires 453.1656).
F r om 14b. A stirring solution of 2 (9.1 mg, 0.022 mmol) in
benzene (220 µL) under Ar was treated with ethoxyacetylene
(14b, 40 wt % solution in hexanes, 27 µL, 0.11 mmol). The
reaction was warmed at 60 °C for 3.5 d before the cooled
reaction mixture was concentrated under reduced pressure.
Flash chromatography (SiO2, 1.5 × 8 cm, 0-10% EtOAc/CH2-
Cl2 gradient elution) afforded 9c (6.6 mg, 66%) as a yellow
solid.
Analogous oxidations of crude 6 enlisting 0.2 equiv (vs 1.5
equiv) of FeCl3 (open to air, 38%), DDQ (1 equiv, THF, 0-23
°C, 20 h), or nitrous gas3 (CH2Cl2, 0 °C, 45 min) provided
comparable conversions.
3,6-Bis(3,4-dim eth oxyben zoyl)-4-ph en ylpyr idazin e (9d).
F r om 8d . A solution of 2 (6.0 mg, 15 µmol) and 1-phenyl-1-
(trimethylsiloxy)ethylene (8d , 4.5 µL, 44 µmol) in toluene (75
µL) was stirred at 23 °C for 2 h during which time the orange
color was discharged and the solution turned yellow. The
solvent was removed in vacuo, and the crude material was
purified by PTLC (SiO2, 2:1 EtOAc/hexanes) to yield 9d (6.0
mg, 86%) as a pale yellow solid: mp 145-147 °C; 1H NMR
(CDCl3, 400 MHz) δ 8.25 (s, 1H), 8.08 (dd, J ) 1.8, 8.2 Hz,
1H), 7.89 (d, J ) 2.0 Hz, 1H), 7.60 (d, J ) 1.8 Hz, 1H), 7.37-
7.45 (m, 6H), 6.97 (d, J ) 9.0 Hz, 1H), 6.86 (d, J ) 8.2 Hz,
1H), 4.00 (s, 6H), 3.95 (s, 3H), 3.93 (s, 3H); 13C NMR (CDCl3,
150 MHz) δ 191.6, 189.6, 159.2, 158.5, 154.7, 154.5, 149.5,
149.2, 141.0, 137.3, 134.4, 133.3, 130.1, 129.4, 128.7, 128.5,
128.3, 128.2, 127.3, 125.3, 112.8, 111.1, 110.3, 110.2, 56.4 (2C),
3,6-Bis(3,4-d im eth oxyben zoyl)-1,2,4,5-tetr a zin e (2). A
stirring solution of 7 (329 mg, 0.793 mmol) in CH2Cl2 (53 mL)
under Ar at 23 °C was treated with Dess-Martin periodinane
(1.01 g, 2.38 mmol). After 3.5 h, the orange slurry was treated
with saturated aqueous NaHCO3 (20 mL) and saturated
aqueous Na2S2O3 (20 mL) and stirred for 15 min at 23 °C
before the layers were separated. The aqueous phase was
extracted with CH2Cl2 (2 × 20 mL), and the combined organic
layers were washed with H2O, dried (Na2SO4), and concen-
trated in vacuo. Trituration with EtOAc (2 × 10 mL) afforded
pure 2 (292 mg, 90%) as a bright orange solid: mp 195-196
°C; 1H NMR (CDCl3, 400 MHz) δ 7.80 (d, J ) 2.0 Hz, 2H),
7.58 (dd, J ) 2.0, 8.5 Hz, 2H), 6.97 (d, J ) 8.5 Hz, 2H), 4.02
(s, 12H); 13C NMR (CDCl3, 125 MHz) δ 185.5, 164.6, 155.8,
150.0, 128.3, 127.3, 111.6, 110.5, 56.6, 56.5; IR (film) νmax 1665,
56.3, 56.2; IR (film) νmax 1661, 1651, 1592, 1514 cm-1
;
MALDIFTMS (DHB) m/z 485.1695 (M + H+, C28H24N2O6
1595, 1515 cm-1; MALDIFTMS (DHB) m/z 433.1104 (C20H18
N4O6 + Na+ requires 433.1118).
-
requires 485.1707).
F r om 14c. A solution of 2 (6.0 mg, 0.015 mmol) and
phenylacetylene (14c, 16.2 µL, 0.15 mmol) in toluene (75 µL)
was warmed at 110 °C for 2 d. PTLC (SiO2, 2:1 EtOAc/hexanes)
afforded 9d (5.0 mg, 70%) as a pale yellow solid.
3,6-Bis(3,4-d im et h oxyb en zoyl)cyclop en t a [d ]p yr id a -
zin e (9a ). F r om 8b. A solution of 2 (8.8 mg, 0.021 mmol) in
toluene (150 µL) under Ar at 60 °C was treated with a solution
of 1-pyrrolidino-1-cyclopentene (8b, 14.4 mg, 0.105 mmol) in
toluene (100 µL). After 30 min, the reaction mixture was cooled
to 23 °C and concentrated in vacuo. Flash chromatography
(SiO2, 1.5 × 8 cm, 0-20% EtOAc/CH2Cl2 gradient elution)
provided 9a (8.5 mg, 90%) as a light yellow solid: mp >250
F r om 12a . Tetrazine 2 (10.4 mg, 0.025 mmol) was treated
with a solution of dimethylhydrazone 12a (20.3 mg, 0.125
mmol) in 1,4-dioxane (250 µL) under Ar and warmed at 95 °C
for 2 h. The cooled reaction mixture was concentrated and flash
chromatography (SiO2, 1.5 × 8 cm, 25-50% EtOAc/hexanes
gradient elution) afforded 9d (5.3 mg, 44%) as a pale yellow
solid. Alternatively, 2 (13.7 mg, 0.033 mmol) was treated with
a solution of 12a (27.1 mg, 0.167 mmol) in DMF (330 µL) under
Ar and warmed at 95 °C for 5 min. The cooled reaction mixture
was concentrated, and flash chromatography (SiO2, 1.5 × 8
cm, 25-50% EtOAc/hexanes gradient elution) afforded 9d (7.0
mg, 44%) as a pale yellow solid.
1
°C dec; H NMR (CDCl3, 400 MHz) δ 7.75 (m, 4H), 6.91 (d, J
) 8.8 Hz, 2H), 3.97 (s, 12H), 3.28 (t, J ) 7.6 Hz, 4H), 2.23 (m,
2H); 13C NMR (CDCl3, 100 MHz) δ 191.2 (2C), 155.7 (2C), 154.5
(2C), 149.4 (2C), 148.1 (2C), 128.9 (2C), 127.9 (2C), 112.2 (2C),
110.2 (2C), 56.4 (2C), 56.3 (2C), 31.8 (2C), 24.1; IR (film) νmax
1642, 1577, 1512 cm-1; MALDIFTMS (DHB) m/z 449.1695
(C25H24N2O6 + H+ requires 449.1707).
F r om 8a . A solution of 2 (10 mg, 0.024 mmol) and 4-(1-
cyclopenten-1-yl)morpholine (8a , 19.6 mg, 0.12 mmol) in
toluene (120 µL) was stirred at 23 °C for 3 h during which
time the reaction solution changed from orange to yellow. A
10% HOAc-benzene solution (v/v) was added to promote
elimination of the amine for aromatization of the Diels-Alder
product. After 16 h, the solvent was removed in vacuo, and
the product was purified by PTLC (SiO2, 2:1 EtOAc/hexanes)
to yield 9a (6.4 mg, 60%) as a light yellow solid.
3,6-Bis(3,4-dim eth oxyben zoyl)-4-eth oxypyr idazin e (9c).
F r om 8c. A stirring solution of 2 (8.9 mg, 0.022 mmol) in
CHCl3 (220 µL) under Ar was treated with ketene diethyl
acetal (8c, 14 µL, 0.11 mmol) and warmed to 45 °C. The
reaction solubilized within 1 min, and the reaction was cooled
F r om 12b. A solution of 2 (10.0 mg, 0.024 mmol) and
acetophenone (O-methyl)oxime (12b, 14.5 mg, 0.097 mmol) in
dioxane (120 µL) was stirred at reflux for 3 h during which
time the orange color was discharged and the solution turned
yellow. The solvent was removed in vacuo, and the crude
material was purified by PTLC (SiO2, 2:1 EtOAc/hexanes) to
yield 9d (6.4 mg, 54%).
3,6-Bis(3,4-d im e t h oxyb e n zoyl)-4-(2-h yd r oxye t h yl)-
p yr id a zin e (9e). A stirring solution of 2 (10.7 mg, 0.026 mmol)
in CHCl3 (260 µL) under Ar was treated with 2,3-dihydrofuran
(8e, 10 µL, 0.130 mmol) and warmed at 50 °C for 5 min. The
cooled solution was maintained at 23 °C for 3 h to allow
complete aromatization before concentration under reduced
pressure. Flash chromatography (SiO2, 1.5 × 8 cm, 0-30%
J . Org. Chem, Vol. 68, No. 9, 2003 3597

Soenen et al.
EtOAc/CH2Cl2 gradient elution) afforded 9e (9.4 mg, 80%) as
an off-white solid: mp 207-209 °C; 1H NMR (CDCl3, 400 MHz)
δ 8.20 (s, 1H), 8.03 (dd, J ) 2.1, 8.5 Hz, 1H), 7.85 (d, J ) 2.1
Hz, 1H), 7.69 (d, J ) 2.1 Hz, 1H), 7.43 (dd, J ) 2.1, 8.5 Hz,
1H), 6.95 (d, J ) 8.5 Hz, 1H), 6.88 (d, J ) 8.5 Hz, 1H), 3.99
(m, 5H), 3.98 (s, 3H), 3.97 (s, 6H), 3.04 (t, J ) 6.2 Hz, 2H),
2.36 (t, J ) 5.3 Hz, 1H); 13C NMR (CDCl3, 100 MHz) δ 192.0,
189.8, 160.0, 158.2, 155.0, 154.5, 149.5, 149.2, 140.6, 129.8,
128.9, 128.3, 128.2, 128.1, 112.8, 111.6, 110.2, 62.0, 56.5, 56.4,
56.4, 56.3; IR (film) νmax 1651, 1592, 1515 cm-1; MALDIFTMS
(DHB) m/z 520.1263 (C27H22ClN3O6 + H+ requires 520.1270).
F r om 10b. A solution of 2 (14.7 mg, 0.036 mmol) in dioxane
(180 µL) was treated with the free base23 of O-methyl (p-
chlorophenyl)imidate hydrochloride (10b , 14.8 mg, 0.072
mmol), and the reaction mixture was stirred at 90 °C for 17
h. The reaction mixture was cooled and the solvent removed
under reduced pressure. Chromatography (SiO2, 1 × 10 cm,
2:1 EtOAc/hexanes) afforded 11 (5.8 mg, 31%).
56.3 (2C), 34.5; IR (film) νmax 3395, 1646, 1580, 1513 cm-1
;
F r om 10c. A solution of 2 (15.4 mg, 0.036 mmol) in dioxane
(185 µL) was treated with the free base23 of p-chlorobenzene-
1-carboximidamide hydrochloride (10c, 14.0 mg, 0.0731 mmol),
and the reaction mixture was stirred at 90 °C for 17 h. The
reaction mixture was cooled and the solvent removed under
reduced pressure. Chromatography (SiO2, 1 × 10 cm, 2:1
EtOAc/hexanes) afforded 11 (6.0 mg, 32%).
MALDIFTMS (DHB) m/z 453.1651 (C24H24N2O7 + H+ requires
453.1656).
3,6-Bis(3,4-dim eth oxyben zoyl)-4-m eth ylpyr idazin e (9f).
Tetrazine 2 (6.0 mg, 0.015 mmol) was dissolved in 2-methoxy-
propene (8f, 105 µL, 1.10 mmol). The mixture was stirred for
4 d at 4 °C, during which time the solution changed color from
orange to yellow. The solvent was removed in vacuo, and the
crude product was purified by PTLC (SiO2, 50% EtOAc/
hexanes) to yield 9f (4.4 mg, 73%) as a pale yellow solid: mp
4-(N,N-Dieth yla m in o)-3,6-bis(3,4-d im eth oxyben zoyl)-
5-m eth ylp yr id a zin e (15). Tetrazine 2 (10 mg, 0.024 mmol)
in dioxane (120 µL) was treated with 1-(N,N-diethylamino)-
1-propyne (14a , 7 µL, 0.048 mmol) at 23 °C. Rapid evolution
of N2 was observed, as the reaction changed from an orange
slurry to a red solution. After 2 h, the solvent was removed in
vacuo, and the crude material purified by PTLC (SiO2, 2:1
EtOAc/hexanes) to yield 15 (11.9 mg, 100%) as a yellow solid:
1
134-135 °C; H NMR (CDCl3, 500 MHz) δ 8.07 (s, 1H), 8.02
(dd, J ) 1.8, 8.6 Hz, 1H), 7.85 (d, J ) 1.8 Hz, 1H), 7.69 (d, J
) 1.8 Hz, 1H), 7.39 (dd, J ) 1.8, 8.4 Hz, 1H), 6.94 (d, J ) 8.6
Hz, 1H), 6.88 (d, J ) 8.4 Hz, 1H), 3.99 (s, 3H), 3.98 (s, 3H),
3.973 (s, 3H), 3.967 (s, 3H), 2.50 (s, 3H); 13C NMR (CDCl3, 125
MHz) δ 193.4, 191.7, 154.9, 154.5, 149.6, 149.2, 138.9, 132.5,
131.2, 129.8, 128.8, 128.4, 128.2, 127.6, 112.8, 111.4, 110.3,
1
mp 65-66 °C; H NMR (CDCl3, 600 MHz) δ 7.71 (d, J ) 1.8
110.2, 56.5, 56.4, 56.3 (2C), 18.4; IR (film) νmax 1650 cm-1
;
Hz, 1H), 7.70 (d, J ) 1.8 Hz, 1H), 7.49 (dd, J ) 1.8, 8.4 Hz,
1H), 7.39 (dd, J ) 1.8, 8.4 Hz, 1H), 6.88 (d, J ) 8.4 Hz, 1H),
6.87 (d, J ) 8.4 Hz, 1H), 3.972 (s, 3H), 3.965 (s, 3H), 3.960 (s,
3H), 3.955 (s, 3H), 3.12 (q, J ) 7.0 Hz, 4H), 2.26 (s, 3H), 1.06
(t, J ) 7.0 Hz, 6H); 13C NMR (CDCl3, 150 MHz) δ 193.0, 191.8,
155.4, 154.7 (2C), 154.4, 149.9, 149.6, 149.4, 131.3, 129.2,
129.0, 127.6, 127.3, 111.5, 111.2, 110.2 (2C), 56.4 (2C), 56.3
(2C), 46.4 (2C), 15.1, 13.7 (2C); IR (film) νmax 1661, 1652, 1593,
1515 cm-1; MALDI-FTMS (DHB) m/z 494.2297 (M + H+,
C27H31N3O6 requires 494.2286).
MALDIFTMS (DHB) m/z 423.1534 (M + H+, C23H22N2O6
requires 423.1551).
3,6-Bis(3,4-d im et h oxyb en zoyl)p yr id a zin e (9g). F r om
8h . A solution of 2 (6.0 mg, 0.015 mmol) and vinyl acetate (8h ,
12.6 µL, 0.014 mmol) in toluene (72 µL) was warmed at 110
°C for 17 h. The solvent was removed in vacuo, and the
material was purified by PTLC (SiO2, 2:1 EtOAc/hexanes) to
yield 9g (6.0 mg, 100%) as a pale yellow solid: mp 210-211
1
°C; H NMR (CDCl3, 500 MHz) δ 8.30 (s, 2H), 8.00 (dd, J )
3,6-B i s (3,4-d i m e t h o x y b e n z o y l)-4-c a r b o m e t h o x y -
p yr id a zin e (16). A stirring solution of 2 (10.5 mg, 0.026 mmol)
in 1,4-dioxane (260 µL) under Ar was treated with methyl
propiolate (14d , 11 µL, 0.128 mmol) and warmed at 95 °C for
5 d. The reaction mixture was concentrated, and the crude
product was purified by flash chromatography (SiO2, 1.5 × 8
cm, 0-5% EtOAc/CH2Cl2 gradient elution) to afford 16 (7.9
mg, 65%) as a yellow oil: 1H NMR (CD3CN, 400 MHz) δ 8.50
(s, 1H), 7.82 (dd, J ) 2.0, 8.5 Hz, 1H), 7.74 (d, J ) 2.0 Hz,
1H), 7.59 (d, J ) 2.0 Hz, 1H), 7.33 (dd, J ) 2.0, 8.5 Hz, 1H),
7.08 (d, J ) 8.5 Hz, 1H), 7.00 (d, J ) 8.5 Hz, 1H), 3.93 (s, 3H),
3.91 (s, 3H), 3.892 (s, 3H), 3.886 (s, 3H), 3.83 (s, 3H); 13C NMR
(acetone-d6, 100 MHz) δ 191.2, 189.9, 164.7, 160.3, 160.0,
155.81, 155.78, 150.6, 150.3, 130.5, 129.5, 128.9, 128.6, 128.5,
127.4, 113.6, 111.9, 111.6, 111.5, 56.4 (2C), 56.3 (2C), 53.9; IR
(film) νmax 1738, 1667, 1651, 1593, 1515 cm-1; MALDIFTMS
(DHB) m/z 467.1445 (C24H22N2O8 + H+ requires 467.1449).
2.0, 8.5 Hz, 2H), 7.83 (d, J ) 2.0 Hz, 2H), 6.95 (d, J ) 8.5 Hz,
2H), 3.99 (s, 6H), 3.98 (s, 6H); 13C NMR (CDCl3, 150 MHz) δ
189.4 (2C), 158.7 (2C), 154.4 (2C), 149.1 (2C), 128.6 (2C), 128.0
(4C), 112.4 (2C), 110.0 (2C), 56.2 (2C), 56.1 (2C); IR (film) νmax
1654, 1593, 1517 cm-1; MALDIFTMS (DHB) m/z 409.1391 (M
+ H+, C22H20N2O6 requires 409.1394).
F r om 8g. A solution of 2 (101 mg, 0.246 mmol) in dioxane
(1.20 mL) was treated with ethyl vinyl ether (8g, 45 µL, 0.48
mmol) and stirred at 35 °C for 48 h. At the end of the reaction,
the solution had changed in color from orange to yellow. The
solvent was removed in vacuo, and the product was precipi-
tated with MeOH and collected by filtration to afford 9g (94.8
mg, 95%).
3,6-Bis(3,4-dim eth oxyben zoyl)-5-(p-ch lor oph en yl)-1,2,4-
tr ia zin e (11). F r om 10a . A solution of 2 (17.9 mg, 0.044
mmol) and the free base23 of S-methyl (p-chlorophenyl)-
thioimidate hydroiodide4 (10a , 27.4 mg, 0.087 mmol) in dioxane
(225 µL) was warmed at 50 °C for 17 h. The solvent was
removed in vacuo, and the product was purified by chroma-
tography (SiO2, 1 × 10 cm, 2:1 EtOAc/hexanes) to yield 11 (13.8
mg, 61%) as a red solid: mp 135-136 °C; 1H NMR (CDCl3,
500 MHz) δ 8.08 (m, 2H), 8.02 (m, 2H), 7.74 (d, J ) 1.8 Hz,
2H), 6.93-6.90 (m, 4H), 3.98 (s, 6H), 3.95 (s, 6H); 13C NMR
(CDCl3, 150 MHz) δ 199.1, 192.9, 170.3, 169.2, 168.8, 160.3,
157.7, 154.6, 152.4, 152.3, 141.3, 135.9, 132.4, 129.7, 129.5,
126.8, 126.7, 118.8, 118.6, 112.8, 112.6, 110.3, 110.2, 56.5 (2C),
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the National Institutes of Health
(CA42056) and the Skaggs Institute for Chemical Biol-
ogy and Bristol-Myers Squibb Corporation for a fellow-
ship for D.R.S. D.R.S. and J .M.Z. are Skaggs Fellows.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
(23) The salt was partitioned between CHCl3 (1.5 mL) and 10%
aqueous NaHCO3 (1.5 mL) and the organic phase was concentrated to
provide the free base.
J O020713V
3598 J . Org. Chem., Vol. 68, No. 9, 2003