Synthesis and characterization of dialkyl esters of 1,2,4,5-tetrazine-3,6- dicarboxylic acid

Article abstract of DOI:10.1135/cccc20080107

Synthesis and characterization of a series of dialkyl esters of 1,2,4,5-tetrazine-3,6-dicarboxylic acid are reported. These compounds were prepared by a two-stage synthesis: re-esterification of dimethyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate in the presence of aluminium triethoxide and subsequent dehydrogenation of dialkyl 1,4-dihydro-1,2,4,5- tetrazine-3,6-dicarboxylates. The structures of the prepared compounds were confirmed by NMR and mass spectra.

Full text of DOI:10.1135/cccc20080107

Dialkyl Esters of 1,2,4,5-Tetrazine-3,6-dicarboxylic Acid
107
SYNTHESIS AND CHARACTERIZATION OF DIALKYL ESTERS OF
1,2,4,5-TETRAZINE-3,6-DICARBOXYLIC ACID
b
Štěpán FREBORTa1, Numan ALMONASYa2,*, Radim HRDINAa3, Antonín LYČKA ,
c1
c2
Miroslav LÍSA and Michal HOLČAPEK
a
Department of Technology of Organic Compounds, Faculty of Chemical Technology,
University of Pardubice, Studentská 95, CZ-532 10 Pardubice, Czech Republic;
e-mail: tramvajak.ostrava@post.cz, numan.almonasy@upce.cz, radim.hrdina@upce.cz
Research Institute for Organic Syntheses, Rybitví 296, CZ-533 54 Rybitví, Czech Republic;
e-mail: antonin.lycka@vuos.com
Department of Analytical Chemistry, Faculty of Chemical Technology,
University of Pardubice, Studentská 95, CZ-532 10 Pardubice, Czech Republic;
e-mail: miroslav.lisa@upce.cz, michal.holcapek@upce.cz
1
2
3
b
c
1
2
Received August 21, 2007
Accepted Decem ber 5, 2007
Syn th esis an d ch aracterization of a series of dialkyl esters of 1,2,4,5-tetrazin e-3,6-dicarboxylic
acid are reported. Th ese com poun ds were prepared by a two-stage syn th esis: re-esterification
of dim eth yl 1,4-dih ydro-1,2,4,5-tetrazin e-3,6-dicarboxylate in th e presen ce of alum in ium
trieth oxide an d subsequen t deh ydrogen ation of dialkyl 1,4-dih ydro-1,2,4,5-tetrazin e-3,6-di-
carboxylates. Th e structures of th e prepared com poun ds were con firm ed by NMR an d m ass
spectra.
Keyw ord s: Tetrazin es; Dih ydrotetrazin es; Esterification ; Alum in ium trieth oxide; Oxidation .
1,2,4,5-Tetrazin es h ave been widely in vestigated because of th e possibility
of a large variety of practical application s. In vestigation s h ave led to th eir
application s as biocides1–6, h igh -n itrogen en ergetic m aterials7,8, an d in ter-
m ediates for syn th eses of a variety of h eterocyclic com poun ds9–12. Over th e
last twenty years, 3,6-disubstituted com pounds of 1,2,4,5-tetrazine type 1 have
been in ten sively studied9–15. Dim eth yl 1,2,4,5-tetrazin e-3,6-dicarboxylate is
th e best-kn own com poun d of th is type from th e viewpoin t of application
an d key in term ediate in in verse-type Diels–Alder reaction 9–15. In th e sen se
of th is reaction , som e dialkyl 1,2,4,5-tetrazin e-3,6-dicarboxylates m ay h ave
poten tial use for im provin g th e sh elf-life of fruits an d vegetables by trap-
pin g th e eth en e gas gen erated by th ese crops16. Eth en e rapidly reacts with
1,2,4,5-tetrazin e com poun ds at room tem perature an d produces dih ydro-
pyridazin e com poun ds with con com itan t form ation of n itrogen gas17
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115
© 2008 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20080107

108
Frebort, Almonasy, Hrdina, Lyčka, Lísa, Holčapek:
(Sch em e 1). A ch aracteristic colour ch an ge is typical of th is reaction : th e
brigh t-red reactan ts turn ed to yellow products.
COOR
COOR
N
N
N
N
N
N
+
CH2=CH2
+
N
N
COOR
COOR
1, R = alkyl
SCHEME 1
1,2,4,5-Tetrazin es are un stable in th e presen ce of water18, wh ich is an im -
pedim en t in th eir application s in m oist or h um id con dition s, as it is th e
case of fruits an d vegetables. Th erefore th e tetrazin es are in corporated in a
h ydrofobic eth en e-perm eable substrate wh ich does n ot con tain h ydroxy
groups. Th e used h ydroph obic polym eric m aterials in clude silicon epoly-
carbon ate, polystyren e, polyeth ylen e an d polypropylen e16. Th e preferred
tetrazin e esters used as active substan ces are dioctyl, didecyl an d dim eth yl
1,2,4,5-tetrazin e-3,6-dicarboxylate16
.
In spite of th e wide ran ge of application s m en tion ed above, th ese com -
poun ds h ave n ot received sufficien t atten tion as far as th eir syn th eses are
con cern ed. Th erefore, our in itial studies were aim ed at fin din g th e opti-
m um m eth od for th eir preparation .
RESULTS AND DISCUSSION
Because of th e in stability of 1,4-dih ydro-1,2,4,5-tetrazin e-3,6-dicarboxylic
acid (3) in acid m edia at en h an ced tem peratures11, we failed to get dialkyl
esters by usin g th e stan dard esterification m eth od. Th erefore, we used th e
procedure described by Boger13 for syn th esis of dim eth yl 1,4-dih ydro-
1,2,4,5-tetrazin e-3,6-dicarboxylate (4a). Accordin g to th is m eth od, th e
diester was prepared by th e th ree-step reaction of eth yl diazoacetate with
sodium h ydroxide in water at 70 °C givin g sodium 1,2,4,5-tetrazin e-
3,6-dicarboxylate (2), wh ich was treated with aqueous HCl to give 3. Th e
followin g reaction of 3 with th ion yl ch loride an d m eth an ol at –30 °C th en
gave 4a (Sch em e 2).
Th e sam e procedure was adopted for preparation of dieth yl, dipropyl,
an d dibutyl esters 4b–4d , but th eir reaction yields were very low (15, 10
an d 6% for 4b, 4c an d 4d , respectively). Th e reaction yields drastically de-
crease with in creasin g alkyl ch ain len gth . So far our efforts to prepare
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

Dialkyl Esters of 1,2,4,5-Tetrazine-3,6-dicarboxylic Acid
109
O
ONa
O
OH
NaOH, H2O
60–80 °C
HCl, H2O
HN
N
N
HN
N
N
N2CHCOOC2H5
NH
NH
NaO
O
HO
O
2
3
O
OH
O
OR
SOCl2, ROH
–30 °C
HN
N
N
HN
N
N
NH
NH
HO
O
RO
O
4
3
a
b
c
d
R
Me Et Pr Bu
SCHEME 2
lon ger-ch ain esters usin g th e sam e m eth od h ave failed. Th erefore, for th e
syn th esis of th ese esters we h ave adopted a m odification of th e com m on
procedure con sistin g in re-esterification of 4a with various alcoh ols at boil-
in g tem perature in th e presen ce of Al(OC2H5)3 as a catalyst. In th is way,
a series of dialkyl 1,4-dih ydro-1,2,4,5-tetrazin e-3,6-dicarboxylates 5a–5i
was prepared (Sch em e 3). Th e equilibrium of th is reaction is usually sh ifted
towards th e product by distillin g off th e m eth an ol form ed.
Gen erally, th e re-esterification reaction s of 4a with lon g-ch ain alcoh ols
were easy an d with out com plication s. However, with lower-boilin g alcoh ols
th e re-esterification was difficult. Th erefore, th e re-esterification of 4a with
sh ort-ch ain , e.g. eth yl, propyl an d butyl, alcoh ols was perform ed in an au-
toclave. Our attem pts at preparin g such esters at n orm al atm osph eric pres-
sure led to very low yields an d con version s. Usually, th e products were
presen t in a yield of ca. 50% togeth er with th e n on -reacted startin g m aterial
4a. Th e use of autoclave was successful: th e reaction tem perature in creased,
an d h en ce also th e yields an d con version s. Neverth eless, th e reaction yields
an d purity of products 5b–5d were n ot as good as th ose of lon g-ch ain esters
5e–5i. Th e reaction course an d purity of products were ch ecked by HPLC
with aceton itrile–water (4:1) as th e m obile ph ase.
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

110
Frebort, Almonasy, Hrdina, Lyčka, Lísa, Holčapek:
Th e oxidation m eth od adopted for all th e dih ydro com poun ds 4 was
iden tical with th e procedures described in th e literature13. Th e syn th esis is
sh own in Sch em e 3. Th e prepared com poun ds were ch aracterized by 1H,
13C, an d 15N NMR spectra, elem en tal an alysis an d m ass spectra. Th e m olec-
ular weigh ts (MW) of all th e studied com poun ds were con firm ed by th e
m easurem en t of th eir atm osph eric-pressure ch em ical ion ization (APCI)
m ass spectra in both polarity m odes. Positive-ion APCI m ass spectra sh ow
m ain ly peaks of proton ated m olecules [M + H]+, wh ile th e n egative-ion
APCI spectra provide com plem en tary in form ation with [M – H]– ion s. In
addition to th ese even -electron ion s, typical of soft ion ization m ass spectra,
th e odd-electron m olecular radical ion s M+ in th e positive-ion m ode an d
M– in th e n egative-ion m ode are observed as well, wh ich is quite un usual in
APCI 19. Th e type of alkyl substituen t can be iden tified on th e basis of n eu-
tral losses of side ch ain , as reported earlier20
.
O
OCH3
O
O(CH2)nCH3
O
O(CH2)nCH3
NOx, CH2
0 °C
HN
N
N
HN
N
N
N
N
N
N
ROH, Al(OEt)3
NH
NH
H3CO
O
CH3(CH2)nO
5a–5i
O
CH3(CH2)nO
6a–6i
O
5,6
a
b
c
d
e
f
g
h
i
SCHEME 3
n
1
2
3
4
5
6
7
8
9
CONCLUSIONS
Alum in ium trieth oxide was used successfully as a catalyst for th e re-
esterification of dim eth yl 1,4-dih ydro-1,2,4,5-tetrazin e-3,6-dicarboxylate
with various alcoh ols. It was foun d th at th e re-esterification with lon ger-
ch ain alcoh ols (C5–C10) is easier th an th at with sh orter-ch ain alcoh ols
(C1–C4). Th e correspon din g dialkyl 1,2,4,5-tetrazin e-3,6-dicarboxylates 6
were obtain ed by oxidation of th e 1,4-dih ydro com poun ds with n itrogen
oxides (NOx).
EXPERIMENTAL
Th e ch em icals used were purch ased from Fluka an d th eir m eltin g poin ts were ch ecked on
a h ot-stage m icroscope. Th e 1H, 13C an d 15N NMR spectra (δ, ppm ; J, Hz) were recorded on
a Bruker Avan ce 500 spectrom eter operatin g at 500.13 MHz for 1H, 125.76 MHz for 13C an d
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

Dialkyl Esters of 1,2,4,5-Tetrazine-3,6-dicarboxylic Acid
111
50.68 MHz for 15N. Th e sam ples were dissolved in deuterioch loroform an d m easured at
room tem perature. Th e 1H an d 13C ch em ical sh ifts were referen ced to in tern al TMS. Th e
15N ch em ical sh ifts were referen ced to extern al n itrom eth an e in coaxial capillary (δ 0.0).
Th e positive values of ch em ical sh ifts den ote sh ifts of sign als to h igh er frequen cies with re-
spect to th e stan dard. Th e 13C ch em ical sh ifts of COO groups were assign ed h avin g an alysed
proton -coupled 13C NMR spectra in wh ich th ese groups form broaden ed triplets due to
3J(13C,H) couplin g con stan ts from OCH2 groups (in con trast to sin glets of arom atic). Mass
spectrom etry an alyses were perform ed on an Esquire 3000 ion trap an alyzer (Bruker
Dalton ics, Brem en , Germ an y). Atm osph eric-pressure ch em ical ion ization (APCI) m ass spec-
tra were recorded in th e m ass ran ge m/z 50–1000 usin g both positive- an d n egative-ion
m odes an d th e followin g settin g of tun in g param eters: target m ass m/z 250 an d com poun d
stability 20% for th e alkyl series C2–C4, m/z 350 an d com poun d stability 100% for th e alkyl
series C5–C10, pressure of th e n ebulizin g gas 103.4 kPa, th e dryin g gas flow rate 4 l/m in , ion
source tem perature 350 °C an d dryin g gas tem perature 300 °C. Th e sam ples were dissolved
in aceton itrile an d delivered in to th e system with in fusion pum p at a flow rate of 5 µl/m in .
Th e an alytical HPLC was perform ed with
a Th erm o separation products PC 1000,
equipped with a reverse-ph ase colum n (250 × 4 m m ) Nucleosil C18. Th e elem en tal an alyses
were perform ed by th e Cen tral An alytical Service of Departm en t of Organ ic Ch em istry,
Un iversity of Pardubice.
1,4-Dih ydro-1,2,4,5-tetrazin e-3,6-dicarboxylic acid (3) an d dim eth yl 1,4-dih ydro-1,2,4,5-
tetrazin e-3,6-dicarboxylate (4a) were prepared by kn own m eth ods12
.
Syn th esis of Com poun ds 5a–5c. Gen eral Procedure
An am oun t of 1 g (5.0 m m ol) of 4a, 60 m l of correspon din g alcoh ol an d a catalytic am oun t
of alum in ium trieth oxide were placed in to an autoclave. Th e autoclave was h eated to 180 °C
un til m axim um con version was reach ed (HPLC, MeCN–H2O 4:1). Durin g 1 h alcoh ol va-
pours were gradually released (45 m l of th e con den sate was collected). Th e autoclave was
th en cooled to room tem perature, th e crude product was filtered off an d purified by
recrystallization from h exan e.
Diethyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5a ; R = C2H5). Yield 46%. M.p.
104–108 °C (ref.1 101–102 °C). 1H NMR: 7.51 (NH); 4.39 (OCH2); 1.39 (CH3). Positive-ion
APCI-MS: m/z 229 [M + H]+ (100%), 228 [M]+, 201 [M + H – C2H4]+•
. Positive-ion
APCI-MS/MS of m/z 229: m/z 201 [M + H – C2H4]+ (100%), 173 [M + H – C4H8]+. Negative-
ion APCI-MS: m/z 226 [M – 2]–• (100%). For C8H12N4O4 (228.2) calculated: 42.10% C, 5.30% H,
24.55% N; foun d: 41.71% C, 5.38% H, 25.14% N.
Dipropyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5b; R = n-C3H7). Yield 39%. M.p.
78–81 °C. 1H NMR: 7.53 (NH); 4.28 (OCH2); 1.76 (CH2); 0.99 (CH3). Positive-ion APCI-MS:
m/z 257 [M + H]+ (100%), 256 [M]+• , 215 [M + H – C3H6]+. Positive-ion APCI-MS/MS of m/z
257: m/z 215 [M + H – C3H6]+ (100%), 173 [M + H – C6H12]+. Negative-ion APCI-MS: m/z 254
[M – 2]–• (100%). For C10H16N4O4 (256.3) calculated: 46.87% C, 6.29% H, 21.86% N; foun d:
46.72% C, 6.27% H, 22.02% N.
Dibutyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5c; R = n-C4H9). Yield 42%. M.p.
56–61 °C. 1H NMR: 7.49 (NH); 4.31 (OCH2); 1.73, 1.43 ((CH2)2); 0.95 (CH3). Positive-ion
APCI-MS: m/z 285 [M + H]+ (100%), 284 [M]+• . Positive-ion APCI-MS/MS of m/z 285: m/z 229
[M + H – C4H8]+ (100%), 173 [M + H – C8H16]+. Negative-ion APCI-MS: m/z 282 [M – 2]–•
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

112
Frebort, Almonasy, Hrdina, Lyčka, Lísa, Holčapek:
(100%). For C12H20N4O4 (284.3) calculated: 50.69% C, 7.09% H, 19.71% N; foun d: 50.76% C,
7.04% H, 20.11% N.
Syn th esis of Com poun ds 5d –5i. Gen eral Procedure
An am oun t of 5 g (24.98 m m ol) of 4a was h eated with 100 m l of correspon din g alcoh ol an d
catalytic am oun t of alum in um trieth oxide. A part of th e alcoh ol form ed was distilled off
from th e reaction m ixture durin g 1 h . Th e rusty brown reaction m ixture was th en allowed
to cool in refrigerator. Th e obtain ed crystals were filtered off, recrystallized from h exan e an d
dried in vacuum at 55 °C.
Dipentyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5d ; R = n-C5H11). Yield 71%. M.p.
50–52 °C. 1H NMR: 7.63 (NH); 4.30 (OCH2); 1.14–1.76 ((CH2)3); 0.89 (CH3). 13C NMR: 158.6
(COO); 138.2 (N=C-N); 67.1 (OCH2); 28.1, 25.2, 22.3 ((CH2)3); 13.8 (CH3). Positive-ion
APCI-MS: m/z 313 [M + H]+ (100%), 312 [M]+• , 243 [M + H – C5H1+0]+, 173 [M + H –
C
10H20]+. Positive-ion APCI-MS/MS of m/z 313: m/z 243 [M + H – C5H10
] (100%), 173 [M +
H – C10H20]+. Negative-ion APCI-MS: m/z 310 [M – 2]– (100%). For C14H24N4O4 (312.4) cal-
culated: 53.83% C, 7.74% H, 17.94% N; foun d: 54.15% C, 7.58% H, 17.73% N.
Dihexyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5e; R = n-C6H13). Yield 70%. M.p.
56–60 °C. 1H NMR: 7.61 (NH); 4.30 (OCH2); 1.34–1.77 ((CH2)4); 0.91 (CH3). 13C NMR: 158.6
(COO); 138.3 (N=C-N); 67.2 (OCH2); 27.9, 27.7, 22.1 ((CH2)4); 13.7 (CH3). Positive-ion
APCI-MS: m/z 341 [M + H]+ (100%), 340 [M]+• , 257 [M + H – C6H12]+. Positive-ion
+
APCI-MS/MS of m/z 341: m/z 257 [M + H – C6H12
]
(100%), 173 [M + H – C12H24]+.
Negative-ion APCI-MS: m/z 338 [M – 2]–• (100%). For C16H28N4O4 (340.4) calculated:
56.45% C, 8.29% H, 16.46% N; foun d: 56.95% C, 8.22% H, 16.20% N.
Diheptyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5f; R = n-C7H15). Yield 66%. M.p.
58–61 °C. 1H NMR: 7.60 (NH); 4.30 (OCH2); 1.24–1.74 ((CH2)5); 0.88 (CH3). 13C NMR: 158.6
(COO); 138.3 (N=C-N); 67.3 (OCH2); 31.5, 29.0, 28.9, 28.3, 25.6, 22.5 ((CH2)5); 14.0 (CH3).
Positive-ion APCI-MS: m/z 369 [M + H]+ (100%), 368 [M]+• , 271 [M + H – C7H14]+. Positive-
+
ion APCI-MS/MS of m/z 369: m/z 271 [M + H – C7H14
]
(100%), 173 [M + H – C14H28]+.
Negative-ion APCI-MS: m/z 366 [M – 2]–• (100%). For C18H32N4O4 (368.5) calculated:
58.67% C, 8.75% H, 15.20% N; foun d: 58.94% C, 8.81% H, 15.27% N.
Dioctyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5g; R = n-C8H17). Yield 70%. M.p.
67–69 °C. 1H NMR: 7.57 (NH-, 1J(15H,1H) = 87.9); 4.30 (OCH2); 1.24–1.74 ((CH2)6); 0.88
(CH3). 13C NMR: 158.6 (COO); 138.3 (N=C-N); 67.3 (OCH2); 31.5, 29.0, 28.9, 28.3, 25.6,
22.5 ((CH2)6); 14.0 (CH3). 15N NMR: –251.4 (-NH-); –102.8 (-N=). Positive-ion APCI-MS: m/z
397 [M + H]+ (100%), 396 [M]+• , 285 [M + H – C8H16]+. Positive-ion APCI-MS/MS of m/z 397:
+
m/z 285 [M + H – C8H16
]
(100%), 173 [M + H – C16H32]+. Negative-ion APCI-MS: m/z 394
[M – 2]–• (100%). For C20H36N4O4 (396.5) calculated: 60.58% C, 9.15% H, 14.13% N; foun d:
60.98% C, 8.99% H, 13.92% N.
Dinonyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5h ; R = n-C9H19). Yield 85%. M.p.
67–70 °C. 1H NMR: 7.59 (NH); 4.30 (OCH2); 1.26–1.76 ((CH2)7); 0.88 (CH3). 13C NMR: 158.6
(COO); 138.2 (N=C-N); 67.1 (OCH2); 31.7, 29.3, 29.1, 29.0, 28.3, 25.6, 22.5 ((CH2)7); 14.0
(CH3). Positive-ion APCI-MS: m/z 425 [M + H]+ (100%), 424 [M]+• , 299 [M + H – C9H18]+.
+
Positive-ion APCI-MS/MS of m/z 425: m/z 299 [M + H – C9H18
C
]
(100%), 173 [M + H –
18H36]+. Negative-ion APCI-MS: m/z 422 [M – 2]–• (100%). For C22H40N4O4 (424.6) calcu-
lated: 62.24% C, 9.50% H, 13.20% N; foun d: 62.58% C, 9.46% H, 13.22% N.
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

Dialkyl Esters of 1,2,4,5-Tetrazine-3,6-dicarboxylic Acid
113
Didecyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (5i; R = n-C10H21). Yield 79%. M.p.
76–77 °C. 1H NMR: 7.54 (NH); 4.30 (OCH2); 1.26–1.76 ((CH2)8); 0.88 (CH3). 13C NMR: 158.6
(COO); 138.3 (N=C-N); 67.2 (OCH2); 31.8, 29.4, 29.3, 29.2, 29.1, 28.3, 25.6, 22.6 ((CH2)8);
14.0 (CH3). Positive-ion APCI-MS: m/z 453 [M + H]+ (100%), 452 [M]+• , 313 [M + H –
+
C
10H20]+. Positive-ion APCI-MS/MS of m/z 453: m/z 313 [M + H – C10H20
]
(100%), 173 [M +
H – C20H40]+. Negative-ion APCI-MS: m/z 450 [M – 2]–• (100%). For C24H44N4O4 (452.6) cal-
culated: 63.69% C, 9.80% H, 12.38% N; foun d: 63.52% C, 9.98% H, 12.41% N.
Syn th esis of Com poun ds 6a–6i. Gen eral Procedure
A solution of 5 g of 5a–5i in 100 m l of dry dich lorom eth an e was cooled to 0 °C. A stream of
n itrous gases was bubbled in to th e reaction m ixture with stirrin g for 30 m in . Th e reaction
m ixture was stirred for an oth er 1 h . Th e clear yellow solution gradually turn ed to brigh t red.
A large part of th e solven t was rem oved, an d th e red precipitate was filtered off.
Diethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6a; R = C2H5). Yield 96%. M.p. 103–106 °C (ref.21
105–106 °C). 1H NMR: 4.69 (OCH2); 1.54 (CH3). Positive-ion APCI-MS: m/z 227 [M + H]+
(100%), 226 [M]+• , 199 [M + H – C2H4]+. Positive-ion APCI-MS/MS of m/z 227: m/z 199 [M +
H – C2H4]+ (100%). Negative-ion APCI-MS: m/z 226 [M]–• (100%).
Dipropyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6b; R = n-C3H7). Yield 94%. M.p. 72–76 °C.
1H NMR: 4.56 (OCH2); 1.95 (CH2); 1.09 (CH3). Positive-ion APCI-MS: m/z 255 [M + H]+
(100%), 254 [M]+• . Positive-ion APCI-MS/MS of m/z 255: m/z 213 [M + H – C3H6]+ (100%).
Negative-ion APCI-MS: m/z 254 [M]–• (100%). For C10H14N4O4 (254.3) calculated: 47.24% C,
5.55% H, 22.04% N; foun d: 47.83% C, 5.45% H, 21.94% N.
Dibutyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6c; R = n-C4H9). Yield 95%. M.p. 50–53 °C.
1H NMR: 4.60 (OCH2); 1.90, 1.45 ((CH2)2); 0.95 (CH3). Positive-ion APCI-MS: m/z 283 [M +
H]+, 282 [M]+• (100%), 227 [M + H – C4H8]+, 171 [M + H – C8H16]+. Positive-ion
APCI-MS/MS of m/z 282: m/z 226 [M – C4H8]+• (100%), 172 [M – C8H14 +• . Negative-ion
]
APCI-MS: m/z 282 [M]–• (100%). For C12H18N4O4 (282.3) calculated: 51.06% C, 6.43% H,
19.85% N; foun d: 50.88% C, 6.56% H, 19.73% N.
Dipentyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6d ; R = n-C5H11). Yield 87%. M.p. 42–45 °C.
1H NMR: 4.62 (OCH2); 1.35–1.92 ((CH2)3); 0.93 (CH3). 13C NMR: 160.2 (COO); 159.3
(N=C-N); 68.4 (OCH2); 28.1, 27.9, 22.3 ((CH2)3); 13.9 (CH3). Positive-ion APCI-MS: m/z 311
+•
[M + H]+, 310 [M]+• (100%). Positive-ion APCI-MS/MS of m/z 310: m/z 240 [M – C5H10
]
(100%), 172 [M – C10H18]
+• . Negative-ion APCI-MS: m/z 310 [M]–• (100%). For C14H22N4O4
(310.4) calculated: 54.18% C, 7.15% H, 18.05% N; foun d: 55.17% C, 7.42% H, 16.83% N.
Dihexyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6e; R = n-C6H13). Yield 94%. M.p. 44–47 °C.
1H NMR: 4.61 (OCH2); 1.31–1.94 ((CH2)4); 0.91 (CH3). 13C NMR: 160.2 (COO); 159.1
(N=C-N); 68.2 (OCH2); 31.3, 28.3, 25.4, 22.1 ((CH2)4); 14.0 (CH3). Positive-ion APCI-MS: m/z
339 [M + H]+, 338 [M]+• (100%), 256 [M – C6H10
m/z 256 [M – C6H10
+• , 254 [M – C6H12
]
+• . Positive-ion APCI-MS/MS of m/z 338:
+•
]
]
+• , 172 [M – C12H22
]
(100%). Negative-ion
APCI-MS: m/z 338 [M]–• (100%). For C16H26N4O4 (338.4) calculated: 56.79% C, 7.74% H,
16.56% N; foun d: 57.05% C, 7.79% H, 16.19% N.
Diheptyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6f; R = n-C7H15). Yield 92%. M.p. 52–55 °C.
1H NMR: 4.61 (OCH2); 1.30–1.92 ((CH2)5); 0.89 (CH3). 13C NMR: 160.2 (COO); 159.3
(N=C-N); 67.2 (OCH2); 31.7, 28.6, 28.4, 25.7, 22.6 ((CH2)5); 14.1 (CH3). Positive-ion
+•
APCI-MS: m/z 367 [M + H]+, 366 [M]+• (100%), 270 [M – C7H12
]
+• , 172 [M – C14H26
]
]
.
.
+•
+•
Positive-ion APCI-MS/MS of m/z 366: m/z 270 [M – C7H12
]
(100%), 172 [M – C14H26
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 1, pp. 107–115

114
Frebort, Almonasy, Hrdina, Lyčka, Lísa, Holčapek:
Negative-ion APCI-MS: m/z 366 [M]–• (100%). For C18H30N4O4 (366.5) calculated: 59.00% C,
8.25% H, 15.29% N; foun d: 59.14% C, 8.17% H, 15.08% N.
Dioctyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6g; R = n-C8H17). Yield 98%. M.p. 63–67 °C.
1H NMR: 4.56 (OCH2); 1.26–1.89 ((CH2)6); 0.86 (CH3). 13C NMR: 160.0 (COO); 159.2
(N=C-N); 68.3 (OCH2); 31.6, 29.0, 28.3, 28.2, 25.7, 22.6 ((CH2)6); 14.0 (CH3). 15N NMR: 14.1
+•
(-N=). Positive-ion APCI-MS: m/z 395 [M + H]+, 394 [M]+• (100%), 284 [M – C8H14
]
]
.
.
+•
+•
Positive-ion APCI-MS/MS of m/z 394: m/z 284 [M – C8H14
]
(100%), 172 [M – C16H30
Negative-ion APCI-MS: m/z 394 [M]–• (100%). For C20H34N4O4 (394.5) calculated: 60.89% C,
8.69% H, 14.20% N; foun d: 61.11% C, 8.59% H, 14.32% N.
Dinonyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6h ; R = n-C9H19). Yield 97%. M.p. 67–69 °C.
1H NMR: 4.61 (OCH2); 1.25–1.92 ((CH2)7); 0.87 (CH3). 13C NMR: 160.1 (COO); 159.3
(N=C-N); 68.4 (OCH2); 31.8, 29.4, 29.2, 29.1, 28.4, 25.7, 22.6 ((CH2)7); 14.0 (CH3). Positive-
ion APCI-MS: m/z 423 [M + H]+, 422 [M]+• (100%). Positive-ion APCI-MS/MS of m/z 422: m/z
+•
298 [M – C9H16
]
(100%), 172 [M – C18H34]
+• . Negative-ion APCI-MS: m/z 422 [M]–• (100%).
For C22H38N4O4 (422.6) calculated: 62.53% C, 9.06% H, 13.26% N; foun d: 62.70% C, 9.04% H,
12.95% N.
Didecyl 1,2,4,5-tetrazine-3,6-dicarboxylate (6i; R = n-C10H21). Yield 89%. M.p. 76–78 °C.
1H NMR: 4.60 (OCH2); 1.24–1.89 ((CH2)8); 0.86 (CH3). 13C NMR: 160.2 (COO); 159.3
(N=C-N); 68.4 (OCH2); 31.9, 29.5, 29.4, 29.3, 29.2, 28.4, 25.8, 22.7 ((CH2)8); 14.0 (CH3).
Positive-ion APCI-MS: m/z 451 [M + H]+ (100%), 450 [M]+• . Positive-ion APCI-MS/MS of m/z
+
451: m/z 313 [M + H – C10H18
]
(100%), 172 [M – C20H38
]
+• . Negative-ion APCI-MS: m/z 450
[M]–• (100%). For C24H42N4O4 (450.6) calculated: 63.97% C, 9.39% H, 12.43% N; foun d:
64.17% C, 9.48% H, 12.35% N.
The authors are grateful for financial support of this research by the VCI Brasil. M. Lísa and
M. Holčapek acknowledge the support of Ministry of Education, Youth and Sports of the Czech
Republic (Grant No. MSM0021627502).
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