Regioselective Inverse Electron Demand Diels-Alder Reactions of N-Acyl 6-Amino-3-(methylthio)-1,2,4,5-tetrazines

Article abstract of DOI:10.1021/jo980795g

The regioselective inverse electron demand Diels-Alder reactions of 6-[(tert-butyloxycarbonyl)-amino]-3-(methylthio)-1,2,4,5-tetrazine (2), 6-(acetylamino)-3-(methylthio)-1,2,4,5-tetrazine (3), and 6-(benzyloxycarbonyl)amino-3-(methylthio)-1,2,4,5-tetrazine (4) are disclosed. All three underwent regioselective [4 + 2] cycloaddition with electron-rich dienophiles to form the corresponding functionalized 1,2-diazines in excellent yields. An order of reactivity with electron-rich dienophiles was observed with both 2 and 3 being more reactive than 3,6-bis(methylthio)-1,2,4,5-tetrazine (1, i.e. 3 > 2 > 1), and both 3 and 4 were shown to be more robust than 2 at the higher temperatures necessary for [4 + 2] cycloaddition with less reactive dienophiles. The cycloaddition regioselectivity is consistent with the polarization of the diene and the ability of the methylthio group to stabilize a partial negative charge at C-3, and the N-acylamino group to stabilize a partial positive charge at C-6. While intermolecular reactions of unactivated alkynes either did not proceed or required high temperatures and long reaction times, intramolecular Diels-Alder reactions utilizing tethered unactivated acetylenes led to five- and six-membered bicyclic 1,2-diazines under mild conditions.

Full text of DOI:10.1021/jo980795g

J . Org. Chem. 1998, 63, 6329-6337
6329
Regioselective In ver se Electr on Dem a n d Diels-Ald er Rea ction s of
N-Acyl 6-Am in o-3-(m eth ylth io)-1,2,4,5-tetr a zin es
Dale L. Boger,* Robert P. Schaum, and Robert M. Garbaccio
Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La J olla, California 92037
Received April 28, 1998
The regioselective inverse electron demand Diels-Alder reactions of 6-[(tert-butyloxycarbonyl)-
amino]-3-(methylthio)-1,2,4,5-tetrazine (2), 6-(acetylamino)-3-(methylthio)-1,2,4,5-tetrazine (3), and
6-(benzyloxycarbonyl)amino-3-(methylthio)-1,2,4,5-tetrazine (4) are disclosed. All three underwent
regioselective [4 + 2] cycloaddition with electron-rich dienophiles to form the corresponding
functionalized 1,2-diazines in excellent yields. An order of reactivity with electron-rich dienophiles
was observed with both 2 and 3 being more reactive than 3,6-bis(methylthio)-1,2,4,5-tetrazine (1,
i.e. 3 > 2 > 1), and both 3 and 4 were shown to be more robust than 2 at the higher temperatures
necessary for [4 + 2] cycloaddition with less reactive dienophiles. The cycloaddition regioselectivity
is consistent with the polarization of the diene and the ability of the methylthio group to stabilize
a partial negative charge at C-3, and the N-acylamino group to stabilize a partial positive charge
at C-6. While intermolecular reactions of unactivated alkynes either did not proceed or required
high temperatures and long reaction times, intramolecular Diels-Alder reactions utilizing tethered
unactivated acetylenes led to five- and six-membered bicyclic 1,2-diazines under mild conditions.
The inverse electron demand Diels-Alder reactions of
electron-deficient heterocyclic azadienes1 with electron-
rich dienophiles have proven useful in the total synthesis
of a number of natural products2 possessing highly
functionalized heteroaromatic systems not easily ac-
cessed by conventional means. In particular, the well-
defined participation of 1,2,4,5-tetrazines in [4 + 2]
cycloadditions, and the ability of the resulting 1,2-
diazines to participate in subsequent intramolecular
Diels-Alder reactions3 or a reductive ring-contraction
reaction to provide the corresponding pyrrole,4 has been
enlisted in the total synthesis of PDE-I/PDE-II and CC-
1065,5 streptonigrin,6 prodigiosin,7 OMP,4 trikentrin A,8
and isochrysohermidin.9 In each case, the reaction of a
symmetrical 1,2,4,5-tetrazine with an electron-rich di-
enophile served as a key transformation. The extension
of these studies to the regioselective cycloadditions of
unsymmetrical 1,2,4,5-tetrazines would extend the flex-
ibility of the approach. However, reports detailing the
preparation and participation of such unsymmetrically
substituted 1,2,4,5-tetrazines in regioselective Diels-
Alder reactions are limited.10-13 Herein, we detail the
preparation of three such azadienes, 6-[(tert-butyloxy-
carbonyl)amino]-3-(methylthio)-1,2,4,5-tetrazine (2), 6-
(acetylamino)-3-(methylthio)-1,2,4,5-tetrazine (3), and
6-[(benzyloxycarbonyl)amino]-3-(methylthio)-1,2,4,5-tet-
razine (4), and an investigation of their inter- and
intramolecular inverse electron demand Diels-Alder
reactions which define their [4 + 2] cycloaddition regio-
selectivity and reactivity. The studies complement the
recent disclosure of Panek and Zhu10 enlisting analogous
solid-supported tetrazines and our own report on 6-meth-
oxy-3-(methylthio)-1,2,4,5-tetrazine.11
Syn th esis of 1,2,4,5-Tetr a zin es 2-4. Tetrazines
2-4 were anticipated to be accessible through the selec-
tive displacement of one methylthio group of 3,6-bis-
(methylthio)-1,2,4,5-tetrazine 13,8,10,11 with the anions of
tert-butyl carbamate, acetamide, or benzyl carbamate in
a one-step procedure. While this route proved successful,
an alternative approach enlisting first the addition of
ammonia and subsequent acylation of the resulting
6-amino-3-(methylthio)-1,2,4,5-tetrazine14 was also ex-
amined thus providing two methods for their preparation
applicable to other unsymmetrically substituted tetra-
zines (Scheme 1).
(1) Boger, D. L.; Weinreb, S. M. Hetero Diels-Alder Methodology
in Organic Synthesis; Academic: San Diego, 1987. Boger, D. L.
Tetrahedron 1983, 39, 2869.
(2) Boger, D. L. Chemtracts: Org. Chem. 1996, 9, 149. Boger, D. L.
Bull. Soc. Chim., Belg. 1990, 99, 599. Boger, D. L. Chem. Rev. 1986,
86, 781.
Thus, treatment of a solution of 1 in DMF cooled to
-40 °C with tert-butyl carbamate (1.2 equiv, 16-18 h),
acetamide (1.2 equiv, 12 h), or benzyl carbamate (1.2
equiv, 29 h) and NaH (1.8 equiv) cleanly provided 2
(3) Boger, D. L.; Sakya, S. M. J . Org. Chem. 1988, 53, 1415. Boger,
D. L.; Coleman, R. S. J . Org. Chem. 1984, 49, 2240.
(4) Boger, D. L.; Coleman, R. S.; Panek, J . S.; Yohannes, D. J . Org.
Chem. 1984, 49, 4405.
(5) Boger, D. L.; Coleman, R. S. J . Org. Chem. 1986, 51, 3250. Boger,
D. L.; Coleman, R. S. J . Am. Chem. Soc. 1987, 109, 2717. Boger, D. L.;
Coleman, R. S. J . Am. Chem. Soc. 1988, 110, 4796. Boger, D. L.;
Coleman, R. S. J . Am. Chem. Soc. 1988, 110, 1321.
(11) Sakya, S. M.; Groskopf, K. K.; Boger, D. L. Tetrahedron Lett.
1997, 38, 3805.
(12) Cioslowski, J .; Sauer, J .; Hetzenegger, J .; Karcher, T.; Hier-
stetter, T. J . Am. Chem. Soc. 1993, 115, 1353. Burg, B.; Dittmar, W.;
Reim, H.; Steigel, A.; Sauer, J . Tetrahedron Lett. 1975, 2897. Mueller,
K.; Sauer, J . Tetrahedron Lett. 1984, 25, 2541. Balcar, J .; Chrisam,
G.; Huber, F. X.; Sauer, J . Tetrahedron Lett. 1983, 24, 1481. Meier,
A.; Sauer, J . Tetrahedron Lett. 1990, 31, 6855. Meresz, O.; Foster-
Verner, P. A. J . Chem. Soc., Chem. Commun. 1972, 950. Sauer, J .;
Heldmann, D. K. Tetrahedron 1998, 54, 4297.
(6) Boger, D. L.; Panek, J . S. J . Am. Chem. Soc. 1985, 107, 5745.
(7) Boger, D. L.; Patel, M. J . Org. Chem. 1988, 53, 1405. Boger, D.
L.; Patel, M. Tetrahedron Lett. 1987, 28, 2499.
(8) Boger, D. L.; Zhang, M. J . Am. Chem. Soc. 1991, 113, 4230.
(9) Boger, D. L.; Baldino, C. M. J . Am. Chem. Soc. 1993, 115, 11418.
(10) Panek, J . S.; Zhu, B. Tetrahedron Lett. 1996, 37, 8151.
(13) Figeys, H. P.; Mathy, A. Tetrahedron Lett. 1981, 22, 1393.
(14) Mangia, A.; Bortesi, F.; Amendola, U. J . Heterocycl. Chem. 1977,
14, 587.
S0022-3263(98)00795-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/14/1998

6330 J . Org. Chem., Vol. 63, No. 18, 1998
Boger et al.
Sch em e 1
F igu r e 1.
1,2,4,5-t et r a zin e (2). The electron-rich alkenes 7a -j
including enamines, ketene acetals, alkyl- and trimeth-
ylsilyl enol ethers, and enamides participated in regio-
selective inverse electron demand Diels-Alder reactions
with 2 providing the 1,2-diazine products 8a -f (Table
1). The cyclic enamines 7a -c underwent rapid cycload-
dition at 23 °C usually within 2 h, with concomitant loss
of N2. As with Diels-Alder cycloadditions of 3,6-bis-
(methylthio)-1,2,4,5-tetrazine (1),3 the slowest step of the
reaction cascade was the aromatization step involving the
elimination of the secondary amine. This could be
facilitated by subsequent addition of an acid catalyst or,
optimally, by treatment with 10% HOAc/C6H6 (v:v). For
example, entry 1 (Table 1) required acidic conditions (v:v
10% HOAc/C6H6) to effect complete elimination of pyr-
rolidine while entry 2 did not require such acid catalysis
to promote the elimination of morpholine. The Diels-
Alder reactions with 1-morpholino-1-cyclohexene 7c (en-
try 3, Table 1) and 3-morpholino-2-pentene 7d (entry 4,
Table 1) also proceeded with excellent yields and only
one regioisomer of the 1,2-diazine product 8c was ob-
tained.
(69%), 3 (65%), and 4 (61%), respectively, accompanied
by lesser amounts of recovered 1. In initial efforts
conducted with the preparation of 2, the use of higher
reaction temperatures (-25 to 25 °C) and prolonged
reaction times (48-72 h) provided only low yields of 2
(<15%) and predominantly 3,6-bis(methylthio)-4-hydroxy-
1,2,4-triazine (48%)15 derived from a formal [4 + 2]
cycloaddition of the carbamate anion with 1 (eq 1). This
competitive reaction was suppressed at the lower reaction
temperatures (-40 °C) and with shortened reaction
times. When subjected to typical workup conditions, 2-4
were found to be unstable to aqueous base extractions,
but surprisingly stable to aqueous acid extractions and
standard SiO2 column chromatography. This practical
stability of 2-4, unlike the instability of the more
electron-deficient 1,2,4,5-tetrazines,16 simplified their
preparation, purification, characterization, and subse-
quent study.
The regioselectivity was initially established by 1D
pulsed field gradient NOE 1H NMR analysis of the
isolable cycloadduct 9 obtained from the reaction of 2
with 7d (dioxane, 70 °C, 2 h) (Figure 1). Strong NOE’s
were observed between the methylthio group and the
methyl protons of the ethyl group as well as the meth-
ylene protons of the morpholine moiety, providing evi-
dence of the cycloaddition regioselectivity. This was later
unambiguously established with a single-crystal X-ray
structure determination of 13b.18
The [4 + 2] cycloadditions of 2 with the morpholino
(7e, entry 5) and pyrrolidino (7f, entry 6) enamines of
acetophenone required higher temperatures and longer
reaction times (100 °C, 12 h) for completion of the initial
cycloaddition step. In both cases, elimination of the
secondary amine required acid catalysis (10% HOAc/
C6H6, 23 °C, 14 h). As with entry 4, only one cycloaddi-
Alternatively, addition of NH3 to 1 (saturated NH3-
CH3OH, 0 °C, 30 min, 100%)14 followed by N-acetylation
with Ac2O (1.5 equiv, 0.1 equiv of DMAP, THF, reflux,
15 h) cleanly provided 3 (71%), unreacted 6-amino-3-
(methylthio)-1,2,4,5-tetrazine (20%), and a trace amount
of bisacetylated product 5 (5%). This provided the
preferred method for the synthesis of 3. Attempts to
extend this approach to the preparation of 2 have not
yet been successful, although it has not been extensively
examined. 6-Amino-3-(methylthio)-1,2,4,5-tetrazine failed
to react with di-tert-butyl dicarbonate (BOC2O) in reflux-
ing THF, and the addition of catalytic amounts of DMAP
(0.1 equiv, 25 °C) or reactions in refluxing dioxane
provided only mixtures of the bis N-acylated tetrazine
617 and unreacted starting material with only traces of
2 detected in the reaction mixture.
(17) For 6-[bis(tert-butyloxycarbonyl)amino]-3-(methylthio)-1,2,4,5-
tetrazine (6): 1H NMR (CDCl3, 250 MHz) δ 2.76 (s, 3H), 1.43 (s, 18H);
13C NMR (CDCl3, 100 MHz) δ 175.5, 161.0, 149.2, 85.1, 27.7, 13.8; IR
(film) νmax 2981, 2935, 1809, 1771, 1734, 1367, 1279, 1243, 1152 cm-1
;
In ver se Electr on Dem a n d Diels-Ald er Rea ction s
of 6-[(ter t-Bu tyloxyca r bon yl)a m in o]-3-(m eth ylth io)-
FABHRMS (NBA-NaI) m/z 366.1212 (M + Na+, C13H21N5O4S requires
366.1242). Preliminary efforts to enlist 6 in Diels-Alder reactions with
electron-rich dienophiles provided mixtures including cycloddition
products bearing N(BOC)2, NHBOC and NH2 substituents.
(18) Details of the X-ray structure determination may be found in
the Supporting Information and have been deposited in the Cambridge
Crystallographic Data Centre.
(15) For 5-hydroxy-3,6-bis(methylthio)-1,2,4-triazine: 1H NMR (DMF-
d7, 250 MHz) δ 14.11 (s, 1H), 2.55 (s, 3H), 2.34 (s, 3H); FABHRMS
(NBA-NaI) m/z 190.2612 (M + H+, C5H7N3OS2 requires 190.2607).
(16) Boger, D. L.; Panek, J . S.; Patel, M. Org. Synth. 1991, 70, 79.

Regioselective Inverse Electron Demand Diels-Alder Reactions
J . Org. Chem., Vol. 63, No. 18, 1998 6331
Ta ble 1. In ver se Electr on Dem a n d Diels-Ald er Rea ction s of
6-[(ter t-Bu tyloxyca r bon yl)a m in o]-3-(m eth ylth io)-1,2,4,5-tetr a zin e (2)
tion regioisomer, 8d , was observed. Significantly longer
resulted in competitive thermal removal of the BOC
group deactivating the tetrazine from further reac-
tion.19,20
Despite their well-behaved participation in inverse
electron demand Diels-Alder reactions with other elec-
tron-deficient 1,2,4,5-tetrazines, the activated acetylenes
1-(diethylamino)-1-propyne and ethoxy acetylene failed
to give the desired [4 + 2] cycloaddition products when
treated with tetrazine 2. The initial attempts with
reaction times (60 °C, 86 h) were also required to effect
the Diels-Alder addition of the trimethylsilyl enol ether
7g, and it also provided 8d as a single regioisomer.
Similarly, 1,1-dimethoxyethylene (7h ) underwent ef-
ficient cycloaddition with 2 requiring elevated tempera-
tures (80 °C, 16 h) to provide 8e as a single regioisomer.
Reactions with N-vinyl pyrrolidinone (7i, entry 9) and
ethyl vinyl ether (7j, entry 10) proceeded effectively but
required more extended reaction times (30-60 h) at
temperatures between 60 and 80 °C. Higher reaction
temperatures (e.g., 100 °C) sustained for prolonged times
(19) 3-Amino-6-(methylthio)-1,2,4,5-tetrazine did not react even with
enamine 7b at 100 °C.
(20) The half-life of 2 in dioxane at 120 °C was 1 day.

6332 J . Org. Chem., Vol. 63, No. 18, 1998
Boger et al.
Ta ble 2. In ver se Electr on Dem a n d Diels-Ald er
Sch em e 2
Rea ction s of
6-(Acetyla m in o)-3-(m eth ylth io)-1,2,4,5-tetr a zin e (3)
1-(diethylamino)-1-propyne led to a color change, but
upon workup provided starting tetrazine. Moreover,
modifications enlisting excess dienophile and variations
in the reaction temperatures failed to provide the desired
products. Recognizing the utility of this ynamine as a
peptide coupling reagent and its potential basicity, a
deprotonation and inactivation of the tetrazine was
suspected to be competitive with the desired [4 + 2]
cycloaddition. Consistent with this interpretation, N-
methylation of 2 to provide 10, followed by treatment
with the ynamine, cleanly gave the expected cycloadduct
11 in 55% (23 °C, 16 h, dioxane) as a single regioisomer
(Scheme 2). J ust as importantly, the successful reaction
of 10 also indicates that the [4 + 2] cycloaddition
reactions of 2-4 do not proceed through the tautomeric
N-acylimines with simple nucleophilic attack by the
electron-rich dienophile, and subsequent collapse of the
zwitterionic intermediates to the formal products of a [4
+ 2] cycloaddition.
Typically, 2 was found to be unreactive toward unac-
tivated acetylenic dienophiles (diphenylacetylene, 2-bu-
tyne) with the exception of phenylacetylene (eq 2) which
provided a 4:1 mixture of diazines 12a and 12b (30%
combined yield). The thermal lability of the tert-butyl-
oxycarbonyl precludes its effective use with such unac-
tivated dienophiles which require sustained reaction
times at the higher reaction temperatures (g100 °C).20
phenylacetylene to provide a 2:1 mixture of regioisomers
13d :14 in 87% yield (eq 3). Unlike 2, the thermal
stability of the N-acetyl group allows its use for sustained
reaction times at such elevated temperatures.
[4 + 2] Cycloa d d ition Rea ction s of 6-(Acetyl-
a m in o)-3-(m eth ylth io)-1,2,4,5-tetr a zin e (3). The Di-
els-Alder reactions of 3 with electron-rich dienophiles
also proceeded effectively providing the 1,2-diazine cy-
cloadducts in good to excellent yields (Table 2). The [4
+ 2] cycloaddition with enamine 7b occurred rapidly (23
°C, 0.1 h) and gave 13a in high yield (88%). Similarly,
the enamine 7d also reacted with 3 at room temperature
(2 h) to provide the single regioisomer of 1,2-diazine 13b
(74%). Higher reaction temperatures were required to
effect cycloaddition with 1-(trimethylsilyloxy)-1-cyclopen-
tene (7k , 110 °C, 48 h), 1,1-dimethoxyethylene (7h , 80
°C, 4 h), and trimethylsilyl enol ether 7g (100 °C, 24 h)
to provide 13a , 13c, and 13d , respectively. Finally, [4
+ 2] cycloaddition with N-vinyl pyrrolidinone (7i) pro-
vided 13e in good yield (100 °C, 24 h, 73%) even when
conducted at 160 °C (3 h, 82%) without evidence of
deacylation.
[4 + 2] Cycloa d d ition Rea ction s of 6-[(Ben zyloxy-
car bon yl)am in o]-3-(m eth ylth io)-1,2,4,5-tetr azin e (4).
Recognizing the thermal instability of 2 and the short-
comings of acetyl as a protecting group for further
functionalization of 3, we prepared a third unsymmetri-
cally substituted tetrazine 4 to combine the robust
thermal stability of 3 with the synthetic versatility of 2
(Scheme 1). The participation of 4 in [4 + 2] cycloaddi-
tions was examined with three representative dienophiles
7b, 7h , and 7i. Tetrazine 4 behaved in a manner
analogous to 3, and the reaction with N-vinyl pyrrolidi-
none (7i, 2.0 equiv, 72%) to provide 15c could be carried
out at 120 °C with no detectable decomposition or
deacylation of the tetrazine (Table 3). However, when
this same reaction was conducted at 160 °C (3 h),
6-amino-3-(methylthio)-1,2,4,5-tetrazine (29%) resulting
from deacylation of 4 was isolated along with 15c (60%).
As with 2, tetrazine 3 was found to be essentially
unreactive toward diphenylacetylene at temperatures up
to 130 °C but did undergo Diels-Alder addition with

Regioselective Inverse Electron Demand Diels-Alder Reactions
J . Org. Chem., Vol. 63, No. 18, 1998 6333
Ta ble 3. Diels-Ald er Rea ction s of
6-[(Ben zyloxyca r bon yl)a m in o]-3-(m eth ylth io)-
1,2,4,5-tetr a zin e (4)
Sch em e 4
Sch em e 3
with alternative dienophiles linked to unsymmetrical
6-amino-3-(methylthio)-1,2,4,5-tetrazines which have been
shown to proceed sluggishly21 or not at all, unless the
amine is N-acylated.21,22
Rea ctivity. A study of the relative reactivity of
tetrazines 1-3 toward N-vinyl pyrrolidinone was under-
taken by competitively subjecting 1 equiv of each com-
ponent to the usual reaction conditions and analyzing the
crude reaction mixture by 1H NMR (Scheme 4).
A
In tr a m olecu la r Diels-Ald er Rea ction s of N-Acyl
6-Am in o-3-(m eth ylth io)-1,2,4,5-tetr a zin es. Although
the intermolecular Diels-Alder reactions of 2-4 were
found to proceed sluggishly with unactivated dienophiles
including simple alkynes, the intramolecular variant was
found to proceed under mild conditions (23-80 °C, 6-16
h, Scheme 3). The 6-(acylamino)-3-(methylthio)-1,2,4,5-
tetrazine 16 possessing a terminal acetylene linked by a
three-carbon tether underwent near quantitative conver-
sion to the bicyclic 1,2-diazine 17 (92%) with formation
of a fused six-membered ring when warmed in dioxane
at 80 °C for 6 h. Likewise, the tethered alkyne 18
underwent a sequential acylation and [4 + 2] cycloaddi-
tion with formation of a fused five-membered ring at 23
°C (16 h) when exposed to BOC2O and catalytic DMAP
in THF affording 19 in superb yield (96%). In this latter
case, N-acylation of the C-6 amino group was sufficient
to activate the tetrazine for room temperature cycload-
dition. Thus, unactivated acetylenes are able to smoothly
undergo [4 + 2] cycloaddition when tethered to the
tetrazine,21-23 and parallel observations have been made
product ratio of 4.5:1 8f:20 was obtained when tetrazines
1 and 2 competed for the available dienophile, indicating
that tetrazine 2 is slightly more reactive than 1. Simi-
larly, the competitive Diels-Alder reaction of 3 and 2
indicated that the reactivity of tetrazine 3 is greater than
that of 2 by a factor of 3.7 (Scheme 4).
This proved consistent with AM1 computational stud-
ies of a full range of substituted 1,2,4,5-tetrazines where
the LUMO of both 2 and 3 are at a higher energy than
that of 3,6-dicarbomethoxy-1,2,4,5-tetrazine but lower
than those of 6-methoxy-3-(methylthio)- or 3,6-dimethoxy-
1,2,4,5-tetrazine accurately reflecting their relative re-
activities (Table 4).11 Similarly, the LUMO for 3 was
significantly lower than that of 2 also accurately reflect-
ing their relative reactivities. In addition, while the
LUMO of 3 was lower than that of 1 mirroring the
experimental observations on relative rate, those of 2 and
1 were not significantly different, and these results less
(22) Benson, S. C.; Lee, L.; Snyder, J . K. Tetrahedron Lett. 1996,
37, 5061.
(23) For examples of intramolecular Diels-Alder additions with
other heteroatom-tethered alkynes, see: Sietz, G.; Go¨rge, L.; Dietrich,
S. Tetrahedron Lett. 1985, 26, 4355.
(21) Seitz, G.; Dietrich, S.; Go¨rge, L.; Richter, J . Tetrahedron Lett.
1986, 27, 2747.

6334 J . Org. Chem., Vol. 63, No. 18, 1998
Boger et al.
Ta ble 4. AM1 Com p u ta tion a l Resu lts (LUMO) of 1,2,4,5-Tetr a zin es
46.0 mL of anhydrous DMF at 0 °C under Ar was treated with
NaH (80% dispersion, 124 mg, 4.1 mmol) in several portions,
and the mixture was stirred for 30 min. The suspension was
cooled to -40 °C, treated with 13 (400 mg, 2.3 mmol) in 7.6
mL of anhydrous DMF dropwise, and stirred for 18 h at -40
°C. The reaction mixture was diluted with Et2O (150 mL),
washed sequentially with 1 N aqueous HCl (150 mL) and
saturated aqueous NaCl (150 mL), dried (Na2SO4), and con-
centrated under reduced pressure. Flash chromatography
(SiO2, 3 × 15 cm, 30% EtOAc-hexane) provided recovered 1
(90.0 mg, 23%) and 2 (384 mg, 69%) as a red oil which slowly
crystallized: mp 65-66 °C (red needles, EtOAc); 1H NMR
F igu r e 2.
closely parallel the experimental observations where 2
was found to be slightly more reactive (ca. 4.5×).
Regioselectivity. The regioselectivity of the cycload-
ditions are consistent with the expectation that the
methylthio group controls the orientation by stabilizing
a partial negative charge at C-3 (Figure 2). The dieno-
phile addition follows an approach predicted by this
stabilization and by the added ability of the acylamino
group to stabilize a partial positive charge on C-6 of the
electron-deficient tetrazine. These intuitive predictions
of the observed regioselectivity are supported by the AM1
computational studies (Table 4). The C-3 position of both
2 and 3 bears a significant partial negative charge
(-0.308 to -0.301) while C-6 is slightly electropositive
(+0.025 to +0.022). Moreover, C-6 bears the largest
LUMO orbital coefficient indicating it dominates the
regioselectivity by preferentially combining with the
dienophile C-2 center which possesses the largest HOMO
orbital coefficient.
(CDCl3, 250 MHz) δ 7.76 (s, 1H), 2.69 (s, 3H), 1.54 (s, 9H); 13
C
NMR (CDCl3, 100 MHz) δ 172.8, 158.6, 149.5, 83.2, 28.0, 13.5;
IR (film) νmax 3225, 2979, 1732, 1537, 1368 cm-1; FABHRMS
(NBA-NaI) m/z 244.0860 (M + H+, C8H13N5O2S requires
244.0868). Anal. Calcd for C8H13N5O2S: C, 39.49; H, 5.39;
N, 28.79; S, 13.18. Found: C, 39.83; H, 5.45; N, 28.79; S, 12.98.
6-(Acetylam in o)-3-(m eth ylth io)-1,2,4,5-tetr azin e (3). 3,6-
Bis(methylthio)-1,2,4,5-tetrazine (1,3 400 mg, 2.28 mmol) in
22.0 mL anhydrous CH3OH at 0 °C under Ar was treated with
22.0 mL of saturated NH3-CH3OH and stirred for 30 min14
before the solvents were removed under reduced pressure. The
orange solid was dissolved in 12.0 mL of anhydrous THF and
treated with Ac2O (320 µL, 3.4 mmol) and DMAP (28.0 mg,
0.2 mmol), and the mixture was warmed at reflux under Ar
for 15 h. The reaction mixture was concentrated under
reduced pressure. Flash chromatography (SiO2, 3 × 15 cm,
0-60% EtOAc-hexane gradient) provided recovered 1 (66.0
mg, 20%), the diacetate 5 (18 mg, 5%) as a bright red oil, and
3 (300 mg, 71%) as a bright red solid: mp 131-133 °C (red
1
needles, EtOAc); H NMR (CDCl3, 250 MHz) δ 8.93 (s, 1H),
2.70 (s, 3H), 2.50 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ 172.9,
169.4, 158.3, 25.0, 13.6; IR (film) νmax 3188, 3054, 1694, 1556,
1347 cm-1; FABHRMS (NBA-NaI) m/z 186.0453 (M + H+,
C5H7N5OS requires 186.0450). Anal. Calcd for C5H7N5OS: C,
32.42; H, 3.81; N, 37.81; S, 17.31. Found: C, 32.78; H, 4.11;
N, 37.73; S, 17.55.
Con clu sion s
1,2,4,5-Tetrazines 2-4 undergo well-behaved and
regioselective inverse electron demand Diels-Alder reac-
tions with a variety of electron-rich dienophiles to provide
the corresponding 1,2-diazines in good to excellent yields.
Thus, simple N-acylation of the unreactive 6-amino-3-
(methylthio)-1,2,4,5-tetrazine provides sufficient activa-
tion to permit the effective usage of these unsymmetri-
cally substituted 1,2,4,5-tetrazines. A limitation of
tetrazine 2 is the thermal instability of the N-BOC group
at temperatures g100 °C for extended periods of time.20
Tetrazines 3 and 4 (3 > 4 > 2) were found to be much
more stable to such vigorous conditions required for
reaction with less reactive dienophiles and, typically, no
deacylation products were observed. The comparative
reactivity of the tetrazines illustrated in Scheme 4
highlights an unusual reactivity pattern where 2-4 (3
> 2, 4) are more reactive than 1 and possess a reactivity
amenable to widespread utilization.
For 6-bis(acetylamino)-3-(methylthio)-1,2,4,5-tetrazine (5):
1H NMR (CDCl3, 400 MHz) δ 2.78 (s, 3H), 2.36 (s, 6H); 13C
NMR (CDCl3, 100 MHz) δ 177.1, 171.3, 161.8, 26.2, 13.7; IR
(film) νmax 1732, 1418, 1350 cm-1; FABHRMS (NBA-NaI) m/z
250.0367 (M + Na+, C7H9N5O2S requires 250.0375).
6-[(Ben zyloxyca r bon yl)a m in o]-3-(m eth ylth io)-1,2,4,5-
tetr a zin e (4). Benzyl carbamate (208 mg, 1.38 mmol) in 23.0
mL of anhydrous DMF at 0 °C under Ar was treated with NaH
(80% dispersion, 62.0 mg, 2.06 mmol) in several portions and
stirred for 30 min. The suspension was cooled to -40 °C,
treated dropwise with 13 (200 mg, 1.15 mmol) in 3.8 mL of
anhydrous DMF, and stirred for 29 h at -40 °C. The reaction
mixture was diluted with Et2O (80 mL), washed sequentially
with 1 N aqueous HCl (80 mL) and saturated aqueous NaCl
(80 mL), dried (Na2SO4), and concentrated under reduced
pressure. Flash chromatography (SiO2, 3 × 15 cm, 30%
EtOAc-hexane) provided recovered 1 (28.0 mg, 14%) and 4
(195.0 mg, 61%) as a red solid: mp 90-92 °C (red needles,
EtOAc); 1H NMR (CDCl3, 250 MHz) δ 8.30 (s, 1H), 7.29-7.39
(m, 5H), 5.27 (s, 2H), 2.67 (s, 3H); 13C NMR (CDCl3, 100 MHz)
Exp er im en ta l Section
6-[(ter t-Bu tyloxycar bon yl)am in o]-3-(m eth ylth io)-1,2,4,5-
tetr a zin e (2). tert-Butyl carbamate (323 mg, 2.75 mmol) in

Regioselective Inverse Electron Demand Diels-Alder Reactions
J . Org. Chem., Vol. 63, No. 18, 1998 6335
δ 172.7, 158.3, 150.7, 134.9, 128.7, 128.6, 128.5, 68.3, 13.5; IR
(film) νmax 3234, 3063, 1755, 1557 cm-1; FABHRMS (NBA-NaI)
m/z 278.0718 (M + H+, C11H11N5O2S requires 278.0712). Anal.
Calcd for C11H11N5O2S: C, 47.64; H, 4.00; N, 25.26; S, 11.56.
Found: C, 48.04; H, 3.81; N, 24.88; S, 11.29.
Similarly, reaction of 2 (18 mg, 0.07 mmol) with 1-pyrroli-
dino-1-phenylethene (7f, 51.0 mg, 0.30 mmol) provided 8d (13.0
mg, 55%).
Similarly, reaction of 2 (54 mg, 0.22 mmol) with 1-(tri-
methylsilyloxy)-1-phenylethene (7g, 136 µL, 0.67 mmol) pro-
vided 8d (37.0 mg, 53%).
Gen er a l P r oced u r e for th e Diels-Ald er Rea ction of
6-[(ter t-Bu tyloxyca r bon yl)a m in o]-3-(m eth ylth io)-1,2,4,5-
tetr a zin e (2), 6-(Acetyla m in o)-3-(m eth ylth io)-1,2,4,5-tet-
r a zin e (3), a n d 6-[(Ben zyloxyca r bon yl)a m in o]-3-(m eth -
ylt h io)-1,2,4,5-t et r a zin e (4) w it h R ep r esen t a t ive Dien -
op h iles 7a -7j. According to the conditions listed in Tables
1-3, 2, 3, or 4 was dissolved in the appropriate solvent (0.5
M) under N2. Dienophile was added, and the reaction was run
at the temperature and for the duration indicated. The solvent
was removed under reduced pressure, and for the indicated
enamines only, the residue was redissolved in 10% HOAc-
benzene and stirred for 14-16 h as indicated to induce
aromatization before reconcentration. Flash chromatography
provided the corresponding 1,2-diazine(s).
6-[(ter t-Bu tyloxyca r bon yl)a m in o]-4-m eth oxy-3-(m eth -
ylth io)-1,2-d ia zin e (8e). Reaction of 2 (31.0 mg, 0.13 mmol)
with 1,1-dimethoxyethylene (7h , 67 µL, 0.77 mmol) followed
by flash chromatography (SiO2, 1.5 × 10 cm, 20% EtOAc-
hexane) provided 8e (26 mg, 75%) as a white solid: mp 161-
162 °C; 1H NMR (CDCl3, 250 MHz) δ 7.58 (s, 1H), 7.50 (s, 1H),
3.95 (s, 3H), 2.60 (s, 3H), 1.50 (s, 9H); 13C NMR (CDCl3, 62.5
MHz) δ 158.3, 156.5, 153.9, 152.4, 95.7, 81.8, 55.8, 28.2, 12.3;
IR (film) νmax 3190, 2973, 1723, 1592, 1513, 1462, 1372 cm-1
;
FABHRMS (NBA-NaI) m/z 272.1077 (M + H+, C11H17N3OS
requires 272.1069). Anal. Calcd for C11H17N3OS: C, 48.79;
H, 6.59; N, 15.17; S, 12.11. Found: C, 48.69; H, 6.32; N, 15.49;
S, 11.82.
6-[(ter t-Bu t yloxyca r b on yl)a m in o]-3-(m et h ylt h io)-1,2-
d ia zin e (8f). Reaction of 2 (47.0 mg, 0.19 mmol) with N-vinyl
pyrrolidinone (7i, 65 µL, 0.58 mmol) followed by flash chro-
matography (SiO2, 1.5 × 10 cm, 20% EtOAc-hexane) provided
8f (27.0 mg, 58%) as a pink solid: mp 112-114 °C (pink
needles, EtOAc); 1H NMR (CDCl3, 400 MHz) δ 8.03 (d, J )
7.6 Hz, 1H), 7.67 (s, 1H), 7.27 (d, J ) 7.6 Hz, 1H), 2.64 (s,
3H), 1.50 (s, 9H); 13C NMR (CDCl3, 100 MHz) δ 157.6, 152.7,
152.2, 127.5, 117.2, 81.9, 28.0, 13.4; IR (CCl4) νmax 3421, 2981,
2931, 1736, 1492 cm-1; FABHRMS (NBA-NaI) m/z 242.0958
(M + H+, C10H15N3O2S requires 242.0963). Anal. Calcd for
1-[(ter t-Bu t yloxyca r b on yl)a m in o]-4-(m et h ylt h io)-6,7-
d ih yd r o-5H-cyclop en ta [d ]p yr id a zin e (8a ). Reaction of 2
(27.0 mg, 0.11 mmol) with 1-pyrrolidino-1-cyclopentene (7a ,
24 µL, 0.16 mmol) followed by flash chromatography (SiO2,
1.5 × 10 cm, 20% EtOAc-hexane) provided 8a (30.0 mg, 96%)
as a light tan solid: mp 97-99 °C (tan prisms, EtOAc); 1H
NMR (CDCl3, 250 MHz) δ 3.03 (t, J ) 7.6 Hz, 2H), 2.81 (t, J
) 7.6 Hz, 2H), 2.69 (s, 3H), 2.08-2.17 (m, 2H), 1.49 (s, 9H);
13C NMR (CDCl3, 62.5 MHz) δ 156.7, 152.8, 150.4, 145.4, 138.1,
81.2, 32.2, 31.0, 28.1, 23.3, 12.7; IR (film) νmax 3181, 2975, 1566,
1493, 1366 cm-1; FABHRMS (NBA-NaI) m/z 282.1272 (M +
C
10H15N3O2S: C, 49.77; H, 6.27; N, 17.41; S, 13.29. Found:
H+, C13H19N3O2S requires 282.1276). Anal. Calcd for C13H19
-
C, 49.54; H, 6.50; N, 17.24; S, 13.11.
N3O2S: C, 55.49; H, 6.81; N, 14.93; S, 11.40. Found: C, 55.43;
H, 6.89; N, 14.98; S, 11.30.
Similarly, reaction of 2 (40.0 mg, 0.16 mmol) with ethyl vinyl
ether (7j, 47 µL, 0.49 mmol) provided 8f (20.0 mg, 50%) and
6-amino-3-(methylthio)-1,2-diazine (2.8 mg, 9%).
Similarly, reaction of 2 (54.0 mg, 0.22 mmol) with 1-mor-
pholino-1-cyclopentene (7b, 53 µL, 0.33 mmol) provided 8a
(58.1 mg, 93%).
6-[(ter t-Bu tyloxycar bon yl)m eth ylam in o]-3-(m eth ylth io)-
1,2,4,5-tetr a zin e (10). A solution of 2 (12 mg, 0.05 mmol) in
anhydrous DMF (200 µL) at -40 °C under N2 was treated with
NaH (3.0 mg of 60% in oil, 0.07 mmol) and stirred for 30 min.
The resulting suspension was treated with CH3I (9 µL, 0.15
mmol) and stirred for an additional 30 min before the reaction
was quenched with the addition of 5% aqueous HCl. The
reaction mixture was extracted with Et2O (3 × 5 mL), and the
combined organic layers were washed with H2O (5 mL) and
saturated aqueous NaCl (5 mL). The solvent was removed
under reduced pressure, and flash chromatography (SiO2, 1.5
× 5 cm, 10% EtOAc-hexane) provided 10 (8.0 mg, 61%) as a
red oil: 1H NMR (CDCl3, 400 MHz) δ 3.53 (s, 3H) 2.72 (s, 3H),
1-[(ter t-Bu tyloxycar bon yl)am in o]-4-(m eth ylth io)-5,6,7,8-
tetr a h yd r op h th a la zin e (8b). Reaction of 2 (25.0 mg, 0.10
mmol) and 1-pyrrolidino-1-cyclohexene (7c, 25 µL, 0.15 mmol)
followed by flash chromatography (SiO2, 1.5 × 10 cm, 20%
EtOAc-hexane) provided 8b (24.0 mg, 79%) as a tan solid:
1
mp 101-103 °C; H NMR (CDCl3, 250 MHz) δ 2.63-2.72 (m,
2H), 2.63 (s, 3H), 2.49-2.60 (m, 2H), 1.80-1.90 (m, 2H), 1.69-
1.71 (m, 2H), 1.48 (s, 9H); 13C NMR (CDCl3, 62.5 MHz) δ 153.4,
136.8, 132.7, 81.1, 28.2, 24.8, 24.2, 21.3, 13.1; IR (film) νmax
3201, 2934, 1726, 1484, 1366 cm-1; FABHRMS (NBA-NaI) m/z
296.1442 (M + H+, C14H21N3O2S requires 296.1442).
1.51 (s, 9H); IR (CHCl3) νmax 3690, 1719, 1601, 1382 cm-1
;
6-[(ter t-Bu tyloxyca r bon yl)a m in o]-4-eth yl-5-m eth yl-3-
(m eth ylth io)-1,2-d ia zin e (8c). Reaction of 2 (15.0 mg, 0.06
mmol) with 3-morpholino-2-pentene (7d , 29.0 mg 0.18 mmol)
followed by flash chromatography (SiO2, 1.5 × 10 cm, 20%
EtOAc-hexane) provided 8c (15.0 mg, 86%) as a pale yellow
oil: 1H NMR (CDCl3, 400 MHz) δ 7.05 (s, 1H), 2.68 (dd, J )
7.6 Hz, 2H), 2.65 (s, 3H), 2.02 (s, 3H), 1.48 (s, 9H), 1.16 (t, J
) 7.6 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ 153.5, 151.9,
141.2, 131.1, 81.3, 28.2, 28.1, 22.2, 13.6, 13.4, 11.5; IR (film)
FABHRMS (NBA-NaI) m/z 280.0839 (M + Na+, C9H15N5O2S
requires 280.0844).
6-[(ter t-Bu tyloxycar bon yl)m eth ylam in o]-4-(dieth ylam i-
n o)-5-m eth yl-3-(m eth ylth io)-1,2-d ia zin e (11). Reaction of
10 (6.0 mg, 24 µmol) with 1-(diethylamino)-1-propyne (5.0 mg,
47 µmol) followed by flash chromatography (SiO2, 1.5 × 10 cm,
10% EtOAc-hexane) provided 11 (4.5 mg, 55%) as a light
yellow oil: 1H NMR (CDCl3, 250 MHz) δ 3.20 (s, 3H), 3.12 (q,
J ) 8.4 Hz, 4H) 2.68 (s, 3H) 2.18 (s, 3H), 1.40 (s, 9H), 1.05 (t,
J ) 8.6 Hz, 6H); 13C NMR (CDCl3, 100 MHz) δ 162.3, 155.1,
154.2, 144.9, 132.4, 80.8, 45.4, 36.2, 29.7, 28.3, 14.9, 14.1, 13.8;
ν
max 3193, 2975, 2931, 1728, 1488, 1392, 1367 cm-1; FABHRMS
(NBA-NaI) m/z 306.1246 (M + Na+, C13H21N3O2S requires
306.1252).
IR (film) νmax 2973, 2928, 1709, 1530, 1429, 1367 cm-1
;
FABHRMS (NBA-NaI) m/z 363.1844 (M + Na+, C16H28N4O2S
requires 363.1831).
6-[(ter t-Bu t yloxyca r b on yl)a m in o]-3-(m e t h ylt h io)-4-
p h en yl-1,2-d ia zin e (8d ). Reaction of 2 (22.0 mg, 0.09 mmol)
with 1-morpholino-1-phenylethene (7e, 26.0 mg, 0.14 mmol)
followed by flash chromatography (SiO2, 1.5 × 10 cm, 20%
EtOAc-hexane) provided 8d (17 mg, 60%) as a pale yellow
6-Am in o-3-(m eth ylth io)-4-p h en yl-1,2-d ia zin e (12a a n d
12b). Reaction of 2 (56.0 mg, 0.23 mmol) with phenylacetylene
(76 µL, 0.69 mmol) followed by flash chromatography (SiO2,
2.5 × 10 cm, 30-100% EtOAc-hexane gradient) provided 12a
and 12b (15.0 mg, 30%) as an inseparable mixture (4:1 by
NMR) and 6-amino-3-(methylthio)-1,2,4,5-tetrazine (18 mg,
54%). For 12a and 12b: 1H NMR (CDCl3, 400 MHz) δ 7.40-
7.50 (m, 5H), 7.00 (s, 1H), 6.60 (s, 1H), 2.59 (s, 3H), 2.49 (s,
3H); FABHRMS (NBA-NaI) m/z 218.0752 (M + H+, C11H11N3S
requires 218.0759).
1
solid: mp 139-141 °C (pale yellow prisms, EtOAc); H NMR
(CDCl3, 400 MHz) δ 8.00 (s, 1H), 7.57 (s, 1H), 7.45 (m, 5H),
2.61 (s, 3H), 1.51 (s, 9H); 13C NMR (CDCl3, 100 MHz) δ 156.6,
152.9, 152.2, 141.3, 135.4, 129.4, 128.7, 116.6, 81.9, 28.1, 14.0;
IR (film) νmax 3189, 3083, 2973, 2927, 1738, 1716, 1574, 1538,
1484 cm-1; FABHRMS (NBA-NaI) m/z 318.1286 (M + H+,
C16H19N3O2S requires 318.1276). Anal. Calcd for C16H19N3-
O2S: C, 60.54; H, 6.03; N, 13.24; S, 10.10. Found: C, 60.54;
H, 6.10; N, 13.20; S, 10.21.
1-(Acetyla m in o)-4-(m eth ylth io)-6,7-d ih yd r o-5H-cyclo-
p en t a [d ]p yr id a zin e (13a ). Reaction of 3 (15.0 mg, 0.08

6336 J . Org. Chem., Vol. 63, No. 18, 1998
Boger et al.
mmol) with 1-(trimethylsilyloxy)-1-cyclopentene (7k , 50.0 mg,
0.32 mmol) followed by flash chromatography (SiO2, 1.5 × 10
cm, 40% EtOAc-hexane) provided 13a (11.0 mg, 61%) as a
white solid: mp 181-182 °C (white needles, CHCl3); 1H NMR
(CDCl3, 300 MHz) δ 3.01 (t, J ) 7.5 Hz, 2H), 2.84 (t, J ) 7.5
2928, 1711, 1685, 1506, 1487, 1410 cm-1; FABHRMS (NBA-
NaI) m/z 184.0551 (M + H+, C7H9N3OS requires 184.0545).
1-[(Ben zyloxyca r b on yl)a m in o]-4-(m et h ylt h io)-6,7-d i-
h yd r o-5H-cyclop en ta [d ]p yr id a zin e (15a ). Reaction of 4
(9.1 mg, 33.0 µmol) with 1-morpholino-1-cyclopentene (7b, 7.8
µL, 49.0 µmol) followed by flash chromatography (SiO2, 1.5 ×
10 cm, 10-25% EtOAc-hexane gradient) provided 15a (10.1
mg, 98%) as a white film: 1H NMR (CDCl3, 250 MHz) δ 7.31-
7.39 (m, 5H), 5.20 (s, 2H), 3.06 (t, J ) 7.5 Hz, 2H), 2.83 (t, J
) 7.5 Hz, 2H), 2.66 (s, 3H), 2.12 (p, J ) 7.5 Hz, 2H); 13C NMR
(CDCl3, 100 MHz) δ 156.8, 154.0, 150.0, 146.0, 138.0, 135.5,
128.6, 128.4, 128.3, 67.7, 32.2, 31.1, 23.2, 12.8; IR (film) νmax
3170, 2928, 1732, 1504 cm-1; FABHRMS (NBA-NaI) m/z
338.0949 (M + Na+, C16H17N3O2S requires 338.0939).
6-[(Ben zyloxyca r b on yl)a m in o]-4-m et h oxy-3-(m et h yl-
th io)-1,2-d ia zin e (15b). Reaction of 4 (14.1 mg, 53.2 µmol)
with 1,1-dimethoxyethylene (7h , 30 µL, 320 µmol) followed by
flash chromatography (SiO2, 1.5 × 10 cm, 30% EtOAc-hexane)
provided 15b (10.8 mg, 64%) as a white solid: mp 134-136
°C (EtOAc); 1H NMR (CDCl3, 400 MHz) δ 7.77 (s, 1H), 7.61 (s,
1H), 7.36 (m, 5H), 5.23 (s, 2H), 3.96 (s, 3H), 2.61 (s, 3H); 13C
NMR (CDCl3, 100 MHz) δ 156.5, 153.5, 153.1, 150.9, 135.4,
128.7, 128.2, 95.7, 67.5, 55.8, 12.3; IR (film) νmax 3410, 2976,
2930, 1735, 1587, 1554, 1500, 1462, 1401, 1380 cm-1; FAB-
HRMS (NBA-NaI) m/z 306.0923 (M + H+, C14H15N3O3S
requires 306.0912).
Hz, 2H), 2.67 (s, 3H), 2.35 (s, 3H), 2.11 (t, J ) 7.5 Hz, 2H); 13
C
NMR (CDCl3, 62.5 MHz) δ 169.8, 156.9, 150.8, 145.3, 138.7,
32.6, 31.1, 23.8, 23.2, 12.7; IR (CHCl3) νmax 3182, 2932, 1696,
1560, 1507, 1481 cm-1; FABHRMS (NBA-NaI) m/z 246.0685
(M + Na+, C10H13N3OS requires 246.0677).
Similarly, reaction of 3 (14.0 mg, 0.08 mmol) with 1-mor-
pholino-1-cyclopentene (7b, 18 µL, 0.11 mmol) provided 13a
(14.8 mg, 88%).
6-(Acet yla m in o)-4-et h yl-5-m et h yl-3-(m et h ylt h io)-1,2-
d ia zin e (13b). Reaction of 3 (10.0 mg, 0.05 mmol) with
3-morpholino-2-pentene (7d , 25.0 mg, 0.16 mmol) followed by
flash chromatography (SiO2, 1.5 × 10 cm, 20% EtOAc-hexane)
provided 13b (9.0 mg, 74%) as a pale yellow solid: mp 125-
1
127 °C (white prisms, CHCl3); H NMR (CDCl3, 400 MHz) δ
2.72 (q, J ) 7.6 Hz, 2H), 2.47 (s, 3H), 2.29 (s, 3H), 2.22 (s,
3H), 1.18 (t, J ) 7.6 Hz, 3H); 13C NMR (CDCl3, 125 MHz) δ
170.8, 160.0, 152.5, 141.6, 131.7, 29.7, 23.5, 22.3, 13.7, 13.6,
11.4; IR (film) νmax 3221, 2972, 2931, 1698, 1548, 1494, 1433,
1392 cm-1; FABHRMS (NBA-NaI) m/z 248.0830 (M + Na+,
C
10H15N3OS requires 248.0834).
Anal. Calcd for C10H15N3OS: C, 53.31; H, 6.71; N, 18.65;
S, 14.23. Found: C, 53.62; H, 7.11; N, 18.28; S, 13.90.
A single-crystal X-ray structure determination conducted
on crystals grown from CHCl3 unambiguously established the
structure of 13b and the regioselectivity of the cycloaddition
reaction.18
6-[(Ben zyloxyca r b on yl)a m in o]-3-(m et h ylt h io)-1,2-d i-
a zin e (15c). Reaction of 4 (15.8 mg, 57.0 µmol) with N-vinyl
pyrrolidinone (7i, 19 µL, 171 µmol) followed by flash chroma-
tography (SiO2, 1.5 × 10 cm, 10-20% EtOAc-hexane gradient)
provided 15c (11.3 mg, 72%) as a white solid: mp 134-136
°C (peach-colored needles, EtOAc); 1H NMR (CDCl3, 400 MHz)
δ 8.23 (br s, 1H), 8.11 (d, J ) 9.4 Hz, 1H), 7.29-7.38 (m, 5H),
7.31 (d, J ) 9.4 Hz, 1H), 5.22 (s, 2H), 2.63 (s, 3H); 13C NMR
(CDCl3, 100 MHz) δ 158.3, 153.0, 152.3, 135.3, 128.7, 128.6,
128.3, 128.1, 117.5, 67.7, 13.4; IR (film) νmax 3171, 2963, 1712,
1531 cm-1; FABHRMS (NBA-NaI) m/z 276.0813 (M + H+,
6-(Acet yla m in o)-4-m et h oxy-3-(m et h ylt h io)-1,2-d ia z-
in e (13c). Reaction of 3 (22.0 mg, 0.12 mmol) with 1,1-
dimethoxyethylene (7h , 50.0 mg, 0.60 mmol) followed by
simple filtration with an EtOAc wash provided 13c (20.0 mg,
78%) as a white solid: mp 253 °C (decomp); 1H NMR (CDCl3,
250 MHz) δ 8.01 (s, 1H), 3.98 (s, 3H), 2.57 (s, 3H), 2.44 (s,
3H); 13C NMR (CDCl3, 100 MHz) δ 171.0, 156.6, 154.9, 150.7,
97.5, 56.0, 25.0, 16.4; IR (CHCl3) νmax 3209, 3029, 2943, 1694,
1587, 1489, 1400, 1373 cm-1; FABHRMS (NBA-NaI) m/z
306.1246 (M + Na+, C13H13N3OS requires 306.1252).
6-(Acet yla m in o)-3-(m et h ylt h io)-4-p h en yl-1,2-d ia zin e
(13d ) a n d 3-(Acetyla m in o)-6-(m eth ylth io)-4-p h en yl-1,2-
d ia zin e (14). Reaction of 3 (32.0 mg, 0.17 mmol) with
phenylacetylene (57 µL, 0.52 mmol) followed by flash chroma-
tography (SiO2, 1.5 × 15 cm, 0-2% CH3OH-CHCl3 gradient)
afforded 13d (26.0 mg, 58%) as a white solid and 14 (13.0 mg,
29%) as a yellow oil. For 13d : mp 188-190 °C (white needles,
C
13H13N3O2S requires 276.0807).
3-Met h ylt h io-6-(4-p en t yn oyla m in o)-1,2,4,5-t et r a zin e
(16). 4-Pentynoyl amide24 (55.0 mg, 0.57 mmol) in 4.0 mL of
anhydrous DMF at -10 °C under N2 was treated with NaH
(18.0 mg of 60% in oil, 0.614 mmol), and the resulting
suspension was warmed to 0 °C over 1 h. The suspension was
recooled to -10 °C, 1 (82.0 mg, 0.472 mmol) was added, and
the reaction mixture was stirred 18 h at 25 °C. Flash
chromatography (SiO2, 2.5 × 15 cm, 40% EtOAc-hexane)
provided 16 (18.0 mg, 14% unoptimized) as a red solid, and
recovered 1 (35.0 mg, 43%). For 16: mp 133-135 °C (EtOAc-
1
1
hexane); H NMR (CDCl3, 250 MHz) δ 8.57 (s, 1H), 3.00 (t, J
EtOAc); H NMR (CDCl3, 300 MHz) δ 10.36 (s, 1H), 8.39 (s,
1H), 7.47 (s, 5H), 2.58 (s, 3H), 2.47 (s, 3H); 13C NMR (CDCl3,
100 MHz) δ 170.7, 157.1, 154.0, 141.9, 135.1, 130.0, 128.5,
118.6, 29.7, 24.9, 14.0; IR (CHCl3) νmax 3204, 3089, 2930, 1696,
1567, 1544, 1480, 1397 cm-1; FABHRMS (NBA-NaI) m/z
260.0855 (M + H+ C13H13N3OS requires 260.0858). Anal.
Calcd for C13H13N3OS: C, 60.21; H, 5.05; N, 16.20; S, 12.37.
Found: C, 60.50; H, 4.94; N, 15.90; S, 12.01.
) 7.1 Hz, 2H), 2.71 (s, 3H), 2.65 (dt, J ) 2.4, 7.1 Hz, 2H), 2.03
(t, J ) 2.4 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ 173.1, 158.1,
82.1, 69.8, 36.4, 29.7, 14.1, 13.6; IR (film) νmax 3194, 2923, 1687,
1455, 1345 cm-1; FABHRMS (NBA-NaI) m/z 246.0434 (M +
Na+, C8H9N5OS requires 246.0426).
3-(Meth ylth io)-5,8-d ih yd r o-6H-p yr id o[2,3-c]p yr id a zin -
7-on e (17). A solution of 16 (11.0 mg, 0.05 mmol) in 3.0 mL
of dioxane under N2 was warmed at 80 °C for 18 h. Flash
chromatography (SiO2, 1.5 × 10 cm, 30% EtOAc-hexane)
provided 17 (9.0 mg, 92%) as a light tan solid: mp 225 °C
For 14: 1H NMR (CDCl3, 400 MHz) δ 7.40-7.50 (m, 5H),
7.28 (s, 1H), 2.69 (s, 3H), 2.30 (s, 3H); 13C NMR (CDCl3, 62.5
MHz) δ 170.4, 160.3, 150.0, 134.4, 131.4, 130.3, 129.5, 129.2,
127.9, 127.6, 127.4, 24.0, 13.4; IR (film) νmax 3174, 3000, 1683,
1538, 1489, 1393 cm-1; FABHRMS (NBA-NaI) m/z 260.0860
(M + H+, C13H13N3OS requires 260.0858).
Similarly, reaction of 3 (17.0 mg, 0.09 mmol) with 1-(tri-
methylsilyloxy)-1-phenylethene (7g, 56 µL, 0.27 mmol) fol-
lowed by flash chromatography (SiO2, 1.5 × 10 cm, 50%
EtOAc-hexane) provided only 13d (20.0 mg, 84%).
6-(Acetyla m in o)-3-(m eth ylth io)-1,2-d ia zin e (13e). Re-
action of 3 (31.0 mg, 0.17 mmol) with N-vinyl pyrrolidinone
(7i, 39 µL, 0.34 mmol) followed by flash chromatography (SiO2,
1.5 × 10 cm, CHCl3) provided 13e (22.3 mg, 73%) as a yellow
solid: mp 208-210 °C (EtOAc); 1H NMR (CDCl3, 400 MHz) δ
10.90 (s, 1H), 8.43 (d, J ) 9.4 Hz, 1H), 7.38 (d, J ) 9.4 Hz,
1H), 2.62 (s, 3H), 2.41 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ
170.8, 158.5, 153.2, 128.6, 119.6, 24.8, 13.4; IR (CCl4) νmax 3035,
1
(decomp); H NMR (CDCl3, 400 MHz) δ 8.27 (s, 1H), 7.14 (s,
1H), 2.95 (t, J ) 7.4 Hz, 2H), 2.70-2.67 (m, 2H), 2.67 (s, 3H);
13C NMR (CDCl3, 125 MHz) δ 169.6, 158.4, 151.6, 125.1, 124.8,
29.5, 23.6, 13.5; IR (film) νmax 3111, 2924, 1726, 1486, 1435,
1364, 1321 cm-1; FABHRMS (NBA-NaI) m/z 196.0551 (M +
H+, C8H9N3OS requires 196.0545). Anal. Calcd for C8H9N3-
OS: C, 49.21; H, 4.65; N, 21.52; S, 16.42. Found: C, 49.04;
H, 4.24; N, 21.32; S, 16.81.
6-(Bu t -3-yn -1-yla m in o)-3-(m e t h ylt h io)-1,2,4,5-t e t r a -
zin e (18). 3,6-Bis(methylthio)-1,2,4,5-tetrazine (1, 200 mg,
1.15 mmol) was added to a solution of 4-amino-1-butyne (95.0
mg, 1.4 mmol) in 5.0 mL of CH3OH and stirred at 23 °C for 24
(24) Prepared by adding NH3(l) to the mixed anhydride of 4-pen-
tynoic acid (ClCO2iBu) in THF at -78 °C.

Regioselective Inverse Electron Demand Diels-Alder Reactions
J . Org. Chem., Vol. 63, No. 18, 1998 6337
h under N2. The solvents were removed under reduced
pressure, and flash chromatography (SiO2, 3.0 × 15 cm, 20%
EtOAc-hexane) yielded 18 (116 mg, 52%) as a red solid: mp
75-76 °C (red needles, EtOAc); 1H NMR (CDCl3, 400 MHz) δ
6.16 (s, 1H), 3.73 (dt, J ) 6.4, 6.4 Hz, 2H), 2.64 (s, 3H), 2.57
(dt, J ) 2.6, 6.4 Hz, 2H), 2.03 (t, J ) 2.6 Hz, 1H); 13C NMR
(CDCl3, 100 MHz) δ 167.5, 160.8, 80.9, 70.7, 39.9, 18.9, 13.6;
IR (film) νmax 3298, 2928, 2855, 1699, 1574, 1557, 1435, 1347
cm-1; FABHRMS (NBA-NaI) m/z 196.0659 (M + H+, C7H9N5S
requires 196.0657). Anal. Calcd for C7H9N5S: C, 43.06; H,
4.65; N, 35.87; S, 16.42. Found: C, 43.37; H, 4.60; N, 35.73;
S, 16.59.
82.2, 46.0, 28.2, 24.4, 13.5; IR (film) νmax 2977, 2927, 1728,
1703, 1622, 1538, 1484, 1418 cm-1; FABHRMS (NBA-NaI) m/z
290.0948 (M + Na+, C12H17N3O2S requires 290.0939). Anal.
Calcd for C12H17N3O2S: C, 53.91; H, 6.41; N, 15.72; S, 11.99.
Found: C, 54.23; H, 6.29; N, 15.95, S, 11.75.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the National Institutes of Health
(CA42056) and the award of an ACS Division of Organic
Chemistry Graduate Fellowship sponsored by Zeneca
Pharmaceuticals (R.M.G.).
7-(ter t-Bu tyloxyca r bon yl)-3-(m eth ylth io)-5,6-d ih yd r o-
p yr r olo[2,3-c]p yr id a zin e (19). A solution of 18 (22.0 mg,
0.113 mmol) in 1.0 mL of anhydrous THF at 23 °C under N2
was treated with BOC2O (98.0 mg, 0.45 mmol) followed by
DMAP (2.0 mg, 0.016 mmol) and the mixture was stirred for
18 h at 23 °C. Removal of solvents under reduced pressure
followed by flash chromatography (SiO2, 1.5 × 15 cm, 20%
EtOAc-hexane) provided 19 (33.0 mg, 96%) as a pale tan oil:
1H NMR (CDCl3, 500 MHz) δ 7.00 (s, 1H), 3.98 (t, J ) 8.0 Hz,
2H), 3.01 (t, J ) 8.0 Hz, 2H), 2.64 (s, 3H), 1.56 (s, 9H); 13C
NMR (CDCl3, 125 MHz) δ 157.7, 155.9, 150.8, 132.3, 122.4,
1
Su p p or tin g In for m a tion Ava ila ble: Copies of H NMR
spectra of 5-hydroxy-3,6-bis(methylthio)-1,2,4-triazine, 5, 6, 8b,
8c, 10, 11, 12a + 12b mixture, 13a , 13c, 13e, 14, 15a -c, 16
and details of the X-ray structure of 13b (29 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
J O980795G