(Pyrrolꢀ1ꢀyl)furazans
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1417
Compounds 2b—e and 2g—k were obtained analogously.
The yields and the physicochemical properties of the products
are given in Tables 1 and 2.
4ꢀ(Pyrrolꢀ1ꢀyl)furazancarboxylic acid (2f). 2,5ꢀDimethoxyꢀ
tetrahydrofuran (1.32 g, 0.01 mol) was added to a stirred suspenꢀ
sion of 4ꢀaminofurazancarboxylic acid 1f (1.29 g, 0.01 mol) in
5 mL of AcOH. The resulting mixture was refluxed for 15 min,
cooled, and concentrated under reduced pressure. The residue
was recrystallized from PriOH—water (9 : 1).
compared to an average value of 1.42 Å found in the CCD
structures. Insofar as the C(6)—C(7) bond (1.504(3) Å) is
ordinary, one can conclude that the ester group is not
involved in conjugation.
As noted earlier,22 the N—O bond lengths in the
furazan ring depend on the character of its substituents.
The N—O bond nearest to a donor substituent is lengthꢀ
ened, while that nearest to an acceptor substituent is shortꢀ
ened. Indeed, the N—O bond lengths in compound 2g
are 1.402(2) Å on the pyrrole side and 1.375(2) Å on the
ester side.
In crystal, molecules form layers running parallel to
the crystallographic plane (100). Neighboring layers are
symmetric about one of the two crystallographically
nonequivalent centers of inversion; for this reason, two
different types of superposition of neighboring layers are
present in crystal. For one type, the distance between
layers is 3.21 Å, while for the other, 3.25 Å. However,
layer superposition is shifted in both cases to give no
shortened contacts between molecules in neighboring
layers.
The O(3) atom is involved in two hydrogen bonds,
namely, intramolecular O(3)...H(1)—C(1) and intermoꢀ
lecular O(3)…...H(4)—C(4), with the following bond paꢀ
rameters: O...H 2.11(2) and 2.39(2) Å, O...C 2.970(3)
and 3.346(3) Å, and the angle C—H—O 147(2) and
161(2)°, respectively. Molecules in layers are united
through intermolecular hydrogen bonding into chains
symmetric about a plane of tangential reflection c.
Hence, the ClausonꢀKaas reaction was found to be
an efficient tool for the synthesis of 3ꢀRꢀ4ꢀ(pyrrolꢀ1ꢀ
yl)furazans. Note that an amino group at a furazan ring
was involved for the first time in construction of a heteroꢀ
cycle.
Reaction of 3ꢀaminoꢀ4ꢀ(formylamino)furazan (1n) with
2,5ꢀdimethoxytetrahydrofuran. 2,5ꢀDimethoxytetrahydrofuran
(1.32 g, 0.01 mol) was added to a stirred suspension of 3ꢀaminoꢀ
4ꢀ(formylamino)furazan 1n (1.28 g, 0.01 mol) in 5 mL of AcOH.
The resulting mixture was refluxed for 15 min, cooled, and
concentrated under reduced pressure. The residue was treated
with CH2Cl2 (30 mL). The insoluble precipitate that formed
was filtered off, washed with CH2Cl2 (3×5 mL), and dried in air
to give 4ꢀ(formylamino)ꢀ3ꢀ(pyrrolꢀ1ꢀyl)furazan (2n) (0.34 g, 19%)
as an amorphous light beige powder, m.p. 142—145 °C, Rf 0.4
(CCl4—MeCN, 3 : 1). Combined mother liquors in CH2Cl2
were washed with 5% NaHCO3 (20 mL) and water (20 mL),
dried over MgSO4, and concentrated in vacuo. The residue was
chromatographed on SiO2 (L100/160 µm) in CH2Cl2 to give
3,4ꢀdi(pyrrolꢀ1ꢀyl)furazan (1m) (0.18 g, 9%) as an oil, Rf 0.9
(CH2Cl2). After compound 1m was isolated, elution was continꢀ
ued in CCl4—MeCN (3 : 1) to give an additional amount of
compound 2n (0.21 g, 11.8%).
4ꢀMethylꢀ3ꢀ(pyrrolꢀ1ꢀyl)methylfurazan (4). Acetic acid
(3.5 mL) was added to a mixture of a hydrochloride of comꢀ
pound 3 (0.41 g, 0.003 mol) and NaOH (0.12 g, 0.003 mol) in
2 mL of water. The resulting solution was stirred at ∼20 °C for
10 min; then 2,5ꢀdimethoxytetrahydrofuran (0.4 g, 0.003 mol)
was added and the reaction mixture was refluxed for 15 min,
cooled, and concentrated in vacuo. The residue was dissolved in
CH2Cl2, washed with 5% NaHCO3 (20 mL) and water (20 mL),
and dried over MgSO4. The solvent was removed, and the resiꢀ
due was dissolved in CHCl3 and passed through a thin layer of
silica gel. The resulting solution was concentrated in vacuo to
give compound 4 (0.42 g, 70%) as a light yellow liquid. 1H NMR
(CDCl3), δ: 2.11 (3 H, Me); 5.24 (CH2); 6.22 (C(3)H); 6.68
(C(4)H). 13C NMR (CDCl3), δ: 7.4 (Me); 41.8 (CH2); 109.5
(C(4)); 120.8 (C(3)); 150.4 (C(1)); 151.2 (C(2)).
Experimental
Melting points were determined on a Kofler stage. Naturalꢀ
isotope 1H, 13C, and 14N NMR spectra were recorded on a
Bruker AMꢀ300 spectrometer (300.13, 75.7, and 21.5 MHz,
respectively). Mass spectra were recorded on Varian MAT CHꢀ6
and Varian MAT CHꢀ111 instruments (70 eV). IR spectra were
recorded on a Specord IR75 spectrometer (in pellets with KBr
for solids and in thin film for liquids). The course of the reaction
was monitored and the purity of products was checked by TLC
on Silufol UVꢀ254 plates; silica gel was used for preparative
chromatography.
3ꢀMethylꢀ4ꢀ(pyrrolꢀ1ꢀyl)furazan (2a). 2,5ꢀDimethoxytetraꢀ
hydrofuran (1.32 g, 0.01 mol) was added to a stirred solution of
3ꢀaminoꢀ4ꢀmethylfurazan 1a (0.99 g, 0.01 mol) in 3 mL of
glacial AcOH. The resulting mixture was refluxed for 15 min,
cooled, and concentrated under reduced pressure. The residue
was dissolved in CH2Cl2 (20 mL) and washed with aqueous
5% K2CO3 (20 mL) and water (2×15 mL). The organic layer was
dried with MgSO4, filtered through a thin layer of silica gel, and
concentrated. The residue was recrystallized from hexane.
Xꢀray diffraction analysis. The crystals of compound 2g at
110 K are monoclinic, space group C2/c, a = 15.161(6) Å, b =
9.819(4) Å, c = 13.368(6) Å, β = 121.52(1)°, V = 1696(1) Å3,
Z = 8, M = 193.17, dcalc = 1.513 g cm–3, µ(MoꢀKα) =
0.119 mm–1, F(000) = 800. The intensities of 6738 reflections
were measured on a Bruker SMART 1000 CCD diffractometer
(λ(MoꢀKα) = 0.710712 Å, ω scan mode for ω = 0.4° and a
20ꢀs exposure for each frame, 2θ < 60°); 2441 independent reꢀ
flections were used in refinement (Rint = 0.0497). Data processꢀ
ing was performed with the SAINT program.23 The structure
was solved by the direct method and refined by the fullꢀmatrix
leastꢀsquares method in the anisotropic approximation. The
H atoms were located from a difference electronꢀdensity map and
refined isotropically. Final discrepancy factors are wR2 = 0.1023
for all independent reflections and R1 = 0.0487 for 1020 reflecꢀ
tions with I > 2σ(I ). All relevant calculations were performed
with the SHELXTL PLUS program package.24 The atomic coꢀ
ordinates and thermal parameters, as well as the bond lengths
and angles, have been deposited with the CCD.
Sheremetev
