Reaction of 4-aryl-2-methylbut-3-yn-2-ols with thiourea as a new approach to the synthesis of 4-arylmethylidene-5,5-dimethyl-4,5-dihydrothiazole-2-amines

Article abstract of DOI:10.1007/s11172-012-0208-1

A reaction of thiourea with 4-aryl-2-methylbut-3-yn-2-ols in refluxing pyridine led to the synthesis of 4-arylmethylidene-5,5-dimethyl-4,5- dihydrothiazole-2-amines.

Full text of DOI:10.1007/s11172-012-0208-1

Russian Chemical Bulletin, International Edition, Vol. 61, No. 8, pp. 1570—1574, August, 2012
1570
Reaction of 4ꢀarylꢀ2ꢀmethylbutꢀ3ꢀynꢀ2ꢀols with thiourea
as a new approach to the synthesis
of 4ꢀarylmethylideneꢀ5,5ꢀdimethylꢀ4,5ꢀdihydrothiazoleꢀ2ꢀamines
D. S. Baranov,a V. I. Mamatyuk,b Yu. V. Gatilov,b and S. F. Vasilevskya
aInstitute of Chemical Kinetics and Combustion, Russian Academy of Sciences,
3 ul. Institutskaya, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 330 7350. Eꢀmail: vasilev@kinetics.nsc.ru
bN. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry,
Siberian Branch of the Russian Academy of Sciences,
9 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation
A reaction of thiourea with 4ꢀarylꢀ2ꢀmethylbutꢀ3ꢀynꢀ2ꢀols in refluxing pyridine led to the
synthesis of 4ꢀarylmethylideneꢀ5,5ꢀdimethylꢀ4,5ꢀdihydrothiazoleꢀ2ꢀamines.
Key words: alkynes, heterocyclization, thiourea, 2ꢀthiazolines.
Synthetic potential of acetylenes significantly broadens
ways arises on the regioselectivity of the process. In the
case of the reaction of alcohols 1 with thiourea, several
pathways of cycloaddition could have been expected
(Scheme 1).
The reaction of alcohols 1a,b with thiourea led to
4ꢀarylmethylideneꢀ5,5ꢀdimethylꢀ4,5ꢀdihydrothiazoleꢀ2ꢀ
amines 2a,b in 31 and 38% yields, respectively. In the case
of 1c, thiazoline 2c (37%) and 5,5ꢀdimethylꢀ4ꢀ(2,5ꢀdiꢀ
methylꢀ4ꢀnitrobenzylidene)thiazolidineꢀ2ꢀthione (3) (10%)
were formed (Scheme 2).
in the presence of acceptor substituents, especially, when
functional groups are placed close to the triple bond. Activatꢀ
ed alkynes in the reactions with nucleophilic agents can unꢀ
dergo various and unusual transformations. In fact, we have
observed such nontrivial rearrangements and heterocyclꢀ
izations earlier in the reactions of periꢀRꢀethynylꢀ9,10ꢀanthraꢀ
quinones with guanidine,1,2 including acetylenic alcohols.3
In continuation of our studies on the reactivity of
activated alkynes with polyfunctional nucleophilic agents,
we investigated a reaction of tertiary acetylenic alcohols
with thiourea.
The Xꢀray diffraction data not only unambiguously
indicated the structure of 2a as 5,5ꢀdimethylꢀ4ꢀ(4ꢀnitroꢀ
benzylidene)ꢀ4,5ꢀdihydrothiazoleꢀ2ꢀamine, but also its
Zꢀconfiguration (Fig. 1).
It should be pointed out that when addition reactions
of nucleophiles at triple bonds are studied, a question alꢀ
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1554—1558, August, 2012.
1066ꢀ5285/12/6108ꢀ1570 © 2012 Springer Science+Business Media, Inc.

Reaction of 2ꢀalkylꢀ4ꢀarylbutꢀ3ꢀynꢀ2ꢀoles
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
1571
Scheme 2
O(2)
N(1)
C(11)
N(3)
C(10)
C(2)
C(12)
N(2)
C(4)
S(1)
C(5)
O(1)
C(9)
C(7)
C(8)
C(13)
C(14)
Fig. 1. Molecular structure of 2a with 50% thermal ellipsoids of
nonhydrogen atoms.
The reaction is accompanied by the formation of large
amounts of poorly separable byꢀproducts, which made puꢀ
rification problematic and led to additional losses of the
final heterocycles 2a—c and 3. It is possible that the preꢀ
dominance of the secondary processes is caused by the
involvement of alkyne 1 in the reactions with numerous
products of thiourea destruction, which inevitably are
formed upon its prolonged heating.
The low solubility of thiourea in many organic solvents
limited their use, meanwhile, it is extremely necessary to
use excess of thiocarbamide. In this case, pyridine turned
out to be more appropriate medium than, for example,
aliphatic alcohols (MeOH, EtOH, BuOH).
The formation of the thiazoline ring can be representꢀ
ed by the addition of the NH2 group of thiourea at the
triple bond with subsequent cyclization of enamine A and
elimination of water (Scheme 3).
The intramolecular cyclization of the enamine interꢀ
mediate A (Scheme 4) resembles the transformations of
Nꢀ(ꢀhydroxyethyl)thioamides, in the course of which, as
it is known, a thiazoline ring is also closed.6
As it was previously mentioned, the thiazoline strucꢀ
ture 2a was reliably confirmed by Xꢀray crystallography.
Comparison of the spectral data (1H and 13C NMR) of
compounds 2a and 2b,c allowed us to state that these
compounds belonged to thiazolines. The signals for the
protons of the exoꢀ=CH—Ar moieties for all three thiazoꢀ
lines 2a—c lie in the narrow range: 5.56 (2a), 5.66 (2b),
and 5.55 ppm (2c).
The Xꢀray diffraction analysis of the crystalline sample
of 2a showed that the ethylidenedihydrothiazole fragment
is planar within 0.038 Å (see Fig.. 1). The plane of the
nitrophenyl group forms with the indicated plane the
angle of 15.97(8). The bond distances in the dihydrothiꢀ
azole ring are close to the analogous values in 2ꢀaminoꢀ4ꢀ
(1,2,2,2ꢀtetrafluoroethylidene)ꢀ5,5ꢀbis(trifluoromethyl)ꢀ
4,5ꢀdihydrothiazole4 and 1ꢀ[4ꢀ(1,2,2,2ꢀtetrafluoroethylꢀ
idene)ꢀ5,5ꢀbis(trifluoromethyl)ꢀ4,5ꢀdihydrothiazoleꢀ2ꢀyl]ꢀ
1Hꢀpyridinꢀ2ꢀone.5 A 3D architecture in the crystal was
formed by the hydrogen bonds N(1)—H...O(1) (H...O
2.12(3), 2.33(2) Å, N—H...O 164(2), 161(2)). Note also
the interactions C(9)—H...(thiazole) and C(11)—H...ꢀ
(phenyl) with the distances H—centroid of 2.87 and 2.89 Å.
It should be noted that for the medicinal chemistry,
compounds containing a thiazoline fragment are of signꢀ
ificant interest, since many of them are antibiotics.6
Scheme 3

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Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
Baranov et al.
Scheme 4
that the closure of the heterocycles 2 and 3 is effected by the
transformation of the common enamine intermediate A
by different pathways. Product 3 can be formed as a result of
cyclization of A to oxazolidinethione, which upon the
action of excess thiourea undergoes thiolation (Scheme 5).
Scheme 5
We also noted that the values of all the corresponding
chemical shifts for carbon atoms of thiazolines 2a—c fell
in the very narrow range in their 13C NMR spectra. Speꢀ
cial attention was drawn by the closeness of chemical shifts
for the key ꢀ and ꢀcarbon atoms at the double bond,
though this is the case when chemical shifts for the carbon
atoms at exoꢀ and endoꢀdouble bond should differ considꢀ
erably: 166.83 (=C<), 105.29 (=CH) for 2a, 166.76 (=C<),
105.68 (=CH) for 2b, and 166.12 (=C<), 102.40 (=CH)
for 2c. As it is seen, the differences for (=C<) are maximꢀ
um of 0.7 ppm, that is insignificant for the 13C NMR
spectra. The difference in the (=CH) values between 2a
and 2b is 0.39 ppm. A slightly larger  (~2 ppm) value for
2c was anticipated: this upfield shift of the signal can be
explained by the presence of two donor methyl groups on
the aromatic ring. Such a difference excludes an alternaꢀ
tive thiazine structure in going from the fiveꢀmembered
thiazoline with the exoꢀmethine fragment to the sixꢀmemꢀ
bered thiazine with the endoꢀmethine fragment. The  value
for the =CHꢀfragment would have been downfield shifted
by 25—30 ppm as compared to the observed values. As to
the recording of 2D NMR spectra, the 2Dꢀcorrelations on
the remote C—H constants for the double bond often are
ambiguous because of complicated dependence of the conꢀ
stant values through the double bond.
Moreover, all three alcohols 1a—c contain strong acꢀ
cepting substituents, they uniformly cause such a polarꢀ
ization when the deficiency of the electron density is
formed on the ꢀcarbon atom of the triple bond (relative
to the aryl substituent), which facilitates attack of the
nucleophile on this carbon atom. In going from comꢀ
pound 1a to acetylenic alcohols 1b,c with more bulky subꢀ
stituents, the steric hindrance from the side of this bulky
substituent makes the attack of nucleophile on the ꢀcarbon
atom (i.e., formation of thiazine) still less probable.
Thus, a combination of 1H and 13C NMR specꢀ
tral data for all the compounds and Xꢀray analysis for the
most representative compound 2a reliably confirms the
structure of products 2a—c as 4ꢀarylmethylideneꢀ5,5ꢀdiꢀ
methylꢀ4,5ꢀdihydrothiazoleꢀ2ꢀamines.
The step of the oxazolidine ring closure can be considꢀ
ered as a type of cyclization described in the works,7,8
since the intermediate A structurally resembles 1ꢀ(2ꢀhydrꢀ
oxyethyl)guanidine and 1ꢀ(2ꢀhydroxyethyl)ꢀ3ꢀnitroꢀ
guanidine.
The structure of compound 3 was confirmed by a comꢀ
bination of analytical and spectral data (IR, mass spectra,
1
and H, 13C NMR spectra). Heterocycles of the type 3
can exist as two tautomeric forms, thiol and thione.9
The presence in the 13C NMR spectra of the signal for
the carbon atom of the C=S group at  193.17 confirms
its thione structure, that also agrees with the presꢀ
1
ence in the H NMR spectra of compound 3 of the sigꢀ
nal for the proton of the NH group as a broad singlet at
 8.18.
The starting alcohols 1a—c were obtained by the crossꢀ
coupling of 2ꢀmethylbutꢀ3ꢀynꢀ2ꢀol with iodides 4a—c unꢀ
der standard conditions of the Sonogashira reaction10 in
91—96% yields (Scheme 6).
Thus, cyclocondensation of thiourea with 4ꢀarylꢀ2ꢀ
methylbutꢀ3ꢀynꢀ2ꢀols 1a—c in refluxing pyridine leads
to yield 4ꢀarylmethylideneꢀ5,5ꢀdimethylꢀ4,5ꢀdihydroꢀ
thiazoleꢀ2ꢀamines 2a—c. The discovered reaction opens
a new approach to the synthesis of substituted 2ꢀaminoꢀ2ꢀ
thiazolines.
The IR spectra of 2a—c contain bands of stretching
vibrations of the NH2 group (3479—3303 cm–1), as well
as strong bands of stretching vibrations of the C=Nꢀbond
of the thiazoline ring (1533—1492 cm–1).
In the reaction of alcohol 1c with thiourea, besides
a predominant thiazoline 2c (37%), a minor product, thiꢀ
azolidinethione 3 (10%), was also formed. It is possible

Reaction of 2ꢀalkylꢀ4ꢀarylbutꢀ3ꢀynꢀ2ꢀoles
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
1573
Scheme 6
formed was subjected to chromatography on Al2O3 in toluene.
The yield was 31%, red crystals, m.p. 212—213 C (toluene).
Found (%): C, 54.89; H, 4.86; N, 15.78; S, 12.21. C12H13N3O2S.
Calculated (%): C, 54.74; H, 4.98; N, 15.96; S, 12.18. 1H NMR
(CDCl3), : 1.73 (s, 6 H, 2 Me); 5.33 (br.s, 2 H, NH2); 5.56 (s, 1 H,
CH=); 7.95 (td, 2 H, Hm, J = 1.80 Hz, J = 2.45 Hz, J = 8.90
Hz); 8.13 (td, 2 H, Ho, J = 1.80 Hz, J = 2.45 Hz, J = 8.90 Hz).
13C NMR (CDCl3), : 31.74 (2 Me); 66.72 (CMe2); 105.29
(CH=); 123.74 (Cm); 128.38 (Co); 144.64, 145.02 (Carom); 166.83
(=C<); 167.37 (CNH2). HRMS, found: m/z 263.0722 [M]+.
C12H13N3O2S. Calculated: M = 263.0723. IR, /cm–1: 3479,
3329 (NH2); 1317, 1516 (NO2); 1492 (C=N).
The reactions of acetylenic alcohols 1b and 1c were perꢀ
formed similarly, in the second case, thione 3 was isolated by
chromatography together with amine 2c.
4ꢀ[(9,10ꢀAnthraquinonꢀ2ꢀyl)methylidene]ꢀ5,5ꢀdimethylꢀ4,5ꢀ
dihydrothiazoleꢀ2ꢀamine (2b). The yield was 38%, red crystals,
m.p. 207—208 C (toluene). Found (%): C, 68.98; H, 4.91;
N, 7.65; S, 9.25. C20H16N2O2S. Calculated (%): C, 68.94;
H, 4.63; N, 8.04; S, 9.20. 1H NMR (CDCl3), : 1.75 (s, 6 H, 2 Me);
5.47 (br.s, 2 H, NH2); 5.66 (s, 1 H, CH=); 7.77 (m, 2 H, H(3),
H(4)); 8.27 (m, 4 H, H(5), H(6), H(7), H(8)); 8.67 (s, 1 H,
H(1)). 13C NMR (CDCl3), : 31.79 (2 Me); 66.70 (CMe2); 105.68
(CH=); 126.67, 127.21, 127.23, 127.60, 129.98, 133.31, 133.61,
133.76, 133.94, 134.08, 134.20, 144.50 (Carom); 166.76 (=C<);
167.40 (CNH2); 182.93, 184.15 (2 C=O). HRMS, found: m/z
348.0930 [M]+. C20H16N2O2S. Calculated: M = 348.0927. IR,
/cm–1: 3344, 3169 (NH2); 1666 (C=O); 1533 (C=N).
Experimental
Elemental analysis was performed on a Carlo Ebra 1106
CHNꢀanalyzer (Italy). IR spectra were recorded on a Bruker
Vector 22 spectrometer in KBr pellets. NMR spectra were reꢀ
corded on a Bruker AV 400 spectrometer in CDCl3 (400.13 (1H)
and 100.61 MHz (13C)). Mass spectra were obtained on a Therꢀ
mo Electron Corporation DFS mass spectrometer (70 eV), using
direct injection, the temperature of the ionization chamber was
220—270 C. Alumina (50—150 m, TU 6ꢀ09ꢀ3916ꢀ75) was used
for column chromatography, TLC monitoring was carried out
on Silufol 60 F254 plates.
2ꢀMethylꢀ4ꢀ(4ꢀnitrophenyl)butꢀ3ꢀynꢀ2ꢀol (1a). A mixture of
1ꢀiodoꢀ4ꢀnitrobenzene (4a) (2.2 g, 9 mmol), 2ꢀmethylbutꢀ3ꢀynꢀ
2ꢀol (0.8 g, 9 mmol), PdCl2(PPh3)2 (20 mg, 0.028 mmol), CuI
(10 mg, 0.052 mmol), and Et3N (7 mL, 38.6 mmol) in toluene
(60 mL) was stirred for 1.5 h at 60 C under argon. The reaction
mixture was cooled and filtered through Al2O3 (d = 25 mm,
h = 20 mm), eluted with toluene. The solvent was evaporated
in vacuo. The compound 1a (1.77 g, 96%) was isolated by subseꢀ
quent recrystallization from a mixture of toluene with hexane,
m.p. 104—105 C (see Ref. 11: m.p. 104.5—105 C (CCl4)).
The alcohol 1b was obtained similarly. The yield was 92%,
m.p. 133—134 C (toluene—hexane; see Ref. 12: m.p. 134—135 C
(benzene)).
4ꢀ(2,5ꢀDimethylꢀ4ꢀnitrophenyl)ꢀ2ꢀmethylbutꢀ3ꢀynꢀ2ꢀol (1c)
was obtained similarly. The yield was 91%, yellow crystals, m.p.
52—53 C (hexane—toluene). Found (%): C, 67.02; H, 6.44; N,
5.82. C13H15NO3. Calculated (%): C, 66.94; H, 6.48; N, 6.00.
1H NMR (CDCl3), : 1.64 (s, 6 H, 2 Me); 2.15 (s, 1 H, OH);
2.42 (s, 3 H, mꢀMe); 2.52 (s, 3 H, oꢀMe); 7.32 (s, 1 H, Hm); 7.82
(s, 1 H, Ho). 13C NMR (CDCl3), : 20.09 (mꢀMe); 20.13
(oꢀMe); 31.53 (2 Me); 65.86 (CMe2); 79.52 (—C); 101.89 (C—);
125.50 (Cm); 128.03 (Cp); 130.97 (Cm—Me); 135.94 (Co); 139.16
(Co—Me); 147.99 (C—NO2). IR, /cm–1: 3460 (OH); 2980,
2931, 2868 (Me); 2226 (CC); 1512, 1335 (NO2).
Zꢀ5,5ꢀDimethylꢀ4ꢀ(4ꢀnitrobenzylidene)ꢀ4,5ꢀdihydrothiazoleꢀ
2ꢀamine (2a). A mixture of 1a (1 g, 4.8 mmol) and thiourea (3.6 g,
48 mmol) in pyridine (24 mL) was refluxed for 9 h. A cooled
reaction mixture was diluted with CH2Cl2 (200 mL) and washed
with H2O (2×250 mL). The organic layer was separated, dried
with Na2SO4, the solvent was evaporated in vacuo, a precipitate
4ꢀ(2,5ꢀDimethylꢀ4ꢀnitrobenzylidene)ꢀ5,5ꢀdimethylꢀ4,5ꢀdiꢀ
hydrothiazoleꢀ2ꢀamine (2c). The yield was 37%, yellow crystals,
m.p. 210—211 C (toluene). Found (%): C, 57.91; H, 5.87;
N, 14.07; S, 11.07. C14H17N3O2S. Calculated (%): C, 57.71;
1
H, 5.88; N, 14.42; S, 11.00. H NMR (CDCl3), : 1.74 (s, 6 H,
2 Me); 2.34 (s, 3 H, mꢀMe); 2.60 (s, 3 H, oꢀMe); 5.40 (br.s, 2 H,
NH2); 5.55 (s, 1 H, CH=); 7.84 (s, 1 H, Hm); 8.20 (s, 1 H, Ho).
13C NMR (CDCl3), : 19.81 (mꢀMe); 21.17 (oꢀMe); 32.09
(2 Me); 66.13 (CMe2); 102.40 (CH=); 126.23 (Cm); 131.57
(Cm—Me); 132.69 (Co); 133.71 (Co—Me); 142.13 (Cp); 145.37
(CNO2); 166.12 (=C<); 166.39 (CNH2). IR, /cm–1: 3420, 3303
(NH2); 2927, 2920, 2858 (Me); 1537, 1325, (NO2); 1492 (C=N).
4ꢀ(2,5ꢀDimethylꢀ4ꢀnitrobenzylidene)ꢀ5,5ꢀdimethylthiazolꢀ
idineꢀ2ꢀthione (3). The yield was 10%, white crystals, m.p.
228—230 C (toluene—ethyl acetate). Found (%): C, 54.80;
H, 5.31; N, 8.91; S, 20.85. C14H16N2O2S2. Calculated (%):
C, 54.52; H, 5.23; N, 9.08; S, 20.79. 1H NMR (CDCl3), : 1.69
(s, 6 H, 2 Me); 2.31 (s, 3 H, mꢀMe); 2.59 (s, 3 H, oꢀMe); 6.40
(s, 1 H, CH=); 7.22 (s, 1 H, Hm); 7.85 (s, 1 H, Ho), 8.18 (br.s, 1 H,
NH). 13C NMR (CDCl3), : 19.31 (mꢀMe); 20.47 (oꢀMe); 29.68
(2 Me); 72.41 (CMe2); 115.15 (CH=); 126.51 (Cm); 131.35 (Co);
131.91 (Cm—Me); 135.27 (Co—Me); 140.00 (Cp); 147.68 (CNO2);
147.96 (=C<); 193.17 (C=S). HRMS, found: m/z 308.0645 [M]+.
C14H16N2O2S2. Calculated: M = 308.0653. IR, /cm–1: 3443,
3126 (NH); 2982, 2930, 2851 (Me); 1516, 1338 (NO2); 1034
(C—N).
Xꢀray diffraction experiment was performed on a Bruker
KAPPA APEX II CCD diffractometer (graphite monochromator,
(MoꢀK) = 0.71073 Å, temperature 150(2) K, ,ꢀscan techꢀ
nique). Monoclinic crystal system, C12H13N3O2S, M = 263.31,
space group P212121, a = 6.8897(5) Å, b = 7.6734(5) Å,
c = 23.5513(17) Å, V = 1245.10(15) Å3, Z = 4, dcalc = 1.405 g cm–3
 = 0.258 mm–1. Intensities of 24568 reflections (2 < 60) were
,

1574
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
Baranov et al.
measured for the sample of 0.06×0.12×0.20 mm in size, from
them 3644 were independent reflections (Rint = 0.0407) and
3169 reflections with I > 2(I), number of refined parameters
173, R1(I > 2(I)) = 0.0328, wR2 = 0.0796 and GOOF = 1.038
(on all the reflections). Allowance for absorption was made usꢀ
ing the SADABS program (Tmin/Tmax = 0.8985/0.9703). The
structure was solved by direct method. Positions and temperaꢀ
ture parameters of nonhydrogen atoms were refined in isotropic
and then in anisotropic approximation by fullꢀmatrix least squares
method. The hydrogen atoms of the amino group were localized
from the differential syntheses and refined in isotropic approxiꢀ
mation, other H atoms were refined using the riding model. All
the calculations were performed using the SHELXTL software.
Atomic coordinates and their temperature parameters were
deposited with the Cambridge Structural Database (CCDC
854823).
3. (a) D. S. Baranov, S. F. Vasilevsky, V. I. Mamatyuk, Y. V.
Gatilov, Mendeleev Commun., 2009, 19, 326; (b) D. S. Baraꢀ
nov, A. A. Ryabichev, V. I. Mamatyuk, Y. V. Gatilov, V. G.
Kartsev, S. F. Vasilevsky, Mendeleev Commun., 2012, 22, 114.
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skaya, Russ. Chem. Bull. (Int. Ed.), 2001, 50, 1072 [Izv. Akad.
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109, 1371.
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This work was financially supported by the Russian
Foundation for Basic Research (Project No. 10ꢀ03ꢀ
00257a) and the Chemical Service Center of the Siberian
Branch of the Russian Academy of Sciences.
11. M. S. Shvartsberg, A. A. Moroz, I. L. Kotlyarevsii, Izv. Akad.
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Received December 6, 2011;
in revised form June 1, 2012